Late stage lowering and drainage of glacial Lake Hitchcock in the Upper Valley Region of Vermont and New Hampshire

At the end of the last ice age, the Connecticut River Valley was filled by glacial Lake Hitchcock (GLH) which was dammed at its southern end near Rocky Hill, Connecticut. After this dam breached at ~16,100 cal yr BP, GLH in the Upper Connecticut River Valley of Vermont and New Hampshire (hereinafter referred to as the Upper Valley) still held water due to isostatic depression. Although it is estimated that GLH did not exist in the Upper Valley subsequent to ~12,500 cal yr BP, little is known about the timing and style of its lowering and final drainage. In order to improve the understanding of the history of GLH, we examined the transition from varved to organic-rich sediments in cores from four modern lake basins. This transition represents the removal of each basin from GLH and the onset of deposition derived from the local catchment area. Prior to post-glacial isostatic uplift, the four lakes were distributed over a range of elevations in the GLH basin and therefore would have been isolated at different times by GLH lowering. To determine an age of the transition from varved to organic-rich sediment, we correlated the varved sediment with the North American Varve Chronology (NAVC) and assigned an age to the uppermost varved sediments. We also applied radiocarbon dating to determine ages of organic macrofossils within and stratigraphically above the uppermost varved sediments. The ages of the transitions in four basins indicate that, in the Upper Valley, GLH lowered by ~60 m between ~13,280 and 12,290 cal yr BP. We also examined varved sediments for patterns of sudden increases in varve thickness which may register lake-level lowering or other events, such as climate changes. We confirmed one sudden increase in varve thickness at ~13,165 cal yr BP as the result of a rapid drainage of GLH. Our results provide a better understanding of environmental conditions during the last deglaciation as registered by lacustrine sediments. The proximity of this study to the Laurentide Ice Sheet (LIS) also provides an analog and point of comparison for modern receding ice sheets, such as the Greenland Ice Sheet.