Seasonal variations in strontium and carbon isotope systematics in the Lower Mississippi River: Implications for chemical weathering

Multiple isotope systematics incorporating paired carbon isotopes (δ13C and ∆14C), strontium isotopes (87Sr/86Sr) and water isotopes (δ2H and δ18O) are used to investigate the coherent relationships among flow paths, chemical weathering regimes, and Sr export fluxes from the Lower Mississippi River. Monthly water samples were collected at a site near Baton Rouge, Louisiana, during 2006–2008 for measurements of water isotopic composition, the concentration and isotopic composition of dissolved inorganic carbon (DIC), and the concentration and isotopic ratio of Sr along with other selected major elements. Both δ2H and δ18O followed a similar seasonal pattern with a steady increase from a minimum in March to a maximum in July, indicating a shift of water sources from the snowmelt-dominant uppermost Upper Mississippi River during spring freshet to rainfall-induced midcontinent surface runoff and groundwater during other seasons. Values of δ13C-DIC ranged from −8.67‰ to −5.96‰ while Δ14C-DIC varied from −56.8‰ to 27.9‰, corresponding to a 14C age from contemporary to 415 yr BP. Generally, Δ14C-DIC increased with increasing δ13C-DIC, suggesting variations in bicarbonate sources in response to the shifts of flow paths and chemical weathering regimes. Depleted Δ14C-DIC and δ13C-DIC values during the wet seasons are likely contributed by carbonate mineral dissolution involving soil-derived CO2, while the higher Δ14C and relatively enriched δ13C-DIC values during the dry seasons mirror the atmospheric CO2 signatures, implying the supply by silicate weathering and a seasonal CO2 exchange between riverwater and the atmosphere which is enhanced by high primary production. Sr concentrations and 87Sr/86Sr ratios averaged 1.80 ± 0.26 μmol L−1 and 0.709866 ± 0.000248, respectively. Both Sr concentrations and 87Sr/86Sr ratios show a significant correlation with δ18O values, supporting a hydrologic control of the Sr provenance in the Mississippi River basin. Indeed, the radiogenic 87Sr from the Archean and early Proterozoic terrain in the uppermost Upper Mississippi River, the Sr released from carbonate-mineral dissolution, and the radiogenic 87Sr from silicate weathering are manifested in the Lower Mississippi River with modifications from the snowmelt flow, rainfall-induced surface runoffs, and subsurface/groundwater, respectively. Overall, our results suggest that different hydrological flow regimes play unique roles in regulating the chemical weathering processes and the