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32nd Annual Arctic Workshop Abstracts
March 14-16, 2002
INSTAAR, University of Colorado at Boulder

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MULTI-PROXY EVIDENCE FOR RAPID AND PRONOUNCED LATE GLACIAL CLIMATE CHANGE IN THE AHKLUN MOUNTAINS, SOUTHWESTERN ALASKA

AUTHORS

BRINER, JASON P. INSTAAR and Geological Sciences, University of Colorado.
Hu, Feng Sheng . Department of Plant Biology, University of Illinois.
Kaufman, Darrell S. Dept. of Geology and Dept. of Environmental Sciences, Northern Arizona University.
Manley, William F. INSTAAR, University of Colorado.
Axford, Yarrow L. National Snow and Ice Data Center, CIRES, University of Colorado.
Werner, Al . Dept. of Earth and Environment, Mt. Holyoke College.
Caffee, Marc . PRIME lab and Dept. of Physics, Purdue University.

Determining the spatial and temporal pattern of abrupt climate events, such as the Younger Dryas event (YD; ~12.9 to 11.6 cal ka), is key to understanding interactions among the components of the climate system and for discerning regional climate teleconnections. Two independent records of late-glacial climate change in the Ahklun Mountains, southwestern Alaska, suggest that this region experienced rapid and pronounced climate oscillations coincident with the YD: 1) An expansion of alpine glaciers during the Mt. Waskey advance produced an extraordinarily well-defined end moraine system. Eleven cosmogenic 10Be and 26Al exposure ages on moraine boulders, combined with radiocarbon ages from a lake core upvalley of one of the moraines, suggest that the advance culminated between 12.4 and 11.0 cal ka, sometime during, or shortly following, the YD. Reconstructed equilibrium line altitudes (ELAs) for the Mt. Waskey advance are 80 30 m below modern values, and are 25 to 40% of the full glacial lowering. 2) Pollen assemblages, biogenic-silica, and organic-carbon contents in a sediment core from Nimgun Lake (~100 km west of Mt. Waskey) indicate pronounced changes in terrestrial vegetation, aquatic productivity, and landscape stability coincident with the YD. For example, Betula shrub tundra abruptly reverted to herb tundra at the onset of the YD, and became re-established at the end of the YD.

The ecological changes recorded at Nimgun Lake likely reflect a climatic cooling and a decrease in effective moisture during the Younger Dryas. Using an empirical relationship between climate and the ELA of modern glaciers, and assuming that it was 30 to 50% drier during the Younger Dryas, then the ELA depression of 80 m for the Mt. Waskey advance could correspond to a temperature depression of ~1.5C to 3.5 C. These values are consistent with published modeling results for the North Pacific region during the YD. Thus, data from the Ahklun Mountains add to a growing body of evidence that the North Pacific region experienced pronounced climate oscillations coincident with the North Atlantic YD. Taken together, these results point towards a tightly coupled ocean-atmospheric system.

FIGURES


Figure 1. Map of Alaska showing the Ahklun Mountains.


Figure 2. Cosmogenic exposure ages of moraine boulders deposited during the Mt. Waskey advance. Error bars span the upper limit of the 1 S.D. range of ages calculated using maximum estimates of snow cover and surface erosion, and the lower limit of the 1 S.D. range using zero erosion and snow cover. Plain text sample IDs represent an average exposure age for samples that have both 10Be and 26Al ages, bold text sample IDs are 10Be ages, and italicized sample IDs are 26Al ages. Also shown is the Younger Dryas chronozone (12.9-11.6 ka), the calibrated radiocarbon age (2 S.D. range) from Waskey Lake (minimum-limiting age for the Mt. Waskey advance), and the maximum 1 S.D. weighted mean moraine stabilization age range.


Figure 3. Comparisons of organic carbon (OC), biogenic silica (BsiO2), total mineral matter other than biogenic silica (MM), selected pollen types, and pollen DCA axis 1 scores from Nimgun Lake with the d18O record from GISP2.

 

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