Provenance of weathering-derived dissolved ions and CO2 balance in the upper Assam Brahmaputra Valley

Rivers draining the Himalaya constitute major atmospheric CO₂ sinks due to exceptionally high chemical weathering fluxes. However, the net CO₂ drawdown depends on both the contribution of end members such as carbonate and silicate weathering, precipitation, and the extent of CO₂ supersaturation and potential emission evasion. Here, we integrate a multi-endmember inverse mixing model with freshwater carbonate-system calculations (CO2SYS) for Brahmaputra main-channel river water samples, suspended particulate matter (SPM) and bed sediments from Assam, India. Size-fractionated silicate leaches (>63 µm, 63–10 µm, 10–2 µm, <2 µm) and SPM plotted on MIA–AF–CN–M ternary cluster toward Mg–Fe–Al–rich silicate fields, consistent with a notable freshly supplied mafic component in the sediment pool (Figure 1a). An inverse framework in charge-equivalent tracer space (χCa, χMg, χNa, χK, χCl,  χSO₄; normalized by Σ⁺ = Ca²⁺+Mg²⁺+Na⁺+K⁺) is used to apportion contributions from precipitation, carbonate, and silicate weathering. Model results indicate carbonate-dominated dissolved ion generation (f_carb ≈ 0.52–0.69) with a smaller, yet consistent silicate contribution (f_sil ≈ 0.21–0.33). An independent forward mass-balance, constrained by regional rain chemistry and published silicate ratios for trans Himalayan and Himalayan rocks, produces comparable silicate and carbonate fractions, thereby corroborating the inverse apportionment. CO2SYS calculations based on measured alkalinity and pH yield pCO₂ ≈ 680–2420 µatm (pH ≈ 7.6–8.2), which indicates persistent CO₂ supersaturation. The ratios of total alkanility to dissolved inorganic carbon (TA/DIC) are consistently less than 1 (TA/DIC ≈ 0.955–0.995). In contrast, a theoretical weathering-only TA/DIC, calculated by balancing sulphuric acid, carbonate, and silicate weathering in charge equivalent space, ranges from 2.0 to 2.9. This DIC excess (theoretical DIC – observed DIC) implies the presence of additional CO₂ along the reach beyond that supplied by weathering. Subansiri, a tributary of the Brahmaputra, shows the highest pCO₂ and DIC excess coupled with the lowest SPM load (0.0075 g/L).  The theoretical weathering framework indicates these sediments as a net CO₂ sink (Figure 1b). The persistently low TA/DIC ratios (<1) and excess DIC relative to theory suggest additional in-stream CO₂ sources, possibly from organic matter decomposition and sulphuric-acid–driven weathering.