SYSTEMATICALLY INHERITED COSMOGENIC 10BE IN LATE HOLOCENE AGE MORAINE BOULDERS IN THE BHAGIRATHI VALLEY, GARHWAL

Excess inventory of (¹⁰Be) cosmogenic nuclides often precludes us to accurately (exposure) date recent geological materials, especially for materials/landforms which are younger than 2 ka. Here, we showed the inherited ¹⁰Be concentration in ten historically dated moraine boulders in one of the largest and erosive glaciers of the Garhwal Himalaya, northern India. We choose 30.2 km-long Gangotri glacier in the Bhagirathi valley to investigate systematically inherited ¹⁰Be concentration during glacier transportation and inclusion by random rockfall events from the valley walls. In conjunction with a detailed geomorphic mapping and cosmogenic ¹⁰Be dating, we presented that even for an erosive, high-velocity glacier (20–44 m/a) with a large ablation area (93 km² or 64%), a minimum of 0.09–0.60 10⁴ atoms/g (i.e., 0.05-0.12 ka) of systematic excess ¹⁰Be concentration is expected, with some outliers could be as old as >4 ka (i.e., >14.15 10⁴ atoms/g). We also applied a numerical rockfall model to simulate inherited ¹⁰Be concentration in boulders/blocks due to random events and compared with our field data. Our results indicate that boulder/block rotation during transportation and/or deposition likely offset some of the excess inventory of (¹⁰Be) cosmogenic nuclides on the sampled surface, particularly for large dynamic glaciers. Several key geomorphic process uncertainties are, therefore, proposed for future inheritance analysis that will likely target young geological materials/landforms. In addition, we also ¹⁰Be dated nine new moraine boulders from the main Gangotri and the tributary Meru glaciers to improve the Late Holocene local glacial chronostratigraphies in the catchment. Morphostratigraphically, from oldest to youngest, the Late Holocene moraines are dated to ~0.8±0.3 and ~0.3±0.1 ka, respectively. We used our inheritance estimates to correct for the possible overestimation of young moraine ages and proposed a robust Holocene local glacial chronology in the valley.