Spatiotemporal Variations of Dissolved Major and Trace Elements in the Potomac River and Chesapeake Bay Estuary

Abstract Geochemical proxies using trace element incorporation into biogenic carbonates have been instrumental in elucidating past environmental conditions and migration histories of estuarine organisms. To develop successful proxies, it is necessary to understand the underlying water chemistry from which biomineralized carbonate precipitates; this is especially so in dynamic environments such as estuaries, where dissolved elements (DEs) may vary in response to different environmental conditions. Here we investigate spatiotemporal variability in DEs that may be useful to develop salinity (Na, Mg, Sr, Ba) or redox (Mn) proxies in estuarine environments. This was accomplished by sampling surface and bottom waters in the summer and winter across the freshwater to marine salinity gradient (0.1–30 psu) of the tidal Potomac River and Chesapeake Bay (PR-CB). Concentrations of Na, Mg, Sr, and Ca were positively correlated with salinity, and did not show spatiotemporal variation. Conversely, Ba was negatively correlated with salinity and exhibited a low-salinity plateau of semi-constant concentrations that varied across sampled seasons. Mn was elevated in bottom waters when dissolved oxygen was < 6 mg/L. Potential salinity proxy DEs were converted to DE/Ca ratios and, when compared to salinity, produced non-linear relationships where the greatest rates of change occurred at salinities < 5–10 psu. Compared to other estuaries, the PR-CB estuary exhibited similar non-linear DE/Ca-salinity relationships, with variations in the lines of best fit and DE/Ca concentrations at a given salinity. The results of this study will inform future proxy development studies of biominerals in estuarine environments, which could improve paleoreconstructions, aid fisheries management, and help predict future environmental change.