Improving the late Pleistocene and Holocene glacial chronology in the central Alaska Range
Young, Nicolas E.
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A chronology of glaciation spanning from the late Pleistocene through the late Holocene is constructed for Fish Lake valley in the central Alaska Range using 10Be surface exposure dating and lichenometry. Ice initially retreated from its Last Glacial Maximum terminal position ∼22 ka and remained close to its maximum ice extent as late as 16.5 ka. Glaciers retreated well into the cirque region by 15-14 ka, before a late Pleistocene re-advance. Culminating by 11.6 ka, limited evidence indicates a late Pleistocene re-advance in Fish Lake valley may be coincident with the North Atlantic Younger Dryas event (12.9-11.6 kya). This adds to the growing amount of evidence in Alaska suggesting that glaciers were advancing in response to Younger Dryas related cooling. Late Holocene moraines pre-dating the Little Ice Age (LIA; ∼AD 1250-1850) are preserved immediately in front of LIA moraines and a minimum age of 3.3-3.0 ka on one moraine marks the oldest Neoglacial moraine preserved in the valley agreeing with regional glacier advances. Subsequent advances occurred in the middle to late First Millennium AD and pulses of LIA glaciation are recorded at AD 1290, 1640, 1860 and 1910. Preservation of the pre-LIA late Holocene moraines suggests that the magnitude of several late Holocene glacial advances were similar. This finding is consistent with moraine records in the Brooks Range, but contradictory to findings in southern Alaska. The innermost moraine crest, lichenometrically dated to AD 1930, indicates that the current glacier terminus has retread approximately 1.3 km to its AD 2007 position. Comparisons of 10 Be and lichenometric ages on late Holocene moraines show remarkable similarity and add confidence for using the regional lichenometric growth curve to date surficial deposits. Results indicate that 10 Be surface exposure dating can be used to constrain the timing of Holocene glacier fluctuations in Alaska; more so when supplemented with well constrained lichenometric ages.