Net CO2 release during chemical weathering in the north-western Himalaya: A dominant role of pyrite oxidation

Abstract

Oxidation of pyrites plays a key role in global chemical and climatic cycles. In particular, interaction of sulfuric acid produced through this process with carbonates releases CO2 to the atmosphere. This CO2 source counterbalances the CO2 consumed during silicate weathering in river basins, and hence, may influence earlier-suggested linkage between silicate weathering and global cooling events. In this study, we have investigated the dissolved major ions and sulfur isotopes of Indus headwaters to quantify the net effect of sulfide oxidation on the CO2 budget. This research is an attempt to evaluate the coupling between global Cenozoic cooling event and Himalaya weathering - ­a hypothesis which overlooked the CO2 supply via sulfide and organic oxidation. Towards this, we have employed sulfur isotopes (δ34S) as a proxy for riverine sulfate sources, mainly due to its distinct composition for the two major end-members [pyrite (~ -12 ‰) and gypsum (~ 17 ‰); [1]]. The average sulfate concentrations for the Indus headwaters are found to be higher than the regional rainfall, global average for rivers, and other major Himalayan rivers (e.g., Ganga and the Brahmaputra). Consistently, the mean δ34S for the Indus headwaters is also depleted with respect to that reported for the Ganga (~ 2 ‰) and Brahmaputra (~ 4 ‰) outflows [1-5]. Also, the sulfur isotopic values for the Indus headwaters are systematically depleted by 3 to 4 ‰ than that reported earlier for Indus mainstream [3]. These lighter δ34S values for the headwaters hint at relatively higher sulfide oxidation in the northwestern (NW) Himalaya compared to central and eastern Himalayas. Also, these processes are found to be more intense in the mountainous regions than in the floodplains. These observations are consistent with the basin lithology dominated by Paleozoic carbonates and organic-rich shales, and higher glacial coverage. Estimation of sulfide-derived cations from carbonate weathering and silicate-derived cations indicate that the chemical weathering in the Indus headwaters serve as a net source of CO2 to the atmosphere. This finding is in contrast with previous suggestion of significant CO2 removal during the Himalaya weathering and hence, challenges the role of land surface processes in the NW Himalaya in regulating the Cenozoic cooling event.  References[1] Burke et al., (2018), Earth Planet. Sci. Lett., 496, 168-177.[2] Chakrapani et al., (2009), J. Asian Earth Sci., 34, 347-362.[3] Karim and Veizer, (2000), Chem. Geol., 170, 153-177.[4] Kemeny et al., (2021), Geochim. Cosmochim. Acta, 294, 43-69.[5] Turchyn et al., (2013), Earth Planet. Sci. Lett., 374, 36-46.