REGOLITH PRODUCTION AND TRANSPORT IN THE SUSQUEHANNA SHALE HILLS CRITICAL ZONE OBSERVATORY: INSIGHTS FROM U-SERIES ISOTOPES

Regolith production contributes to important Critical Zone processes such as nutrient cycling, carbon sequestration, and erosion. Over the long term, the rates of regolith production and erosion combine with tectonic uplift to control the evolution of landscapes. Uranium-series isotopes offer one of the few available but powerful tools to quantify regolith production rates and timescales. Here, we present a study of U-series isotopes in regolith developed on shale bedrock at the Shale Hills Critical Zone Observatory in central Appalachian region. To investigate the timescales of regolith formation on hillslopes with contrasting topographic aspect, we measured U-series isotopes in regolith profiles from north- and south-facing hillslopes. The north-facing hillslope has a slope gradient of ~20°, slightly steeper than the south-facing hillslope (~15°). The regolith samples display significant U-series disequilibria resulting from shale weathering. Based on the U-series data, the rates of regolith production on the two ridgetops are indistinguishable (40±22 vs. 45±12 m/Ma). However, at downslope positions, the regolith profiles on the south-facing hillslope are characterized by faster regolith production rates (50±15 to 52±15 m/Ma) and shorter weathering durations (12±3 to16±5 ka) than those on the north-facing hillslope (17±14 to 18±13m/Ma and 39±20 to 43±20 ka). The south-facing hillslope is also characterized by faster chemical weathering rates inferred from major element chemistry in regolith. These results suggest an influence of aspect on regolith formation: aspect affects such variables as temperature, moisture content, and evapotranspiration in the regolith zone, causing faster chemical weathering and regolith formation rates on the south-facing side of the catchment. Such a difference is inferred to have been especially significant during the periglacial period (~15 ka). At that time, the erosion may have denuded the south-facing hillslope of regolith but not quite stripped the north-facing hillslope. A linear mass transport model of hillslope evolution and response timescales shows that the ridge tops of Shale Hills are in geomorphologic steady state but the current landscape on the hillslopes is likely disturbed by the climate shift of the Holocene periglacial conditions.