Variation in the moisture regime of northeastern interior Alaska and possible linkages to the Aleutian Low: inferences from a late-Holocene δ18O record

Abstract

Analyses of oxygen and carbon isotopes in bulk carbonate and Chara-stem encrustations, X-ray diffraction, and sediment composition from Keche Lake offer new information on climatic change over the past ~3,400 years in northeastern interior Alaska. The δ18O and δD values of water samples from the lake and its inlet streams suggest that evaporation plays an important role in determining the isotopic composition of Keche Lake water at present. However, evaporative enrichment does not appear to be a major driver of the pronounced fluctuations in the bulk-carbonate δ18O record on the basis of comparison with Chara-δ18O values. The δ18O values of bulk carbonate in the Keche Lake sediments vary by up to 10 ‰ over the past 3,400 years, with maximum values of −12 ‰ around 3,400 cal BP and between 2,100 and 1,500 cal BP. High δ18O peaks are associated with sediments dominated by quartz, feldspar, and clay minerals suggesting the influence of detrital carbonate. Multi-millennial patterns of δ18O variation at Keche Lake appear to be linked with changes in watershed and sediment-depositional processes, which may be driven by varying moisture abundance associated with the position of the Aleutian Low (AL). The increasing trend of carbonate δ18O from 3,400 to ~2,100 cal BP probably reflects the increasing importance of a westerly AL, and the high frequency of δ18O spikes ~2,100–1,500 cal BP may have resulted from the prevalence of a westerly AL position. Predominance of a westerly AL likely increased snowfall and winter temperature in the region. Such conditions would have promoted soil erosion and thermokarst activity during spring snowmelt, resulting in episodic large influxes of detrital carbonate to Keche Lake and elevating bulk-carbonate δ18O. Over the past 1,500 years, bulk-carbonate δ18O remained relatively high at Keche Lake but variation was much less pronounced than before. A broad δ18O peak centered ~400 cal BP may be related to enhanced winter moisture during the Little Ice Age, although our chronology is inadequate for a rigorous assessment of this interpretation. This study contributes a new δ18O record and offers additional information on past moisture-regime shifts associated with changing atmospheric-circulation patterns.