Active CO2 emissions from thermal springs in the Karakoram fault system and adjacent regions, western Tibetan Plateau

Abstract

Large-scale intra-continental strike-slip faults provide the pathways for groundwater circulation and the release of CO2-rich fluids through thermal springs, which is a significant type of deep CO2 emissions into the atmosphere and hydrosphere. India-Asia collision created numerous strike-slip faults and thermal springs in the Tibetan Plateau and adjacent regions, but their CO2 origins and fluxes are still not well understood. Here, we present a quantitative study on CO2 emissions from thermal springs in the Karakoram fault system (KKFS) and adjacent regions, western Tibetan Plateau, with the aim of revealing the origins of CO2-rich fluids and estimating the dissolved and gaseous CO2 fluxes. Hydrogen and oxygen isotopic data show that the geothermal waters were recharged by local precipitation (e.g., rainfall and snow-melt water) and experienced variable degrees of water-rock interaction, giving rise to the dominant Na–HCO3 type and minor Na–Cl and Ca–HCO3 type waters discharging from thermal springs. Spatially, the thermal springs within or close to the KKFS (Group 1 and Group 2) have larger circulation depths of 1.3–5.1 km, while those away from the KKFS (Group 3) exhibit shallow (0.6−0.8 km) groundwater circulation. Mass balance calculations reveal high deeply-sourced carbon inputs (60.4−94.3%) to dissolved inorganic carbon (DIC) of the Group 1 and Group 2 samples, while the Group 3 samples have higher shallow carbon contributions (50.4−60.9%) to DIC via dissolution of carbonate and organic carbon. This is consistent with higher Δ14CDIC values (−788.6‰ to −692.1‰) in Group 3 samples and the lack of radioactive 14C in Group 1 and Group 2 samples (Δ14CDIC = −999.9‰ to −985.9‰). Based on chemical compositions and fluxes of spring water and bubbling gases, the total CO2 output from thermal springs (including dissolved and gaseous CO2) in the KKFS is estimated to be ∼2332 t a−1, in which the endogenic carbon such as metamorphic CO2 accounts for ∼1863 t a−1. Our results provide the first estimate of deep CO2 flux for the KKFS and would contribute to a better understanding of active CO2 emissions facilitated by strike-slip faulting processes in continental collision zone.