Millennial Scale Climatic Variability

Transcription

1 INTRODUCTION

2 Millennial Scale Climatic Variability Over the last 500,000 years, periodic ice sheet advances and associated temperature shifts have left a signature in records of stable oxygen isotope ratios collected from both ice and ocean sediment cores. These records display a 100,000 year cyclical transition from glacial to interglacial conditions associated with changes in the physical relationship between the Earth and the Sun. The periodicity s of the precession of the equinox, the angle of the Earth on its axis (obliquity) and the Earth Sun distance (eccentricity) are all evident in the proxies used to identify variations in the Earth s climate on a glacial/interglacial timescale. The more recent discovery of very rapid fluctuations in the earth s climate, occurring on sub-orbital timescales, has drawn attention to the need for further exploration into the mechanism that is forcing climate change with a periodicity not explained by the Milankovitch orbital cycles. The successful retrieval of long ice cores from both Greenland and Antarctica have dramatically changed the timescales considered when studying climatic change (Blunier et al., 2001; Chappellaz et al., 1993; Dansgaard et al., 1993; Jouzel et al., 1993; Petit et al., 1999; Petit et al., 1990; Ram et al., 1997; Taylor et al., 1993). Proxy measurements of polar temperature from the Greenland Ice Sheet Project (GISP) ice core, indicate that the glacial climate was punctuated with temperature swings of up to 10ºC, which are superimposed on the glacial/interglacial sequence (Dansgaard et al., 1993). These sub-orbital or millennial-scale events, known as Dansgaard-Oeschger (D-O) events have a frequency near 1500 years (Bond et al., 1993; Bond et al., 1997) that is highly evident during glacial periods but identifiable with a smaller magnitude during the last interglacial as well as throughout the Holocene (Bond et al., 1997). The rapidity of the D-O event transitions from warm interstadial periods to cool stadials brought about a paradigm shift in the field of climate change. Previously global temperature was believed to swing like a slow pendulum, now climate is thought to switch dramatically from warm to cold conditions over a only decade or so (Severinghaus et al., 1999). Not only has temperature been found to vary on sub-orbital time-scales, but so has the atmospheric concentration of greenhouse gases. Measurements of methane (CH 4 ) and nitrous oxide (N 2 O) made on gas bubbles extracted from polar ice indicate that the atmospheric concentrations of these greenhouse gases has varied dramatically between D-O warm stadials and cold interstadials with the transitions between these 2

3 two states occurring within years (Blunier et al., 1998; Brook et al., 1999; Chappellaz et al., 1993; Flückiger et al., 2004; Flückiger et al., 1999; Raynaud et al., 1993; Sowers et al., 2003). Carbon dioxide (CO 2 ) measurements also suggest millennial-scale variability, however large amplitude of stadial/interstadial transitions seen in Greenland ice are not replicated in Antarctic samples. The close correlation between atmospheric temperature and greenhouse gas concentrations suggests that these gases act as amplifiers of orbital forcing, significantly contributing to glacial/interglacial variations in global climate (Petit et al., 1999). Marine Records of Millennial Scale Climatic Variability A comparison between the rapid temperature fluctuations measured in Northern Hemisphere ice sheets and the determination of North Atlantic sea surface temperatures for the past 90 kyr confirmed the predictions that marine sediments would be imprinted with the signature of D-O events (Bond et al., 1993). The connection between Greenland temperature and sea surface conditions in the North Atlantic has been expanded to include sites from the Scottish margin (Kroon et al., 1997) and off the Norwegian coast (Lehman et al., 1992) both indicating that ocean surface waters warmed as Greenland temperature increased. In addition to D-O type variability, sediments from the North Atlantic show distinct episodes of Ice Rafted Debris (IRD), referred to as Heinrich Events, which occurred approximately every 7-10 thousand years during the last glacial period and correspond with extreme cold temperatures in the Northern Hemisphere (Bond et al., 1992; Bond et al., 1995; Elliot et al., 1998; Heinrich, 1988). Between kyr before present, sea level changes of approximately 15 m have been measured from exposed coral terraces in the Equatorial Pacific and are believed to be associated with the ice rafting of Heinrich Events (Chappell, 2002; Lambeck et al., 2002; Yokoyama et al., 2001). There has been a global proliferation of climate records that identify clear climatic signals that correlate with the timing of Heinrich Events (An, 2000; Arz et al., 1998; Porter, 2001; Ram et al., 1997) and are distinct from the higher frequency D-O variations. The evidence that the climate proxies determined from the North Atlantic sediments capture the rapid climate variability identified in the Greenland ice cores, has spurred the collection of high-resolution paleo-climate records globally. Changes 3

