FORAMINIFERA FROM COASTAL ENVIRONMENTS AS INDICATORS OF PALEOSTORMS AND TSUNAMIS

Coastal risk assessment requires datasets on centennial and millennial timescales in order to capture the variability and multiple occurrences of the largest, but infrequent, events. Coastal sediments from low-energy depositional environments archive geologic evidence of storms and tsunamis, making it possible to assess patterns of intensity and recurrence over sufficiently long timescales. Many of the best reconstructions of these events are derived from foraminiferal assemblages because they indicate the transport of marine sediment into an otherwise terrestrial setting. We argue that foraminifera-based reconstructions of storms and tsunamis are enhanced by including information concerning the taphonomic (or surface) condition of individual foraminifera because it provides important information about sediment provenance and transport that cannot be obtained from the taxonomic assemblages alone. We used a combined taxonomic and taphonomic approach (unaltered, fragmented, corraded tests) to document storm deposits from Vanuatu and the Philippines, as well as tsunami deposits from Japan, Oman and Indonesia. Storm deposits from Vanuatu and the Philippines could not be discriminated from underlying sediments using only their taxonomic assemblages. Foraminifera contained within the storm deposits and underlying beach sediments were nearly identical as they were sourced from the same location, but by different processes. However, their taphonomic assemblages were distinct. Sediments deposited by both storms contained 80-100% unaltered foraminifera, while sandy sediments from the underlying layers were 90-100% corraded. Similarly, sediments deposited by tsunamis in Japan, Oman, and Indonesia contain relatively high abundances of unaltered foraminifera because the tsunami waves scoured marine sediment from protected subtidal and offshore locations. The combined use of taxonomy and taphonomy identified a mixed source for the 2011 Tohoku tsunami deposit, where corraded foraminifera were sourced from the beach and dunes and minor abundances of unaltered foraminifera were sourced from the offshore. Based on these examples, we argue that taphonomy strengthens paleoecological interpretations and will be useful to studies that aim to document paleooverwash deposits using microfossils.