Rates of rockwall slope erosion in the upper Bhagirathi catchment, Garhwal Himalaya

Abstract

Rockwall slope erosion is defined for the upper Bhagirathi catchment using cosmogenic Beryllium‐10 (10Be) concentrations in sediment from medial moraines on Gangotri glacier. Beryllium‐10 concentrations range from 1.1 ± 0.2 to 2.7 ± 0.3 × 104 at/g SiO2, yielding rockwall slope erosion rates from 2.4 ± 0.4 to 6.9 ± 1.9 mm/a. Slope erosion rates are likely to have varied over space and time and responded to shifts in climate, geomorphic and/or tectonic regime throughout the late Quaternary. Geomorphic and sedimentological analyses confirm that the moraines are predominately composed of rockfall and avalanche debris mobilized from steep relief rockwall slopes via periglacial weathering processes. The glacial rockwall slope erosion affects sediment flux and storage of snow and ice at the catchment head on diurnal to millennial timescales, and more broadly influences catchment configuration and relief, glacier dynamics and microclimates. The slope erosion rates exceed the averaged catchment‐wide and exhumation rates of Bhagirathi and the Garhwal region on geomorphic timescales (103−105 years), supporting the view that erosion at the headwaters can outpace the wider catchment. The 10Be concentrations of medial moraine sediment for the upper Bhagirathi catchment and the catchments of Chhota Shigri in Lahul, northern India and Baltoro glacier in Central Karakoram, Pakistan show a tentative relationship between 10Be concentration and precipitation. As such there is more rapid glacial rockwall slope erosion in the monsoon‐influenced Lesser and Greater Himalaya compared to the semi‐arid interior of the orogen. Rockwall slope erosion in the three study areas, and more broadly across the northwest Himalaya is likely governed by individual catchment dynamics that vary across space and time. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons, Ltd.

Rockwall Slope Erosion in the Northwestern Himalaya

“…Confidential manuscript submitted to Journal of Geophysical Research: Earth Surface Orr et al (2019) were able to identify a tentative positive relationship between periglacial rockwall slope erosion and precipitation in the northwestern (NW) Himalaya by comparing erosion rates inferred from cosmogenic nuclide concentrations of medial moraine sediment across three glacier systems. Higher rates of erosion were determined for catchments with enhanced monsoon precipitation.…”

“…In this study, we seek to better define the distribution and magnitude of periglacial rockwall slope erosion in the NW Himalaya by building upon the work of Orr et al (2019) and quantifying erosion rates for a suite of 12 catchments. Rates of rockwall slope erosion are derived from cosmogenic nuclide 10 Be concentrations measured in sediment from medial moraines.…”

“…Rates of rockwall slope erosion are derived from cosmogenic nuclide 10 Be concentrations measured in sediment from medial moraines. Our new erosion dataset is combined with existing rockwall slope erosion records from Seong et al (2009), Scherler and Egholm (2017) and Orr et al (2019). This regional rockwall erosion dataset is compared to records of catchment-wide erosion and exhumation for the NW Himalaya to evaluate the extent to which rockwall slope erosion may differ from other records of landscape change, which have been averaged across various spatial and temporal scales.…”

“…Climatic variables included mean annual precipitation (weather stations [as referenced in Table 1] and TRMM [1998−2009]) and temperature (weather stations and CRU2.0 [as referenced in Table 1]), mean rockwall slope temperature and minimum catchment temperature. Catchment specific temperatures were calculated using an adiabatic lapse rate of 7°C/km and methods outlined in Orr et al (2019).…”

“…Catchment-wide rates for the Lahul-Spiti and Kullu districts are unavailable because much of the region remains glaciated (Owen and Dortch, 2014). Orr et al (2019) caution that comparing these erosion datasets can be problematic as they refer to landscape change through a variety of erosional processes and across various spatial and temporal scales. Nevertheless, the order of magnitude difference in these rates shows that erosion at catchment headwaters in the NW Himalaya largely outpace the entire drainage basins (Oskin and Burbank, 2005;Naylor and Gabet, 2007), and that erosion can vary significantly across short distances downstream .…”