Constraining Laurentide Ice Sheet meltwater routing changes in eastern New York State using berillyum-10 dating
Porreca, Charles J.
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As the Laurentide Ice Sheet (LIS) retreated north of the Adirondack Mountains of eastern New York State near the end of the Late Pleistocene glaciation, the ice dam at Covey Hill, which held back Glacial Lake Iroquois (GLI) waters at its northeastern margin was breached. This event caused a shift in LIS meltwater outflow from GLI via the Mohawk valley spillway at the southeastern margin of the lake to its northeastern margin at Covey Hill. From Covey Hill, the meltwater then flowed southward, and eventually down the Hudson River valley. The extreme increase in discharge associated with the meltwater re-routing event caused a jökulhlaup (flooding) event that discharged into the Atlantic Ocean where it may have altered oceanic circulation. When the re-routing event occurred is not certain. I applied cosmogenic Beryllium-10 (10Be) exposure dating to a strath terrace on Moss Island, along the Mohawk Spillway in order to constrain the timing of abandonment of the strath terrace, thought to be formed by GLI outflow. By dating the age of abandonment of outflow along the Mohawk Spillway, I aim to constrain the timing of the meltwater re-routing event. Based on pothole distribution across Moss Island, I determined that a strath terrace was formed by turbulent GLI outflow and fluvial scouring, whereas the upper elevations of Moss Island escaped fluvial scouring. Six samples of bedrock surfaces collected from the strath terrace yield an average 10Be age of 14.3 ± 0.8 ka. These ages are older than a previous estimate of 13,350 cal yr BP for the meltwater re-routing event based on confining radiocarbon ages of GLI sediments, glacial lake sediments in the Champlain basin, and a musk ox fossil from the New York – New Jersey continental shelf, as well as the onset of the Intra-Allerød Cold Period (Donnelly et al., 2005). Six additional samples collected from above the strath terrace on Moss Island yield older 10Be ages that average 24.6 ± 1.3 ka, excluding two older outliers. These ages are difficult to explain. Regional deglaciation estimates around Moss Island are ~16.9 ka (Ridge et al., 2004). Thus, the samples collected from the upper level surfaces of Moss Island likely contain inherited cosmogenic 10Be due to weak (<2 m) glacial erosion during ice sheet overriding. Further study and application of the cosmogenic 10Be exposure dating method in the region may lead to tighter chronologic constraints of this GLI meltwater outflow re-routing event.