Coastal and dune evolution in south east India revealed by optically stimulated luminescence dating

Transcription

1 Coastal and dune evolution in south east India revealed by optically stimulated luminescence dating Reconstruction of sediment dynamic, event history, climatic and environmental change for the last 3500 years Der Fakultät Umwelt und Technik der Leuphana Universität Lüneburg zur Erlangung des Grades Doktor der Naturwissenschaften - Dr. rer. nat. - vorgelegt von Jan-Alexander Kunz geb in Ückermünde 1

2 1. Gutachter und Betreuer: Prof. Dr. Brigitte Urban 2. Gutachter: Prof. Dr. Manfred Frechen Eingereicht am: Tag der Disputation:

3 Ausdauer wird früher oder später belohnt......meistens aber später. Wilhelm Busch 3

4 Table of contents List of figures List of tables Summary Chapter 1 Introduction 1.1. Background and aims of the study Method Dating methods Numerical dating methods Relative dating methods Luminescence dating Physical background of luminescence dating Measurement procedures Quality checks of the OSL SAR measurements Dosimetry Age calculation Outline of the thesis References Chapter 2 Revealing the coastal event-history of the Andaman Islands (Bay of Bengal) during the Holocene using radiocarbon and OSL dating Abstract Introduction Geological setting Red Skin Island North Cinque Island (section 1) North Cinque Island (section 2) Methods Optically stimulated luminescence dating

5 Gamma spectrometry: sample preparation and measurement Radiocarbon dating Results Discussion Red Skin Island North Cinque Island, section North Cinque Island, section Conclusions Acknowledgments References Chapter 3 Luminescence dating of Late Holocene dunes showing remnants of early settlement in Cuddalore and evidence of monsoon activity in south east India Abstract Introduction Regional setting Materials and methods Sample preparation Dosimetry Water content OSL measurement and experimental details Age calculation Results Discussion Correlation of dunes Discussion of results and interpretation Climate and human impact Sea level change Conclusion Acknowledgements References Chapter 4 Periods of recent dune sand mobilisation in Cuddalore, south east India Abstract Kurzfassung

6 4.1. Introduction Regional setting / geological setting Materials and methods Sample preparation OSL measurement and experimental details Dosimetry and water content Age calculation Results Discussion Conclusions Acknowledgements References Chapter 5 Conclusions Acknowledgements Curriculum Vitae List of publications Selbständigkeitserklärung Erklärung

7 List of figures 1.1 Location of the study areas in India Age range of the most common dating methods Dating methods and the suitable material With optically stimulated luminescence dated event Illustration of the processes in the crystal leading to luminescence MAAD and SAR growth curves Luminescence signals of quartz and feldspar detected during optical stimulation Idealized diagrams from a preheat plateau test and a thermal transfer test Gamma spectrum of a natural sample from south east India Decay chains for 238 U, 232 Th and 40 K Attenuation of cosmic dose with depth in dependence of latitude and altitude Map showing the location of the Andaman Islands Lithological description of the sediment sequence from Red Skin Island Eroded shells from a Stromboidea and an oyster found at the section on Red Skin Island Upper part of the section on Red Skin Island with possible tsunami layers Lithological description of section 1 from North Cinque Island Upper part of section 1 on North Cinque Island Lithological description of section 2 from North Cinque Island Section 2 on North Cinque Island Results for the preheat test and recycling ratios for samples LUM 999 and LUM Results of the dose recovery tests for samples LUM 999 and LUM Summary of the dating results from sections on Red Skin and North Cinque Island Grain-size distribution of samples from Red Skin Island, North Cinque Island section 1 and North Cinque Island section Map showing the distribution of important dune areas in India Study area between Cuddalore and Porto Novo Sections of the Temple-Dune and Sherd-Dune Upper part of the Temple-Dune with beds DS1, DS2 and DS3a Locations of the sections 1 and 2 in the Sherd-Dune

8 3.6 Detailed view of section 1 from the Sherd-Dune Detail of the event layer DS4 in section 2 of the Sherd-Dune Dose distributions of the investigated samples Results of the preheat plateau test for samples from the Temple-and Sherd-Dune Results of the dose recovery test for samples from the Temple- and Sherd-Dune Comparison of the results of this study with other studies about the monsoon history in India Distribution of dune areas in India and the location of the study area Geomorphological map of the area between Cuddalore and Porto Novo Section of the Palm-Dune, Charburner-Dune and Little Sister Lower part of the section in the Palm-Dune showing a cross-bedded layer Section in the Charburner-Dune Detail of the water escape structure in the Charburner-Dune section Section in the Little Sister dune Luminescence properties of the investigated samples OSL ages of this study compared with the rainfall during the instrumental period at Madras Observatory

9 List of tables 1.1 Generalized single aliquot regenerative (SAR) dose protocol Energies used to calculate the activity of 238 U and 232 Th Results of the OSL dating and values used to calculate the age SAR protocol for feldspar used for the preheat tests SAR protocol for feldspar used for the measurement of both luminescence samples Results of the radiocarbon dating from the sections on South Andaman Island Results of the gamma spectrometry, OSL measurements and the age calculation Results for the dose recovery test Single aliquot regenerative (SAR) protocol used for the measurement of the samples Single aliquot regeneration (SAR) protocol used for the measurement of the samples Statistical parameters for the samples from the Palm-Dune, Charburner-Dune and Little-Sister dune Results of the gamma spectrometry and the age calculation

10 Summary Sediments in drylands or coastal areas are good archives for palaeoenvironmental studies. They preserve and reflect changes in climate, events like storms or tsunamis and human impact. The application of dating methods allows establishing a geochronological framework which can be used, e.g., to correlate periods of dune and soil formation to past dry and wet climate conditions or to estimate a recurrence interval for events like storms, tsunamis and earthquakes. The investigation of coastal areas is important as coasts are one of the most dynamic landforms and in many parts of the world densely populated. The impact of events like storms and tsunamis or climatic change has severe consequences on ecosphere, geosphere and anthroposphere. The knowledge about coastal processes and their timing can help in developing plans for coastal protection and risk assessment. The aim of this study was to establish a reliable chronological framework for the coastal development of the Andaman Islands and south east India. This framework was used to understand the timing of coastal processes and to reconstruct the genesis of sediments and connect them with events which are indicative for environmental and climatic changes. The dating was done using optically stimulated luminescence (OSL) and radiocarbon. Based on the physical properties of the OSL dating method it is an excellent tool to determine the depositional age of sediments. On the Andaman Islands coastal sediments have been investigated to find evidence for palaeotsunamis and palaeoearthquakes and to reconstruct the recurrence interval for strong events like the Indian Ocean tsunami from December Sediment material from event-layers was dated using OSL and radiocarbon dating. The results show evidence for strong earthquakes at around 1000 and 3000 years before present and they reflect the storm activity for the last 1000 years. Also the complex pattern of co- and postseismic uplift and subsidence of the Andaman Islands could be reconstructed. At the south east coast of India dunes in the Cuddalore area have been investigated. These dunes form a belt running parallel to the coast. A transect from the coast to the most western dune inland was investigated. The dunes show sedimentological features like unconformities, changes in the direction of bedding, erosional features, water escape structures and remnants of human settlement and soil-like horizons which are indicative for environmental changes. The results from the dunes show a connection between the monsoon activity for the last 3500 years and periods of sand movement and stabilisation of dunes. The younger dunes show a connection between periods of reduced rainfall and sand mobilisation for the last 200 years. The investigation of the younger dunes shows also that the dune system in the study area reacts very sensitive to changes in rainfall and disturbances in the landscape. Based on the dating and sedimentological results of this thesis it can be concluded that the coastal areas are very dynamic with rapidly occurring environmental changes. This research draws a clearer picture of the dynamics of coastal environments in south east India and the Andaman Islands and thus provides useful information for coastal zone management planning. 10

11 Chapter 1 Introduction 1.1. Background and aims of the study This thesis is part of the interdisciplinary research project Ecological inter-linkages between terrestrial and coastal environments in the Indian Western Pacific Region funded by the German Federal Ministry of Education and Research (BMBF) and the Indian Department of Science and Technology (DST). The central aim of this research project is the investigation of the impact of environmental changes on coastal areas due to geogenic processes or human influence. In this study sediments which can act as archives for past events, from the coasts of the Andaman Islands (Bay of Bengal in the Indian Ocean) and from south east India were investigated. These deposits were lithologically studied and dated using radiocarbon and optically stimulated luminescence (OSL) dating to establish a more reliable chronological frame. This frame was used to reconstruct the development of the sediments and connect them with events which are indicative for environmental and climatic changes. Coastal areas are one of the most dynamic landforms. They are influenced by the interaction of the sea and human activities. Changes occur on different time scales. Sea level rise or fall takes hundreds to thousands of years (Siddall et al. 2007) and the ecosystems along the coasts have enough time to adjust to the changing conditions. On the other hand storms, tsunamis or earthquakes occur over a very short time with a strong impact on the coast. They alter the coast by erosion, e.g. removing of spits, small barrier islands, coastal dunes, vegetation or buildings. The eroded sediment is deposited along the coast over wide areas forming new geomorphic features, such as sand sheets or sand bars. Flooding of low lying areas can create new lagoons or disturb the hydrology when saltwater inundates into groundwater (Illangasekare et al. 2006; Kitagawa et al. 2006; Mascarenhas and Jayakumar 2008; Morton 1988; Nelson and Leclair 2006; Pari et al. 2008). Ground movement due to an earthquake can lead to emergence or submergence of mangroves or coral reefs and destroy them (Bahuguna et al. 2008; Ray and Acharyya 2007). In densely populated areas the impact of events like storms, tsunamis or earthquakes cause severe damage with loss of many lives and economic loss as seen by the M w 9.3 earthquake and the resulting tsunami December 2004 in the Bay of Bengal (Dasgupta 2007; Papadopoulos et al. 2006; Satake et al. 2006). The study of coastal areas helps to understand the processes during and after the impact of events and the aftermath on geosphere, ecosphere and anthroposphere. This knowledge can be used to develop plans for coastal protection and risk assessment. To understand long term processes a chrono- 11

12 logical frame for the coastal deposits is essential. This will give information about the recurrence of events like earthquakes and tsunamis but also about the influence of climate change and human impact. Besides the direct impact of events, long-term climatic changes are also important. Coastal dunes and forests play an important role in protecting the coastal areas against storms or tsunamis (Danielsen et al. 2005; Kathiresan and Rajendran 2005; Mascarenhas and Jayakumar 2008). Eyewitnesses reported that in south east India coastal dunes acted as a barrier against the inundating wave and protected the hinterland. Stabilisation and preservation of the dunes is sensitive to changes in precipitation and vegetation cover. As India receives most of the annual rainfall from the SW-monsoon (Rao 1981), the failure or strengthening of monsoon activity will have strong impact on ecology, geomorphology and economy (Mooley and Parthasarathy 1983). Natural barriers (mangroves, forests, dunes) will be disturbed or destroyed. Two areas in India with different geological environments are investigated (Fig. 1.1). On the Andaman Islands coastal sediments are studied which can give evidence for storms, tsunamis, earthquakes and sea level changes. In Cuddalore (Tamil Nadu) coastal dunes are investigated. Dunes are archives for palaeoclimate changes, sea level changes if they are situated close enough to the coast and they reflect human activities. In many geological environments around the world dating methods have been applied to study palaeoclimate and palaeoenvironment. For example in Australia dunes are investigated to reconstruct the climate over the last 70 ka (Fitzsimmons et al. 2007) or sea level changes (Murray-Wallace et al. 2002). In North America dunes are investigated to reconstruct droughts and the Figure 1.1: Location of the two study areas in India. Area 1 is in Cuddalore (see chapters 3 and 4). Area 2 is on South Andaman Island (see chapter 2). Black dots show locations of capital cities. 12

13 influence of human activities over the last 200 years (Forman et al. 2001; Wolfe et al. 2001). In Europe dunes are used to reconstruct sea level changes (Clemmensen et al. 2001; Madsen et al. 2007). In India research was concentrated on dunes in the Thar Desert to investigate the sedimentary dynamics, sand accumulation rates, and palaeoclimate reconstruction (Chawla et al. 1992; Juyal et al. 2003; Singhvi et al. 1994; Singhvi et al. 1982; Thomas et al. 1999). The reconstruction of monsoon fluctuations and flood events in India using sediment archives was done by Kale (1999), Kale et al. (2000, 2004), Nigam and Khare (1994), Prasad and Enzel (2006) and Thomas et al. (2007). Detailed chronological investigations of coastal sediments are not done yet at the east coast of India and the Andaman Islands. Coastal deposits are mostly assigned as Holocene based on the geomorphic position of them between Late Holocene palaeostrandlines and the present sea level. In several studies the palaeostrandlines are dated with radiocarbon. This was done for the Godavari Delta and the Krishna Delta by Brückner (1988), Mahender Reddy and Shah (1991) and Nageswara Rao (2006) and giving ages of maximum 6.5 ka BP. For the Cauvery Delta radiocarbon ages of beach ridges are only available from the south with a maximum age of 6 ka BP (Ramasamy 2006). Ages for the northern part, where one of the study areas is located, are missing. This thesis provides new data and interpretation about the genesis of coastal sediments, changes in monsoon intensity during the Holocene, sea level changes, the recurrence of tsunamis and earthquakes and the influence of human activities in Tamil Nadu and on the Andaman Islands in coastal areas. As the study area in Cuddalore is one of the most affected areas of the tsunami 2004 in India (Srinivasan and Nagarajan 2007), this study will also help to provide data about the timing of coastal processes which can help in developing strategies in coastal and environmental protection plans Method There is a great variety of different dating methods available for dating deposits from the Quaternary. These methods differ in their principle mechanism and the dateable material. Figures 1.2 and 1.3 show the most important methods used for dating, the age ranges they cover and which material is suitable for dating. In general there are two groups of dating methods: numerical and relative methods. Numerical methods are based on counting of repeated physical or chemical processes with known duration. These can be e.g. atomic disintegrations with known half lifes or annual layers in trees, lake sediments or ice. Numerical dating methods give reliable numerical ages. Relative methods do not give numerical ages; they give information what is older or younger in relation to each other. For example in lithostratigraphy the succession of beds starts from the oldest at the bottom and ends with the youngest on top. With dating of samples from these sediment layers using numerical methods an age can be obtained and a chronostratigraphy established. 13

14 Figure 1.2: Age range of the most common dating methods. Hatched areas indicate age ranges where the method has limitations or is in experimental use. Dating methods are explained in chapter The geological time scale is based on the International Stratigraphic Chart published by the International Commission on Stratigraphy in the year ESR = Electron spin resonance, tpl = Pliocene, tmi = Miocene, a = year, ka = thousand years, Ma = million years. Age ranges are based on Bradley (1998), Geyh (2005) and Wagner (1995). Figure 1.3: Dating methods and the suitable material. ESR = Electron spin resonance, AAR = Amino acid racemisation. Figure modified from Aitken (1998) Dating methods In this chapter the most common dating methods will be introduced shortly. Detailed descriptions about the principles, usable material, limitations and age ranges can be found in many textbooks e.g. Bradley (1999), Geyh (2005), Wagner (1995) or publications. A detailed introduction about luminescence dating, which is used in this thesis, will be given in chapter

