No major increase in erosion rates in Central Himalayas during the late
Cenozoic, revealed by 10Be in the newly dated Valmiki Siwalik section.
Abstract
The Glaciations impacted erosion during the Late Cenozoic but no
consensus has emerged whether they led to increased erosion rates
globally. In the Himalayas, recent work used past sediment
concentrations of the terrestrial cosmogenic nuclide (TCN) 10Be and
demonstrated that erosion rates have not permanently increased in the
Himalayas. However, for the Quaternary, the published sedimentary
records suffer from provenance uncertainties which prevent to elaborate
on the causes of steady erosion rates. Here, we document the new,
4,000-m thick Valmiki Section (VS) to address this question. In the
remote Valmiki Tiger Reserve, the VS consists of Siwalik sediment
deposited in the Himalayan foreland basin by the Narayani River, a major
river of Central Himalayas. To quantify past Himalayan erosion rates
from TCN 10Be measurements, we determine: (1) the magnetostratigraphic
deposition age model, (2) provenance using major elements and Sr-Nd
isotopes, and (3) the recent cosmic exposure related to Siwalik
exhumation using TCN 36Cl measurement in feldspar. The VS records
Himalayan erosion from 7.5 to 1.25 Ma. Our 10Be results confirm steady
erosion rates, close to modern values, 1.4-2.3 mm/y, with a brief
increase by 35% at 2 Ma, possibly due to sustained glacial erosion of
the high peaks as suggested by the geochemical signature. The Narayani
Catchment may be more sensitive to the onset of the Glaciations because
of larger glacial cover (presently ~10%) than elsewhere
in the Himalayas. Despite this sensitivity, our results support that
over long timescales, rather than climate, tectonics control Himalayan
erosion.