DO DIFFERENT BIVALVE MOLLUSK SPECIES RECORD THEIR SHARED ENVIRONMENT IN THE SAME WAY?

Bivalve mollusks record environmental variations experienced during shell growth. Paleoenvironmental reconstructions derived from these biogeochemical archives rely on the assumption that these animals faithfully recorded environmental conditions. It is generally accepted that bivalves precipitate oxygen isotopes (ẟ¹⁸O) in equilibrium with water, and carbonate ẟ¹⁸O values reflect temperature and the oxygen isotope composition of the water. Less well understood, however, is the role of species-specific growth preferences. Variable growth patterns can obscure original environmental variation in the same way an antique windowpane blurs the view outside. Our previous work suggests that the clam, Mercenaria mercenaria, and the oyster, Crassostrea virginica, record slightly different aspects of their shared environment. ẟ¹⁸O values from M. mercenaria reflect preferential growth during the warmest hours of the day, while C. virginica do not show this bias. To investigate this idea, we collected 22 clams and 36 oysters of various sizes living in the same locality (Jarrett Bay, Carteret County, North Carolina). All specimens were sectioned along the axis of maximum growth. A single ~75 μg carbonate sample was drilled from each specimen using a 300-μm drill bit. M. mercenaria specimens were drilled at the commissure, whereas C. virginica samples were drilled from the ventral margin of the resilifer. In all cases, carbonate samples represent the most recent growth prior to collection. Our preliminary results suggest ẟ¹⁸O values from M. mercenaria shells have a lower mean, smaller standard deviation, and narrower range than those from C. virginica. In both clams and oysters, ẟ¹⁸O values from smaller specimens showed less variability than from larger specimens. These patterns reflect a combination of species-specific preferential growth temperatures and sample time-averaging reflecting different growth rates through ontogeny. We also calculated water temperatures from all specimens using several aragonite (clam) and calcite (oyster) paleotemperature equations. Assuming a constant ẟ¹⁸O water value, our reconstructed temperatures range over at least 5℃. Taken together, our results suggest reconstructed paleotemperatures are highly dependent on taxon, specimen size, and paleotemperature equation.