15 Numerical dating methods Radiocarbon The radiocarbon dating method is based on disintegration of 14 C which is constantly produced in the atmosphere. Living organisms incorporate the radioactive 14 C. The ratio of 14 C to 12 C in the atmosphere and hence in the living organism is the same. After death of an organism no new 14 C is incorporated and the remaining 14 C starts to decay. From the activity of 14 C in organic material the age can be calculated which equals the time since the organism is dead. The age range for radiocarbon dating is from 350 years to years (Geyh 2005). Uranium Series Dating This dating method is based on the disintegration of 235 U and 238 U and disturbances in their decay chains. Normally the decay chain is in equilibrium which means that all daughters have the same activity as the parent nuclide. If this equilibrium is disturbed it needs some time to establish again. The difference between the activities in the disturbed decay chain and in the decay chain in equilibrium is used to calculate the time elapsed when the disturbance occurred. For uranium series dating different nuclide ratios are used: 230 Th/ 234 U, 231 Pa/ 235 U, 234 U/ 238 U and 230 Th/ 231 Pa. This method is applicable to a variety of materials which contain at minimum 0.1 µg g -1 uranium. These are e.g. corals, stalagmites, stalactites, travertine, bones, teeth, shells and peat. The age range depends on the nuclides used and ranges from a few hundred years to 350 ka and with newer methods to 500 ka (Wagner 1995). Lead-210 ( 210 Pb) Lead-210 dating is a special form of the uranium series dating. The decay of 210 Pb is used, which is a nuclide with short half life of t 1/2 = 22.3 years and a product from the decay of 222 Rn. Radon is a gas and is produced from the decay of 238 U found in rocks at the surface. 210 Pb remains a few days in the atmosphere and then falls out on the earth surface. The accumulation of this short living nuclide produces an excess in the sediment which can be differentiated from the normal level of radioactive lead. From the activity and the half life of the remaining 210 Pb in the sediment the age of deposition can be calculated. Lead-210 dating is used to determine the age for marine or lacustrine clayey, silty sediments which are deposited in the last 250 years (Bradley 1998). Potassium/Argon (K/Ar) This dating method is based on the decay of 40 K to 40 Ar and the age is obtained from the ratio of Potassium to Argon. There are two techniques resulting from the decay mode of Potassium to Argon and Calcium: 40 K/ 40 Ar and 40 K/ 40 Ca whereas the latter is not common, because there is to much natural occurring 40 Ca. A special form of K/Ar-dating is the Argon-Argon-technique which is based on the decay of 40 Ar to 39 Ar. The K/Ar dating method is a common and widely used numerical dating 15

16 method. It covers an age range from around 50 ka up to 4.6 Ga and is applicable to all minerals containing potassium from magmatic, metamorphic and sedimentary rocks. The age represents the time since crystallisation, cooling, sedimentation or metamorphoses. For dating Quaternary deposits the K/Ar-method is applied mostly on volcanic rocks (basalt and tephra) to date the time of eruption (Geyh 2005; Wagner 1995). Fission Track Dating The decay of uranium releases heavy particles with high kinetic energy. These particles leave tracks in minerals or glasses which can be detected using a microscope after etching. As each track represents one decay, the amount of tracks is a measure for the age of the surface. Fission track dating covers a large age range from 300 ka to 10 Ma (Geyh 2005). Electron Spin Resonance Dating (ESR) In crystal lattices natural defects from crystal growing and defects from radioactive radiation from uranium and thorium occur. The amount of radiogenic defects increases with the duration of radioactive radiation. Electrons accumulate in the natural and radiogenic defects. But the electrons in the radiogenic defects are unpaired and behave different than the paired electrons in natural defects. In a magnetic field the spin from the paired electrons is compensated. Unpaired electrons start to adjust like magnets. The spin starts to vibrate in the external magnetic field. Using microwaves with the same wavelength as the vibration of the spin it will come to resonance and the rotating direction of the spin changes. This spin flip needs the energy of the microwave which is absorbed in that moment. The wavelength of the resonance identifies the type of the absorption centre and the rate of absorption is equivalent to the amount of trapped electrons indicative of the duration of radioactive radiation. The ESR dating method is still more or less in an experimental stage and routinely application should be made carefully. This method covers an age range from a few hundreds of years to 100 ka and in some cases up to 1 Ma (Geyh 2005; Wagner 1995). An advantage of this method is the wide range of dateable material, e.g. stalagmite, stalactite, travertine, shells, bones, teeth, corals, foraminifers, flint (Wagner 1995) and that the measurement of the same sample can be repeated many times without losing the age-information. Amino Acid Racemisation (AAR) For dating with amino acid racemisation the effect is used, that L- and D-amino acids are isomers. These are identical but mirror-inverted molecules. In living tissue nearly only L-amino acids occur. During aging of dead tissue the L-amino acids transform to D-amino acids. The ratio of L- and D- amino acids represents the time since the organism is dead. The age range of the AAR-dating methods is from a few years to some 100 ka depending on the type of amino acid used (Wagner 1995). 16

17 Obsidian Hydration Obsidian is a volcanic glass. If the fresh glass surface is exposed, moisture from air or soil diffuses into the glass and forms a narrow band or rind. This hydration rind grows with time. From the growth rate and the thickness the age since the surface is exposed can be determined. This method covers an age range from a few 100 years to 1 Ma (Geyh 2005). Dendrochronology The counting of annual layers in trees gives the age of wood. Each year a new ring is formed during the growth period in spring and summer. The thickness of a ring depends on the precipitation and temperature resulting in thick and thin rings forming a characteristic pattern. The rings can be correlated with those of other trees from the same species and region. Fossil wood is used to extend the tree ring chronology back. The pattern of tree rings from wood with unknown age can be compared with the tree-ring chronology and dated. With radiocarbon dating an absolute age can be assigned to the tree-ring-chronology. The chronology depends on the region and tree species and ranges from the present back to 11 ka (Wagner 1995). Varve Chronology Varves are characteristic sediment layers deposited in lakes in front of glaciers. During summer coarse grained clastic material is deposited and during winter fine grained material (silt and clay) is deposited in the lake. Each year is represented by two sediment layers. Counting of these layers gives the depositional age of the sediment. Based on characteristic thicknesses different varve profiles can be correlated in a region. The oldest varve chronology is known from Scandinavia and covers an age range from the present back to 15 ka (Wagner 1995). Ice-layer-counting Snow is accumulated on glaciers during winter and transformed slowly to ice. In summer the snow melts partly. In the next winter new snow is accumulated and annual snow/ice-layers develop. Counting of these layers will give the age of the ice. Additionally dust and chemical elements like calcium or nitrates are incorporated into the snow. These elements show annual fluctuations and can be used for identification of single layers and correlation. Chemical analysis is used for older ice, where layers due to compaction are not visible anymore. The combination of ice layer counting and chemical analysis makes a chronology possible from the present to 14 ka back (Wagner 1995). 17

18 Relative dating methods Palaeomagnetism Past reversals of the earth magnetic field are used for dating. The magnetic field switches from normal to reversed polarisation where the magnetic north pole moves to the south. The reversal takes up to years. The stable periods differ in duration and last to a few million years. The magnetisation is saved in magmatic rocks but also in sediments. For the earth history there is a characteristic pattern of normal and reversed polarisation which can be used to date rock sequences of unknown age. Biostratigraphy Biostratigraphy uses fossils to establish a relative chronology of lithological units. The basic concept is that specific fossils always occur in the same lithological unit at different locations. Hence the units containing the same characteristic fossil, which are known as index fossils, have the same age. The smallest biostratigraphical unit is a zone and equals the life span of a species. A succession of zones can be used to define a relative chronology of lithological units. The relative age of zones is defined using phylogenetic successions where the evolution from older to younger species is visible. Palynology As an interdisciplinary science palynology is a part of geological and biological sciences, in particular of botany. This method uses pollen, spores and microfossils (e.g. dinoflagellates, acritarchs, scolecodonts) as well as micro-charcoal. Palynomorphs like pollen, spores or other microfossils form assemblages which are characteristic for ecological communities and climatic zones. Assemblage zones are defined by the first (beginning of a zone) and last (end of a zone) occurrence of species. In earth sciences palynology is used in biostratigraphic research, palaeoecology and reconstruction of palaeoclimate. This method allows for example distinguishing between glacial and interglacial periods by the occurrence of pollen and spores from plants that grow under cold or warm-temperature climatic conditions. It furthermore enables identifying different interglacials by their characteristic vegetation development and marker species. Palynology is used successfully to define human impact on the environment during the Quaternary with emphasis on the Holocene (Moore et al. 1991). Lithostratigraphy and Tephrochronology These methods use rocks and tephra layers to obtain relative ages based on the position to each other. They are based on the general concept in geology that sediment layers are build up from bottom to top, so that the oldest rocks are found at the bottom and the youngest at the top. The same concept is used for tephra layers, where the bottom most layer is normally the oldest layer. Geochemical characteristics of tephra layers are used to identify and correlate them. Using numerical methods, e.g. K/Ar dating, an absolute age for the lithological units can be determined. 18

19 Luminescence dating The luminescence dating method is a numerical dating method and belongs to the group of radioisotopic methods which are based on effects caused by the atomic disintegration in the surrounding sediment. The first description of luminescence from minerals was made by Sir Robert Boyle in the year 1663 (Aitken 1998). He observed a glimmering from a diamond in the dark which he took in bed and hold on his naked body. This effect is called thermoluminescence (TL) and can be observed from many minerals. Until the 1940s luminescence was only used in geosciences to identify minerals. With the development of sensitive photomultipliers in the 1950s, the relationship between the luminescence intensity and the exposure to nuclear radiation has been investigated. During this time a suggestion was made to use TL for age determination in geology and archaeology. During the 1960s TL has been subsequently developed for dating in archaeology, mainly for pottery. The application of TL to other materials e.g. burnt flint, burnt stone or speleothem started in the 1970s. At this time first attempts were made to use TL dating for sediments from different depositional environments. However, the first more reliable ages of sediments were obtained in the year of 1985 by Huntley et al. (1985) using optically stimulated luminescence (OSL) (Aitken 1985). Since then, luminescence excitation and emission spectra for quartz and feldspar and the luminescence characteristics of these minerals have been studied in order to develop routine measuring protocols. In the end of the 1990s until the beginning of the new millennium the single aliquot regenerative (SAR) dose protocol was developed (Murray and Wintle 2000; Wintle and Murray 2006). This made the determination of reliable ages from quartz and the use of the OSL dating method in geosciences possible. In numerous studies OSL dating was applied to well bleached aeolian and shallow marine sediments. Methodological research in the last years has focused on the development of protocols for feldspar and improvements in dating quartz from environments with problematic luminescence properties, e.g. fluvial and glacial deposits (Fuchs and Owen 2008; Rittenour 2008). The OSL dating method uses mainly quartz and potassium-rich feldspar grains for dating. These minerals are widely distributed and occur in nearly all geological environments. Both minerals have advantages and disadvantages. The measurement procedures for quartz are fully developed and resulting in robust ages. Especially for well bleached sediments like dunes or loess, quartz is the mineral of the first choice. Also for environments with insufficient bleaching, e.g. fluvial or glacial deposits, quartz gives good results with single grain measurements (Olley et al. 2004). However, the age range for quartz compared to the age range from feldspar is limited. Maximum ages for quartz are between ka (Duller 2008) and for feldspar up to 250 ka without fading correction (Wallinga et al. 2007). Unfortunately, there are areas in the world where quartz shows a very low sensitivity and is not suitable for OSL dating; e.g. the Alps (Klasen et al. 2007), Hungary (Novothny 2008), Scotland (Lukas et al. 2007), Peru (Steffen et al. 2009), Argentina (Spencer and Robinson 2008) and the Andaman Islands in the Bay of Bengal (Kunz et al. 2010a). The reason for this effect is not yet clear. One hy- 19

20 pothesis is that the rocks, from which the quartz derived, are geologically young and the quartz has not undergone enough cycles of bleaching and irradiation (Moska and Murray 2006; Pietsch et al. 2008). In these cases only feldspar is available for dating which is not affected by this problem. The one big problem of using feldspar is anomalous fading. This is a non-thermal loss of charge from kinetically stable traps due to quantum mechanical tunnelling which results in age underestimation (Spooner 1994; Visocekas 2002; Visocekas et al. 1994; Wintle 1973). A quantitative correction method for samples younger than 20 ka was proposed by Huntley and Lamothe (2001). The optically stimulated luminescence (OSL) dating method is an excellent tool to determine the deposition age of sediments which lack other dateable material like charcoal for radiocarbon dating. The obtained age is the time elapsed since the last exposure of the mineral grains to sunlight (Fig. 1.4). Figure 1.4: The obtained age is the time since last exposure to sunlight during transport. All previous events are not recorded in the signal anymore because each exposure to light during erosion and transport erases the luminescence signal. Compared to radiocarbon dating, which is a common dating method using organic matter for geological and archeological questions, OSL dating has some advantages: The age range for OSL dating is from a few ten years (Kunz et al. 2010c) to 700 ka (Wang et al. 2006). Radiocarbon dating covers an age range from 0.35 to 50 ka years depending on the quality of the sample (Geyh 2005). Because of the complexity of the 14 C production curve over the last 350 years, non-unique solutions are found leading to multiple ages, whereas the OSL dating method gives always one age (Duller 2004). The physical mechanism leading to luminescence allows determining the depositional age of sediments directly. Radiocarbon dates the time since an organism is dead and is useful only for autochthonous material. Contamination of sample material for OSL dating is negligible; only sufficient bleaching prior to deposition is important. 20

21 Physical background of luminescence dating The luminescence signal measured in the laboratory from quartz or feldspar is the result of the interaction of alpha, beta, gamma and cosmic radiation with the crystal structure. The radiation in the sediment derives mainly from the decay of 235 U, 238 U, 232 Th and their daughters in the decay chain; 40 K and 87 Rb. The cosmic radiation has a small contribution, depending on the altitude, latitude and the thickness of the sediment cover. Due to the radiation the electrons are ionized and detached from the parent nuclei in the crystal lattice. When an electron moves from the valence band to the conduction band it leaves a hole (Fig. 1.5, irradiation). This hole is the conceptual opposite of an electron and describes the absence of an electron in the valence band. Electron and hole, which diffuse in the crystal lattice, are trapped by lattice defects (L and T in Fig. 1.5). The number of trapped electrons increases with the duration of nuclear radiation. These trapped electrons can be only released from their traps when the electrons get additional energy (E). This can be given by photons (light) in the case of OSL or by heat in the case of TL. The evicted electrons diffuse in the crystal lattice until a recombination centre (e.g. hole) is found (Fig. 1.5, eviction). The recombination emits light which is then detected with a photomultiplier in the laboratory and used for age determination. Figure 1.5: Schematic illustration of the processes in the crystal leading to luminescence (modified from Aitken (1998)). For explanation see text. The intensity of the emitted light is proportional to the number of trapped electrons and hence to the accumulated energy that has been absorbed from the flux of nuclear radiation during a specific time. The age of a sample is calculated using the formula: D age = D e R Gy Gy a D e is the equivalent dose measured in the lab from luminescence and D R is the annual dose rate from the surrounding sediment including the contribution from the cosmic radiation. More detailed descrip- 21

