Abstract
ABSTRACTRockwall 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.
Highlights
Glaciation and glacial erosion are central to the topographic evolution of high-altitude mountain belts such as the Himalayan-Tibetan orogen, by influencing rates of sedimentation and localized incision, limiting relief production and elevation, and offsetting tectonic uplift (Brozović et al, 1997; Whipple et al, 1999; Mitchell and Montgomery, 2006; Wulf et al, 2010, 2011; Scherler et al, 2014)
The variability in grain size distributions in the samples is insufficient to draw any conclusions about the transport and sorting of medial moraine sediment in upper Bhagirathi
No significant relationship is observed between grain size distribution and proximity of the sample to the glacier snout or margin (Supporting Information Item 2)
Summary
Glaciation and glacial erosion are central to the topographic evolution of high-altitude mountain belts such as the Himalayan-Tibetan orogen, by influencing rates of sedimentation and localized incision, limiting relief production and elevation, and offsetting tectonic uplift (Brozović et al, 1997; Whipple et al, 1999; Mitchell and Montgomery, 2006; Wulf et al, 2010, 2011; Scherler et al, 2014). The contributions of periglacial erosion at the catchment head to the denudation budgets of Himalayan glacierized catchments have been largely overlooked, with the exception of studies by Heimsath and McGlynn (2008) in the Nepal High Himalaya and Seong et al (2009) in the Central Karakoram of Pakistan. This is surprising given that lateral slope erosion via periglacial processes are shown to exceed. Erosion of rockwall slopes in the Himalayan-Tibetan orogen has broad implications for its morphological development and the distribution of precipitation (Burbank et al, 2003; Gabet et al, 2004; Anders et al, 2006; Bookhagen and Burbank, 2006)
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