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A 1000-YEAR VARVED SEDIMENT RECORD FROM THE BROOKS RANGE, ALASKA

OSGOOD-KUTCHKO, BARBARA G  University of Pittsburgh.
Abbott, Mark B  University of Pittsburgh.
Finney, Bruce  University of Alaska.

Geomorphologic evidence of late Holocene glacial advances shows the central Brooks Range is sensitive to changes in temperature and moisture balance. Proxy records from the region detailing the middle to late Holocene are sparse. Blue Lake is a small (<0.5 km^2), shallow (4.7 m) glacier-fed lake set on the crest of the Brooks Range (6805.3 N, 15027.8 W) in north-central Alaska at an altitude of 1265 m. The 4-km2 watershed contains a small cirque glacier set on the north face of the 1890 m high headwall on the north side of the continental divide. Field observations and air photos indicate that melt-water from the glacier contributes a substantial quantity of fine-grained sediment to the lake. Sediment cores recovered in August 1999 contain mm-scale laminations comprised of couplets of thick, light colored silt to fine sand laminae overlain by a thin, darker clay cap. Thin-sections were prepared to study the laminations using a shock-freeze (sublimation) technique and embedded with low viscosity epoxy resin under vacuum. Laminae counts and thickness measurements were made using digital image analysis techniques. Scanning electron microscopy coupled with energy dispersive spectroscopy were used to characterize mineralogy and grain-size. In thin-section, the laminae couplets appear as alternating light and dark bands and exhibit the classic mode of varve formation in a glacial basin consisting of a succession of fine sands to silts deposited during the summer months, followed by a well-defined winter clay cap. In addition to annual variability in varve thickness, long-term trends in thickness were observed and compared to the historical climate record. Overall, the varve measurements from Blue Lake correlate with regional cooling and warming trends described for the late Holocene. Blue Lake records the glacial response to late Holocene climate phenomena such as the Little Ice Age (cooling), Medieval Climatic Anomaly (warming), and the 20th century warming trend and provide a record for panarctic comparison.


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