APPLICATION OF SALT-MARSH AND TIDAL-FLAT FORAMINIFERA TO IDENTIFY LATE HOLOCENE LAND-LEVEL CHANGES CAUSED BY MEGATHRUST EARTHQUAKES AT SITKINAK ISLAND, ALASKA
The diversity of foraminifera on Sitkinak and Simeonof islands is low, but distinctive assemblages with different elevational ranges constitute viable sea-level indicators. Three assemblages are recognized along five transects. The high marsh (> -0.2 m mean high water (MHW)) is dominated by Balticammina pseudomacrescens, Miliammina fusca characterizes the low-marsh (< 0 m MHW) environment, and a tidal-flat assemblage (< -0.5 m MHW) is recognized by high abundance of Cibicides lobatulus. Multiple radiocarbon-dated stratigraphic contacts of the past 1000 years in tidal sections fringing a lagoon on Sitkinak Island may indicate coseismic uplift (peat overlies mud) or subsidence (mud overlies peat). For example, foraminiferal assemblages at a pre-16th-century contact indicate rapid uplift, with a facies consisting of >95% B. pseudomacrescens replaced by a freshwater marsh facies barren of foraminifera. The record of land-level changes identified by foraminiferal assemblages will improve the understanding of recurrence intervals of great Alaska-Aleutian earthquakes and associated tsunamis.