Microfossils from coastal environments as indicators of paleo-earthquakes, tsunamis and storms

Abstract

Coastal risk assessment and hazard mitigation require datasets on centennial and millennial temporal scales to capture natural variability and multiple occurrences of the largest, but least frequent, events. Coastal sediments from low-energy depositional environments archive geologic evidence of paleo-earthquakes, tsunamis, and storms. Many of the best reconstructions of these events are derived from changes in microfossil (diatoms, foraminifera, and pollen) assemblages. In this review we explain how microfossils are used to reconstruct records of paleoearthquakes by quantifying the amount of coseismic and interseismic vertical land movements along tectonically active coastlines. Examples from the United States (Alaska and the Pacific Northwest), Japan, and Chile show that microfossil-based transfer functions may provide continuous records of vertical land movement during earthquake deformation cycles. We discuss how microfossil habitat preferences and taphonomic character are used to constrain sediment provenance (e.g., beach, nearshore, or offshore sources) and identify overwash deposits, and how this information can be used to reconstruct the recurrence of tsunamis and storms. Analysis of overwash deposits from Thailand and Malaysia indicates the ability of microfossils to resolve individual waves within tsunami sediments, and an example from the Sendai coastal plain in Japan uses foraminifera to ascribe a beach to nearshore provenance for the 2011 Tohoku tsunami deposit. Finally, we present recent examples from the Gulf of Mexico on the use of foraminifera to estimate the volume and distance of transport of storm overwash from hurricanes.