BIOLOGICALLY DRIVEN ISOTOPIC FRACTIONATIONS IN BIVALVE GENERA DOSINIA AND MACTRA BETWEEN 43°S AND 53°N

The δ¹⁸O, δ¹³C, Δ₄₇, and recently Δ₄₈ composition of mollusk shells has been widely used to reconstruct past climate. The outer shell layers (OSL) of fossil bivalves are especially useful for paleoenvironment, as the isotopic values tend to reflect water temperature and δ¹⁸O of water at the time of shell growth. However, recent work has shown that the composition of the inner shell layer (ISL) can be different from the OSL in any or all of these isotopic parameters. These differences can be attributed to different biologically-controlled precipitation mechanisms governing formation of each layer. These biologically driven isotopic fractionations (BioDIFs), or “vital effects,” are typically avoided in paleoclimate studies, but new tools (paired δ¹⁸O-Δ₄₇ and Δ₄₇-Δ₄₈ measurements) and an integrated understanding of biology and geochemistry can help illuminate the mechanisms driving these fractionations. Preliminary data suggests that these BioDIFs may be characteristic of particular genera, however there are few systematic measurements of BioDIFs that control for phylogeny and location/environment. We measured δ¹⁸O, δ¹³C, Δ₄₇, and Δ₄₈ in modern bivalves of two different genera (Dosinia spp. and Mactra spp.) to characterize BioDIFs. We sampled 10 species of genus Mactra and 13 species of genus Dosinia (total n = 48) over a latitudinal range of 43°S to 53°N. This can test whether BioDIFs are the same among species of a certain genus. Considering latitude and growth environment may reveal how or whether certain external factors influence BioDIFs in bivalves. This could give insight into how bivalve shells record environmental information, which can strengthen paleoclimate interpretation. Differences in BioDIFs may reveal information about how the organism regulates its internal environment during the biomineralization process in response to environmental conditions. Characterizing these relationships in modern bivalves can help us interpret BioDIFs in fossil bivalves as a paleophysiological proxy. This can help address questions of evolutionary relationships, responses to environmental stress, and survivorship during episodes of climate change.