22 tions about the physical background of luminescence in quartz and feldspar can be found in e.g. Aitken (1998), Krbetschek et al. (1997), Preusser et al. (2009) Smith and Rhodes (1994) and Wintle (1997, 2008) Measurement procedures There are two general ways to measure the equivalent dose (D e ): the multiple aliquot additive dose (MAAD) protocol and the single aliquot regenerative (SAR) dose protocol. Both protocols can be applied to quartz and feldspar. Some other protocols are also available but not generally used, e.g. single aliquot additive dose (SAAD) (Murray et al. 1997) or the single aliquot regeneration and added dose (SARA) protocol (Mejdahl and Bøtter-Jensen 1997). For the MAAD protocol many aliquots (subsamples) are to be measured to obtain one D e -value. In contrast the SAR protocol only needs one aliquot to obtain one D e -value, resulting in a set of many D e -values for one sample. This dataset is then used to calculate the burial dose (D b ; see chapter ). This is one of the advantages of the SAR protocol over the MAAD protocol. A second advantage of the SAR protocol is that the D e -value is calculated from the dose points by interpolation and not extrapolation of the growth curve (Fig. 1.6). Despite these disadvantages of the MAAD protocol there is one important advantage. The number of heat treatment and stimulation for an aliquot during the measurement is less in the MAAD than in the SAR and hence the risk of sensitivity changes are low. In a SAR measurement the aliquots are subjected to many more heating and optical stimulation leading to higher sensitivity changes. To minimize these problems the sensitivity change is monitored and used for correction in the SAR. Figure 1.6: Diagram a shows the MAAD growth curve from a feldspar sample from Croatia. Diamonds are the natural signal and the circles show the signal after giving artificial irradiation doses to the aliquots. The equivalent dose (D e ) is obtained by extrapolating the fitting curve (dashed line) to the intercept with the x-axis. For the measurement of one D e in this example 31 aliquots were needed. The extrapolation depends strongly on the quality of the curve fitting. Diagram b shows the growth curve from a SAR measurement of a quartz sample from south east India. The diamond shows the natural signal (L n /T n ) and circles are showing the signals from the artificial doses. Filled circle (L 5 /T 5 ) is the zero dose that represents the recuperation. Blue circle (L 1 /T 1 and L 6 /T 6 ) is the repeated dose point and represents the recycling ratio. Only one aliquot was used to obtain the D e - value. For detailed description see text. 22

23 In this thesis all samples were measured using the SAR protocol which is explained below. Each measuring cycle produces one dose point and is made up of six or seven steps (Tab. 1.1). Table 1.1: Generalized single aliquot regenerative (SAR) dose protocol. Step Treatment Observed 1 Give regenerative dose D i 2 Preheat x C for 10 s 3 OSL with blue light for quartz (40 s at 125 C) or infrared light for feldspar (100 or 300 s at 50 C) L i 4 Give test dose D t 5 Cutheat x C for 0 s for quartz and for 10 s for feldspar 6 OSL with blue or infrared light like step 3 T i 7 OSL with blue (40 s at 290 C) or infrared light (40 s at 280 C) For the natural sample i = 0 and D 0 = 0. The whole sequence is repeated for several regenerative doses, including a zero dose and a repeat dose. Regenerative doses D i depending on the expected age of the sample. Regeneration doses are chosen to bracket the natural dose. The ratio L i /T i is used for the calculation of the equivalent dose. The OSL signal from step 7 (hotbleach) is not used. The preheat and cutheat temperature x are determined by the preheat test and normally ranges from 160 C to 300 C. The measurement starts with preheat of the sample (Tab. 1.1, step 2). The function of preheat is to empty any light sensitive shallow traps which are normally filled during laboratory irradiation and give an unwanted contribution to the luminescence signal. After applying the preheat the natural luminescence signal (L n ) is measured (Tab. 1.1, step 3) by stimulating the quartz minerals with blue light (wavelength 470 nm) or feldspar minerals with infrared light (wavelength 875 nm). During the stimulation the traps are emptied and the luminescence signal is detected as an exponential decay curve (Fig. 1.7a). The value for L i or T i (luminescence signal of the testdose) is the integrated intensity over the first second of stimulation for quartz and 10 s of stimulation for feldspar (Fig. 1.7b) subtracted by the integrated intensity over the last 10 s of stimulation (background subtraction). Figure 1.7: (a) Luminescence signals of quartz and feldspar detected during optical stimulation. Quartz minerals show always a very fast decay. After 5 seconds of stimulation the traps are emptied. Feldspar shows a much slower decay and therefore the stimulation time is longer to empty all traps. (b) Enlargement of the decay curves showing the areas used for the calculation of the L i or T i value, respectively. 23

24 The luminescence measurement is followed by the application of an artificial dose known as testdose (D t ) (Tab. 1.1, step 4). The testdose is applied to normalise the measurements among the different SAR cycles and makes the comparison between different aliquots possible. During a SAR measurement the testdoses for one sample are always the same. Before measuring the luminescence (T n ) of the testdose (Tab. 1.1, step 6) a so called cutheat is applied. The cutheat (Tab. 1.1, step 5) has the same function as the preheat. The cutheat temperature should be close to or same as the preheat temperature. In a last step (7 in Tab. 1.1) which is optional and used for samples with high recuperation (will be explained later) so called hotbleach is applied. During the hotbleach the minerals are stimulated with light at elevated temperatures to transfer and simultaneously remove charge that is thermally transferred from any light-sensitive traps into the main OSL trap during preheat or cutheat. After the last step (6 or 7) the SAR cycle starts again with irradiation (Tab. 1.1., step 1 D i ). And the same steps are performed again measuring the luminescence of the irradiated aliquot (L i ) and the response of the testdose (T i ). This is repeated several times with increasing artificial doses (D i ). The ratio of L i /T i is plotted against the applied dose in a graph (Fig. 1.6b) which is fitted to a linear function for young samples and to a saturating exponential for old samples. From the curve the natural luminescence signal (L n /T n ) is interpolated and the equivalent dose (D e ) is obtained (Fig. 1.6b). When measuring quartz an additional step is included to check for any contamination with feldspar. Feldspar gives also a luminescence signal when stimulated with blue light and the intensity is higher than that of quartz. Hence a contaminated sample would give wrong ages. The check is done using the depletion ratio based on Duller (2003). After a complete SAR cycle the sample is irradiated with a known dose and the L blue1 /T blue1 is measured with blue light stimulation. Then the sample is irradiated again with the same dose. Before measuring the L blue2 /T blue2 the sample is stimulated with infrared light. In the case of feldspar contamination the L blue2 /T blue2 from the second blue light stimulation is lower because the infrared stimulation depletes the feldspar signal. Quartz is not affected by infrared stimulation. Therefore L blue2 /T blue2 represents the response only from quartz. An IR depletion ratio ((L blue1 /T blue1 )/(L blue2 /T blue2 )) between 0.8 and 1.0 is regarded that the sample has no feldspar contamination. A ratio lower than 0.8 suggests a contamination with feldspar. Some samples of this studies showed a weak contamination with feldspar. These samples underwent a second 1-h etching with concentrated hydrofluoric acid (40%) to remove all feldspar. The samples are measured again and showed no feldspar contamination Quality checks of the OSL SAR measurements To monitor the quality of a SAR measurement a repeated dose point (recycling ratio) and a zero dose point are measured (Fig. 1.6b). For the recycling ratio, a repeated dose which is identical to the first regeneration dose is applied again at the end of the SAR measurement. As sensitivity changes are 24

25 progressive the first and last measurement will show the widest spread in sensitivity change. The ratio between (L i /T i )/(L 1 /T 1 ) is termed recycling ratio and should be in the range between 0.90 and 1.10 (Murray and Wintle 2000). Having a good recycling ratio means the sensitivity correction using test dose functions and the same dose can be reproduced during the measurement. The measurement of the zero dose point should give no OSL signal. However, the transfer of charge from deeper traps caused by the previous irradiation, stimulation and heating results in a signal which is above zero. This signal is termed recuperation and should not exceed 5% of the natural signal (Murray and Wintle 2000). Before starting the measurement of a sample with OSL using the SAR protocol some tests have to be made to check if the luminescence properties of the mineral grains are suitable for the application of the OSL method and to obtain a well working SAR protocol. As there are no standard settings for the SAR protocol for each sample the preheat and cutheat temperatures have to be adapted. This is done by the preheat test. A cutheat test is not necessary because the cutheat temperature is normally the same as the preheat temperature. Only in some special cases the quartz or feldspar properties require different cutheat and preheat temperatures (Novothny 2008). The preheat test shows if there is any unwanted transfer of charge to optically sensitive traps during thermal treatment. This charge transfer would result in an increased equivalent dose and hence in a wrong age. For the preheat test the sample is split into eight groups with three aliquots each. Every group is treated with a different preheat temperature with a 20 C interval starting from 160 C. The measured D e -values are plotted against the preheat temperature (Fig. 1.8a). An increase of the D e -values with increasing temperature indicates transfer of charge. The preheat temperature which is taken for measurement should be in a plateau. In general a low preheat ( C) is used for young samples and a high preheat temperature ( C) is applied for old samples. The thermal transfer test is an additional test regarding the thermal treatment especially for young quartz samples. The phenomenon of thermal transfer derives from charge which is thermally released from shallow but optically insensitive traps. These charges are recaptured subsequently during heating by deeper OSL traps (Wintle and Murray 2006). Shallow trap means here, that the thermal activation energy is lower than for the main OSL trap. The problem for young samples is that a charge derived from a very large residual dose could be trapped from these shallow traps (because the shallow trap is light insensitive), whereas the OSL trap has a very small dose. During the first preheat before measuring the natural luminescence, a large amount of charge is transferred from the shallow trap to the OSL trap and increases the signal leading to an overestimated age. The critical preheat temperature where thermal transfer starts can be detected using the thermal transfer test. The test is similar to the preheat test but the aliquots are bleached before, using sunlight or a solar simulator to empty all light-sensitive traps. For any preheat temperature between C the D e -values should be zero with the absence of thermal transfer. But in the case of thermal transfer the D e -values are increasing slightly toward higher preheat temperatures (Fig. 1.8b). Every preheat temperature where the D e -values are zero can be used for the SAR measurement. 25

26 Figure 1.8: Idealized diagrams from a preheat plateau test (a) and a thermal transfer test (b). For each temperature the mean of the aliquots and the standard deviation is shown. The preheat test shows stable equivalent doses in the temperature range between 160 and 220 C. The thermal transfer test shows, that from temperatures higher than 200 C thermally induces transfer of charge occurs. In this example the preheat temperature for the SAR measurement should be in the range between 160 to 200 C. After determining the correct preheat temperature a dose recovery test is made, to check if the SAR protocol can obtain the correct equivalent dose. The sample is bleached with sunlight or in a solar simulator to empty all traps and then irradiated with a known dose. The applied dose should be in the range of the expected natural dose. This dose is measured using the SAR protocol applied for the sample. For a good working SAR protocol the ratio between applied and measured dose should be unity (Murray and Wintle 2003) or at least in the range between 0.90 and It is important to note, that the dose recovery test and the recycling ratio are not the same although both recover a known dose. For the dose recovery an unheated sample is used and the natural irradiation from the sediment is simulated. On the other hand the recycling ratio monitors for sensitivity changes of a heated sample during the SAR measurement and if the protocol can recover a dose (Wintle and Murray 2006) Dosimetry As mentioned before the luminescence signal is the result of the interaction of the radiation from the surrounding sediment with the crystal structure. The radiation derives from the natural radioactivity in the sediment. This radioactivity is mainly the result of the decay of 238 U, 235 U, 232 Th, 40 K, 87 Rb and their daughter nuclides. The decay of these nuclides produces alpha, beta and gamma radiation with different ranges which has strong impact for luminescence dating. Alpha particles are 4 He nuclei 4 2+ ( 2 He ). Due to their relatively high mass, the maximum range of alpha-particles in air is 10 cm and in matter a few µm (Aitken 1998). This complicates luminescence dating using coarse grains (diameter 100 to 200 µm). The alpha particles penetrate only the outer rim of a grain producing parts with higher influence of alpha radiation and parts with no influence from alpha radiation. Stimulating these grains with light would result in a mixed luminescence signal with different intensities from these 26

27 parts. In this case for the calculation of the total dose rate the influence of the alpha radiation has to be modelled. One way to solve this problem is to remove the alpha influenced outer rim of the mineral grains by etching with hydrofluoric acid. As a result the alpha-contribution for quartz grains is negligible. If using coarse grain feldspar grains or polymineral fine grains (diameter 4 to 11 µm) the alpha contribution has to be included into dose rate calculation. However, the contribution of beta and gamma radiation is more important for all grain sizes. The range of both radiation types is large enough to penetrate a grain completely and move charge which contributes to the luminescence signal. The range of beta particles depends on their energy and is in air a few centimetres to metres and in sediment up to three millimetres (Aitken 1998). Gamma rays have the highest energy and hence the highest range: a few metres in air and tens of centimetres in sediment. For the calculation of the dose rate from the sediment (D R ) the finite matrix is assumed. This means for a volume having dimensions greater than the range of radiation, that the rate of energy absorption is equal to the rate of energy emission (Aitken 1998). The dose rate of the sediment is calculated from the activity (disintegrations per second per mass = Bq/kg) of 238 U, 232 Th and 40 K in the sample. The activity is measured using gamma spectrometry. Alternatively, the activity can be determined by alpha-counting or calculated from the concentrations of U, Th and K obtained chemically by ICP-MS (Inductively Coupled Plasma Mass Spectrometry). Both methods have their issues. Alphacounting is difficult regarding sample preparation and measurement due to the very short range of alpha particles. Furthermore potassium cannot be detected by alpha-counting because 40 K has only beta-decay. ICP-MS does not show any radioactive disequilibrium in the decay chains of U and Th. Gamma spectrometry is easy to use and an effective method for measuring the radiation activity of natural sediment. It uses the effect, that most of the alpha- and beta-disintegrations are accompanied by the release of gamma rays. Each nuclide has its characteristic gamma energies. For example 137 Cs decays to 137 Ba by the release of an electron (beta-minus decay) and the release of one gamma ray with the energy of kev. The gamma energies are like a fingerprint of a nuclide and used to identify them in spectra from natural samples with mixtures of different nuclides. Figure 1.9 shows a gamma spectrum of natural sediment from south east India. The gamma spectrometer counts the gamma rays released per disintegration and classifies them according to their energies. The activity of a nuclide is represented by the area below the peak (Fig. 1.9). 27

