Abstract 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 therefore the seasonal variations in isotope systematics and in the concentrations and export fluxes of both DIC and Sr from the Mississippi River.

中文翻译：

---

密西西比河下游锶和碳同位素系统学的季节性变化：对化学风化的影响

摘要 包含成对碳同位素（δ13C 和 ∆14C）、锶同位素（87Sr/86Sr）和水同位素（δ2H 和 δ18O）的多同位素系统学用于研究流动路径、化学风化机制和 Sr 输出通量之间的相干关系。密西西比河下游。2006 年至 2008 年期间，在路易斯安那州巴吞鲁日附近的一个地点每月收集水样，用于测量水同位素组成、溶解无机碳 (DIC) 的浓度和同位素组成，以及 Sr 的浓度和同位素比以及其他选定的主要元素。δ2H 和 δ18O 都遵循类似的季节性模式，从 3 月的最小值稳定增加到 7 月的最大值，表明水源从春季新鲜期间以融雪为主的密西西比河上游上游转移到其他季节降雨引起的大陆中部地表径流和地下水。δ13​​C-DIC 的值范围从 -8.67‰ 到 -5.96‰，而 Δ14C-DIC 的范围从 -56.8‰ 到 27.9‰，对应于从当代到 415 年 BP 的 14C 年龄。一般来说，Δ14C-DIC 随 δ13C-DIC 的增加而增加，表明碳酸氢盐来源的变化响应流动路径和化学风化状态的变化。雨季消耗的 Δ14C-DIC 和 δ13C-DIC 值可能是由碳酸盐矿物溶解引起的，涉及土壤衍生的 CO2，而旱季较高的 Δ14C 和相对富集的 δ13C-DIC 值反映了大气 CO2 特征，这意味着通过硅酸盐风化和河水与大气之间的季节性 CO2 交换来供应，而高初级生产增强了这种交换。Sr 浓度和 87Sr/86Sr 比率平均分别为 1.80 ± 0.26 μmol L-1 和 0.709866 ± 0.000248。Sr 浓度和 87Sr/86Sr 比率均显示与 δ18O 值显着相关，支持对密西西比河流域 Sr 源的水文控制。事实上，来自密西西比河上游最上游的太古代和元古代早期地形的放射成因 87Sr、碳酸盐矿物溶解释放的 Sr 以及硅酸盐风化产生的放射成因 87Sr 都出现在密西西比河下游，经过融雪流、降雨量的修改- 分别引起地表径流和地下/地下水。全面的，