LATE HOLOCENE GREAT EARTHQUAKES AND RELATIVE LAND- AND SEA-LEVEL CHANGE: ESTABLISHING REGIONAL CORRELATIONS BETWEEN COOK INLET AND THE PACIFIC COAST OF SOUTH CENTRAL ALASKA

Multiple peat-silt couplets in tidal marsh sediment sequences around Cook Inlet suggest recurring great earthquakes during the last 3300 years associated with marsh subsidence over large areas, comparable in extent to that recorded in AD 1964. Potential difficulties in interpreting evidence include sediment mixing and lack of modern analogues for applying quantitative diatom transfer functions to parts of the fossil record. A field experiment to simulate co-seismic subsidence identifies limited mixing at the top of submerged marsh sediment and indicates how mixing must be shown to differ from evidence for pre-seismic relative sea-level rise. Analysis of diatom assemblages from ice, frozen intertidal sediment and melt-out sediment demonstrates the importance of winter processes in transporting diatoms and their interpretation in fossil sequences. Winter processes may also be important in transporting organic material that leads to significant differences, ~120 - 2800 yr, between radiocarbon ages from plant macrofossils and bulk peat samples. This necessitates a new approach for establishing between-site correlations, recurrence intervals and spatial extent of great Holocene earthquakes in south central Alaska, including Copper River Delta and Cape Suckling.