Assessing the Rates of Post-Depositional Change Within 2004 Indian Ocean Sediments: Implications for Long-Term Records of Paleotsunamis

Foraminifera are commonly used to examine patterns of tsunami inundation occurring over centennial to millennial timescales, but the impacts of post-depositional change on geologic reconstructions are unknown. In Sumatra, the taphonomic character (i.e., test surface condition) of a foraminifer can deteriorate over time, rendering them unidentifiable, and even dissolve them entirely. Here I investigate the rates of post-depositional change of foraminiferal assemblages found within the 2004 Indian Ocean Tsunami (IOT) deposit over a 15-year time interval in Aceh, Indonesia in a vegetated open coastal plain (Site 1: Pulot) and an unvegetated protected coastal cave (Site 2). I identified two zones of taphonomic change (ZTC), where the Pulot foraminiferal assemblage became more corraded (dissolved, abraded) by the final sampling period in the upper 6cm (+28 % corraded individuals) and in lower 6 cm of the IOT deposit (+39 % abundance in corraded individuals). In contrast, the coastal cave contained high abundances of unaltered individuals (49-52% of total assemblage). At Pulot, Partitioning Around a Mediod (PAM) cluster analysis was successful in discriminating depths with better preservation from those that had experienced higher post-depositional change throughout the entire time series, indicating the stability of the assemblage with time. My results present good evidence that microfossils can be used as reliable indictors of tsunami origin and to investigate tsunami characteristics over long time frames. Selecting a protected field site with sufficient accommodation space, low vegetation and lacking marine or fluvial inputs was the most important factor to minimize post-depositional change in samples. Sampling is recommended in the middle depths of a tsunami deposit where the least post-depositional change occurs