28 Figure 1.9: Gamma spectrum of a natural sample from south east India. The green peaks represent nuclides from the decay chain from 238 U. Red peaks represent nuclides from the decay chain from 232 Th. The inset shows an enlargement of the 40 K-peak. The area below the blue curve represents the activity of the nuclide. For a detailed explanation of gamma spectra see Gilmore and Hemingway (1995). The measurement of the activity from 40 K is easy as there is one gamma energy at kev released by the beta-decay from 40 K to 40 Ar. For 238 U and 232 Th the measurement of the activities is more complex. Both nuclides release gamma rays with a low probability and in a low energy range which is difficult to detect. Low probability means, that there are many alpha or beta disintegrations needed to release one gamma ray. To solve this problem, gamma rays released from the decay of the daughter nuclides in the decay chains are measured (Fig. 1.10). In case of a radioactive equilibrium the activities for all daughter nuclides in the decay chain are the same as the activity from the parent nuclide. To obtain the activity from 238 U the activities from 214 Pb, 210 Pb, 234 Th and 214 Bi are measured. And for 232 Th the activities from 228 Ac, 212 Pb and 208 Tl are measured. This approach also allows monitoring for radioactive disequilibrium in the decay chains from 238 U and 232 Th. 28

29 Figure 1.10: Decay chains for 238 U (green), 232 Th (red) and 40 K (inset). Bold nuclides showing start and end of a decay chain. 238 U decays to 206 Pb, 232 Th decays to 208 Pb and 40 K decays to 40 Ar and 40 Ca, respectively. Half lifes (T 1/2 ) are based on the Decay Radiation Database (version 12/11/2009) T 1/2 are given in million years (Ma), thousand years (ka), years (a), days (d), hours (h), minutes (m) and seconds (s). The measurements of the dose rates from the samples were done using a high-purity germanium (HPGe) N-type coaxial detector from Canberra. The efficiency and energy of the detector was calibrated using standard material (RGU-1 for 238 U, RGTh-1 for 232 Th and RGK-1 for 40 K) with known activities provided by the International Atomic Energy Association (IAEA). Before starting the measurement the sample was dried, homogenised and filled into a Marinelli beaker. The beaker was sealed to avoid exchange with atmosphere and any exchange of radon. The beaker was stored for at least four weeks so that the radon disequilibrium could adjust. Each sample was measured for 24 hours. After determining the activities for 40 K and 228 Ac, 208 Tl, 212 Pb for the 232 Th-decay chain and 234 Th, 214 Bi, 214 Pb, 210 Pb for the 238 U-decay chain the element concentrations for 40 K, 232 Th and 238 U are calculated. From these element concentrations the dose rates derived from alpha-, beta- and gamma radiation are calculated using these formulas: D D D α ( dry) β ( dry) γ ( dry) = C = b = C U Uα ( C E + C E + C E ) U E U E Uγ ak Uβ U + C C U, C Th and C K are the element concentrations for 238 U in ppm, for 232 Th in ppm and for 40 K in %. E are the dose rate conversion factors for each nuclide ( 238 U, 232 Th, 40 K) and type of radiation (α, β,γ) based on Adamiec and Aitken (1998). A radon loss of 20% is included into the dose rate conversion factors which represents the natural radon loss (Murray 1981). + C Th Th E Th Thγ E Thβ Thα + C ak K Th K E Kγ Kβ 29

30 a is the value for the alpha efficiency based on Rees-Jones (1995), for etched coarse grain quartz a = 0. k U and k Th are conversion factors for the alpha efficiency; k U = 0.8 and k Th = b is the attenuation of the energy of beta particles which depends on grain size, values are taken from Aitken (1985). The dry dose rates are corrected by the application of the water content w using the formulas based on Aitken (1998): D Dα ( dry) = w α ; D Dβ ( dry) = w β ; D γ Dγ ( dry) = w The cosmic radiation D cosmic contributes to the dose rate in the sediment and depends on altitude, latitude and thickness of sediment cover (Fig. 1.11). For the calculation of the cosmic dose the approach of Prescott and Hutton (1988) and Prescott and Stephan (1982) is used. Figure 1.11: Attenuation of cosmic dose with depth in dependence of latitude and altitude. Graphs calculated using formulas from Prescott and Hutton (1988) and Prescott and Stephan (1982). The total annual dose rate from the sediment is then calculated by: D + R = Dα + Dβ + Dγ D cosmic. If dating coarse grain quartz were the outer rim is removed by etching, the formula simplifies to: D + R = Dβ + Dγ D cosmic Age calculation In luminescence dating there are different approaches to calculate the burial dose (D b ) for a sample from a set of single equivalent doses (D e ) obtained by SAR measurement. The methods depend on 30

31 the distribution of the D e -values which in turn depends on the bleaching of the samples prior to the deposition and hence on the geological and sedimentary environment. Samples from aeolian deposits like dunes or loess are transported for a long distance and are fully bleached. These samples show an excellent normal distribution. For these samples the burial dose (D b ) can be calculated using the mean, weighted mean or the central age model (CAM) based on Galbraith et al. (1999). Incompletely bleached samples are found in fluvial, marine and glacial environments. The amount of bleaching is controlled by water depth, turbulence, sediment load, distance and speed of transport and the timing of the transport (day or night-time). The TL signal is not well bleached in water but the OSL signal is more sensitive and can be bleached to zero. However, zeroing takes longer than in aerial transport. Depending on the water depth and turbulence it takes up to 20 hours until the signal is completely set to zero (Gemmel 1985; Rendell et al. 1994). This is important for dating material from environments where the deposition occurs very fast, e.g. storm- or tsunami-laid deposits. Ditlefsen (1992) found that sediment load or transport in a suspension blocks the light more effective leading to an incomplete bleaching of the OSL signal. In environments where the transport distance is short, mixing of sediment with different amount of bleaching can occur (Fiebig and Preusser 2007; Gemmel 1985). In glacial and glaciofluvial environments incomplete bleaching is common. Most of the sediment is transported in a ground moraine below the ice and deposited in situ. From the moraines the material is transported by meltwater only a short distance (Alexanderson 2007; Gemmel 1999). The periods where bleaching occurs are very short. Samples with incomplete bleaching show a wide scatter of equivalent doses. This makes the decision for a burial dose (D b ) that represents the true depositional age very difficult. In cases of incomplete bleaching the leading edge model (Lepper and McKeever 2002) or minimum age model (MAM) (Galbraith et al. 1999) provide good estimates for the burial dose (D b ). Both models are based on the assumption that the lowest D e -values represent the depositional age. Mineral grains with the lowest dose are most likely completely bleached during transport. There are also situations where mineral grains are completely bleached during transport but after deposition they are mixed, e.g. by bioturbation, resulting in a polymodal distribution of the D e -values. In this case the finite mixture model (FMM) based on Galbraith and Green (1990) is a good approach to separate the different populations of a dataset. But this model can only be applied to datasets which are obtained by single-grain measurement (Arnold and Roberts 2009). Different populations of equivalent doses (D e ) from multi-grain aliquots do not represent different depositional ages. In this thesis between 26 and 30 aliquots are measured for each sample depending on the amount of available sample material. Before starting any statistical treatment the aliquots which do not fulfil the quality criteria (recycling ratio outside 1.0 ± 0.1 and recuperation > 5%) were rejected. The remaining dataset was then checked for outliers. An outlier is a value which is outside the range x ± 4s, where x is the mean without the suspected outlier and s is the standard deviation. The final dataset was then checked for normal distribution using the Kolmogorov-Smirnov-Test. In this study the data 31

32 for every sample showed a normal distribution and the burial dose for the sample (D b ) was calculated using the central age model (Galbraith et al. 1999) or the weighted mean. The age was then calculated using the D b -value from the sample divided by the dose rate of the surrounding sediment (D R ): age = D α + D β Db + D γ + D cosmic Outline of the thesis This thesis is composed of five chapters. Chapters 2, 3 and 4 are written as articles to peerreviewed journals. Two of them are in press; chapter 2 in International Journal of Earth Sciences (former Geologische Rundschau) and chapter 3 in Quaternary International. Chapter 4 is accepted in Zeitschrift der Deutschen Gesellschaft für Geowissenschaften (ZDGG). Chapter 5 provides a conclusion of all the results. Chapter 2: Revealing the coastal event-history of the Andaman Islands (Bay of Bengal) during the Holocene using radiocarbon and OSL dating (Kunz et al. 2010a). In this chapter the study of coastal sediments on the Andaman Islands is presented. A chronology of storm- and tsunami-events is reconstructed. This was done by radiocarbon dating of organic material and OSL dating of sediments. Three sections in southern Andaman Islands have been investigated. For radiocarbon dating 9 samples are taken and 2 samples for OSL dating. Additional 7 samples are taken for sedimentological studies. It was found, that the quartz grains in that region are not suitable for OSL dating. But feldspar was successfully used for OSL dating. Before measuring the samples with the SAR protocol, it had to be modified for the application on feldspars. Different approaches were tested. The dating results are used to reconstruct the recurrence interval of storms, tsunamis and earthquakes. The results are compared with other studies in the eastern part of the Bay of Bengal. Chapter 3: Luminescence dating of Late Holocene dunes showing remnants of early settlement in Cuddalore and evidence of monsoon activity in south east India (Kunz et al. 2010b). In this study dunes at the south east coast of India were investigated. This area was also strongly affected by the Indian Ocean tsunami in December A chronological frame of the development of the western part of the dune belt was established using OSL dating of quartz. In contrast to the material from the Andaman Islands the quartz from mainland India has very good luminescence properties. Thus it was possible to date very young deposits with a high accuracy. In this study three sections have been investigated. For sedimentological and mineralogical studies 65 samples and for OSL dating 12 samples have been taken. The results show different periods of sand movement, stabilised land surface and flood events. These periods are connected with fluctuations of the monsoon intensity in 32

33 India. There is also evidence of sea level changes during the Holocene which are reflected in the morphology of the dune belt. Chapter 4: Periods of recent dune sand mobilisation in Cuddalore, south east India (Kunz et al. 2010c) This work is the continuation of the study presented in chapter 3. In chapter 4 the eastern part of the dune belt was investigated. Three sections are studied and 19 samples for OSL dating and 56 samples for sedimentological and mineralogical studies are taken. The results show young periods of sand movements; not older than 200 years. The sand movement periods coincide with dry periods in southern India. This work shows a very good agreement between precise OSL dating and the instrumental rainfall record. These young dunes also demonstrate the sensitivity of the dune system to changes in the environmental conditions References Adamiec, G., Aitken, M. (1998): Dose-rate conversion factors: update. Ancient TL 16: Aitken, M.J. (1985): Thermoluminescence Dating. Academic Press: London. Aitken, M.J. (1998): An introduction to optical dating. Oxford University Press: Oxford. Alexanderson, H. (2007): Residual OSL signals from modern greenlandic river sediments. Geochronometria 26: 1-9. Arnold, L.J., Roberts, R.G. (2009): Stochastic modelling of multi-grain equivalent dose (D e ) distributions: implications for OSL dating of sediment mixtures. Quaternary Geochronology 4: Bahuguna, A., Nayak, S., Roy, D. (2008): Impact of the tsunami and earthquake of 26 th December 2004 on the vital coastal ecosystems of the Andaman and Nicobar Islands assessed using RE- SOURCESAT AWiFS data. International Journal of Applied Earth Observatorion and Geoinformation 10: Bradley, R.S. (1999): Paleoclimatology - reconstructing climates of the Quaternary. Academic Press: San Diego. Brückner, H. (1988): Indicators for formerly higher sea levels along the east coast of India and on the Andaman Islands. In Neue Ergebnisse der Küstenforschung. Schipull, K., Thannheiser, D. (eds.). Institut für Geographie und Wirtschaftsgeographie der Universität Hamburg: Hamburg; Chawla, S., Dhir, R.P., Singhvi, A.K. (1992): Thermoluminescence chronology of sand profiles in the Thar desert and their implications. Quaternary Science Reviews 11: Clemmensen, L.B., Andreasen, F., Heinemeier, J., Murray, A. (2001): A Holocene coastal aeolian system, Vejers, Denmark: landscape evolution and sequence stratigraphy. Terra Nova 13:

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39 Wintle, A.G. (2008): Luminescence dating: where it has been and where it is going. Boreas 37: Wintle, A.G., Murray, A.S. (2006): A review of quartz optically stimulated luminescence characteristics and their relevance in single-aliquot regeneration dating protocols. Radiation Measurements 41: Wolfe, S.A., Huntley, D.J., David, P.P., Ollerhead, J., Sauchyn, D.J., MacDonald, G.M. (2001): Late 18 th century drought-induced sand dune activity, Great Sand Hills, Saskatchewan. Canadian Journal of Earth Sciences 38:

40 Chapter 2 International Journal of Earth Sciences 99 (2010) Revealing the coastal event-history of the Andaman Islands (Bay of Bengal) during the Holocene using radiocarbon and OSL dating Alexander Kunz a,b, *, Manfred Frechen b, Ramachandran Ramesh c, Brigitte Urban a a Leuphana University, Suderburg, Germany b Leibniz Institute for Applied Geophysics, Hannover, Germany c Institute for Ocean Management, Anna University, Chennai India * corresponding author: jan-alexander.kunz@liag-hannover.de Abstract Earthquakes that trigger tsunamis are of great geological, ecological and socio-economic importance. The knowledge of the recurrence interval of these events will give information about the hazard for a region. Coastal sediments on the Andaman Islands located in the eastern Bay of Bengal were investigated to find evidence for palaeotsunamis and palaeoearthquakes. Fieldwork was conducted on Red Skin Island and North Cinque Island, south of South Andaman. Sediment material from eventlayers was dated by optically stimulated luminescence and radiocarbon dating method. The results show evidence possibly for one earthquake at about 1000 or 3000 years before the present together with deposits from possible tsunamis and storms. The complex pattern of co- and post-seismic uplift and subsidence of the Andaman Islands is reflected in the investigated sections and made it possible to reconstruct an event-history for the last 3000 years. Keywords: Palaeostorm, Palaeotsunamis, Palaeoearthquakes, Andaman Islands, OSL dating, Radiocarbon dating 40

