GEOCHEMICAL VARIATION IN MICROSTRUCTURAL LAYERS OF BIVALVE SHELLS (MERCENARIA CAMPECHIENSIS)

Mercenaria campechiensis, the southern quahog, is an estuarine clam (salinity range: 20-30 ppt) that has been used in paleoenvironmental, paleoclimate, and paleoecological studies, but has not yet been calibrated geochemically. Its aragonitic shell may preserve seasonal change in ambient temperature, salinity, and dissolved inorganic carbon as variation in Sr:Ca ratios, d¹⁸O, and d¹³C, respectively. Early geochemical studies of quahog shells focused on the outer prismatic layer. Because of innovations in microsampling techniques, recent studies have focused on the middle cross-lamellar layer, providing high temporal resolution. Do both microstructural layers record similar profiles of d¹⁸O, d¹³C, and Sr:Ca variation? If not, which layer more faithfully records ambient conditions? Here, we provide a preliminary d¹⁸O calibration and test the hypothesis that both microstructural layers record similar geochemical information.

Quahogs were collected alive near Bokeelia, Pine Island in Charlotte Harbor, Florida. The USGS maintained fortnightly to monthly temperature and salinity records at this site. These data, together with the mixing equation of d¹⁸O-WATER and salinity, were used to construct a predictive model shell. The last 2 years of shell growth were sampled at 0.1-0.5 mm intervals yielding ~100 mg of material that was split for isotopic and elemental analyses.

Comparison of the predicted and observed shells confirms that d¹⁸O is in isotopic equilibrium with ambient water. Values of predicted and observed shells range from -2.5 to +2‰. Profiles of d¹⁸O from outer and middle layers are nearly identical. In contrast, d¹³C of the middle layer is more variable and can be as much as 2.4‰ more positive than the outer layer. Sr:Ca ratios from the middle layer can be as much as 0.4 mmol/mol higher than the outer layer.

We conclude that either microstructural layer can be used to study variation in d¹⁸O because both layers preserve nearly identical profiles. Observed offsets in d¹³C and Sr:Ca ratios between microstructural layers can potentially complicate calibration of these geochemical proxies and environmental and climate reconstructions. The next phase of our research is to understand the mechanisms that control these offsets and determine which microstructural layer is better suited for geochemical study.