COMPARISON BETWEEN CONVENTIONAL IRMS AND SIMS FOR ANALYZING δ18O VALUES IN BIOGENIC CARBONATES FOR PALEOTEMPERATURE RECONSTRUCTIONS

The oxygen isotope (δ¹⁸O) ratios of serially sampled mollusc shells are often used to reconstruct temperature at sub-monthly resolution. However, conventional micromilling techniques combined with isotope ratio mass spectrometry (IRMS) can notably diminish and obscure the temporal resolution along ontogeny, especially when working with species that are relatively small or that contain growth increments at very shallow angles. This challenge can be overcome by applying an alternative technique with significantly finer sampling resolution capabilities, such as the secondary ion mass spectrometry (SIMS). SIMS affords ultra-high sampling resolution, which minimizes time averaging despite increasing analytical error. This study investigates the δ¹⁸O values of the archaeological limpet Patella candei retrieved from shell middens in the Canary Islands using SIMS with the ultimate goal to reconstruct seasonal temperature variations during the Medieval Climate Anomaly (MCA; 1000-1300 AD) in the subtropical eastern North Atlantic Ocean. Preliminary SIMS δ¹⁸O values suggest a seasonal variation in coastal temperatures of about 9°C during the MCA. Published studies indicate that δ¹⁸O values measured via SIMS may be lower than paired samples measured using IRMS, resulting in warmer temperatures than expected. While this offset appears to be consistent, it seems to vary among taxa. For example, the offset is larger for calcitic foraminifera (−0.9±0.1 ‰) than for aragonitic otoliths (about −0.5‰). Because the cause and magnitude of this offset has not been established for calcitic Patella candei shells, future work will compare the δ¹⁸O values of limpet shells analyzed by both SIMS and IRMS methods. Preliminary results already suggest a negative offset of up to ~0.6‰ between δ¹⁸O values measured at the growth margin using SIMS relative to IRMS. Future work will evaluate the consistency and cause of this offset in modern and archaeological limpets to calibrate and validate SIMS δ¹⁸O values of Patella candei and other smaller and thinner mollusc species difficult to analyze by conventional IRMS.