Uranium in aragonitic marine bivalve shells

Uranium concentrations in foraminiferal and coral skeletons track oceanic [CO32 −] and can be useful as a proxy of ocean acidification, but this pH proxy is yet to be investigated in bivalves. Two Saxidomus giganteus shells from the industrialized Puget Sound (WA, USA) and one from the more pristine Kodiak Island (AK, USA) were sampled through ontogeny for U/Ca using laser ablation inductively coupled mass spectrometry. All three shells show a similar pattern of seasonal U/Ca cycles during the first six years of life, followed by a sharp decrease to below the detection limit for the remainder of the clams life (10–20 years), consistent with a biological or ontogenic forcing (vital effect). However, analyses along a growth-line (carbonate formed at the same time) show a decrease in U/Ca from the outside of the shell toward the inside, consistent with diagenesis. Clearly U/Ca is not under environmental control in these aragonite shells, but the cause of the variability is not currently clear.