Silicate weathering rates decoupled from the 87Sr/ 86Sr ratio of the dissolved load during Himalayan erosion

Abstract

This study establishes that carbonate weathering dominates the supply of Sr with elevated 87Sr/ 86Sr ratios in the Bhote Kosi–Sun Kosi river system, central Nepal, a major Himalayan tributary to the Ganges. The dissolved load of the Bhote Kosi–Sun Kosi displays a rapid increase in the 87Sr/ 86Sr ratio in its lower section, downstream of the Main Central Thrust, a feature common to other Himalayan tributaries of the Ganges. Mass balance analysis 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. This confirms the presence of a highly soluble source of radiogenic strontium and suggests that diffusive exchange during a protracted metamorphic history has homogenised Sr isotopes between silicate and carbonate phases within the assemblage. Weathering of the carbonate component of such rocks has yielded unusually high 87Sr/ 86Sr at significant Sr fluxes in the dissolved load. Approximately 60% of the impact of the Bhote Kosi–Sun Kosi on the marine 87Sr/ 86Sr ratio can be traced to weathering of this carbonate. Since similar lithologies outcrop within the catchments of most Himalayan tributaries that feed the Ganges and Brahmaputra rivers, the presence of such radiogenic carbonates undermines the inference that the impact of these rivers on Sr isotopes in seawater is indicative of high silicate weathering rates that result from the uplift of Tibet and the Himalaya.