USING GEOCHEMISTRY TO CHARACTERISE TSUNAMI DEPOSITS AND IDENTIFY THE TSUNAMI INUNDATION LIMIT

Much advance has been made in the last 10 years or so in tsunami research, mostly as a response to the devastating events that have also provided the opportunity to study large tsunamis and their impact in different settings. We are thus now better equipped to identify tsunami deposits in the geological record, and this should help us being better prepared for the future. Geochemistry is one of the recent proxies which are being increasingly adopted by tsunami researchers. However, its value is not always recognised, and this mostly because the information it provides is not often straightforward.

Chemical signatures can reflect the occurrence of shell hash and/or microfossils; they can also be salinity indicators, thus helping to identify the marine source of the deposit. The chemical composition of sandy deposits also reflects their mineralogy and is thus site-specific. The preservation potential of salinity indicators in particular is dependent on the presence of organic matter and fine material as salts are leached readily through the porous sand and will be diluted by rainfall. Studies of recent events show that they can be retained in the underlying soil, thus corroborating the marine source of the overlying sandy deposit.

Another recent advance has been the use of geochemical signatures as markers of the tsunami inundation limit, beyond the extent of the visible (sandy) deposit. Until recently, researchers had relied on the extent of sandy units within soil and mud, to estimate the inundation limit of previous tsunamis. This has led to an underestimation of the magnitude of these precursor events, with catastrophic outcomes. Geochemical signatures, including stable isotopes, can help distinguish the mud left behind by a tsunami from the terrestrial mud in which it was deposited. This will allow us to redraw tsunami inundation maps, with important implications for tsunami risk assessment and preparedness.

Nevertheless, it is recommended to use geochemistry in association with other proxies. While geochemistry can provide the missing clue, we most often need a full suite of diagnostic criteria to be able to gain a better understanding of previous events.