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.