The development of palaeotsunami research

The development of palaeotsunami research Sahra Skripsky, Undergraduate Student, Dalhousie University Abstract Over the last thirty years, the study of palaeotsunamis has received increasing attention. A palaeotsunami is a tsunami that happened in the distant past that there is no written record of. This paper will review the progress achieved and obstacles encountered in this field of palaeotsunamis. It will review how techniques, such as optical dating and radiocarbon dating, are used on coastal sediments to expand our understanding of palaeotsunamis. The main study sites discussed in this paper are located in New Zealand and British Columbia because these regions have different coastal deposits. By studying palaeotsunamis, researchers are able to better model and predict future tsunamis. Future ambition for this field of study is using palaeotsunami data to create a worldwide tsunami risk assessment, and being able to distinguish between sediments produced by palaeotsunamis or palaeostorms. 1. Introduction A palaeotsunami is defined as a tsunami, which is a large sea wave caused by the displacement of a large volume of water, that occurred prior to a historical record or for which there are no written observations (Goff et al. 2012). Palaeotsunamis were produced by a variety of events including earthquakes, terrestrial and submarine landslides, volcanic eruptions and other volcanogenic processes, jökulhaups (glacier outburst floods), meteorological events, methane hydrate release and the impact of extra- terrestrial objects (Goff et al. 2012). While current tsunami research mainly focuses on modeling coastal hazards, palaeotsunami research focuses on the identification, mapping, and dating of past tsunamis. Since the study of palaeotsunamis is a relatively new research area there aren t a wide range of data available. The goal of this paper is to review the strengths and weaknesses of palaeotsunami research. It will also explain how coastal sediments are used in this field of study, and the techniques used at the study sites. This paper will focus on research sites located in New Zealand and British Columbia to compare the developing research in two different geological areas. Research in British Columbia has mainly focused on studying the deposits using multiple techniques. Three sites near Tofino and Ucluelet (Vancouver Island, British Columbia) have deposits that have an optical age of between 260 and 335 years old (Huntley and Clague 1996). Meanwhile New Zealand researchers have concentrated on developing a palaeotsunami database that has records of at least 40 tsunamis dating back about 80,000 years (Goff et al. 2010). The main foundation of palaeotsunami research is using coastal sediment deposits because these deposits contain a geological record of past tsunamis. The 10

identification of palaeotsunami deposits can expand our knowledge about the source, magnitude, and frequency of palaeotsunamis (Goff et al. 2001). Multiple techniques, such as optical dating and radiocarbon dating, are used to identify palaeotsunamis. However, these techniques are unable to differentiate between palaeotsunami sediment deposits and other coastal processes, such as palaeostorms that are caused by high winds and severe weather events (Goff et al. 2010). Presently, tsunamis are one of the deadliest natural disasters for human kind. Thus, it is important to study and learn from palaeotsunamis, so we can better prepare for the future. If the opportunity to study palaeotsunamis is lost, the chance to improve modeling of future tsunamis is also lost. 2. Deposit dating techniques Figure 1: The arrow is pointing to a palaeotsunami deposit exposed in a pit on the tidal marsh near Tofino, British Columbia (Huntley and Clague 1996). In New Zealand and British Columbia a variety of techniques are used to extract information from coastal deposits. The two main techniques used are optical dating and radiocarbon dating. One technique that can be used to determine the age of palaeotsunamis from a deposit, such as the one in Figure 1, is optical dating. Optical dating determines the time that has passed since the sediments were last exposed to daylight (Huntley and Clague 1996). The key requirement for this technique is that the sediments in the deposits have been reworked and exposed to daylight by tidal currents, waves, or wind during the years before (but not during) the palaeotsunami occurred (Huntley and Clague 1996). Optical ages are obtained when sunlight energizes the electrons within the mineral sample out of their electron traps, which 11