4 in sea surface temperature (Bond et al., 1993; Cacho et al., 1999; Hendy et al., 2000; Kroon et al., 1997), ocean circulation (Behl et al., 1996; Broecker et al., 1989; Hendy et al., 2002; Paillard et al., 1994), terrestrial vegetation (Grimm et al., 1993`) as well as changes in the tropical hydrological cycle and monsoons (Altabet et al., 2002; Baker et al., 2001; Burns et al., 2003; Dunbar, ; Leuschner et al., 2000; Peterson et al., 2000; Schulz et al., 1998; Suthhof et al., 2001; Wang et al., 1999; Wang et al., 2001) all show high frequency changes related in some manner to the D-O events of the Greenland ice core records and/or the Heinrich events of the North Atlantic. As more records become available, mechanisms of sub-orbital climate change that are focussed solely in the North Atlantic are less able to explain the global signals evident in the paleo-records. Identifying local, regional and global responses to sub-orbital climatic change is therefore critical to our understanding of the mechanisms involved. Mid and Low Latitude records showing D-O Variability Recently, climatic variability in tropical paleo-records has been associated with millennial-scale changes in the northern polar region (Altabet et al., 2002; Baker et al., 2001; Burns et al., 2003; Fleitmann et al., 2003; Gupta et al., 2003; Haug et al., 2001; Jung et al., 2002; Leuschner et al., 2000; Schulz et al., 1998; Suthhof et al., 2001) identifying a need to understand sub-orbital mechanisms of heat transfer between the high and low latitude regions. Millennial-scale changes in the position of the Intertropical Convergence Zone have been identified over the last glacial periods as well as the Holocene (Burns et al., 2003; Fleitmann et al., 2003; Haug et al., 2001; Peterson et al., 2000; Wang et al., 1999; Wang et al., 2001) and model studies suggest that the equatorial Pacific play a role in determining global temperatures (Cane et al., 1999; Clement et al., 1999). As atmospheric water vapour is the most significant greenhouse gas and it is mediated by changes in the tropical hydrological cycle (Pierrehumbert, 2000), understanding these changes on a millennial-scale is a critical step, necessary to move the field of climate change forward. The aim of this contribution is to reconstruct sub-orbital changes in the intensity of the Arabian Sea monsoon using both productivity proxies and geochemical analysis of two sediments cores, one from the Somali margin and one from the Indian margin. The development of a high-resolution productivity record from the Somali margin that is free from sedimentary dilution effects is critical in 4

5 determining the relationship between monsoon wind-induced upwelling and sedimentary records of organic carbon. To this end, Ba/Al measurements provide an independent record of biological production and nitrogen isotope measurements indicate past changes in the strength of the oxygen minimum zone. Additionally, elemental measurements on the Somali core provide a dust record, highlighting changes in continental aridity that are related to variations in the intensity of the summer monsoon. Together these records create a robust proxy of millennial-scale changes in the intensity of the summer monsoon. The Indian margin core complements this reconstruction by providing a history of monsoon related environmental changes over southwestern India. The Indian margin in under-represented in the paleoclimate literature and this highresolution record, covering the last 70 kyr, provides a unique opportunity to reconstruct climatic change in an area that today is critically reliant on the timing, duration and intensity of the summer monsoon. A background discussion of the monsoon climate, the regional winds, precipitation patterns, ocean circulation patterns and water masses that are present in the Arabian Sea Basin is presented in Chapter 2. Chapter 2 also provides an introduction to the geochemical proxies that are used in this study to determine past changes in the regional environment. In order to compare the paleo-records from these two sites, age models were constructed using both AMS 14 C dating, and a correlation of the δ 18 O and δ 15 N records. The details of the chronologies developed for this study are discussed in Chapter 3. Chapter 4 is an investigation of the millennial-scale variability and the monsoon proxies the Somali Margin core. The proxy records of productivity and lithogenic content are used to generate a wellrounded picture of changes in the summer monsoon winds, upwelling, productivity and continental aridity over the past 90 kyr. The role of the tropics as a regulator of atmospheric water vapour, dust and greenhouse gas emissions is discussed. Chapter 5 investigates the cyclicity and possible forcing mechanisms of Holocene variability and changes in the intensity of the summer monsoon over the past 10 kyrs. In Chapter 6, variations in continental aridity and monsoon strength are used to identify a proxy of sea level change during Heinrich Events. Chapter 7 introduces the climate record from the Indian margin. Using the monsoon reconstruction developed in Chapter 1 details of the lithogenic records from the Indian margin are developed. The unique 5

6 local environment, dominated by highly leached and chemically weathered soils, produces geochemical signals are significantly different from average crustal values. Therefore the local terrestrial input must be considered in the interpretation of biogeochemical records from this area. Chapter 8 compares the productivity records from the Western and Eastern Arabian Sea and determines local versus regional variations in productivity and denitrification. The relative influence of the summer and winter monsoon are discussed. Chapter 9 provides an overview of the main concepts developed in chapters 3-8 and a brief summary of this contribution. A description of the analytical methods employed for this study are included in Appendix A. All of the measurements produced during this study are presented in Appendices B and C. References: Altabet, M.A., Higgins, M.J. and Murray, D.W., The effects of millennial-scale changes in Arabian Sea denitrification on atmospheric CO2. Nature, 415: , An, Z., The history and variability of the East Asian paleomonsoon climate. Quaternary Science Reviews, 19: , Arz, H.W., J., P. and Wefer, G., Correlated millennial-scale changes in surface hydrography and terrigenous sediment yield inferred from last-glacial marine deposits off northeastern Brazil. Quaternary Research, 50: , Baker, P.A. et al., Tropical climate changes at millennial and orbital timescales on the Bolivian Altiplano. Nature, 409: , Behl, R.J. and Kennett, J.P., Brief interstadial events in the Santa Barbara basin, NE Pacific, during the past 60 kyr. Nature, 379: , Blunier, T. and Brook, E.J., Timing of millennial-scale climate change in Antarctica and Greenland during the last glacial period. Science, 291(5501): , Blunier, T. et al., Asynchrony of Antarctic and Greenland climate change during the last glacial period. Nature, 394(6695): , Bond, G. et al., Correlations between climate records from north-atlantic sediments and Greenland ice. Nature, 365(6442): , Bond, G. et al., Evidence for massive discharges of icebergs into the North Atlantic ocean during the last glacial period. Nature, 360: ,

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