October 12 , 2011 Matthew Wooller, An ~11,200 cal yr BP record of changes in hydrology and limnology at Quartz Lake, Alaska: evidence primarily provided by stable isotope analyses, University of Alaska Fairbanks

An ~11,200 cal yr BP record of changes in hydrology and limnology at Quartz Lake, Alaska: evidence primarily provided by stable isotope analyses

Matthew Wooller, University of Alaska Fairbanks

Wetlands and lakes in the Tanana Valley, Alaska, have provided important resources for early humans who inhabited this region. These wetlands still provide important resources for subsistence today. We examine an ~11,200 cal yr BP record of past environmental, hydrological and limnological changes from Quartz Lake in the middle Tanana Valley. We analyzed the stable carbon and nitrogen isotope composition of total organic matter from a ~7m sediment core, coupled with stable oxygen and carbon isotope analyses of Pisidiidae shells (finger nail clams), in addition to chironomid (non-biting midge) assemblage changes. Radiocarbon dating of material preserved the core provided chronological control. Lacustrine sediments began to accumulate at ~11,200 cal yr BP. Initially, low levels of autochthonous primary production and a negligible amount of allochthonous organic input occurred between 11,000 and 10,500 cal yr BP and were likely associated with relatively cool conditions at Quartz Lake recorded at ~10,700 cal yr BP. After 10,500 cal yr BP, autochthonous primary production was higher coincident with a decrease in chironomid assemblages dominated by taxa associated with cooler climates. A decrease in stable carbon isotope values of total organic carbon (TOC) and organic content of the sediment between 9,000 and 4,000 cal yr BP may indicate declining autochthonous primary production. This period ended with an abrupt (~7‰) decrease in the stable oxygen isotope values from Pisidiidae shells at ~3,000 cal yr BP, which we hypothesize represented an episodic connection (flood) of the lake with flow from the nearby (~6 km) Tanana River. Our findings coincide with evidence for major flooding at other locations connected to the Tanana River and further afield in Alaska. From ~3,000 cal yr BP Quartz Lake subsequently appeared to become a relatively closed system, as indicated by the stable oxygen and carbon isotope values of Pisidiidae that are positively correlated and generally higher. This evidence of a progressively stronger evaporative influence on the lake's closed hydrology after ~3,000 cal yr BP is consistent with our modern oxygen and hydrogen isotope water data from Quartz Lake that plot along a local evaporative line we base on isotopic measurements from other local lakes and rivers. Stable isotope analyses provided a unique opportunity to examine the development of the Quartz Lake habitat present today.