41 2.1. Introduction The Andaman Islands are located in the eastern part of the Bay of Bengal (Fig. 2.1) in a tectonically active area. Numerous strong earthquakes in the recent past give evidence for this high tectonic activity. The area of interest is affected by strong tropical storms and tsunamis altering the coastal area and causing serious damage. The most recent strong tsunami in December 2004 following the M w 9.3 earthquake off the west coast of northern Sumatra caused severe damage along the coasts of the Andaman Islands and the whole Bay of Bengal. This historically unprecedented earthquake and tsunami suggest that the Indo-Andaman plate boundary ruptures in variable modes (Satake and Atwater 2007; Stein and Okal 2007). To evaluate the behaviour of this plate boundary, we need a more reliable chronology of earthquakes or the tsunamis they generate. The Holocene coastal deposits are thought to contain a detailed sediment archive of past cyclones and tsunamis. The Andaman Islands were chosen to study this sediment archive due to its proximity to the subduction zone west of the Indonesian Archipelago, which is the main driving force for the tectonic activity. Even weak tsunamis could have deposited or eroded sediments on the nearby Andaman Islands. No written records are known about ancient tsunamis, such as those recorded in Japan, but some evidence suggests that tsunamis do recur. For example, indigenous people living on islands off the coast of Thailand know about tsunamis from oral traditions and used that knowledge to escape from the 2004 tsunami (Arunotai 2006). Indigenous people live also on the Andaman Islands and behaved similarly. Historical tsunami events are recorded for the past 260 years along the coasts of the Bay of Bengal (Manimaran and Chacko 2006). Bilham et al. (2005) summarized historical records of earthquakes and tsunamis along the Indo-Andaman plate boundary, including events in 1847 (M w ), 1881 (M w 7.9) and 1941 (M w 7.7). The last two of these triggered tsunamis recorded in tide gauges in India. Little is known about the chronology of terrestrial Quaternary sediment sequences and the occurrence and cyclicity of strong tsunami events preceding the seventeenth century in this area. Identifying ancient tsunamis is one of the main roles of the young and fast-growing field of tsunami geology (Rhodes et al. 2006). Geologists have discovered stratigraphic evidence for prehistoric tsunamis on shores of the North Atlantic (Bondevik et al. 2005, 1997; Dawson et al. 1988; Grauert et al. 2001; Tuttle et al. 2004; Williams and Hall 2004), the Pacific Northwest (Atwater et al. 2005; Kelsey et al. 2005), western South America (Cisternas et al. 2005), New Zealand (Chagué-Goff et al. 2000; Goff et al. 2001), Japan (Minoura and Nakata 1994; Nanayama et al. 2003) and Kamchatka (Pinegina et al. 2003). The records described in this paper do fill important gaps on our knowledge about ancient tsunamis. Such records provide opportunities to develop models of deposition and preservation of tsunami deposits in tropical settings. They can also help to estimate the recurrence of plate-boundary megathrust earthquakes along the Indo-Andaman plate boundary. The structure, thickness, lateral and inland extent of palaeotsunami deposits will help to reveal when the northern Sunda Trench ruptured wholesale producing large earthquakes (M w > 9) and when it broke in segments producing smaller 41

42 Figure 2.1: Map showing the location of the Andaman Islands (AND). Arrows indicate the sampling positions on Red Skin Island and North Cinque Island (sections 1 and 2). Bathymetry is given in meters below present sea level. The dark grey areas on North Cinque Island represent sandbars. AFG = Afghanistan, IND = India, MYA = Myanmar, NIC = Nicobar Islands, PK = Pakistan, RC = China. earthquakes (M w < 8). A palaeotsunami record would also inform earthquake and tsunami hazards preparation and mitigation efforts that will reduce losses from future disasters (Sieh 2006). A main issue in palaeotsunami research is the identification of the deposits. Studies of modern analogs provide geological criteria for identifying palaeotsunamis. First studies were done with surveys of the 1946 Aleutian tsunami in Hawaii (Shepard et al. 1950) and the 1960 Chile tsunami in Japan (Konno et al. 1961), and now include a wide range of geomorphic and stratigraphic evidences (Atwater et al. 2005; Dawson and Shi 2000; Dawson and Stewart 2007; Srinivasalu et al. 2007). Tsunami deposits are identified because they are anomalous in many settings, such as beds of marine sand within sequences of terrestrial peat or freshwater lake sediments (e.g. Burney 2002; Bondevik et al. 2005; Dawson et al. 1991; Jankaew et al. 2008; Moore 2000). Rarely the lithology and other character- 42

43 istics of the beds are unique. For example storm surges may also deposit marine sand several meters above normal tidal levels (Collins et al. 1999; Dawson and Shi 2000; Donnelly et al. 2001; Liu and Fearn 1993; Liu and Fearn 2000; Nelson and Leclair 2006; Parsons 1998). Studies comparing modern tsunami and storm deposits help to document the processes and impacts of sediment erosion and deposition during these events (e.g. Gelfenbaum and Jaffe 2003; Kortekaas and Dawson 2007; Maramai et al. 2005a, b; Morton et al. 2007; Nanayama et al. 2000). Several studies showed that it is possible to distinguish tsunami from storm deposits. In New Zealand Goff et al. (2004) compared sediments deposited by a tsunami in the fifteenth century and a large storm in the year The two types of deposits show clear differences in their sedimentology, bed continuity and inland extent. Kortekaas and Dawson (2007) compared deposits from the tsunami associated with the Lisbon earthquake in 1755 with storm deposits at the same site in Martinhal, SW Portugal. The main differences for the nearly similar deposits are the rip-up clasts and boulders in the tsunami deposit and the larger extent of the tsunami deposit. Tuttle et al. (2004) compared tsunami deposits in southern Newfoundland resulting from the 1929 Grand Banks earthquake and submarine slides, with deposits in New England resulting from the 1991 Halloween storm. These studies suggest that two fundamentally different processes lay down tsunami and storm deposits. Tsunamis are long wavelength ( km) impulse-generated waves. This leads to relatively long inundation and outwash time intervals (minutes to hours) and successive waves can last for hours and days. Tsunamis are erosional and depositional with high energy levels in run-up and backwash. Storms consist of a surge with short wavelength wind waves (ten to several hundreds of meters) induced by changes of barometric pressure and wind stress. The frequency of run-up and outwash is high (10-20 s periods). Because of the surge, waves can reach much further inland than the normal location of wave reworked sediments. They can be both erosional and depositional, with energy levels that are highly variable. In order to investigate the timing and frequency of past tropical storms and tsunami activity, the coastal area of the South Andaman Islands was explored. Fieldwork was carried out to find suitable geological outcrops. Sampling was done on Red Skin Island and North Cinque Island located in the southwest and in the southeast of South Andaman, respectively. Samples were taken for radiocarbon and optically stimulated luminescence (OSL) dating to investigate the suitability of modern and submodern sediments for OSL dating. As additional information to characterise the sediment, grain size analysis are done for bulk samples from distinctive horizons using the particle analyser PartAn2001L from AnaTec. The aim of this paper is to set up a more reliable chronological frame for tsunami and storm events on the Andaman Islands. 43

44 2.2. Geological setting The Andaman Islands are the northern part of the Andaman-Nicobar Island Arc located in the Bay of Bengal at E and between N and N (see Fig. 2.1). The arc stretches from the north-western tip of Sumatra to the delta of the Ayeyarwady River in Myanmar. The Andaman Islands are about 1400 km away from the mainland of India and about 550 km away from Thailand. The three main islands, South, Middle and North Andaman, are 260 km long and less than 30 km wide. More than 300 small islands make the entire Andaman Island group. Most of these islands are surrounded by large coral reefs. Associated with the island arc are the extinct volcano Narcondam in the east of North Andaman Island and the still active volcano Barren Island in the east of Ritchies Archipelago. The topography of the Andaman Islands is hilly. The highest point is Saddle Hill at 750 m a.s.l. on North Andaman. The mountain ridges and valleys are mostly north-south orientated reflecting the geotectonic setting of the island. From a geotectonic point of view the Andaman Islands form a part of the great Indonesian Island Arc system (Pandey et al. 1992) which continues to the north into the Indo-Burma Range. To the south the Andaman-Nicobar Arc continues to the Mentawi Group islands of Indonesia. The Andamans are separated into the outer non-volcanic island arc and the inner volcanic arc including Barren, Narcondam and other seamounts. The inner arc connects with the central volcanic line of Central Burma to the north and of Sumatra and Java to the south (Acharyya 1997). The evolution of the Andaman- Nicobar Islands took place through a complex history of tectonics associated with the subduction of the Indian plate along the boundary with the West Burmese plate (Pandey et al. 1992). The geotectonic setting of the Andaman Islands has a major influence on the deposition of the sediments and also the behaviour of the minerals used for luminescence dating. The Andaman Islands are built up of Cretaceous and predominantly Tertiary and Quaternary rocks. These are dominated by alternating sequences of shales, siltstones and sandstones. They reflect the development of the forearc with flysch sedimentation from the Cretaceous to the Oligocene and later uplift during the Miocene (Pandey et al. 1992). Pleistocene deposits are dominated by calcareous rocks and Holocene sediments are widely distributed over the Andaman Islands. Rajshekhar and Reddy (2003) identified Holocene alluvium, raised beaches, terraces, wave cut platforms, coral rags, calcareous tuffa, shell limestone and beach rock. The investigated sites on South Andaman are of Holocene age. These are unconsolidated beach sediments Red Skin Island Red Skin Island ( N; E) is located about 5 km south east of South Andaman Island (Fig. 2.1). A 2.5-m thick sediment section was situated on the beach approximately 10 m 44

45 from the water line. The base of this section is composed of more than 30 cm of alternating layers of grey fine sand and black fine sand rich in organic material (part A in Fig. 2.2). From the organic-rich layers, two bulk sediment samples for radiocarbon dating were taken (Hv and Hv 25467). Overlying this basal sequence and separated by a sharp contact, is a 30-cm-thick weakly fining upwards sequence (part B in Fig. 2.2). The lowermost part of the sequence consists of a thin, poorly compacted, shell layer with mostly complete conches mixed with grey medium-grained sand. These deposits are covered by a horizon of fine to coarse-grained sand with bioclastic material. It is obvious from field evidence that the grain size decreases in the higher part of the layer. About 2 m below the surface charcoal was found within a light grey to yellow fine-grained sand layer, 65-cm-thick (lower part C in Fig. 2.2). The charcoal appears only in the lowermost 10 cm of this horizon and was sampled for ra- Figure 2.2: Lithological description of the sediment sequence from Red Skin Island. Details of the upper part (dashed rectangle) of this section are shown in Fig Samples for OSL dating are marked with LUM, samples for radiocarbon dating are marked with Hv. RSI-1 and 2 are the samples for grain-size analysis. Legend is shown in Fig The height was measured above the mean sea level. 45

46 diocarbon dating (Hv 25465). The yellow sand continues to the top into a 50-cm-thick horizon of finegrained sand which contains a lot of shell fragments and plant remnants (leaves and branches cut into small pieces) in the upper part (upper part C in Fig. 2.2). The uppermost 2 cm of this horizon is composed of medium to coarse grained sand which is weakly cemented in contrast to the loose sediment of the remainder of the sequence. The latter is succeeded by an about 20-cm-thick massive, diverse shell and coral fragment layer (part D in Fig. 2.2), with the shells including nearly 10-cm-large fragments of Nautilus, about 8-cm-large oysters and conches from Stromboidea and Tridacna. It is remarkable that the shells from Stromboidea are not damaged, especially the spikes are not broken. The outer surfaces of the shells from the oysters and Stromboidea reveal features of erosion (Fig. 2.3). It seems, that the shells are washed together by a stronger event. The position of the oysters and Stromboidea is similar to their life positions. One of the Tridacna shells was sampled for radiocarbon dating (Hv 25468). The upper parts of the section on Red Skin Island (part E in Fig. 2.2) are composed of a 40-cm-thick horizon of homogenous brownish fine-grained sand containing lots of roots from growing mangrove trees. From that layer a sample was taken for OSL dating (LUM 1000) and for grain size analysis (RSI-2). Separated by an erosional unconformity, two storm or tsunami derived layers complete the section (part F in Fig. 2.2). Each of the both layers is about 10 cm thick with a 2-cm-thick coarse-grained basal part and a fine-grained-8-cm-thick upper part (Fig. 2.4). The basal part contains a lot of coral and shell fragments and few millimetre thick layers of fine-grained material are intercalated. In the higher part of the 8-cm-thick upper part are dark around 2-mm- thick layers of silty material intercalated. An OSL sample was taken (LUM 999) from the lower storm or tsunami deposited layers. Grain-size analyses were made from the lower of these two layers (RSI-1). Figure 2.3: This picture shows the eroded shells from a Stromboidea (A) and an oyster (B) found at the section on Red Skin Island. Nearly all shells from this horizon are showing this feature. 46

47 Figure 2.4: Photograph showing the upper part (part E and F in Fig. 2.2) of the section on Red Skin Island. The right picture (A) shows an enlargement of the upper most part (part F in Fig. 2.2) with the possible tsunami layers. The coarse-grained basal parts are marked with dashed lines. The arrows indicate the dark silty layers. For detailed description of the section see text. Scale is in centimetres and vertical and horizontal scales are the same North Cinque Island (section 1) North Cinque Island is composed of three rock formations connected by coral reefs and sand bars (Fig. 2.1). The section under study ( N; E) was exposed in a small cliff of a creek about 20 m from the northern coastline situated in the middle part of the island. The creek is bordered by calcareous rocks with intense karst formations on the western side, whereas on the eastern side the rocks dip gently and disappear into the middle part of the island. There is water in the creek only during the rainy seasons. The valley of the creek was filled mostly by marine bioclastic sediments with a light yellowish colour. Only a small channel was eroded into the deposits by the creek. The top of this deposit is about 2.5 m above sea level. The section is split into two different depositional types including beach and river deposits in the basal part (part A in Fig. 2.5) and a thick tsunami or storm-laid deposit in the upper part (part B in Fig. 2.5). At the base of the section a horizon with alternating layers of greenish sandy silt and sandy finegrained pebbles occurs. This unit is more than 21 cm thick and contains fragments of shells, corals and organic material. Shells and corals were sampled for radiocarbon dating (Hv 25462). This shell-rich unit grades upwards into a 70-cm-thick fine to medium grained homogenous sand layer. In the basal part of this sand layer many small charcoal pieces were found. Grain-size analyses were done from 47

48 Figure 2.5: Lithological description of section 1 from North Cinque Island. Samples for radiocarbon dating are marked with Hv. NCI-1A and 1B are the samples for grain-size analyses. The legend is shown in Fig The height was measured above the mean sea level. this sand. The sand is covered by an about 1-cm-thick layer of weakly decomposed leaves, which is overlain by a layer of grey sand with well rounded cobbles up to 20 cm in diameter. The sediments from the basal part are covered by a 130-cm-thick deposit with an abrupt basal contact showing loading structures (Fig. 2.6). The sediment appears like a massive sandy body composed of medium to coarse grained bioclastic material with some layers of pebbles. The uppermost 22 cm of this layer shows a weak fining upward sequence. The middle part, approximately 98 cm in thickness, consists of fine to medium grained sand with intercalated fragments of gravel size and alternating thin layers of gravels and coral detritus. Grain-size analyses are done for a bulk sample from this horizon. The basal part of this deposit is dominated by mostly sandy material. A remarkable feature of this deposit is the uppermost 15 cm. There are two layers of coarse-grained sand to small pebbles (maximum 1 cm in diameter) separated by fine-grained material without a visible erosional contact (Fig. 2.6). 48

