Silicate and carbonate weathering in the drainage basins of the Ganga-Ghaghara-Indus head waters: contributions to major ion and Sr isotope geochemistry

Abstract

The role of silicate and carbonate weathering in contributing to the major cation and Sr isotope geochemistry of the headwaters of the Ganga-Ghaghara-Indus system is investigated from the available data. The contributions from silicate weathering are determined from the composition of granites/ gneisses, soil profiles developed from them and from the chemistry of rivers flowing predominantly through silicate terrains. The chemistry of Precambrian carbonate outcrops of the Lesser Himalaya provided the data base to assess the supply from carbonate weathering. Mass balance calculations indicate that on an average ~ 77% (Na + K) and ~ 17% (Ca + Mg) in these rivers is of silicate origin. The silicate Sr component in these waters average ~40% and in most cases it exceeds the carbonate Sr. The observations that (i) the ⁸⁷Sr/⁸⁶Sr and Sr/Ca in the granites/gneisses bracket the values measured in the head waters; (ii) there is a strong positive correlation between ⁸⁷Sr/⁸⁶Sr of the rivers and the silicate derived cations in them, suggest that silicate weathering is a major source for the highly radiogenic Sr isotope composition of these source waters. The generally low ⁸⁷Sr/⁸⁶Sr (< 0.720) and Sr/Ca (~ 0.2 nM/μM) in the Precambrian carbonate outcrops rules them out as a major source of Sr and ⁸⁷Sr/⁸⁶Sr in the headwaters on a basin-wide scale, however, the high ⁸⁷Sr/⁸⁶Sr (~ 0.85) in a few of these carbonates suggests that they can be important for particular streams. The analysis of ⁸⁷Sr/⁸⁶Sr and Ca/Sr data of the source waters show that they diverge from a low ⁸⁷Sr/⁸⁶Sr and low Ca/Sr end member. The high Ca/Sr of the Precambrian carbonates precludes them from being this end member, other possible candidates being Tethyan carbonates and Sr rich evaporite phases such as gypsum and celestite. The results of this study should find application in estimating the present-day silicate and carbonate weathering rates in the Himalaya and associated CO₂ consumption rates and their global significance.