Erosion rates before and after the Mid-Pleistocene Transition in periglaciated central Appalachia

Abstract:

Glaciation associated with Quaternary cooling changed dominant erosion processes and rates, but we understand less about how the pace and pattern of erosion may have differed beyond the margins of continental ice. Periglaciated hillslope deposits in slowly eroding landscapes provide terrestrial records of climate-driven erosion throughout the Quaternary to address this question. Here, we quantify the timing and magnitude of erosion and colluvial deposition from an 18 m-deep upland sediment core in central Pennsylvania ~100 km south of the maximum extent of the Laurentide Ice Sheet. We measured bulk geochemistry and cosmogenic ¹⁰Be and ²⁶Al concentrations in matrix sand to determine hillslope erosion timing and processes. By using a forward model to interpret the concentrations of nuclides in the core, we find that the initiation of periglacial deposits characterized by coarse colluvial deposits coincides with the onset of longer, colder glacial cycles beginning at the Mid-Pleistocene Transition (MPT; 1.2-0.8 Ma). Intensification of glaciation likely resulted in a sustained increase in long-term erosion rates after the MPT compared to before. Since the mid-Pleistocene, episodes of permafrost thaw have instigated rapid stripping of hillslope material and accumulation of thick coarse debris in Appalachian headwater valleys and streams, leading to long-term and pervasive sediment storage. Increases in erosion rates coinciding with the onset of cycles of repeated periglaciation may indicate permafrost and periglacial processes efficiently erode hillslopes in landscapes where stable temperate climates would otherwise promote slower, diffusive processes.