PERIGLACIAL LANDSCAPE CHANGE OF THE UPPER BHAGIRATHI CATCHMENT, NW HIMALAYA: CONSTRAINING BEDROCK SLOPE EROSION RATES USING 10BE

Lateral erosion of glacier valley bedrock slopes via periglacial and glacial processes can exceed vertical rates of glacial incision in alpine landscapes, and affect the configuration and migration of catchment divides. Questions remain about the rates and drivers of bedrock slope erosion throughout the Himalaya-Tibetan orogen, and the possible geomorphic impacts of climatic change in these active glacierized environments. We use ¹⁰Be concentrations of ablation dominated medial moraines from the Gangotri glacier to constrain bedrock slope erosion rates for the upper Bhagirathi catchment, Uttarakhand, of the NW Himalaya in northern India. Be-10 concentrations range from ~1 to 2.7 x 10⁴ atoms/g/quartz, which yield erosion rates of 2.1−5.3 mm/year. Although these rates are not unusual for mountain landscapes, they are higher than catchment-wide averaged rates and long term exhumation rates for the region. Geochemical, geomorphic and sedimentological analyses of the medial moraines confirm that these landforms share a supraglacial transport history, and that inherent complexities within the supraglacial system cause variability in the sediment characteristics and derived erosion rates, within and between the moraines. We begin to discuss to what extent climate-topography interactions govern rates of periglacial landscape change in the upper Bhagirathi catchment. Constraining bedrock slopes erosion rates serves to improve our understanding of the nature of sediment production and transfer within high-altitude glaciated catchments, and highlights the importance of periglacial rockfall processes in landscape evolution.