49 Figure 2.6: Photograph of section 1 on North Cinque Island showing the upper part (part B in Fig. 2.5). For detailed description see text and also Fig (A) Enlargement of the uppermost layers with the coarse-grained and gravely horizon. (B) Base of the deposit with an irregular contact to the grey fine grained sand. Scale is in centimetres and vertical and horizontal scales are the same North Cinque Island (section 2) Section 2 ( N; E) is located on the northern coastline of North Cinque Island (Fig. 2.1) approximately 400 m west of section 1. A tombolo-like structure about 300 m long and less than 50 m wide connects the limestone rock formation of the central part of this island with the one in the most western part of the island. This tombolo is built up of mostly bioclastic material derived from the coral reefs surrounding the island. In the eastern part of the transition to the bedrock, a dune-like morphology occurs. The section under study was exposed in a small beach-cliff about 10 m from the present coastline. This cliff has formed by wave erosion. At the margin of the cliff there is no evidence for deposits resulting from the 2004 tsunami, which most likely were eroded shortly after the tsunami. Based on field observations, it is very likely that the cliff was formed by high-energy events like storms or tsunamis. Three main layers can be distinguished in the sequence (Fig. 2.7). The sediment has a low content of siliciclastic material, with nearly 90% of the sediment fragments being derived from corals and shells. The lower part of the section (part A in Fig. 2.7) is composed of more than 50 cm of mediumgrained sand. The unit is homogenous apart from subangular black siliciclastic rock fragments up to 4 mm in size and occasional 5-cm sized coral fragments and 2 cm shell fragments. The base of the sediment sequence is not exposed. The middle part of the sequence (part B in Fig. 2.7 and Fig. 2.8) 49

50 Figure 2.7: Lithological description of section 2 from North Cinque Island. Samples for radiocarbon dating are marked with Hv. NCI-2A, NCI-2B and NCI-2C are samples for grain size-analyses. The legend is also valid for Figs. 2.2 and 2.5. The height was measured above the mean sea level. consists of a dark yellow, brown to black unit which is about 20 cm thick. This horizon has the same sedimentological features as the horizon above, but contains a lot of charcoal. Charcoal pieces are up to 8 mm in diameter and well rounded, indicating reworking under marine conditions. The boundary of this middle unit to the upper and lower unit is irregular, but there is no erosional unconformity visible. This horizon likely correlates to a palaeosoil and former stable land surface. The uppermost 70 cm of the sequence (part C in Fig. 2.7) consists of light grey to white medium-grained bioclastic material. The topmost 20 cm consist of a modern soil covering a non-bedded layer of medium to fine-grained light grey to white material with numerous black fine-sand-sized grains. Samples for radiocarbon dating were taken from the lowermost (corals Hv 25591) and middle part of this section (charcoal Hv 25459, corals Hv 25460, conch Hv 25471). Grain-size analyses were done for bulk samples from part A, B and C of the section. Sampling points are shown in Fig

51 Figure 2.8: Photograph of section 2 on North Cinque Island with the darker horizon in the middle part (marked with dashed lines). A, B and C are the same horizons as described in the text and shown in Fig Scale is in centimetres and vertical and horizontal scales are the same Methods Optically stimulated luminescence dating The optically stimulated luminescence (OSL) dating method is one of the dosimetric dating methods used in determining the time elapsed since the last light exposure of mineral grains, which equivalents the time since the deposition of the sediments. The physical model is based on the accumulation of electrons, induced by the natural radioactivity of the surrounding sediment, in defects of the crystal lattice of mineral grains. Due to this ionizing radiation, electrons are detached from the parent nuclei and diffuse in the crystal lattice. If the electron is in the vicinity of a defect in the lattice it will be trapped. From this trap it can only be released by additional energy. This could be heat that causes the crystal lattice to vibrate (thermoluminescence) or light (OSL). The electrons evicted from the traps diffuse into the lattice until a recombination centre is found. During the recombination, light is emitted and this light is measured by a photomultiplier in the laboratory. The intensity of the emitted light is proportional to the number of trapped electrons and increases with time since burial. 51

52 The age or time since deposition is determined by dividing the equivalent dose (measured by OSL) by the dose rate of the sediment (measured with gamma spectrometry). A more detailed description of the physical background is given in Aitken (1998), Smith and Rhodes (1994), Singhvi and Krbetschek (1996) and Krbetschek et al. (1997). For OSL dating quartz and feldspar grains are used as dosimeters because they are widely distributed. Luminescence dating methods are applied to aeolian sediments like loess (Frechen et al. 2003) or dunes (Kunz et al. 2010), to fluvial sediments (Wallinga et al. 2001; Schokker et al. 2005) and glacial sediments (Alexanderson 2007). It is even possible to date very young sediments with depositional ages of less than 10 years (Ballarini et al. 2003) and very old sediments with ages up to years (Wang et al. 2006, 2007). The optically stimulated luminescence dating method extends the dating range for younger and older ages far beyond the dating limit of radiocarbon. The errors for the OSL ages are between 5 and 10% (Jacobs 2008; Wintle 2008). Samples for OSL dating were collected in opaque plastic tubes hammered into the sediment after cleaning the wall in daylight. The cylinders were sealed to protect the samples from light exposure. Additional material, approximately 1 kg, was taken for gamma spectrometry. The sample containers were opened under subdued red light in the laboratory. The light-exposed outer parts of each sample, about 2 cm on each side, were removed and discarded. The rest of the sample was dried at 50 C for one or more days depending on the moisture content of the sample. The drying was only undertaken as long as necessary, because feldspars are sensitive to higher temperatures (Wallinga et al. 2001). After drying, the samples were sieved and the grain-size fraction between µm was taken. Carbonates and organic material were removed by treatment with hydrochloric acid and hydrogen peroxide, respectively. The quartz and potassium-rich feldspar minerals were separated using sodium polytungstate as a heavy density liquid. A detailed description of the sample preparation is given in Aitken (1998) and Ujházy et al. (2003). The quartz grains (10 g) were subjected to 1-h etching in concentrated hydrofluoric acid, as described in Zander (2000). This procedure is necessary to remove remnants of other minerals like feldspars and the outer rim of quartz grains, which is affected by alpha-radiation The prepared mineral grains were mounted with silicon oil on stainless steel discs with a diameter of 10 mm. During the measurement the discs were checked for feldspar contamination with the so-called depletion ratio of Duller (2003). A Risø Reader TL/OSL-DA-15 was used for the measurement of the luminescence signal. The quartz samples are stimulated with blue LEDs with a wavelength of 470 nm and a maximum power of 40 mw/cm². The feldspar grains are stimulated with infrared LEDs with a wavelength of 875 nm and a maximum power of 135 mw/cm². The emitted light is detected with a bialkali EMI 9235QA photomultiplier tube with a detection window between nm. Optical filters are mounted in front of the PM-tube to avoid the detection of the stimulation light. For the measurement of quartz a 7-mmthick Hoya U-340 filter with a transmission in the range of nm was used. A 6.5-mm-thick 52

53 combination of a Schott BG 39 and a Corning 7-59 filter was used for the measurement of feldspar. Both filters give a transmission in the range of nm. Irradiation of the samples was done by an attached 90 Sr/ 90 Y beta source with a dose rate of Gy/s Gamma spectrometry: sample preparation and measurement The natural luminescence signal is mainly the result of the natural radioactivity from uranium chains, thorium chains and potassium-40 and a minor portion of some other radioisotopes and cosmic radiation. For the age calculation it is necessary to know the dose rate of the sediment, which is measured by gamma spectrometry with an HPGe (high-purity germanium) N-type coaxial detector in the laboratory. For the dosimetry the infinite matrix approach is assumed (Aitken 1998). This means for a volume having dimensions greater than the range of radiation, that the rate of energy absorption is equal to the rate of energy emission. The dose rate is calculated from the activity of 238 U, 232 Th and 40 K (Aitken 1998). For the measurement 700 g of the dried sample were homogenised. The material was placed into a Marinelli beaker and the cap was sealed to avoid loss of 222 Rn in the 238 U decay chain. The beaker was stored for a minimum of 4 weeks so that the radon disequilibrium could adjust again. Only potassium is measured directly from the decay of 40 K to 40 Ar by release of gamma rays. The activity of 238 U and 232 Th cannot be measured directly by gamma counting because both nuclides have an alpha decay without release of detectable gamma rays. The activity can be calculated from the gamma rays released by the daughter nuclides in the decay chain of 238 U and 232 Th, respectively. 210 Pb, 214 Pb, 234 Th and 214 Bi were taken to calculate the activity of 238 U and 212 Pb, 208 Tl and 228 Ac to calculate the activity of 232 Th. Table 2.1 summarizes the nuclides and the energies used for the calculation. This method allows the monitoring for radioactive disequilibrium. If there is a radioactive disequilibrium, then the activities for the daughter nuclides from uranium or thorium are not equal. Table 2.2 (on page 61) shows the results of the gamma spectrometry for the luminescence samples from the Andaman Islands. For these samples radioactive equilibrium could be observed supported by the equivalent activities of the nuclides in the decay chains of 238 U and 232 Th, respectively. The contribution by alpha particles for dating of quartz grains is negligible due to the etching of the outer rim of the grains. For potassium-rich feldspar the alpha contribution has to be corrected by a factor: the k value (Aitken 1998) or the alpha-efficiency as an equivalent correction factor. For the age calculation of the samples here, an alpha-efficiency of 0.08 ± 0.02 proposed by Wallinga et al. (2001) was used. Additionally, the internal radioactivity of the potassium-rich feldspar grains has to be kept in mind. For this correction the internal potassium content of 12.5 ± 0.5% of Huntley and Baril (1997) was used. The cosmic radiation, which depends on the latitude, longitude and the thickness of the sediment cover, is calculated following the approach of Prescott and Hutton (1994). 53

54 Table 2.1: Energies used to calculate the activity of 238 U and 232 Th. Uranium-238 Thorium Pb (kev) 214 Pb (kev) 234Th (kev) 214 Bi (kev) 212 Pb (kev) 208 Tl (kev) 228 Ac (kev) The activity is represented by the area under the energy-peak Radiocarbon dating In this study charcoal, corals, conches, shells from snails and organic-rich sediment were taken for radiocarbon dating. The specific activity of 14 C was measured radiometrically by proportional counters (Geyh 2005). The conventional ages are given with a 2σ standard deviation and were calculated into calibrated ages using Calib based on Stuiver and Reimer (1993). For terrestrial material the dataset intcal04 (Reimer et al. 2004) and for marine material the dataset marine04 (Hughen et al. 2004) was used. The marine reservoir correction (ΔR) for marine material was applied. For the Andaman and Nicobar Islands ΔR values were published by Dutta et al. (2001) and Southon et al. (2002). From these values the weighted mean and the average uncertainty was calculated and a ΔR value of 16 ± 31 for the correction was used. All calibrated 14 C-ages are in the 2σ range. Radiocarbon ages allow a resolution in the best case of ±15 years for Holocene material. Normally the resolution is between ±40 and ±50 years. It increases to ±2000 years for the maximum age at BP (Geyh 2005). Because of the complexity of the 14 C production curve over the last 600 years, non-unique solutions are found leading to multiple ages, whereas the OSL dating method gives always one age in that time period (Duller 2004) Results Three sections on the Andaman Islands were sampled for OSL, radiocarbon dating and grain size analyses, whereas the section on Red Skin Island was sampled for OSL dating only. The material from the sections on North Cinque Island was not suitable for OSL dating owing to the very low content of siliciclastic material. First tests were made with quartz using the single aliquot regenerative (SAR) dose protocol after Murray and Wintle (2000, 2003). The results of the preheat tests showed that the luminescence properties of quartz were not suitable for dating material from the Andaman sites. The 54

55 natural signal was very weak, hardly distinguishable from background. Even after applying higher beta-doses of 3.5 Gy, the luminescence signal of the quartz did not increase. In this case we decided to use the potassium-rich feldspar for OSL dating applying the SAR protocol. Different preheat tests, following the work of Blair et al. (2005), Fattahi and Walker (2007) and Wallinga et al. (2000, 2007) were carried out to obtain the appropriate protocol. It was found that the SAR protocol of Wallinga et al. (2007) provided the most suitable results for this kind of material. Based on this protocol, the preheat test was performed. Heating of quartz or feldspar can generate transfer of charge. This test shows that, if there is any temperature-dependent transfer of charge, it can cause erroneous determination of the equivalent dose D e (Wintle and Murray 2006). The aliquots of one sample were split into groups of three. Each group was treated with different preheat temperatures and measured in steps of 20 C from 160 to 300 C. In contrast to previous SAR protocols (Wallinga et al. 2000) we applied the same cutheat-temperature and preheat-temperature in this study. Table 2.3 shows the steps used for this measurement. Transfer of charge is detected by an increase of D e with increasing preheat temperature. Fig. 2.9 shows the preheat tests for samples LUM 999 and LUM Both samples show a plateau from 160 to 240 C. For higher temperatures the D e -values increase. Based on this result a preheat temperature of 180 C was used for both samples. Table 2.3: SAR protocol for feldspar used for the preheat tests. Step Treatment Observed 1 a Give regenerative dose D i 2 Preheat C for 10 s (5 C/s heating rate) 3 IRSL 100s at 50 C L i 4 Give test dose D t 5 Cutheat C for 10 s (5 C/s heating rate) 6 IRSL 100 s at 50 C T i 7 IRSL 40 s at 285 C a For the natural sample i = 0 and D 0 = 0. The whole sequence is repeated for several regenerative doses, including a zero dose and a repeat dose. After determination of the appropriate preheat temperature, dose-recovery tests based on the protocol, as described by Murray and Wintle (2003) were carried out. This test determines whether an artificially applied dose can be recovered by the SAR protocol. Ten aliquots were bleached in the Risø Reader for 300 s by infrared-light at room temperature. After 2-h delay the bleaching step was repeated followed by irradiation with a fixed beta-dose (0.35 Gy for both samples) which is close to the expected equivalent dose. This dose was recovered using the SAR protocol developed for this sample (Table 2.4). The ratio between the recovered dose and the applied dose should give a value of 1.0 ± 0.1 for a good working SAR protocol. For sample LUM 999, nine aliquots could reproduce the applied dose within 10% deviation from unity and one aliquot within a deviation of 15% (Fig. 2.10a). For sample LUM 1000 five discs could reproduce the applied dose within 10% deviation from unity, three aliquots within 15% deviation and one aliquot had a deviation of more than 20% (Fig. 2.10b). 55

