PLEISTOCENE CLIMATE-MODULATED EROSION: INTERPRETATIONS FROM COSMOGENIC NUCLIDE CONCENTRATIONS OF AN 18 M SEDIMENT CORE IN CENTRAL APPALACHIA

Preserved colluvial deposits in slowly eroding landscapes provide insight into long-term erosion trends and can indicate how hillslopes responded to climate changes throughout the Quaternary. Here, we quantify the timing and magnitude of chemical and physical weathering from an 18 m sediment core near Bear Meadows Bog, a breached anticline basin in central Pennsylvania 100 km south of the maximum extent of the Laurentide Ice Sheet. In core sediment, we measured geochemistry and cosmogenic ¹⁰Be and ²⁶Al concentrations in matrix sand to determine how hillslopes responded to the onset of Pleistocene climate cycles.

Low nuclide concentrations, material texture, and similarity of immobile-element chemistry to local bedrock imply that the deepest stratigraphic unit (6-18 m depth) is saprolite. Based on numerical simulations of steady-state soil denudation using concentrations and ratios of ²⁶Al and ¹⁰Be, this saprolite reflects paleoerosion rates of 1-5 m/Myr before it was partially eroded and buried. The overlying unit (0-6 m depth) exhibits higher nuclide concentration and different chemistry than the saprolite; texture and grain size trends suggest the overlying material represents periglacial debris derived from upslope. Nuclide concentrations constrain the timing of deposition of the lower periglacial unit (3-6 m below the surface) to no later than 750 ka, coeval with extensive regional glaciation during the mid-Pleistocene. Subsequent periglaciations created a composite regolith profile and landscape morphology by reworking deeply weathered regolith into solifluction lobes that characterize the present-day critical zone. The upper periglacial unit (0-3 m below the surface) shows no isotopic evidence of burial. There are few contrasts in geochemistry between older and younger periglacial units. Cosmogenic nuclide concentrations from the periglacial debris imply erosion rates of 6-10 m/Myr after periglacial onset, consistent with other erosion rates integrating over the late Pleistocene. Increases in erosion rates coinciding with the onset of periglaciation may indicate cold-climate processes efficiently denude hillslopes in landscapes where background rock uplift rates would otherwise promote slower, diffusive processes.