Erosional modulation of the balance between alkalinity and acid generation from rock weathering

The balance between alkalinity generation by carbonate and silicate weathering and sulfuric acid generation by sulfide weathering controls the effect of weathering on atmospheric pCO2 over geologic timescales. How this balance varies across environmental gradients remains poorly constrained. Here, we analyze this balance across an erosional gradient of two orders of magnitude in the Three Rivers (Yangtze, Mekong, and Salween Rivers) Headwater Region (TRHR), eastern Qinghai-Tibet Plateau (QTP). By employing major element chemistry and multiple isotopes (delta³⁴SSO4, delta¹⁸OSO4, and delta¹⁸OH2O) coupled with forward and inverse approaches, we unmix contributions of silicates, carbonates, evaporites, and sulfides to the total weathering budget. Across the TRHR, riverine SO4^2- is derived mainly from a mixture of an evaporite source with uniform values of delta³⁴SSO4 and delta¹⁸OSO4, and a sulfide source that contributes highly variable values of delta³⁴S (-12.2 parts per thousand to +4.1 parts per thousand) and delta¹⁸O (-17.7 parts per thousand to -1.6 parts per thousand). Contributions of sulfide oxidation to riverine SO4^2- vary from 16 % to 94 %, and sulfuric acid consumes 6 % to 63 % of the alkalinity produced by weathering. The fractions of weathering alkalinity derived from carbonate weathering range from 36 % to 98 % relative to silicate weathering. The combination of silicate, carbonate, and sulfide weathering suggests that the instantaneous weathering fluxes of most sampled catchments in the TRHR act as a sink of atmospheric CO2 over timescales shorter than marine carbonate burial (similar to 10⁴ years), but as a carbon source over timescales longer than carbonate burial and shorter than sulfide burial (similar to 10⁷ years). The spatial variability of the balance between alkalinity and acid generation, and, thus, the relationship between chemical weathering and atmospheric pCO2, are largely dependent on lithology. However, within comparable lithologic settings, sulfide and carbonate weathering rates rise with increasing erosion, whereas silicate weathering rates remain constant. Consequently, plateau weathering shifts from a sink to a source of atmospheric CO2 with increasing erosion. These findings suggest that sulfide weathering is more sensitive to erosion than carbonate and silicate weathering, and that it could play an important role in the long-term carbon cycle during mountain building.