Chronology and paleoclimatic implications of the upper Ganga catchment floods since Marine Isotopic Stage-2

The present study aims at understanding the chronology and pattern of extreme flood events in the upper Ganga catchment. Towards this, we explored relict alluvial sequences, particularly the sand bodies overlying the fluvial gravels and underlain by the source proximal alluvial fans and debris flows, supported by geochemistry, and optical chronology. Based on the accretionary pattern of sand bodies, the progressive increase in the flood magnitude since the beginning of Marine Isotopic Phase-2 (MIS-2) has been identified. The oldest flood is dated to 30 ± 4 ka and the subsequent floods are dated to the post-glacial transitional climate (16 ± 4 ka) and during the early Holocene (between 14 ± 2 ka and 12 ± 2 ka), and the late mid-Holocene (7 ± 1 ka). Based on the temporal changes in the major element geochemistry (detrital and weathering proxies) the study indicate that during early to mid-MIS-2, the upper Ganga catchment witnessed high frequency-low magnitude floods. Further, the absence of flood sediment, the development of calcrete, and a sharp decrease in the detrital proxies, a significant reduction in high-magnitude floods during the Last Glacial Maximum (LGM) is inferred. The isotopic fingerprinting, based on 87Sr/86Sr and ɛNd(0), indicates that the majority of the floods during the MIS-2 originated from the Higher Himalayan lithologies, whereas the inner Lesser Himalayan lithology dominates the mid to late Holocene floods. Climatically, the floods were triggered neither during too-wet nor too-dry climatic conditions, but instead during the transitional climate caused due to the mesoscale atmospheric coupling between Indian Summer Monsoon (ISM) and mid-latitude westerlies.