TERRESTRIAL RESPONSE TO DEGLACIAL CLIMATE VARIATION IN INTERIOR ALASKA RECORDED IN ANCIENT SEDIMENTARY DNA

The unglaciated Alaskan interior saw major climatic changes during deglaciation, undergoing a transition from a cool, dry climate and herb-dominated late-Pleistocene vegetation through a period of increasing, but fluctuating, levels of warmth and moisture to near-modern boreal-forest conditions. Previous isotopic and lake-level studies and climate modelling confirm major switches in moisture levels and moisture sources across this transition. These changes were experienced by early human populations, who entered the region just prior to the onset of the transition (between 15,000 and 14,000 years ago). We used a recently developed paleoecological approach—analysis of ancient sedimentary DNA (sedaDNA)—at Chisholm Lake (Tanana River valley, interior Alaska), which lies near the oldest known archaeological sites, to obtain a high-resolution record of terrestrial and aquatic changes through this transition (ca 15,000-9500 cal. yr BP). The taxonomically rich record shows episodes of rapid turnover in terrestrial and aquatic taxa, a complex succession of plant communities and major changes in lake status not revealed by conventional palaeoecological techniques. Topographic heterogeneity at the landscape scale likely maintained a mosaic of deciduous shrubland/woodland and open areas over a ca. 5000-yr period, during which key subsistence herbivore populations persisted in the Tanana Valley. At ca. 9500 cal. yr BP moisture availability increased for a final time, lake level rose and Picea (spruce) forest expanded across the landscape, changes that coincided with mammalian turnover and changes in the archaeological record.