Oxygen isotope disequilibrium in the juvenile portion of oyster shells biases seawater temperature reconstructions

For decades, bivalve shells have constituted one of the most common supports for paleoclimatic archives based on stable isotope approaches. In this work, we conducted chemical marking and recapture techniques to study the fluctuation of δ18O values of oyster shells of the species Magallana gigas reared in natural environment in Normandy (France) for two years. The results were compared to the continuous monitoring of temperature and salinity and monthly records of seawater δ18O. Isotopic measurements were performed on the hinge area that regroups the whole life of the oysters. Here we demonstrate that oysters mineralized their shells with no significant growth breaks during the two-years experiment, even at temperatures below 6 °C. The results confirm that adult oysters (i.e. > 1 yr) mineralized their shells at equilibrium. However, juvenile specimens exhibit a strong isotopic disequilibrium, with a maximum shift of the δ18O values of 3‰ in winter, likely due to kinetic isotope effects. This corresponds to a reconstructed temperature up to 13 °C warmer than expected. This work indicates that although these oyster shells can be used as an accurate archive of (paleo)environmental conditions, the shell portion mineralized during the juvenile stage (i.e. < 1 yr) should be avoided for paleotemperature reconstructions. Given the wide use of bivalve shells as environmental archives, similar studies on others species are required

New insights into oyster high-resolution hinge growth patterns

While oyster shells are one of the most common mollusks used for the analysis of (paleo)environmental and (paleo)climatic records based on geochemical proxies, high-resolution growth rate changes still need to be determined. Promising previous works are restricted to small portions of shell sections due to difficulties in continuous growth increment revelation. Based on a mark and recapture experiment of Magallana gigas specimens reared in an intertidal area of Normandy (France) for 22 months, and a sclerochronological approach using cathodoluminescence microscopy, this study provides the longest high-resolution record of growth increments in oyster shells to date. Different growth patterns were identified likely related to the oyster age. After age 1 year, the formation of growth increments follows an expected tide-related model, leading to the mineralization of ~ 2 calcitic increments per day, together with growth rate changes at lunar and semi-lunar periodicities, and a seasonal trend with occasional growth breaks during winter when temperatures fall below ~ 6 °C. However, for oysters younger than 1 year, i.e., before reaching their sexual maturity, the growth increment analysis reveals unconventional patterns. In this case, oysters’ growth is associated with either a large number (~ 5) or less than one increment per day depending on the period. This pattern is also associated with frequent growth cessations, although the growth rate of the shell is high at this period. Our results illustrate that the high-resolution sclerochronological approach is required for accurate paleoenvironmental reconstructions based on oyster shells