Laurentide ice sheet meltwater routing along the Iro-Mohawk River, eastern New York, USA

Abstract

The rerouting of meltwater as the configuration of ice sheets evolved during the last deglaciation is thought to have led to some of the most significant perturbations to the climate system in the late Quaternary. However, the complex pattern of ice sheet meltwater drainage off the continents, and the timing of rerouting events, remains to be fully resolved. As the Laurentide Ice Sheet (LIS) retreated north of the Adirondack Uplands of northeastern New York State during the last deglaciation, a large proglacial lake, Lake Iroquois, found a lower outlet that resulted in a significant flood event. This meltwater rerouting event, from outflow via the Iro-Mohawk River valley (southern Adirondack Mountains) to the spillway at Covey Hill (northeastern Adirondack Mountains), is hypothesized to have taken place ~13.2ka and disturbed meridional circulation in the North Atlantic Ocean. However, the timing of the rerouting event is not certain because the event has not been directly dated. With improving the history of Lake Iroquois drainage in mind, we obtained cosmogenic 10Be exposure ages on a strath terrace on Moss Island, along the Iro-Mohawk River spillway. We hypothesize that Moss Island's strath terrace became abandoned during the rerouting event. Six 10Be ages from the strath surface average 14.8±1.3ka, which predates the previously published bracketing radiocarbon ages of ~13.2ka. Several possibilities for the discrepancy exist: (1) the 10Be age accurately represents the timing of a decrease in discharge through the Iro-Mohawk River spillway; (2) the age is influenced by inheritance. The 10Be ages from glacially sculpted surfaces on Moss Island above the strath terrace predate the deglaciation of the site by 5 to 35ky; and (3) the abandonment of the Moss Island strath terrace relates to knickpoint migration and not the final abandonment of the Iro-Mohawk River as the Lake Iroquois spillway. Further study and application of cosmogenic 10Be exposure dating in the region may lead to tighter chronologic constraints of meltwater history of the LIS.