56 Figure 2.9: Graph showing the results for the preheat test and the recycling ratios for sample LUM 999 (A) and LUM 1000 (B). For each temperature the mean of three discs was calculated. Based on these tests for both luminescence samples a SAR protocol with 180 C preheat and the same cutheat-temperature was used. Table 2.4 shows the steps in this SAR protocol. The mean D e was calculated using the central age model of Galbraith et al. (1999). The equivalent dose for sample LUM 999 is ± Gy and ± Gy for sample LUM Based on this dataset and an estimated water content of 20 ± 2% for each sample, the ages are calculated. The calculated age for sample LUM 999 is 10 ± 2 and 63 ± 8 years for the stratigraphically older sample LUM 1000 (Table 2.2 on page 66). Table 2.4: SAR protocol for feldspar used for the measurement of both luminescence samples. Step Treatment Observed 1 Give regenerative dose D a i ( Gy) 2 Preheat 180 C for 10 s (5 C/s heating rate) 3 IRSL 100s at 50 C L i 4 Give test dose D t = 0.35 Gy 5 Cutheat 180 C for 10 s (5 C/s heating rate) 6 IRSL 100 s at 50 C T i 7 IRSL 40 s at 285 C a For the natural sample i = 0 and D 0 = 0. The whole sequence is repeated for several regenerative doses, including a zero dose and a repeat dose. Uranium, thorium and potassium content range from 2.71 to 3.60 ppm, to ppm and 0.78 to 0.91%, respectively (Table 2.2 on page 66). The dose rate of the sediment is between 2.86 and 3.3 Gy/ka. Radiocarbon ages were calculated for samples from all three sections under study (Table 2.5). A Tridacna from the massive shell layer on Red Skin Island yielded a calibrated radiocarbon age of cal AD (anno domini). The shells from the lower part of the section taken at a depth of 0.9 m below sea level gave a calibrated age of cal BC (before Christ). A similar calibrated age of cal BC was obtained from the shells in the lowermost part of this section. The organic 56

57 Figure 2.10: Graph of the results of the dose recovery tests for sample LUM 999 (A) and LUM 1000 (B). Applied dose was 0.35 Gy for both samples. The dashed lines are the 15% limit for the recovered dose. material from this horizon has a slightly higher age of cal BC. On North Cinque Island the shells and corals from the lower part of section 1 (part A in Fig. 2.5) gave a calibrated age of cal AD. The charcoal, corals and conch from the brownish horizon in section 2 on North Cinque Island yielded calibrated ages of cal AD, cal AD and cal AD, respectively. The corals from the horizon below gave a calibrated age of cal AD. The dating results are summarized in Fig showing the relation of the deposits according to their elevation. Deposits with a young age can be found in lower elevation as deposits with a higher age indicating reworking of sediment in recent times. Grain-size analyses were done for two horizons from the section on Red Skin Island, for two horizons from section 1 from North Cinque Island and for three horizons from section 2 from North Cinque Island. The grain-size distribution is shown in Fig All samples show a bi- or poly-modal distribution. The upper sample from the section in Red Skin Islands (RSI-1) shows a bimodal distribution with peaks at and phi (fine- to very fine-grained sand). The sample from the horizon below (RSI-2) shows a polymodal distribution with one dominant peak at phi (fine-grained sand) and two peaks at phi (medium-grained sand) and phi (very fine-grained sand). At section 1 on North Cinque Island the sediment in the upper part (NCI-1A) shows a polymodal distribution over a wide range from phi (gravel to fine sand). There is a dominance of material at phi (pebbles) and phi (coarse- and medium-grained sand). The sediment below this deposit (NCI- 1B) shows a clearly different grain-size distribution. It has also a polymodal distribution but only in the coarse- to fine-grained sand fraction ( phi). Two peaks at phi (medium-grained sand) and phi (fine-grained sand) are dominant. In section 2 on North Cinque Island the grain-size distribution of the samples (NCI-2A, NCI-2B and NCI-2C) shows a bimodal distribution in the range phi with two peaks at phi (coarse-grained sand) and 1-2 phi (medium-grained sand). The coarse sand fraction increases with depth. The amount of material in the middle sand fraction is more or less constant. 57

58 Figure 2.11: Summary of the dating results from the sections on Red Skin and North Cinque Island. The height was measured in above the mean sea level. Samples indicated with Hv are dated by conventional radiocarbon dating. Conventional radiocarbon years ( 14 C year BP) are years before 1950 AD. The calibrated ages are presented in brackets; AD anno domini and BC before Christ. Samples indicated with LUM are dated by optically stimulated luminescence dating. The year (yr) and age range in brackets are related to the year of sampling in 2006 AD. 58

59 Figure 2.12: Grain-size distribution of the samples from Red Skin Island (left), North Cinque Island section 1 (middle) and North Cinque Island section 2 (right). All samples show a bi- or poly-modal distribution. Grain categories are given for the sand fraction and based on Wentworth (vc = very coarse sand, c = coarse sand, m = medium sand, f = fine sand, vf = very fine sand). Grain category from -5 to -1 phi is assigned as gravel and from 4-5 phi as silt Discussion Red Skin Island The deeper part of this section (parts A and B in Fig. 2.2) reflects tectonic activity resulting in the subsidence of this part of the Andaman archipelago. The shells and organic material are now in a depth of about 1 m below present sea level. But the deposition of the material was at sea level or higher due to wave action. The radiocarbon ages show, that the material was deposited earliest in the year 1154 BC (Table 2.5). Since that time somewhen subsidence occurred. The uplift and subsidence of the Andaman Islands is episodic following strong earthquakes (Rajendran et al. 2007). On Interview Island, west of North Andaman Islands, and on Little Andaman, coral terraces are exposed above sea level. The younger terraces are dated to 1042 ± C year BP (before present = years before

60 AD) and 3286 ± C year BP (Rajendran et al. 2007). These marine terraces are related to strong earthquake events in the past. The southern parts of the Andaman Islands are subsiding. In the area of Port Blair evidence for past submergence is described, as evidenced by tree trunks and peat layers yielding 710 ± C year BP and 3070 ± C year BP (Rajendran et al. 2007). Compared to the vertical movement of the Andaman Islands after the earthquake in December 2004 (Shishikura et al. 2005), this layer might also relate to a strong earthquake in the past. It is not clear which of these two events about 1000 and 3000 years before present reported by Rajendran et al. (2007) is reflected here. Kayanne et al. (2007) show that there is a coseismic subsidence and a postseismic uplift of the South Andaman Islands. This movement follows a complex pattern. Especially in the southern part of the Andamans the movement occurs in a NE-SW oriented direction, like a syncline. The central part of this synclinal structure lies in the area of Port Blair (Kayanne et al. 2007). After the 2004 earthquake the co- and post-seismic movements resulted in a subsidence of nearly 1 m. In the outer parts of this subsiding synclinal-like structure the resulting movement was zero. Red Skin Island is situated on the western flank of this structure. If we assume, that this pattern of tectonic movement exists since or even prior to the Holocene the sediments here have been subsided and uplifted in different modes after the earthquakes in the last 3000 years. This complicates the interpretation of the event-history here. Evidences for younger events are missing here. The massive shell layer in part D in Fig. 2.2 indicates storm events which washed the material together. It seems that several storm events are recorded here. There is a great variety of species and a mixture of littoral-living organisms (e.g. Stromboidea, marine gastropoda, Tridacna and other bivalvia) with pelagic-living organisms (broken shells from Nautilidae). Indicated by the presence oysters, there are also species that live on other substrates as found here along the beach. The material is very likely washed together over a long period. The earliest time of deposition is given by the radiocarbon age of the shell from Tridacna (Hv 25468) in cal AD. But later deposition and redeposition is very likely. The erosional features of the shells (Fig. 2.3) indicate a period of exposure on the beach in the surf zone. The shells are covered half with sand and the rest was eroded slowly by wave action. The OSL ages reflect a quite recent deposition of the two horizons at the top of the section on Red Skin Island. Additionally, a broken glass bottle found in the horizon of sample LUM 1000 gives evidence of a young depositional age. The errors of the OSL ages with 20% for sample LUM 999 and 12.7% for sample LUM 1000 are unusually high for the SAR protocol. Normally the errors are much smaller and in the range of less than 10% (Nielsen et al. 2006). It seems that, for young samples, such large errors are common (Ballarini et al. 2003; Madsen et al. 2007). One reason is the bad ratio of signal to background resulting in a large uncertainty. A second reason is the incomplete bleaching of the mineral grains. This is a problem for rapidly deposited sediments like storm deposits or tsunami sediments. In this study incomplete bleaching can be excluded because the aliquots do not show any scatter in data and there is just one population. For incomplete bleached samples a polymodal distribution of the equivalent dose D e is expected (Lukas et al. 2007). 60

61 Table 2.2: Results of the OSL dating and values used to calculate the age. Sample Grain size (µm) Water Depth (%) a (m) Potassium (%) Uranium (ppm) Thorium (ppm) Cosmic dose (mgy/ka) Equivalent dose De (Gy) LUM ± ± ± ± ± ± ± 2 LUM ± ± ± ± ± ± ± 8 a Water content is an estimate Age (years) Table 2.5: Results of the radiocarbon dating from the sections on South Andaman Island. Location Sample Height/depth m a.s.l. Material δ 13 C 14 C yr BP Calibrated age Red Skin Island Hv Shell (Tridacna) ± cal AD a Red Skin Island Hv Shells ± cal BC a Red Skin Island Hv Organic material ± cal BC Red Skin Island Hv Shells ± cal BC a North Cinque Island (section 1) Hv Shells, corals ± cal AD a North Cinque Island (section 2) Hv Charcoal ± cal AD North Cinque Island (section 2) Hv Corals ± cal AD a North Cinque Island (section 2) Hv Conch ± cal AD a North Cinque Island (section 2) Hv Corals ± cal AD a The conventional radiocarbon years ( 14 C year BP) and calibrated ages are in the 2σ range. AD = anno domini, BC = before Christ, BP = years before present (before 1950 AD). a Marine reservoir correction was applied for calibrated ages. 61

62 The OSL age of the deposit (part E in Fig. 2.2) above the shell layer (part D in Fig. 2.2) shows a deposition 63 ± 8 years ago. As sampling was carried out in the year 2006 the deposition could have happened already in the year For 1941 a strong tsunami (Manimaran and Chacko 2006) is recorded. It could be that this horizon was deposited during this event. There are no other records of events for this time period for the South Andaman Islands. As the field observations do not provide clear evidence for a tsunami deposit, this conclusion is nevertheless constrained. From our appraisal, the deposit might well have originated from a strong storm event. A clear differentiation and assignment of these two kinds of events and the resulting geomorphological and sedimentological features is rather challenging, as the generated deposits are quite similar (Kortekaas and Dawson 2007). The layer, studied at the section of Red Skin Island, looks homogenous and contains a broken glass bottle, indicating a fast deposition during modern times. The thickness of 40 cm is unusual for a tsunami deposit. As the base of this horizon is covered by groundwater, no indicative sedimentological features like, e.g. an erosional unconformity could be identified. Rip-up clasts and silty layers in the higher parts are also missing. The latter are typical features of tsunami deposits (Morton et al. 2007; Tuttle et al. 2004). This sand layer was deposited later than the shell layer (part D in Fig. 2.2) because of the erosional features of the shells. Hence, both layers are not related to the same event. The two layers on top of the section (Fig. 2.4) could have been deposited by a tsunami or by a tropical storm in the last 10 years. Goff et al. (2004), Kortekaas and Dawson (2007), Morton et al. (2007), Tuttle et al. (2004) and Nanayama et al. (2000) provide criteria for distinguishing tsunami and storm deposits. When we apply these criteria on these deposits, it is very likely that these are tsunami deposits. It has a continuous lateral extend about 100 m inland and we observed it for about 300 m along the coast. The material overlies the organic rich sand with a sharp contact. The deposit consists of two layers, each about 10 cm thick. Storm deposits show normally more layers or foresets and they are thicker and do not reach so far inland. Other important features are the dark, thin, silty layers in the upper parts of the deposit. They indicate a stand still of the inundating wave and a deposition of very fine-grained material from the water column (Morton et al. 2007). Such very fine-grained layers are not observed in storm deposits. The hydraulic conditions during a storm do not allow the sedimentation of fines (Tuttle et al. 2004). The OSL age of 10 ± 2 years for this deposit supports the correlation to the Indian Ocean tsunami from December It is very likely, that these deposits are younger than 10 ± 2 years. Bishop et al. (2005) investigated deposits from the tsunami 2004 from Thailand by OSL. These deposits should have an age of zero years. But a residual OSL signal was measured and the calculated ages range from about ten years up to hundreds of years. The very likely reason is incomplete bleaching during the fast transport in sediment loaded water. The same effect very likely took place on Red Skin Island. The occurrence of two layers is supported by eyewitness reports describing up to three waves reaching the Andaman Islands in December 2004 (Rajendran et al. 2007). The relevant layers are situ- 62

63 ated near the coast at low elevation but higher than the high tide level. The inundation height of the tsunami for the Wandoor region ranges between 2.2 and 3.4 m (Shishikura et al. 2005). Based on the data from the Joint Typhoon Warning Center more than eight tropical storms passed the Andaman Islands in the last 10 years. Between December 2004 and spring 2006 no heavy tropical storm hit South Andaman Islands. This can explain the preservation of these sediments North Cinque Island, section 1 It was first thought, that the thick deposit in the upper part of this section (part B in Fig. 2.5) was deposited from the Indian Ocean tsunami in December But applying the criteria from Kortekaas and Dawson (2007), Morton et al. (2007) and Tuttle et al. (2004) the sediments correlate very likely to a storm deposit. There are a few criteria supporting this. The thickness of about 130 cm is more common for storm deposits than for tsunami deposits with thicknesses smaller than 30 cm (Jaffe et al. 2003; Morton et al. 2007; Tuttle et al. 2004; Srinivasalu et al. 2007). A large lateral extent, which is typical for tsunami deposits, was not observed on North Cinque Island. It is only limited to the creek and reaches not more than 50 m inland. The sedimentological structures are more typical for storm deposits. It appears more homogeneous, no fining upward sequences and no rip-up clasts are observed. The last two are typical features of tsunami deposits (Kortekaas and Dawson 2007). The grain size and sorting are no definite criteria for a tsunami deposit, because it depends strongly on the available material and the speed of deposition (Morton et al. 2007). This deposit here is poorly sorted and coarse grained (Fig. 2.12) which is in contrast to the tsunami deposit observed on Red Skin Island (part F in Fig. 2.2, 2.4) which is well sorted and fine grained (Fig. 2.12). The basal contact is abrupt and shows loading structures. Both are related to tsunamis and storms (Kortekaas and Dawson 2007; Morton et al. 2007). It is not clear whether the two coarse grained and gravely layers on top of this deposit (Fig. 2.6) are related to the storm deposit below or to another later deposition. It is very likely, that these two layers belong to the storm deposit because there is no erosional contact visible and the gravel appears to develop from the finer material. Underlying the storm deposit, the previous land surface was detected. At a height of 0.5 m above sea level shells and corals gave 710 ± C year BP ( cal AD), indicating an earliest possible deposition in the year 1444 AD. But it is very likely that this material was redeposited. Compared to the sedimentary dynamics reflected in the section on Red Skin Island, a stable period for about 700 years is unlikely. Traces of redeposition could not be observed at this section, also there were no traces of other storm events. It is possible, that the river eroded the sediments. 63

