Stable Isotopes of Dissolved Inorganic Carbon as a Tracer of Chemical Weathering Sources

Rivers play a central role in the carbon cycle by mediating the transfer of carbon and alkalinity from the continents to the ocean. Alkalinity generated by chemical weathering of silicate minerals serves as a carbon sink on geologic timescales. The relative contributions of silicate weathering to dissolved inorganic carbon (DIC) and alkalinity fluxes is typically calculated by inverting water solute data, which is limited by assumptions of conservative solute transport. Instead, the analysis of ẟ13C in DIC enables the apportionment of DIC from organic matter, carbonate, and atmospheric sources. Modeling DIC in streamflow as a linear mixture of such endmembers may induce bias because most rivers are supersaturated with respect to atmospheric CO2 and the ẟ13C signature of DIC is fractionated by CO2 degassing.

In this study, we simulate the co-variation of river pCO2 and ẟ13CDIC induced by degassing. Our model is calibrated to groundwater and stream samples along the Little Deschutes River, OR, which is underlain by volcanic rocks and contains little to no carbonate minerals. Samples from different geomorphic settings (springs, headwaters, main stem) have been degassed to variable extents, supported by a wide range in estimated pCO2. Groundwater springs show DIC isotope signatures that indicate equilibration with respired organic matter (ẟ13CDIC of -18.1‰ to -15.8‰) whereas adjacent stream samples show a more enriched signature (ẟ13CDIC of -13.5‰ to -4.3‰) that varies predictably with pCO2. After testing the degassing model on this nominally carbonate-free watershed, we explore whether the DIC isotope signature of "un-degassed" groundwater can be determined using only a spatially distributed set of stream samples. We additionally apply our model to several watersheds in Iceland that have hypothesized carbonate weathering inputs to test the applicability of our degassing approach for calculating geogenic contributions to DIC fluxes.