Proxy climate indicators
J of Paleolimnology special issue
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Synthesis and data-model comparison
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Hallet and Greyling Lakes are located along the crest of the northeastern Chugach Mountains, about 50 km north of Valdez and within the drainage of the Copper River. Both lakes are proglacial; each fed by several cirque glaciers ranging from 0.1 to 1.0
The lakes are oligotrophic, and sedimentation is dominated by glacially-derived minerogenic mud.
Because the conditions are harsh, and the growing season is short, biota at the lakes are particularly sensitive to shall changes in climate. Sedimentary biogenic silica concentration at Hallet Lake is used to infer temperature over the past 2000 years (McKay et al., 2008). Because the lake sediments is dominantly glacial and minerogenic, the physical parameters of the sediments, such as organic-matter content, dry bulk density, grain size, and sedimentation rate, are strongly linked to the activity of the glaciers in the head water, and are used along with glacial landform evidence to infer glacier variability during the Holocene.
The biogenic-silica-inferred temperature reconstruction from Hallet Lake shows synchronous changes with numerous independent paleoclimatic proxies, with clear evidence of First Millennium AD cooling from 600-900 AD, warmth from 1300-1500 AD, Little Ice Age cooling between 1500 and 1900 AD, and rapid, recent warming beginning around 1900 AD.
The multiproxy lake sediment and glacial landform evidence suggests a warm early Holocene from 10 to 6 ka, followed by the onset of Neoglaciation in the two drainage basins, beginning between 4.5 and 4.0 ka. During the past 2 ka, the glacial landforms and lacustrine sediments from the two valleys record a remarkably similar history of glaciation, with two primary advances, one during the first millennium AD, from ~500-800 AD, and the second during the Little Ice Age (LIA) from ~1400-1900 AD. Differences between the summer temperature inferred from the biogenic-silica content and the evidence for glacial advances and retreats suggest a period of increased winter precipitation from 1300-1500 AD, and reduced winter precipitation from 1800-1900 AD, likely associated with variability in the strength of the Aleutian Low.
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