HIGH-RESOLUTION RECONSTRUCTIONS OF LITTLE ICE AGE CLIMATE, GLACIAL RESPONSE, AND PARAGLACIAL SEDIMENTATION FROM GLACIER-DAMMED ICEBERG LAKE, SOUTHCENTRAL ALASKA

Varves are often examined to document Holocene climate. In Alaska, however, proglacial lakes that might preserve these records have a distinct shortcoming: they rarely survive the downvalley ice advances associated with cold periods like the Little Ice Age. At Iceberg Lake, a glacier-dammed proglacial lake in Alaska's Chugach Mountains, we have discovered a rare exception to this rule. Since 1999, the historically-stable lake has been drained completely by three outburst floods; each time, rapid post-outburst incision by the main inlet stream provided subaerial access to much of the lake's neoglacial sedimentary record. Over three field seasons we documented and collected lacustrine sediments that record nearly two millennia of continuous sediment deposition.

The longest stratigraphic section has at least 1760 laminations, and independent dating from both cesium-137 spikes and AMS carbon-14 ages confirms the interpretation of these laminations as annual varves. Importantly, the record begins several centuries before commencement of Little Ice Age cooling, and we found no sedimentary evidence of prehistoric outburst floods. From well before the Little Ice Age until only 4 years ago, then, the large glacier that impounds Iceberg Lake must have been sufficiently advanced to continuously dam the lake. Modern breaches of the ice dam are unprecedented in neoglacial time.

This varve thickness chronology promises a regionally unique record of interannual variability in spring/summer temperatures. The preliminary record correlates most strongly with a published 400 year southcentral Alaskan tree-ring record when lagged behind by several decades, suggesting that the sediment yield reflected in our varve chronology is also a function of glacial processes that respond to climate forcing with a characteristic low frequency lag. We are using a new lichenometry technique to date moraines in the lake's watershed to account explicitly for these processes as we reconstruct the original climate forcing.