An ~11,200 year paleolimnological perspective for emerging archaeological findings at Quartz Lake, Alaska

摘要

Wetlands and lakes in the Tanana Valley, Alaska, have provided important resources for prehistoric humans who inhabited this region. We examine an ~11,200 cal yr BP record of environmental and paleolimnological changes from Quartz Lake in the middle Tanana Valley. Our data are also presented in the context of recent archaeological findings in the lake's general vicinity that have 18 associated AMS ~(14)C dates. We analyzed the stable-carbon and nitrogen isotope composition of total organic matter from the core, coupled with oxygen and carbon isotope analyses of Pisidiidae shells (fingernail clams), in addition to chironomid assemblage changes. Lacustrine sediments began to accumulate at ~11,200 cal yr BP. Initially, autochthonous production was low and allochthonous organic input was negligible between 11,000 and 10,500 cal yr BP, and were associated with relatively cool conditions at Quartz Lake at ~10,700 cal yr BP. After 10,500 cal yr BP, autochthonous production was higher coincident with a shift to chironomid assemblages dominated by taxa associated with warmer summer climates. A decrease in δ ~(13)C 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 δ ~(18)O 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 δ ~(18)O _(Pisidiidae) and δ ~(13)C _(Pisidiidae) data that are positively correlated and generally higher, which also correlates with a shift to moderately higher abundances of littoral chironomids. The cause of the transition to closed-basin conditions may have been geomorphic rather than climatic. 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 δ ~(18)O and δD water data from Quartz Lake that plot along a regional evaporative line we base on isotopic measurements from other local lakes and rivers.