A GLACIAL VARVE CHRONOLOGY ALONG THE SOUTHERN LAURENTIDE ICE SHEET REVEALS THE ICE MARGINAL RESPONSE TO HOLOCENE WARMING

Glacial varves can detail ice­­-margin positions and provide a proxy for meltwater discharge at resolutions comparable to the Greenland ice core archives, and thus they can be critical paleorecords for assessing ice sheet response to climate change. Here we describe a ~1500-year varve chronology from glacial Lake Agassiz that straddles the Younger Dryas (YD)–Holocene boundary. The chronology is pinned on AMS radiocarbon-dated terrestrial macrofossils at the base of a widespread, red-clay bed deposited during flooding from the Lake Superior basin. We illustrate the utility of this record by examining ice-margin retreat and melting through the late Younger Dryas and across the Holocene boundary. The ice margin receded at a constant rate, not only during the late YD, but for at least 300 years after the onset of the Holocene. In contrast, varve thicknesses increased at the boundary. In addition, a sand and gravel moraine formed over a 50-yr period, perhaps in response to the warming climate. Series of thicker varves, ranging between 30 and 100 years, have been associated with the formation of similar moraines in glacial lakes Ojibway and Minong (Superior), but this particular series in Lake Agassiz is the most tightly constrained yet, and supports earlier interpretations. Given their timing, one possibility is that rapid warming triggers the formation of these moraines that typify the Canadian Shield.