Current Projects

Boron isotopes in estuarine environments: A new proxy for paleosalinity
We propose the possibility of using the boron isotopic composition of carbonate shell material as a new proxy for paleosalinity in estuarine environments and demonstrate the theoretical basis for this approach. A model, based on expected conservative B and δ¹¹B mixing relationships, pH, and salinity-dependent boric acid and borate ion dissociation constant variations, predicts the δ¹¹B variations of borate ion in estuarine environments as a function of salinity. Three scenarios are simulated in which the pH of a fresh water river endmember determines the pH-salinity relationships in the estuary. The salinity-boron-δ¹¹B relationships were investigated for two estuaries (Newport estuary, NC and Venice lagoon, Italy). The model shows that the estuarine mixing effect is minimal for δ¹¹B in water due to the relatively high concentration of B in seawater, but that pH and salinity combine to control the fraction of borate ion in the solution, and consequently, the δ¹¹B of the borate ion. Because borate ion is the primary boron species incorporated into CaCO3, we show that for estuaries with riverine inflow of low (6.5) and medium (7.5) pH, a lower δ¹¹B in carbonates with decreasing salinity is expected. For low and medium pH systems, our model predicts a continuous decrease in δ¹¹B in carbonate skeletons from ~20‰ at seawater salinity to ~12‰ at salinity of 5 ppt. Analytical precision on δ¹¹B measurements in carbonate skeletons is approximately ±0.5‰. Thus the δ¹¹B of estuarine calcareous micro- and macrofossils has the potential to serve as a sensitive paleosalinity proxy.