DECONVOLVING SEASONAL TEMPERATURES AND SALINITIES FROM CRASSOSTREA VIRGINICA USING A δ13C AND δ18O MULTIPROXY APPROACH

Researchers have attempted to reconstruct estuarine environmental temperatures and salinities from mollusk shell geochemistry for decades. Because shell carbonate δ¹⁸O is controlled by both temperature and δ¹⁸O_water (a salinity proxy), a second geochemical proxy is needed in order to uniquely solve for temperature and constrain salinity. Shell calcite Mg/Ca (a temperature proxy) or Ba/Ca (a salinity proxy) have been combined with δ¹⁸O measurements to solve this conundrum in other calcifying groups such as foraminifera. Unfortunately, the use of elemental ratios in mollusk shells to reconstruct ambient salinity and temperature has been elusive.

Here we present environmental reconstructions using the Eastern North American oyster, Crassostrea virginica, and water samples from the James River, Virginia that describe estuarine salinity and temperature conditions across four centuries. Water samples were collected from several localities across an annual cycle and analyzed for salinity, δ¹⁸O_water and δ¹³C_DIC. James River C. virginica shells of known provenance were sampled along their hinge margins for δ¹⁸O and δ¹³C measurements. James River water data display linear S vs δ¹⁸O_water and S vs δ¹³C_DIC relationships. We combine these environmental relationships with measured δ¹³C_shell and δ¹⁸O_shell values to compute the ambient James River δ¹⁸O_water values for the environmental chronology recorded by an oyster throughout its life. Converting the δ¹³C_shell values to salinity using the local S vs δ¹³C_DIC relationship yields ambient salinities that agree remarkably well the environmental data. We conclude that δ¹³C_shell can be used to compute ambient salinity and, in combination with δ¹⁸O_shell and a local S vs δ¹⁸O_water relationship, compute environmental temperatures in this mesohaline estuarine region. We present environmental salinity and temperature reconstructions from 5 recent (2002-2006), 4 historical (Jamestown, 1608-1612 AD) and two fossil Pleistocene C. virginica shells from the nearby Yorktown Formation to demonstrate the application of this multiproxy approach for deconvolving environmental salinity and temperatures from C. virginica shell geochemistry.