Late Pleistocene Climatic Conditions Inferred from Sediment Infill in a Thermal‐Contraction Wedge in Shale Bedrock Near Carlisle, Pennsylvania
Gross, Evan Charles
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The Laurentide Ice Sheet covered a substantial portion of North America 26,500 to 19,000 years ago. When the ice sheet achieved its maximum southerly extent, a time known as the Last Glacial Maximum (LGM), it impinged onto northern Pennsylvanian landscapes. A periglacial environment developed adjacent to the ice sheet and encompassed the rest of Pennsylvania’s landscape, as well as other Mid-Atlantic landscapes around the LGM. With subsequent warming, the Laurentide Ice Sheet retreated and the permafrost that once dominated Mid-Atlantic periglacial landscapes thawed. However, various relict periglacial landforms persist throughout Pennsylvania, New Jersey, Maryland, and Delaware. They resemble those prevalent throughout modern periglacial environments. Therefore, analysis of such relict landforms can yield valuable paleoclimate information, which can then be used to inform our understanding of climate change occurring in modern periglacial environments. In this study, we inferred local paleoclimatic conditions from sediment infill of a relict thermal-contraction wedge, forming polygonal ground, recently discovered near Carlisle, Pennsylvania. The sediment was analyzed to determine its bedrock provenances, modes of transportation, and burial age. Particle size analyses, scanning electron microscopy and energy dispersive x-ray spectroscopy, x-ray powder diffraction, optical mineralogy and petrography, and optically-stimulated luminescence dating were among the techniques employed during this study. The results suggest that the ancient feature was a sand wedge that underwent thermal-contraction cracking approximately 14,900 ± 1,800 years ago (weighted mean age ± 1 for two OSL ages) in an arid periglacial landscape with continuous permafrost. The formation of the feature within highly frost-shattered shale bedrock indicates a mean annual air temperature of at most - 4 °C (25 °F) at the time of cracking. Its sediment infill originated from both proximal and distal upwind bedrock exposures and was transported toward the wedge via aeolian processes (i.e. suspension, saltation, creep). Direct aeolian infilling was supplemented by other processes, such as slumping. Based on the bedrock provenances of many infill grains, the dominant paleowind direction in central Pennsylvania was from the north or northwest. Katabatic winds, generated at the ice sheet’s margin, likely swept across the landscape around the time of thermal-contraction cracking.
Franklin and Marshall College Archives, Undergraduate Honors Thesis 2017
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