Reconstructing Late Pleistocene and Holocene glacier fluctuations using cosmogenic 10Be exposure dating and lacustrine sediment, Brooks Range, arctic Alaska
Ceperley, Elizabeth G.
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Due to cryosphere-albedo feedbacks mechanisms, climate change is amplified in the Arctic, making it sensitive to changes in temperature. Alpine glaciers grow and retreat depending on climate, and are excellent recorders of past climate fluctuations. By analyzing the landforms and sediment deposited by glaciers, high-resolution climate chronologies can be constructed and past glacier fluctuations can be inferred. 10 Be ages and physical properties of lake sediment are used here to reconstruct Late Pleistocene and Holocene glacier activity from Alapah River valley and Shainin Lake in the north-central Brooks Range. 10 Be ages from moraine boulders in Alapah River valley in the north-central Brooks Range were used to reconstruct the maximum glacier extent during the LGM. After eliminating outliers, the 10 Be ages from a terminal moraine deposit in the Alapah River valley indicate that the local LGM culminated at 21.0 ± 0.8 ka. This new 10 Be chronology is the first to firmly constrain the timing of the local LGM in the Brooks Range, and is in agreement with LGM moraine records from other sites in Alaska and the globe. Two 10 Be ages from boulders located on bedrock 14 km upvalley from the Itkillik II terminal moraine give an age of deglaciation in Alapah River valley of 18.2 ± 0.8 ka. This indicates rapid retreat after the LGM and shows that deglaciation is synchronous with sites in Alaska but was initiated earlier than the age of 17 ka previously proposed for onset of LGM deglaciation in the western US. Physical and geochemical properties of lake sediment from a proglacial lake in Alapah River valley, Shainin Lake, were analyzed to investigate any glacial signals recorded in the lake sediment. Age-depth models for each core were established using 14 C ages and analytical methods included magnetic susceptibility, wet bulk density (WBD), scanning X-Ray fluorescence (ITRAX) and visible scanning reflectance spectroscopy. The WBD record from Shainin Lake may serve as a proxy for glacial history of Alapah and Kayak Creek valleys. If interpreted correctly, glacial activity increased from 12,700 to ~10,000 cal yr, decreased from ~10,000 to ~5700 cal yr BP, then increased from ~5700 cal yr BP to the present. This indicates that there is evidence for early Holocene glacial activity, the retreating or stagnating glaciers in the middle Holocene until ~5700 cal yr BP, followed by expanding ice.