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Loso, M.G., 2004

Late Holocene climate and glacier response reconstructed using stratigraphy and lichenometry at Iceberg Lake, Alaska

Bibliographic Reference

Loso, M.G., 2004, Late Holocene climate and glacier response reconstructed using stratigraphy and lichenometry at Iceberg Lake, Alaska: University of California, Santa Cruz, Ph.D. dissertation, 167 p.

Abstract

In the summer of 1999, glacier-dammed Iceberg Lake drained catastrophically and completely, exposing to subaerial erosion a lacustrine sediment package that records more than 1,500 years of continuous deposition. Situated in south-central Alaska at the heart of the world's largest nonpolar icefields, this record offered an unprecedented opportunity to characterize at high resolution the history of climate and glacier response for a remote, understudied region previously known only by a 50-year historic/instrumental record, 500+ years of tree rings, and patchy records of late Holocene glacier advances. Combining Iceberg Lake's unique and well-exposed stratigraphic record with additional evidence that includes results from a newly developed lichenometric dating technique, this dissertation presents chronologies of clastic sediment yield, alpine glacier terminus retreat, and varve thickness. At the decadal and centennial timescales considered here, glacier sediment yield and erosion rates (which average 1.6 mm/yr) have remained roughly constant, but a combination of increased sediment transport capacity (by meltwater streams) and diminished lake size (related to shrinkage of the glacier-dam) drives varve thickness variations that we show to be positively correlated with summer temperatures. In the late 20th century, varve thickness (and, by inference, summer temperatures) in Iceberg Lake reached high levels unprecedented in the last 1,500 years. After developing and testing a new, process-based lichenometric technique that relies upon explicit biological models to provide surface ages with robust confidence intervals, we use the method to date terminal moraines that show alpine glacier retreat rates accelerating in response to 20th-century warming. Although the Medieval Warm Period (MWP, 1000-1250 A.D.) shows up clearly in the varve chronology as a period of prolonged warmth preceding the Little Ice Age, two lines of evidence suggest the 20th-century warming is more intense, and accompanied by more extensive glacier retreat. First, varve thicknesses during the MWP never approached 20th century values. Second, lacustrine stratigraphy strongly supports the contention that repeated catastrophic drainage events since 1999 are unprecedented in the 1,500+ year history of the lake, showing that contemporary shrinkage of the glacier dam was unmatched during the MWP.

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