are impurities or structural defects in the minerals, and then the environmental radiation after burial during the tsunami slowly puts the electrons back in their electron traps again. Exposure to light in research laboratories ejects the trapped electrons that have accumulated, and the resulting light emission provides a measure of the radiation dose since the palaeotsunami (Huntley and Clague 1996). The age of the palaeotsunami and other information can be obtained from the radiation dose s intensity and concentration (Huntley and Clague 1996). Optical dating can be used on deposits that lack suitable material for radiocarbon dating. Radiocarbon dating is a method that can be used to date any organic material (crushed vegetation, shells, or wood) within the palaeotsunami deposits (Goff et al. 2012). This method can determine the age of a palaeotsunami deposit by looking at the proportion of radioactive isotope of carbon (radiocarbon) remaining in the sample (Goff et al. 2012). The study sites in British Columbia found that the accuracy of optical dating was superior to the technique of radiocarbon dating (Huntley and Clague 1996). Furthermore, the deposits from the sites near Tofino and Ucluelet exhibited enormous variability (Dawson and Shi 2000). In some sites, only a single layer of sand defined the palaeotsunami. Other times there were chaotic sediment layers containing abundant evidence of past tsunamis, such as microfossils, volcanic ash, or organic material (Dawson and Shi 2000). The sites located in New Zealand relied on multidisciplinary research to identify palaeotsunami deposits (Goff et al. 2001). Instead of using just radiocarbon dating or optical dating, standard analyses included micropalaeontology, macropalaeontology and archaeology (Goff et al. 2001). These are not the only techniques used on palaeotsunami deposits in the studied regions but they are the main ones. For the identification of palaeotsunamis to be accurate a wide range of multidisciplinary techniques are required to be used on each study site. 3. Strengths of palaeotsunami research New Zealand is a prime study location for palaeotsunami research; the coastal sediments found there have provided evidence of numerous past tsunamis. Figure 2 shows a record of multiple palaeotsunami deposit sites (Kapiti, Wairoa, Abel Tasman National Park and Palliser Bay) in New Zealand and the approximate age of the deposits studied there. The deposits from these sites give physical evidence of palaeotsunamis. The deposits range from large boulders (may be 750 m 3 or larger) to fine mud (Goff et al. 2012). They contain the details about the waves that transported them there within crushed vegetation, volcanic ash, or other organic material (Goff et al. 2012). The fundamental development in this field of study is that researchers have worked out how to understand and read the information within these deposits. For example, these deposits hold the evidence of the generating events of the palaeotsunamis (Goff et al. 2010). Events such as an 12

earthquake, landslide or volcanic eruption leave identifiable signals in the sediments, like subsided coastal layers, landslide scars, and ash layers. However, it is not always possible for the source of the palaeotsunami to be known. Figure 2: Map of New Zealand showing the study sites, the approximate ages of palaeotsunami deposits, and relevant references (Goff et al. 2001). The use of multidisciplinary techniques, and the acknowledgment that to precisely identify palaeotsunami deposits hinges on the use of as many diagnostic characteristics as possible, has been an advancement in palaeotsunami research (Goff et al. 2001). Research being done in New Zealand has contributed significantly to the development of diagnostic and descriptor characteristics for palaeotsunami deposits (Goff et al. 2001). By using multidisciplinary approaches to investigate the sites located in Kapiti, Wairoa, Abel Tasman National Park and Palliser Bay, researchers in New Zealand have begun archiving palaeotsunami data describing between 35 and 40 palaeotsunamis (Goff et al. 2010). The second site, which is located in British Columbia, is also a prime palaeotsunami research area. In the tidal marshes near Tofino and Ucluelet (Vancouver Island, British Columbia) there are sand sheets containing marine foraminifera and plant fossils. Figure 3 shows the different components of the elevation layers within ten deposit sites. The sand layers in each site are considered to be the palaeotsunami deposits (Dawson and Shi 2000). The layers ranged in thickness from a few millimeters to 0.3 m, and progressively thinned out as they extended further inland (Dawson and Shi 2000). Within these deposits are fragments of bark, twigs, branches, stumps, cones and other plant material that can be used to gain information on the tsunami and of the environment at the time of the tsunami by 13

using either radiocarbon dating or optical dating (Dawson and Shi 2000). In between the sites, there is peat (organic material composed of partially decayed vegetable matter) that was submerged by an earthquake 100-400 years ago (Dawson and Shi 2000). These stratigraphic deposits give information on the sediment erosion, transport and deposition associated with the individual waves from the tsunami (Dawson and Shi 2000). Figure 3: Representative stratigraphy of coastal sediment in Tofino and Ucluelet tidal marshes, Vancouver Island, British Columbia, Canada (Dawson and Shi 2000). While the compositions of the deposits in New Zealand are different than the deposits in British Columbia, similar diagnostic characteristics and techniques are used to date and understand past tsunamis. The palaeotsunami database that New Zealand has developed has extended the record of tsunami deposits back to ~80 000 years ago (Goff et al. 2010). Whereas, the research done at the tidal marshes near Tofino and Ucluelet has conclusive evidence that can be used to reconstruct the individual waves from palaeotsunamis (Dawson and Shi 2000), but dates back only hundreds of years. Using a combination of dating techniques and descriptors in both palaeotsunami research areas has enhanced and strengthened the understanding of the magnitude and frequency of past tsunamis (Goff et al. 2012). 4. Weaknesses of palaeotsunami research While there have been advancements in palaeotsunami research, there are still many gaps in our understanding of palaeotsunamis. Since palaeotsunami research has received little attention, there has only been a limited amount of work done, mainly in New Zealand and in British Columbia. This is the main weakness of palaeotsunami research. Currently, researchers cannot confidently identify the palaeotsunami origin from deposits simply because there is not enough evidence (Goff et al. 2012). Also, it is a problem for researchers to completely distinguish 14

palaeotsunami deposits from palaeostorm deposits because both events leave similar geological signatures. In both New Zealand and British Columbia, there is evidence of palaeotsunamis and palaeostorms within the deposits (Dawson and Shi 2000). A palaeostorm deposit could have been moved due to high winds and waves because of harsh weather conditions or a hurricane. While, a palaeotsunamis deposit could be transported by earthquakes, landslides, volcanic eruptions or meteorological events. However, it has been noted that storm waves result in the deposit of a discrete sedimentary layer, while tsunamis deposit continuous and discontinuous sediment layers over wider areas and further inland (Dawson and Shi 2000). Another knowledge gap in palaeotsunami research is that erosion can occur on the existing deposits and on occasion, can lead to the complete removal of the deposits (Dawson and Shi 2000). This loss of evidence can leave holes when constructing the chronology of a palaeotsunami. Palaeotsunami research has the potential to improve our knowledge of past tsunamis, but there are still obstacles that need to be overcome. This field of study is starving for data. Future experiments and research need to be done in a wider range of geographic locations to gain a better understanding of palaeotsunamis. 5. Conclusion In New Zealand and British Columbia there have been both advancements and setbacks in palaeotsunami research, but it is a promising field of study. The strongest development in palaeotsunami research is using coastal sediment deposits to gain information about the source, aftermath, magnitude and frequency of tsunamis that occurred in the New Zealand and British Columbia regions prior to a historical record (Goff et al. 2012). These improvements in palaeotsunami research have led to a more accurate modeling system, which can reduce the risks associated with future tsunamis (Goff et al. 2012). Enhancements in the palaeotsunami database on the Pacific coast have provided the foundation for a more meaningful disaster risk reduction for British Columbia (Goff et al. 2012). The biggest weakness within this research is that because the study of palaeotsunamis is fairly new, there is not enough comparative data. Researchers are not yet able to identify the origin of palaeotsunamis or differentiate between palaeotsunami and palaeostorm deposits (Dawson and Shi 2000). Solving this particular problem is a priority for future research. Researchers in both study regions used optical dating, radiocarbon dating, and many other techniques to extract data from deposits (Goff et al. 2012). The information gained can be applied to numerical modeling and tsunami risk assessments all over the world (Goff et al. 2012). A promising research area that focuses on the techniques used to investigate tsunami deposits is the study of microfossils (Dawson and Shi 2000). 15

A future application of palaeotsunami research will be the widespread inclusion of palaeotsunami data into numerical modeling and tsunami risk assessment all over the world. Tsunami risk assessments can then produce a more comprehensive understanding of the longer- term hazards (Goff et al. 2012). This can only be achieved through more wide range, comparative research spanning field sites all over the world. Overall, palaeotsunami research is a developing field of study and will hopefully soon evolve to reach its highest potential. References Dawson AG, Shi SZ. 2000. Tsunami Deposits. Pure Appl Geophys. 157(6): 875-897. Goff J, Chague- Goff C, Nichol S. 2001. Palaeotsunami deposits: a New Zealand perspective. Sediment Geol. 143(1): 1-6. Goff J, Chague- Goff C, Nichol S, Jaffe B, Dominey- Howes D. 2012. Progress in palaeotsunami research. Sediment Geol. 243: 70-88. Goff J, Nichol S, David K. 2010. Development of a palaeotsunami database for New Zealand. Nat Hazards. 54(2): 193-208. Huntley D, Clague J. 1996. Optical dating of tsunami- laid sands. Quaternary Res. 46(2): 127-140. 16