Abstract:
The present study attempts to understand the geomorphic response in the upper Ganga catchment to the
mid-late Holocene (neoglacial) climate variability. The study infers five major phases of millennial-scale
climate variability with centennial-scale inversions using geochemical and magnetic proxies from relict
Lesser Himalayan Lake sediments. Phase-1 (6e4 ka) is marked by enhanced precipitation/runoff
(increased allochthonous contribution) under a stronger Indian Summer Monsoon (ISM). The prominent
reversal in the trend between ~5 and 4 ka includes global arid events such as 4.2 ka. Phase-2 (4e2.2 ka)
shows a declining precipitation/runoff (decreased allochthonous input) under declining ISM with a
prominent dip after ~3 ka. After phase-2 the climate reversals are distinct and of shorter (centennial)
duration. For example, in Phase-3 (2.2e1.4 ka) improved ISM is inferred; Phase-4 (1.4e1.0 ka) is marked
by a sharp decline in the ISM, and Phase-5 (<1.0 ka) includes centennial-scale events of Medieval Climate
Anomaly (MCA) and the onset of Little Ice Age (LIA). The relative increase (decrease) in the concentration
of geochemical and magnetic proxies is indicative of strengthened (weakened) ISM where relatively drier
phases are in sync with the North Atlantic climate perturbations. We observed clustering of optically
dated flood events around 6.5, 4.5, 2.6, 1.4, 0.8, and 0.4 ka which corresponds to periods of moderate ISM
thus, suggesting a coupling between warm-humid monsoon and relatively dry westerlies. The relatively
higher concentration of micro-charcoal in the lake sediments indicates widespread forest fires around
5.9e5.3, 4.5e4.3, 3.4e3.0, 2.0e1.5 and ~1 ka. Given the archaeological evidence of sedentary settlements
since ~3 ka in the upper Ganga catchment, the study speculatively argues anthropogenic forcing for
forest fires after 3 ka. Further, the highest probability flood phases succeed the fire events and may be
indicative of enhanced vulnerability of the catchment to floods due to vegetation loss (enhanced erosion
and surface runoff).
