GRAIN SIZE DISTRIBUTIONS OF SOILS, TEPHRAS, AND CULTURALLY-MODIFIED SEDIMENTS RECORD PREHISTORIC HUMAN RESILIENCE TO GEOLOGIC PROCESSES IN THE ISLANDS OF FOUR MOUNTAINS, AK

The Holocene coastal environment of the Islands of Four Mountains (IFM), in the Aleutian Archipelago, is prone to volcanism, earthquakes, tsunamis, changing sea levels, glaciation, and paraglacial processes that have challenged long-term human occupation. From circa 3,800 cal yr BP until 1741 CE, the Unangax (Aleut) people lived on sediment fans of Carlisle and Chuginadak Islands. Three stratigraphic profiles were collected from beach cliffs at eroded fan toes: two from village strata (AMK-0003 Unit 3, n=27; AMK-0003 Unit 4, n=46), and a comparative profile of undisturbed volcanic ashes (CR-N, n=15). Grain size analyses of the fine sediment fraction combined with archaeological records and field observations permits attribution of one or more processes. The base of the fans are composed primarily of debris flow deposits associated with paraglacial conditions. Overlying fine-grained material (up to 3 m) is associated with tephra, fluvial, and cultural deposition. Cultural layers were identified by artifactual materials and features such as charcoal lenses. Evaluation of grain size distribution (GSD) for basal non-volcanic debris flow deposits have unimodal peaks with broad distribution of grain size and a higher sand fraction than other strata. Layers interpreted as volcanic ashes shows defined unimodal peaks with a rapid drop off at the coarse end of the spectra (mode range: 48-923 µm, D10: 13-42 µm, and a D90: 196-1470 µm). Analyses of these data reveal information about source proximity and explosivity of volcanic events. Seven culturally-modified strata feature broad, flat-topped peaks that are slightly bimodal in GSD (mode range: 49-439 µm, D10 range: 16-21 µm, D90 range: 465-694 µm). These data show evidence of human occupation during periodic volcanic eruptions and reoccupation after displacement by a larger eruption (ca. 1,050 BP), illustrating the resilience of the people who inhabited these islands.