Silicate weathering rates decoupled from Sr isotope ratios in the Himalayas

The dissolved loads of many Himalayan tributaries of the Ganges display a rapid increase in the ⁸⁷Sr/⁸⁶Sr ratio, downstream of the Main Central Thrust. Both principal component and mass balance analyses of the contributing sources to the dissolved load identifies the weathering of Palaeoproterozoic impure carbonates (comprised largely of dolomite and mica) from the Lesser Himalaya as the most significant lithological control upon elevated ⁸⁷Sr/⁸⁶Sr ratios. In situ laser ablation analysis of the dolomite component yields ⁸⁷Sr/⁸⁶Sr ratios of 0.93 to 1.11, indistinguishable from bulk rock. We infer that a protracted metamorphic history has homogenised Sr isotopes between silicate and carbonate phases within the assemblage, generating a highly soluble source of radiogenic strontium. Approximately 60% of the impact of Himalayan tributaries on the marine ⁸⁷Sr/⁸⁶Sr ratio can be traced to weathering of these calc-silicates implying that the post-40 Ma increase in marine ⁸⁷Sr/⁸⁶Sr ratios is decoupled from any increase in silicate weathering rates in response to uplift of Tibet and the Himalaya. Prior to exposure of the Lesser Himalaya, we estimate that the ⁸⁷Sr/⁸⁶Sr ratio of dissolved Sr at the mouth of the Ganges-Brahaputra rivers would be reduced from ∼0.731 to <0.720, allowing for detrital weathering in the foreland basin and assuming that the relative values of contributing Sr fluxes have not changed significantly. A reduction of this magnitude could account for at least 57% of the rise in seawater values over the passed 8 Ma. In general, riverine ⁸⁷Sr/⁸⁶Sr ratios in the dissolved load do not provide a proxy for silicate weathering rates where ancient calc-silicate rocks are exposed in the catchments. However, since orogenesis potentially exposes old metamorphosed carbonates, as well as silicates, marine ⁸⁷Sr/⁸⁶Sr ratios may be indicative of orogenic events, even when decoupled from silicate weathering fluxes.