Spatiotemporal Variation of Trace Elements and Stable Isotopes in Subtropical Estuaries: I. Freshwater Endmembers and Mixing Curves

Understanding spatial and temporal variation in chemical constituents across salinity gradients is essential for using proxies to track riverine inflow, mixing processes, and migration of mobile fauna. We analyzed the freshwater endmembers and mixing curves of a suite of trace element concentrations (Mg, Ca, Mn, Sr, and Ba) and stable isotope ratios (delta O-18 and Sr-87/Sr-86) in tributaries and estuaries of the south Texas coastal bend region. While Sr/Ca and Sr-87/Sr-86 were consistent across tributaries due to the dominance of carbonate bedrock underlying the region, Ba/Ca showed some river-specific variation perhaps due to upstream urbanization. In contrast, both Mn/Ca and delta O-18 showed a high degree of variability between 2 years due to their sensitivity to redox conditions and net evaporation, respectively. Mixing curves for Ca and Sr were moderately curvilinear, suggesting the potential importance of alternative processes altering mixing relationships, such as submarine groundwater discharge (SGD). Mixing of Mg was linear and consistent with conservative behavior, while Ba showed a low salinity peak and there was no apparent mixing relationship for Mn. Mixing of Sr-87/Sr-86 was curvilinear as expected, although the steepness of the mixing curve was lower than expected under linear mixing assumptions, further indicating the possible influence of SGD. Unexpectedly, delta O-18 values were elevated above marine values across the entire salinity gradient, perhaps due to drought conditions during the sampling period. Together, these results point to the individualistic spatiotemporal dynamics of chemical constituents and illustrate the care that must be taken when choosing an appropriate proxy for salinity.