T51 Looking to the Past to Understand the Future: Glacial Chronology and Process

Sunday, 22 October 2017
1:30 PM - 5:30 PM
Chelan 2

THE COSMOGENIC ISOTOPE CONCENTRATION OF A DEVONIAN SANDSTONE AS MEASURE OF GLACIAL EROSION ON THE NORTHERN EDGE OF THE APPALACHIAN BASIN

Paper number: T51-4
Program: Technical Sessions
Time: 2:20 PM – 2:35 PM

Authors

Abstract

The landscape of Central New York was subjected to multiple Pleistocene glacial advances and is dominated by glacial landforms. With each episode of glacial erosion, the underlying material moves towards the surface and eventually enters the zone where significant cosmogenic nuclides are produced. Thus, the concentration of cosmogenic nuclides, such as 10Be and 26Al that build up at slow rates tens of meters deep in bedrock due to muon production, can be used to track the amount of glacial erosion that has occurred in the area. In this study we exploit the presence of a regionally extensive quartz sandstone, the Oriskany Sandstone, to infer the amount and timing of glacial erosion along the escarpment of the Appalachian Basin.

We collected five samples from active quarries where the Oriskany Sandstone is exposed over an E-W distance of ~ 140 km and an additional sample collected from an interstate road cut. Sampling depths below the modern day surface range between 35 and 8 m, a depth where cosmogenic nuclides are overwhelmingly produced by interactions with muons rather than spallation. The surface production rate of of 10Be by muons is ~0.4 atoms/(g*yr) at the surface and becomes > 0.001 atoms/(g*yr) at 110 m depth. All samples contain <6000 atoms/g, representing concentrations that barely detectable above laboratory backgrounds.

Forward modeling of 10Be concentrations allows us to estimate the amount of glacial erosion that occurred over the last 1 Ma. We assume that Marine Isotope Stages 2, 6, 10, 12 and 16 correspond to periods when New York State was covered by the Laurentide Ice Sheet. Our forward model explores a model space that randomly prescribes an initial sample depth below the collection depth and an amount of glacial erosion at the end of each of the five glacial cycles. We assume complete shielding (no production of 10Be) due to ice thickness during the glacial intervals considered.

Our modeling results suggest at least 50 m of erosion across the northern edge of the Appalachian Plateau during MIS 2, with the greatest amount of erosion in the area between the two largest Finger Lakes. The modeling predicts modest erosion during previous glaciations. Local relief suggests that additional glacial work prior to 1 Ma is necessary.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-303091

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