Oxygen Isotope Values and Schlerochronology of Crassostrea virginica Shells from Chesapeake Bay: Applications to Historical Paleoclimate

Reconstruction of regional climate variability in estuarine systems is challenging, particularly in our study area due to a lack of pre-instrumental era climate records for the Chesapeake Bay. The oyster, Crassostrea virginica, is a promising archive for reconstruction of pre-industrial climate in the Chesapeake Bay region. C. virginica has the potential to provide seasonal climate records that can inform how climate influenced history. Here, we present 18Osw and 18Oshell values from four modern C. virginica shells to better understand how variability in 18Oshell values of C. virginica calcium carbonate shells reflect changes in surface water temperature and salinity in Chesapeake Bay. We apply our results to six historic oyster shells collected stratigraphically from infilled slave quarters at Stratford Hall, Virginia (14C dated to ~1756 CE bottommost shell to ~1779 CE topmost shell). These oysters provide records of seasonal variability on multidecadal timescales near the end of the Little Ice Age that could have played a role in the demise of Stratford Halls working plantation. All modern and historical oysters were sampled along their banded growth structure, providing seasonal resolution for isotopic analysis. Additionally, we photographed shells using a scanning electron microscope and measured growth rates between major growth bands, creating a growth band- 18Oshell model. We found that the 18O values of the modern shells precipitate in isotopic equilibrium with the 18Osw. In the historical oysters, older shells had a smaller range in 18O (-2.8 to -7.3VPDB) and smaller growth averages during both the cold and warm seasons (-0.84 and -1.3 mm/season; respectively). The youngest shell range of 18O values was -2.1 to -7.4VPDB and exhibited the largest growth during both cold and warm seasons (1.12 and 1.74 mm/season; respectively). These results suggest a change from a colder, harsher climate to a warmer climate with more seasonal variability, in agreement with previous studies that show an increase in seasonality on multidecadal timescales near the end of the Little Ice Age- a change that may have impacted Stratford Halls plantation. Our work highlights how C. Virginica has potential to provide seasonal-resolution paleoclimate reconstructions and inform shifts in major historical events.