Sulfate-isotope and marker-gene evidence for microbial overprinting of pyrite oxidation in terrestrial environments

Pyrite oxidizes aerobically or anaerobically to generate dissolved sulfate (SO₄^2-) and acidity (H⁺ ions) in rivers – the latter drives the chemical weathering of carbonate rocks. The isotopic composition of sulfate (δ³⁴S_SO4 and δ¹⁸O_SO4) has been utilized to resolve the sources of riverine dissolved SO₄^2- – pyrite oxidation and evaporite weathering. Furthermore, the triple oxygen isotopic composition (Δ’¹⁷O) of marine sulfate deposits is used as a proxy for reconstructing past atmospheric conditions (pO₂/pCO₂) and gross primary productivity—an approach that requires that terrestrial pyrite oxidation consumes atmospheric O₂ without subsequent secondary modification. However, sulfate isotopes may not be conservative tracers of pyrite oxidation if microbial sulfate reduction (MSR) in anoxic environments, such as those in soils and aquifers, overprints the pyrite-derived sulfate isotopic composition. Hence, to derive fresh insights into pyrite oxidation and MSR in terrestrial environments, we analyze the δ³⁴S_SO4, δ¹⁸O_SO4, δ¹⁸O_H2O, major ions, and microbial marker gene abundances of dissimilatory sulfite reductase subunit B (dsrB) and the 16S rRNA gene in a suite of river samples across an elevational and erosional gradient in the headwaters of the Ganga in the Himalayas. We find that dissolved SO₄^2- primarily derived from pyrite oxidation is extensively modified by MSR, which is maximized in low-erosion catchments with moderate mean annual precipitation (MAP) – a combination of factors that promotes longer fluid residence times in aquifers and in the vadose zone. By extending our framework to a global compilation of concomitant δ³⁴S_SO4, δ¹⁸O_SO4, δ¹⁸O_H2O, and major ions measurements, we find that MSR is as important as lithological variability in setting the isotopic composition of terrestrially derived SO₄^2-. As such, we argue for explicit constraints on terrestrial MSR when inferring relative contributions of pyrite and evaporite weathering to riverine SO₄^2- and when utilizing Δ’¹⁷O in marine sulfates to infer past atmospheric conditions.