Grain size distribution analysis 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 east-central 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 debris flows on Carlisle and Chuginadak Islands. 86 samples from three stratigraphic profiles were collected from beach cliffs eroded from alluvial/paraglacial fan toes. Two sample profiles intersected prehistoric village sites (AMK-0003 Unit 3, n=32; AMK-0003 Unit 4, n=42), and one consisted of a comparative profile of undisturbed volcanic ashes (CR-N, n=12). Grain size analyses of the fine sediment fraction combined with archaeological records and field observations permits attribution of one or more emplacement or alteration processes (such as debris flow, airfall tephra, human modified, etc.) involved with each layer of strata. The base of the fans are composed primarily of debris flow deposits associated with paraglacial conditions. Overlying fine-grained layers (collectively up to 3 m thick) are associated with tephra, fluvial, and cultural deposition. Cultural layers were identified by features such as artifactual materials, charcoal lenses, and chipped stone tools. Grain size distributions (GSD) for basal, non-volcanic debris flow deposits show bimodal or unimodal peaks with a broad distribution of grain size (mode range: 33.1-665 µm, D10: 6.06-19.48 µm, and a D90: 90.7-834 µm). Layers interpreted as volcanic air-fall ashes show defined unimodal peaks with either a fine skew or coarse skew (mode range: 48-923 µm, D10: 13-42 µm, and a D90: 196-1470 µm). Some tephra layers were interpreted as pyroclastic density currents (PDCs) (mode range: 211-923 µm, D10: 6.95-42.4 µm, and a D90: 440-1470 µm). Analyses of these data reveal information about source proximity and explosivity of volcanic events. Twelve culturally-modified strata feature distinctly 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, kurtosis: 2.921-14.826). 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.