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POST LITTLE ICE AGE RECORD OF FINE AND COARSE CLASTIC SEDIMENTATION IN AN ALASKAN PROGLACIAL LAKE

LOSO, MICHAEL G  Department of Eaerth Sciences, UC Santa Cruz, Santa Cruz, CA 95064.
Anderson, Robert S  INSTAAR and Department of Geological Sciences, CU, Boulder, CO 80309.
Anderson, Suzanne P  INSTAAR, CU, Boulder, CO 80309.

The coastal mountains of Southern Alaska are characterized by the highest glacial erosion rates in the world, but most quantitative assessments of glacial sediment yield (in Alaska and elsewhere) have traditionally focused on measurements of only the fine fraction carried in suspension by glacial rivers. Because bedload accounts for 30-70% of sediment discharge in proglacial streams, estimates of total yield based upon suspended sediment alone are subject to large errors; they also provide no insight into the relative contributions of glacial quarrying and abrasion. Here, we document coarse and fine-grained sediment yield over the past 200 years from Iceberg Lake, a proglacial lake in the Chugach Range of Alaska, using a well-dated suite of deltaic and lacustrine sediments. Iceberg Lake has an area of 4.4 km2 and is impounded by one tributary of the Tana Glacier, a large northward-flowing outlet of the Bagley Icefield in Wrangell-St. Elias National Park. It receives runoff and sediment from smaller alpine glaciers that cover 52% of the land surface in its 66 km2 watershed. While Iceberg Lake is glacier dammed, a spillway cut in a bedrock spur adjacent to the ice dam has controlled its level. The lake drained completely in a jökulhlaup in 1999, the only event of this kind evident in the record preserved in the lake sediments. Four discrete strandlines ring the lake above its 1999 shoreline. All but the oldest are associated with Gilbert deltas at the main inlet. The lake level dropped in discrete events associated with abandonment of one spillway for a new lower one, apparently in response to lowering of the ice dam. Varved lacustrine sediments record lake-lowering events as transient increases in the basin-wide deposition of unusually fine-grained sediments, and show that all events occurred within the last 200 years. Using a combination of subaerial sediment exposures, sediment cores, differential GPS surveys, and aerial photo interpretation, we calculated volumes of impounded deltaic and lacustrine sediments. These allow independent estimates of specific yield for the four time intervals bracketed by lowering events in 1825, 1834, 1867, and 1957 AD and the 1999 drainage event. The time-weighted average sediment yield is 4317 tons/km2/yr, and ranged from 14% above to 12% below this value. In each interval, 81-86% of the sediment is found in deltaic deposits, which we interpret to be derived from bedload. A wide range of evidence suggests a close coupling between sub-glacial source and lacustrine sink in this small, heavily glacierized basin. The time-averaged yields should therefore accurately reflect upstream glacial erosion. The average effective erosion rate of 1.6 mm/yr is well within the expected range for basins of comparable size. The texture of the deposits clearly attest to the long-term significance of bedload as a dominant component of glacial sediment output. Further, they provide a robust long-term estimate of the importance of quarrying in glacial erosion. Assuming that the silts and clays in the varved sedimentary record of Iceberg Lake are produced equally from abrasion of the bed and of quarried blocks embedded in the glacier sole, and that the sands and gravels in the deltas of Iceberg Lake are produced primarily by crushing of quarried blocks, then quarrying accounts for as much as 90% of the glacial erosion in this system.


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