WHY SMALL CAN BE BETTER FOR DECIEFERING CLIMATE-DRIVEN GLACIAL RESPONSE IN PROGLACIAL LAKE SEDIMENTS

Mountain glaciers are sensitive barometers of climate. The records they leave, though, are often ambiguous and difficult to interpret. Whereas direct evidence of past glacier changes (moraines, till, striae) is generally incomplete, proglacial lake sediments offer more continuous records of glacier fluctuations. Coring efforts have resulted in a handful of impressive records interspersed with many complex and confounding records. Based on several decades of experiences, I have concluded that the most accurate lake records of glacier fluctuations come from lakes in small, geomorphically simple basins.

The concept behind coring proglacial lakes is attractively simple: as glaciers expand, they flow across greater amounts of bedrock and produce progressively greater amounts of fine glacial rock flour; as they shrink, less rock flour is produced. Proglacial lakes act as efficient traps for this suspended sediment, and should effectively record these fluctuations in clastic sedimentation. Although studies have validated this concept in some small basins, the link becomes increasingly muddled as the watershed feeding a lake grows in size and complexity. For example, studies in the Canadian Cordillera indicate that the maximum rock flour flux to proglacial lakes occurs during retreat, lagging the glacial maximum; this idea contrasts with my own studies in the mountains of the western US where there seems to be little if any lag. The difference appears to reflect the sizes of the glacier systems; those in Canada are mostly substantial valley glaciers with large lateral moraines, whereas the glaciers in my studies are small cirque glaciers. As the large valley glaciers retreat, the inner slopes of the lateral moraines fail and send a large pulse of stored rock flour downstream into any proglacial lakes. Conversely, small cirque glaciers produce small moraines dominated by coarse rockfall with relatively little rock flour; as these glaciers thin and retreat, the flux of rock flour decreases nearly instantaneously.

Larger basins introduce other complexities to the clastic sediment records of proglacial lakes: they have increased potential for landslides, extreme rain events, or fires to dilute the rock flour record. Small glaciers in simple stripped bedrock basins minimize these problems.