Calibration of δ18O values in mollusc shells analyzed via SIMS for paleotemperature reconstructions

The oxygen isotope ratios (δ¹⁸O) of serially sampled mollusc shells are often used to reconstruct temperature at sub-monthly resolution. However, when working with species that are relatively small or that contain growth increments at very shallow angles, conventional micromilling techniques combined with isotope ratio mass spectrometry (IRMS) can result in considerable time averaging. This challenge can be overcome using an alternative technique with ultra-high sampling resolution capabilities, such as secondary ion mass spectrometry (SIMS). Previous studies established that limpet Patella candei precipitates its calcite shell in predictable, consistent disequilibrium with ambient waters, though shallow growth increment angles make sub-monthly sampling difficult using conventional techniques. Hence, this study calibrates the use of SIMS δ¹⁸O analyses in archaeological P. candei retrieved from shell middens in the Canary Islands in the subtropical North Atlantic Ocean. Published studies indicate that δ¹⁸O values measured via SIMS are more negative than paired samples measured using IRMS, resulting in warmer calculated temperatures than expected. However, this offset is not well understood and is complicated by the disparate sampling resolutions achievable by SIMS and micromilling. Previous studies report the offset is larger for calcitic foraminifera (−0.9±0.1‰) than for aragonitic otoliths (−0.5±0.4‰). Therefore, we will compare the δ¹⁸O values of calcitic limpet shells analyzed by both SIMS and IRMS to evaluate the utility of SIMS δ¹⁸O values as recorders of paleotemperature. Preliminary results indicate a negative offset of up to ~0.6‰ between δ¹⁸O values measured at the growth margin using SIMS relative to IRMS which equates to an error of about ~3°C in paleotemperature calculations. However, future work will assess the consistency of this SIMS-IRMS offset in limpets to calibrate and validate SIMS δ¹⁸O values of Patella candei as a paleotemperature proxy during periods of abrupt climate change, such as the Medieval Climate Anomaly.