Stable Isotopes and Bayesian Tracer Mixing Model Reveal Chemical Weathering and CO2 Release in the Jiulongjiang River Basin, Southeast China

Abstract

AbstractLinks between erosion, chemical weathering, and atmospheric CO2 can modulate the global climate over geologic times, but the role of chemical weathering on Earth's long‐term carbon cycle is still unclear. Although silicate weathering by carbonic acid could sequester CO2, the pyrite oxidation coupled to carbonate dissolution could provide a transient source of CO2 at the same time. Landscapes with low physical erosion rate cover the majority of exposed land area, however, the concomitant weathering of silicate, carbonate, and sulfide occurring in the low erosion rate terrains is not well understood. Here, we investigate these questions by examining the river water chemistry from a catchment characterized by complex lithology and low erosion rate. Using a Bayesian tracer mixing model, elemental and isotopic ratios, we quantitatively identify the proportion of cations from silicate and carbonate weathering as well as the acid sources during weathering. We propose that the sulfuric acid from pyrite oxidation boosts both silicate and carbonate weathering, and the coupling between pyrite oxidation and silicate weathering is more pronounced as the pyrite oxidation rates increases. In doing so we determine the effect of weathering on the CO2 balance in our study area, and find that the CO2 release from pyrite oxidation coupled to carbonate dissolution overwhelms the CO2 drawdown from silicate weathering on the time scale between marine carbonate precipitation and sulfate reduction (approximately 106–107 years). Our analysis indicates that the lithology and distribution of minerals play an important role on the chemical weathering in low erosion rate terrains.