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Revised age of deglaciation of Lake Emma based on new radiocarbon and macrofossil analyses

Published online by Cambridge University Press:  20 January 2017

Scott A. Elias ,
Paul E. Carrara ,
L. J. Toolin  and
A. J. T. Jull
Scott A. Elias
Affiliation:
Institute of Arctic and Alpine Research, Box 450, University of Colorado, Boulder, Colorado 80309 USA
Paul E. Carrara
Affiliation:
U.S. Geological Survey, P.O. Box 25046, Denver Federal Center, Mail Stop 913, Denver, Colorado 80225 USA
L. J. Toolin
Affiliation:
National Science Foundation Facility for Radioisotope Analysis, University of Arizona, Tucson, Arizona 85721 USA
A. J. T. Jull
Affiliation:
National Science Foundation Facility for Radioisotope Analysis, University of Arizona, Tucson, Arizona 85721 USA
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Abstract

Previous radiocarbon ages of detrital moss fragments in basal organic sediments of Lake Emma indicated that extensive deglaciation of the San Juan Mountains occurred prior to 14,900 yr B.P. (Carrara et al., 1984). Paleoecological analyses of insect and plant macrofossils from these basal sediments cast doubt on the reliability of the radiocarbon ages. Subsequent accelerator radiocarbon dates of insect fossils and wood fragments indicate an early Holocene age, rather than a late Pleistocene age, for the basal sediments of Lake Emma. These new radiocarbon ages suggest that by at least 10,000 yr B.P. deglaciation of the San Juan Mountains was complete. The insect and plant macrofossils from the basal organic sediments indicate a higher-than-present treeline during the early Holocene. The insect assemblages consisted of about 30% bark beetles, which contrasts markedly with the composition of insects from modern lake sediments and modern specimens collected in the Lake Emma cirque, in which bark beetles comprise only about 3% of the assemblages. In addition, in the fossil assemblages there were a number of flightless insect species (not subject to upslope transport by wind) indicative of coniferous forest environments. These insects were likewise absent in the modern assemblage.

Type
Research Article
Information
Quaternary Research , Volume 36 , Issue 3 , November 1991 , pp. 307 - 321
Copyright
University of Washington

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