HR: 09:15h
AN: B21A-06    [PDF]
TI: Oxygen and Carbon Stable Isotope and Sr:Ca Records in Outer and Middle Microstructural Layers of Bivalve Shells ({\it Mercenaria campechiensis})
AU: * Surge, D
EM: donna@iastate.edu
AF: Iowa State University, Geological & Atmospheric Sci 253 Science I, Ames, IA 50011-3212 United States
AU: Owens, S
EM: sowens@iastate.edu
AF: Iowa State University, Geological & Atmospheric Sci 253 Science I, Ames, IA 50011-3212 United States
AB: $\delta$18O, $\delta$13C, and Sr:Ca records contained in the aragonitic shells of the northern and southern quahogs ({\it Mercenaria mercenaria} and {\it M. campechiensis}, respectively) have been used in paleoclimate and paleoecological studies, although these environmental proxies have not yet been calibrated. Early geochemical studies of quahog shells focused on the outer prismatic layer. Because of innovations in microsampling techniques, recent studies focus on the middle cross-lamellar layer, providing high temporal resolution. Do both microstructural layers record similar profiles of $\delta$18O, $\delta$13C, and Sr:Ca? If not, which layer more faithfully records ambient conditions? Here, we provide a preliminary calibration of $\delta$18O and $\delta$13C using shells of {\it M. campechiensis} and test the hypothesis that both microstructural layers record similar geochemical information. Quahogs were collected alive near Bokeelia, SW Florida. The USGS maintained fortnightly to monthly temperature and salinity records at this site. These data, together with mixing equations of $\delta$18O$_{WATER}$- and $\delta$13C$_{DIC}$-salinity, were used to construct predictive model shells. The last 2 years of shell growth were sampled at 0.1-0.5 mm intervals yielding $\sim$100 $\mu$g of material that was split for isotopic and elemental analyses. Comparison of model and observed shells reveals a predictable offset in $\delta$18O, where the model is offset by +1.3$\permil$ relative to the observed shell. When corrected for the offset, $\delta$18O shell closely matches the model. Values range from -2.5 to +2$\permil$. Profiles of $\delta$18O from outer and middle layers are nearly identical. $\delta$13C of the middle layer is more variable and can be as much as 2.4$\permil$ more positive than the outer layer. Observed $\delta$13C in the middle layer follows the general trend of the model, but winter-spring shell growth is offset by -1.7$\permil$ relative to the model. 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 $\delta$18O because both layers preserve nearly identical profiles. Observed offsets in $\delta$13C and Sr:Ca ratios between microstructural layers can potentially complicate calibration and environmental/ 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.
DE: 0400 Biogeosciences
DE: 1045 Low-temperature geochemistry
DE: 3344 Paleoclimatology
DE: 4870 Stable isotopes
SC: Biogeosciences [B]
MN: 2003 Fall Meeting