EVALUATING OXYGEN AND CARBON ISOTOPE RATIOS IN RANGIA CUNEATA AS A PALEOENVIRONMENTAL PROXY

Oxygen isotope ratios (δ¹⁸O) in biogenic carbonates are the most common proxies for reconstructing seawater temperature. The utility of this proxy is inherently more complicated in estuarine environments because of the likely simultaneous variability in temperature and δ¹⁸O_water values. One of these variables must be held constant to interpret the δ¹⁸O values recorded in shells. The bivalve Rangia cuneata from the Neuse River Estuary, NC, can potentially serve as a useful environmental archive because, as we will show, the δ¹⁸O_water value does not vary much during the growing season in the upper and middle estuary (i.e., we can constrain the δ¹⁸O_water value). This study tests whether R. cuneata precipitates its shell in isotopic equilibrium with ambient conditions. Water samples were collected fortnightly from the upper estuary (UE), upper-middle estuary (UME), and middle estuary (ME) from June 2016 through July 2017 and analyzed for δ¹⁸O values and δ¹³C values of dissolved inorganic carbon (DIC). Average δ¹⁸O_water values were similar at all sites (UE = –4.0 ±1.0‰, UME = –3.5 ±1.1‰, ME = –3.0 ±1.1‰). Average δ¹³C_DIC values were more negative in the UE (–12.7 ± 2.1‰) than in the UME (–7.8 ±2.1‰) and ME (–6.7 ±2.6‰). Expected shell profiles were calculated using δ¹⁸O_water values, in situ water temperature, and established paleotemperature equations. Expected values were compared with measured shell values sampled at submonthly resolution. Measured δ¹⁸O_shell values were consistent with expected values at all sites, indicating that R. cuneata precipitate their shells in oxygen isotope equilibrium. However, preliminary results suggest that growth rates are highly variable with long periods of growth cessation in winter. We also observe periods of slowed or no growth in early spring and late fall. Measured δ¹³C_shell values are more complicated and cannot be used to track seasonal variations in δ¹³C_DIC values. In contrast, average δ¹³C_shell values reflect average δ¹³C_DIC values in the UME and ME, but not in the UE where freshwater has more of an influence. Future work will further investigate potential causes of growth cessation and the use of shell δ¹³C values as a proxy for δ¹³C_DIC values in R. cuneata.