RELATIONSHIP OF D18O AND D13C OF FORAMINIFERAL TESTS TO BOTTOM-WATER SALINITY IN AN ATLANTIC COASTAL LAGOON, DELAWARE, USA

We examined a spatial array of sites in Rehoboth Bay, a lagoonal estuary along the Atlantic Coast of Delaware (USA), to evaluate the possible use of stable isotopes of benthic foraminiferal tests to track the relative proportions of river versus marine water input. Samples were taken at 29 sites in Rehoboth Bay in August 2005. The bay is shallow (mostly less than 2 m) and microtidal. During the summer, a number of small tidal rivers provide an input of lower salinity, higher temperature water; in contrast, marine input is through a single inlet, providing lower temperature, higher salinity waters. Given these temperature and salinity relationships, we would expect that foraminiferal tests from sites closer to fresh water sources should have lower d18O values than samples from sites located farther away. To test the potential of estuarine foraminifera in the Rehoboth Bay for paleoenvironmental reconstruction, we have selected living and dead tests of two calcareous foraminiferal species: Ammonia beccarii and Elphidium poeyanum. Both species yield consistent results in that d18O values are indeed lower at the sites closer to the river and increase with distance from it. Specifically, highest statistical significance between test geochemistry and modern conditions is observed between d18O values of living E. poeyanum and salinity (r2=0.72). For both d13C and d18O the dead E. poeyanum show better correlation with temperature (d18O, r2= 0.54; d13C, r2=0.19) than the living ones. For A. beccarii, ¬d18O values of living individuals also show better correlation with salinity (r2=0.49) and temperature (r2=0.36) than dead ones (r2=0.31; r2=0.30); however the d13C from the dead individuals show better correlation with salinity (r2= 0.17) and temperature (r2=0.24) than the living ones. The d13C values show similar relationship to d18O values, which is consistent with modern conditions as river water is generally enriched in 12C with respect to open marine water. These cores top results suggest that stable isotopes of tests of these foraminiferal species reflect mixing of marine and fresh waters in this lagoon, and that a down core study to evaluate salinity history in this estuary is feasible.