DETANGLING STABLE CARBON ISOTOPES IN GIANT CLAM SHELLS: DISCREPANCIES BETWEEN SHELL LAYERS AND THE SUESS EFFECT

Giant clams, Tridacna, are sentient bivalves that live in symbiosis with algae in shallow, tropical coral reefs in the Pacific and Indo-Pacific Oceans. Their aragonitic shells mineralize at a fast rate, producing robust seasonal records of their ambient environment. Stable isotope ratios are often measured from the carbonate of these shells as oxygen is known to be in isotopic equilibrium with seawater, thus useful for reconstructing sea surface temperatures. During the collection of δ¹⁸O values, δ¹³C is also obtained, yet often not discussed or interpreted. Giant clams are composed of two distinct shell layers, inner and outer, from which δ¹³C values are not equivalent. Here, we present a comparison of δ¹³C and δ¹⁸O values from both shell layers of T. derasa and T. squamosa collected from Palau in 2017 and T. maxima collected from American Samoa in 1982. Values of δ¹⁸O are used to calibrate growth rate between layers and show that growth is not always proportional. Profiles of δ¹³C from the inner shell layer of T. derasa are typically 0.5 ‰ greater than the outer shell layer while the difference is about 1.0 ‰ for T maxima. Values of δ¹³C from T. squamosa are nearly equal between the shell layers, although there is a trend towards lower values in the outer layer with age. On the other hand, previous studies of T. gigas show that the outer layer is more enriched in ¹³C than the inner layer. Values of δ¹³C from the inner layer of contemporaneous specimens are not statistically different across species. These discrepancies suggest there are species specific fractionations occurring in the outer shell layer, implying that it is important to note which layer is analyzed, especially for climatological assessments. Additional biochemical studies are required to determine the cause of fractionation in the outer shell layer or any effects from symbiotic algae. A temporal comparison of these and all published inner layer δ¹³C profiles shows depletion at the same rate as the depletion of atmospheric CO₂ due to the Suess Effect. Under the assumptions that 1) mineralization of the inner shell layer is similar among species, 2) there are no overriding environmental effects and 3) the atmosphere is well mixed, we find that the inner shell layer of the giant clam is an adequate record of the Suess Effect.