HR: 16:00h
AN: PP44A-01    [Abstracts]
TI: Sediments Exposed by Drainage of a Collapsing Glacier-Dammed Lake Show That Contemporary Summer Temperatures and Glacier Retreat Exceed the Medieval Warm Period in Southern Alaska
AU: * Loso, M G
EM: mloso@alaskapacific.edu
AF: Department of Environmental Science, Alaska Pacific University, Anchorage, AK 9a9508, United States
AU: Anderson, R S
AF: Department of Geological Sciences and INSTAAR, University of Colorado, Boulder, CO 80309, United States
AU: Anderson, S P
AF: Department of Geography and INSTAAR, University of Colorado, Boulder, CO 80309,
AU: Reimer, P J
AF: 14CHRONO Centre for Climate, Environment, and Chronology, Queen's University, Belfast, BT7 1NN, United Kingdom
AB: In the mountains of southcentral Alaska, recent and widespread glacier retreat is well-documented, but few instrumental or proxy records of temperature are available to place recent changes in a long-term context. The Medieval Warm Period in particular, is poorly documented because subsequent Little Ice Age glacier advances destroyed much of the existing sedimentary record. In a rare exception, sudden and unexpected catastrophic drainage of a previously stable glacier-dammed lake recently revealed lacustrine stratigraphy that spans over 1500 years. Located near the Bagley Icefield in Wrangell-St. Elias National Park and Preserve, Iceberg Lake first drained in A.D. 1999 and has not regained a stable shoreline since that time. Rapid incision of the exposed lakebed provided subaerial exposure of annual laminations (varves, confirmed by radiogenic evidence) that record continuous sediment deposition from A.D. 442 to A.D. 1998. We present a recalculated master chronology of varve thickness that combines measurements from several sites within the former lake. Varve thickness in this chronology is positively correlated with northern hemisphere temperature trends and also with a local, ~600 year long tree ring width chronology. Varve thickness increases in warm summers because of higher melt, runoff, and sediment transport, and also because shrinkage of the glacier dam allows shoreline regression that concentrates sediment in the smaller lake. Relative to the entire record, varve thicknesses and implied summer temperatures were lowest around A.D. 600, high between A.D. 1000 and A.D. 1300, low between A.D. 1500 and A.D 1850, and highest in the late 20th century. Combined with stratigraphic evidence that contemporary jokulhlaups are unprecedented since at least A.D. 442, this record suggests that late 20th century warming was more intense, and accompanied by more extensive glacier retreat, than the Medieval Warm Period or any other time in the last 1500 years. We emphasize that the chronology presented here does not include the entire sedimentary history of the lake. Deeper sediments unexposed by the subaerial exposures we examined may extend this record of summer temperatures back to the onset of significant glaciation in this region. Traditional coring techniques could capture this record before ongoing erosion of the dry lakebed exports it to the Gulf of Alaska.
DE: 0720 Glaciers
DE: 0746 Lakes (9345)
DE: 1105 Quaternary geochronology
DE: 1621 Cryospheric change (0776)
DE: 3305 Climate change and variability (1616, 1635, 3309, 4215, 4513)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting