Constraining sulfate sources and its implications on atmospheric pCO2 in Arid Regions: Evidence from the Tarim Basin

Abstract

The pyrite oxidation-driven weathering has important implications for carbon cycle over geological timescales. Despite extensive researche using SO₄²⁻ as information carriers across diverse climatic zones, studies in arid regions remain limited, underscoring the urgency for research in these areas. This study collected and analyzed water samples from rivers and lakes in the Tarim Basin, one of the driest regions worldwide and China's largest endorheic basin. The samples were analyzed for physical, chemical and isotopic parameters e.g., water temperature, pH, electrical conductivity, concentrations of major anions and cations, and isotopic compositions of SO₄²⁻ (δ³⁴S_SO4 and δ¹⁸O_SO4). The results reveal pronounced spatial variations in δ³⁴S_SO4 and δ¹⁸O_SO4 values, which ranged from −2.4 to +14.8 ‰ and + 0.6 to +16.8 ‰, respectively. Post-simulation with a Monte Carlo inversion model suggests that carbonates (i.e., calcite and dolomite) weathering, evaporites (i.e., gypsum and halite) dissolution and pyrite (i.e., FeS₂) oxidation predominantly control water chemical compositions, contributing ∼72.9 % to 97.7 % (median 91.8 %) to dissolved solutes. The findings indicate that pyrite oxidation and gypsum dissolution are the primary sources of SO₄²⁻, accounting for 32.7 % to 80.9 % (median 55.9 %) and 13.4 % to 66.5 % (median 39.7 %), respectively. The estimated SO₄²⁻ fluxes in the study area amount to 0.108 Tmol/year, representing ∼3.2 % of the global SO₄²⁻ flux, underscoring the significance of SO₄²⁻ in arid regions for global sulfur cycle. Furthermore, the saturation indices of calcite, aragonite and dolomite are all greater than 0, indicating carbonate precipitation occurring in current environment. The high evaporation under arid conditions and wide distribution of evaporites (e.g., gypsum) in the study area can promote carbonate precipitation, enhancing the carbon sink capacity of rock weathering. This research provides critical data for elucidating the role of pyrite oxidation-driven weathering and evaporite dissolution on carbonate budget in arid regions influenced by the Westerlies, offering novel empirical evidence on continental weathering.