64 North Cinque Island, section 2 The cliff, where section 2 is located, is formed by erosion of a beach ridge. The samples from section 2 were taken from higher elevations and gave younger ages than those of the other sections (Fig. 2.11). In the brownish horizon (part B in Fig. 2.7) the three radiocarbon samples show a mixture of ages indicating redeposition. This is supported by the very well-rounded nature of the charcoal fragments indicating a period of transport along the beach. The ages span a time interval from the year 1409 AD to 1895 AD. The earliest possible time for deposition is hence the year 1409 AD. The radiocarbon age from the coral taken from the horizon below (part A in Fig. 2.7) suggests an earliest deposition in the year 464 AD. Compared to the descriptions of Otvos (2000) and Tanner (1995) the origin of the ridge is very likely by storm surges. Thick storm deposits are also observed at section 1 on North Cinque Island and at the section on Red Skin Island. It is likely that in the ridge at least three storm events were preserved. The brownish horizon (part B in Fig. 2.7) could be indicative for a short period of stabilized land surface. The age of the topmost horizon (part C in Fig. 2.7) is unknown. But the deposition seems some time ago, supported by the soil horizon and the grass vegetation on top. The top of the ridge is about 3.5 m above the mean water level. The mean high tide is +1 m and cannot reach the cliff. Hence, the erosion of the cliff must be by waves from high-energy events like storms or tsunamis. Due to the stabilized surface no strong storm in the past occurred. A tectonic uplift of North Cinque Island as described in Kayanne et al. (2007) and Shishikura et al. (2005) is also possible but cannot be shown here Conclusions We investigated three sections on the South Andaman Islands in the eastern part of the Bay of Bengal. These sections are all located near the present shore line and were sampled for radiocarbon dating and OSL dating. In this study a time range from the present back to 3000 years was covered. The investigated sections show deposits which are indicative of former earthquakes and high-energy events like tsunamis and storms. The geochronological investigations reveal the event-history of the South Andaman Islands for the late Holocene. Sediments related to an earthquake about 3000 years or 1000 years ago are recorded in the section on Red Skin Island. The recently deposited sediments with an OSL age of 63 ± 8 years and the shell layer give evidence for storm events in the last 800 years. On Red Skin Island the topmost layers of the section with an OSL age of 10 ± 2 years were very likely deposited by the tsunami From Bishop et al. (2005) it is known, that recently accumulated sediments are not completely bleached and hence could have a residual OSL signal leading to a higher age. In this study the OSL results show no scattering in data, hence an incomplete bleaching of the 64

65 sediments is unlikely. The sedimentology, the geomorphological position higher than the high tide level, eyewitnesses which reported up to three tsunami waves reaching the coast of the Andaman Islands in 2004 and the fact, that no heavy tropical storms occurred in the period from 2004 to 2006 support the conclusion of the deposition by the 2004 tsunami. The deposits in sections 1 and 2 on North Cinque Island reflect storm events for the last 1,000 years. Deposits of palaeotsunamis and from the Indian Ocean tsunami in December 2004 are not found here. Due to the high storm activity traces of tsunami events are very likely eroded. Another explanation for missing palaeotsunamis could be, that no tsunamis occurred for the last 3000 years in the Bay of Bengal. In Chile (Cisternas et al. 2005) and Kamchatka (Nanayama et al. 2003) tsunami deposits are related to strong earthquakes. For the Bay of Bengal, Stein and Okal (2007) propose that tsunamis are triggered by earthquakes with magnitudes greater 9 (M w 9). Since the year 2004, no earthquakes were observed with that magnitude along the Andaman and Nicobar Islands (Chandra 1977; Rao and Rao 1984; USGS 2008). Rajendran et al. (2007) related the uplift of North Andaman and the subsidence of South Andaman to earthquakes released by the subsidence of the Indian plate below the Sunda plate. They dated uplifted terraces on North Andaman Island. The recurrence interval for earthquakes with large magnitudes calculated from his work is greater than 1000 years. Bilham et al. (2005) also predict a recurrence interval of about 1000 years. Also for northern Sumatra long recurrence intervals are reported by Monecke et al. (2008). This long recurrence interval for large earthquakes which can produce ocean-wide tsunamis is also reflected by the absence of ancient records about these events (Arunotai 2006; Bilham et al. 2005; Manimaran and Chacko 2006). It is still unclear how often earthquakes and tsunamis occur in the Bay of Bengal. Keeping in mind the destructions from the earthquake and the tsunami in December 2004, it remains important to investigate the coastal areas to establish a chronology of past events. Summarizing, the fieldwork and the geochronological work reveal a first approach to reconstructing event-history represented by subsidence and uplift after earthquakes, by periods of stability with erosion and reworking of coastal sediments and by high-energy wave events like tsunami and storms for the Andaman Islands Acknowledgments This research was done with the cooperation of the Department of Ocean Management, Anna University, Chennai, India and LEUPHANA University Lüneburg, and Leibniz Institute for Applied Geophysics, Hannover, Germany andwas financed by the German Federal Ministry of Science and Education (BMBF) and by the Indian Department of Science and Technology (DST), which is highly appreciated. We cordially thank Prof. Peter Kershaw from the Monash University, Australia, for valu- 65

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73 Chapter 3 Quaternary International 222 (2010) Luminescence dating of Late Holocene dunes showing remnants of early settlement in Cuddalore and evidence of monsoon activity in south east India Alexander Kunz a,b, *, Manfred Frechen b, Ramachandran Ramesh c, Brigitte Urban a a Leuphana University, Suderburg, Germany b Leibniz Institute for Applied Geophysics, Hannover, Germany c Institute for Ocean Management, Anna University, Chennai India * corresponding author: jan-alexander.kunz@liag-hannover.de Abstract In this study dunes from south east India were dated using optically stimulated luminescence (OSL) to reconstruct the depositional history. A belt of dunes has developed parallel to the coast between Pondicherry and Karikal in Tamil Nadu, south east India. In the area between Cuddalore and Porto Novo the dune belt is 5 km wide. A transect from the coast to the most western dune inland was investigated. Changes in the environmental conditions are recorded in the dunes. They show features including unconformities, changes in the direction of bedding, erosional features, water escape structures and remnants of human settlements. The OSL results show that strong changes in the environmental conditions occurred about 100 and 300 years ago. The latter event marks also the termination of settlement in this place. The settlement period started about 1500 years previously. The periods of sand mobility and stabilisation of the land surface by soil formation correlate with changes in the precipitation record of India. The investigated dunes very likely reflect fluctuations in the monsoon activity during the last 3500 years in south east India. 73

74 3.1. Introduction Optically stimulated luminescence (OSL) dating is an excellent tool to determine the deposition age of sediments, which is the time elapsed since the last exposure of the mineral grains to sunlight. OSL dating has been applied to a variety of different depositional environments such as aeolian deposits (e.g., Bateman et al., 2004; Roberts, 2008), fluvial deposits (e.g., Rittenour, 2008; Rodnight et al., 2006) and glacial deposits (e.g., Bateman, 2008; Fuchs and Owen, 2008). Due to the physical mechanism of the OSL method, reliable and precise results for well bleached aeolian loess or dunes can be determined. Jacobs (2008), Lancaster (2008) and Singhvi and Porat (2008) give summaries of the application of OSL dating to coastal and marine sediments, the development and the dynamics of desert dunes, and to geomorphological and palaeoclimatological research in dry lands. In India, large dune fields occur only in the Thar Desert situated in the northwestern part of Rajasthan, close to the border with Pakistan. Along the eastern coast of peninsular India, narrow belts of sand dunes, coastal dunes and beach ridges are present (Fig. 3.1). They are connected to the low lying deltaic and coastal plains. Larger areas covered with dunes are located along the coast of West Bengal Figure 3.1: Map showing the distribution of important dune areas in India. The dark grey shaded areas indicate the zones of coastal dunes and sand dunes in the low lying coastal areas. Typical desert sand dunes are only found in the Thar Desert in the north-east of India. AP = Andrha Pradesh, CG = Chattisgarh, GJ = Gujarat, JK = Jammu and Kashmir, KA = Karnataka, KL = Kerala, MH = Maharashtra, MP = Madhya Pradesh, OR = Orissa, RJ = Rajasthan, TN = Tamil Nadu, UP = Uttar Pradesh, WB = West Bengal. 1 = Penner River Basin, 2 = Tapi River Basin, 3 = Narmada River Basin, 4 = Luni River Basin. in the northeast of India, along the coast of Andhra Pradesh in the delta plains of the Krishna River and the Godavari River, and along the coast of Tamil Nadu in the southeast, especially in the area of the Cauvery Delta and the southeastern-most part along the coast of the Gulf of Mannar. On the west 74

75 coast of India, beach ridges and dunes occur only in a narrow belt along the south coast of Kerala. Jayappa and Vijaya Kumar (2006) summarize the morphological features of the coast of India. Luminescence dating was applied on dunes in the Thar Desert (Chawla et al., 1992; Juyal et al., 2003; Singhvi et al., 1994; Singhvi et al., 1982; Thomas et al., 1999) to investigate the sedimentary dynamics of dunes, sand accumulation rates, and palaeoclimate reconstruction. The chronology of dunes in the low lying deltaic and coastal plains in India has not yet been investigated. Sedimentology and mineralogy of the dunes and beach ridges in the region between Pondicherry and Porto Novo (Fig. 3.2) have been studied to find resources for heavy minerals (Chandrasekharan and Murugan, 2001; Mohan, 1995; Mohan and Rajamanickam, 2000). This study presents the first OSL ages for the dunes in the Cuddalore region. This geochronological framework is used to reconstruct the depositional history of the dunes. The investigation of the dunes in the Cuddalore region gives new insights into periods of sand mobilisation, which are connected to climate conditions, and the age of dune formation. The influence of human activities and the role of sea level changes and tectonic activities as mechanisms for the development of coastal dunes in this place will be discussed briefly Regional setting The study area (Fig. 3.2) is located at the Coromandel Coast in Tamil Nadu, approximately 50 km south of Pondicherry. The cities of Cuddalore to the north and Porto Novo to the south are both 20 km away from the area of investigation. The area is characterized by a belt of coastal dunes running parallel to the coast. This dune belt is about 12 km wide in the south near Porto Novo and narrows to the north, until it is less than 5 km wide near Cuddalore. This place was chosen for research because it is sparsely populated. Villages have been built mostly on the dunes. The areas between the dunes are used for agriculture. The accessibility to the dunes in the wide southern part of the dune belt is limited due to the periphery of Porto Novo which is densely populated. The coastal dunes and sand dunes are separated by slacks and swales and partly stabilized by vegetation. Some of the dunes and adjacent areas are covered by cashew trees. There are also dunes that are still moving and recently constructed houses are buried by large dunes. The height of the dunes varies. Near the coast, they are no more than a few metres high; further inland, the dunes reach heights of more than 10 m. During fieldwork, five dunes at three localities were investigated: one dune situated directly on the coast, two dunes approximately 2 km, and two dunes about 5 km away from the coast. The results of the optically stimulated luminescence (OSL) dating study for both of these latter dunes are presented in this work. These two dunes are located at the most western position within the dune belt (Fig. 3.2). Behind this point, no dunes are observed. The dune at sampling position CUD-1 is designated Temple-Dune because there is a small Hindu temple nearby. The dune at the sampling position CUD-2 is named Sherd-Dune because there is a horizon with plenty of potsherds and bricks. 75

76 Figure 3.2: Map showing the study area between Cuddalore and Porto Novo. The sample locations are in the western part of the study area and indicated with a star. CUD 1 is named Temple-Dune due to the close position to a small Hindu temple. CUD 2 is named Sherd-Dune because it contains numerous potsherds. The geomorphology is based on remote sensing using IRS IC-LISS III data and field observations. The Temple-Dune has an altitude of 13 m above sea level and is one of the highest dunes in the study area. A 4-m deep section from top to bottom was excavated, and the layers found have been termed DS1, DS2, DS3a, DS3b and DS5b (Fig. 3.3). The nomenclature and correlation of the different beds are based on lithology and the geochronological results. The dune body is built up mostly of fine to medium grained sand. The grain size was measured with a Camsizer from samples taken every 10 cm. The sand at the base (DS5b in Fig. 3.3) is light grey and is more compact than the rest of the dune. This layer contains numerous artefacts including potsherds and fragments from bricks. It is overlain by homogeneous yellowish sand (more than 280 cm thick). In the lower part (DS3b in Fig. 3.3), this sand is dark-yellow and the higher part (DS3a in Fig. 3.3) is yellowish grey. This sand shows no sedimentological structures. Separated by an erosional unconformity, an 80 cm thick cross-bedded sand layer occurs (DS2 in Fig. 3.3). It shows alternating layers of yellowish-grey and brown sand. 76

77 Each layer is about 10 cm thick and has thin laminae (Fig. 3.4). The dip angle is about 24 and increases in the higher part. The top of the dune is made up of a 10 cm thick sand layer (DS1 in Fig. 3.3) with an erosional unconformity at the base. This sand is dark brown and contains many roots. Figure 3.3: Sections of the Temple-Dune (right) and the Sherd-Dune (middle and left). Both dunes are plotted together to visualise the correlation of the beds. They are not just one morphological form. The lithological description of the beds is given in 3.2. Regional setting. The correlation of the beds is based on lithology and geochronology and described in Correlation of dunes. The OSL ages are calculated using the Central Age Model. Samples for OSL dating are taken in the year 2006 AD and calculated ages are related to the sampling date. Unconformable boundaries visible in the outcrop are indicated by triangles. Open triangles indicate possible unconformities based on the geochronological results. Artefacts are only intermingled in bed DS5 in all investigated sections. In Figs. 6 and 7 the position of larger artefacts is shown. 77

78 Figure 3.4: Upper part of the Temple-Dune showing the beds DS1, DS2 and DS3a. The Sherd-Dune is located 330 m south of the Temple-Dune. The section is located in an outcrop at the foot of the stoss slope of the dune where sand was excavated. The dune itself is located to the south and is stabilised with large palm trees. Two sections separated by 25 m were studied in detail (Fig. 3.5). The beds are found are termed DS1, DS2, DS4, DS5a, DS5b, DS5c and DS6 (Fig. 3.3). Figure 3.5: Locations of the sections 1 and 2 in the Sherd-Dune. The beds DS1, DS2, DS5a and DS5b are indicated in the middle part. 78