Paper No. 97-2
Presentation Time: 8:15 AM
STABLE CARBONATE ISOTOPES IN GIANT CLAM SHELLS
Giant clams (Cardiidae: Tridacninae) are excellent bioarchives of their local low latitude reef ecosystems because they are sedentary, grow rapidly and have longevity. Stable oxygen isotopes from shell carbonate are in equilibrium with seawater, thus useful archives. However, stable carbon isotopes are often measured simultaneously but disregarded as they have not been well-associated with environmental or vital factors. Stable isotope ratios from the internal layer of the aragonitic shells have previously been measured in fourteen modern and ten fossil individuals. This study presents a thorough literature review of all published stable isotope profiles of Tridacna to compare species, localities, and changes through time as well as present original isotope profiles from a modern Palauan T. derasa. Results suggest that δ18O is sensitive to very local conditions, thus cannot be accurately compared on an oceanic scale. However, median δ13C values have become depleted in giant clam shells since the early 1960’s, the oldest modern measurement, additionally supported by δ13C of T. derasa collected from Palau in 2016. This trend is similar to that of δ13C of atmospheric CO2, which has been becoming depleted in 13C due to the anthropogenic burning of fossil fuels which are isotopically light. Since the evolution of C4 plants (isotopically heavy) in the Miocene, atmospheric CO2 has become heavier, thus as fossil fuels burn, they are introducing isotopically light CO2 from C3 sourced fossil fuels. Atmospheric CO2 is mixed relatively quickly and incorporated into seawater. Subsequently it is included in the shell aragonite. Although there are other influences on carbon in giant clam shells, it appears that δ13C values can be a useful analogy to δ13C from atmospheric CO2.