LAST INTERGLACIAL CLIMATE FROM DENDOSTREA FRONS OYSTERS, VERRILL ISLAND, BERMUDA

The islands of Bermuda lie in the North Atlantic Ocean and preserve carbonates from several glacial and interglacial intervals. In the Great Sound, ~110 m long Verrill Island exposes shell lag from the Last Interglacial period (~125,000 years ago). This was broadly a time of similar or slightly warmer climate than today, which can perhaps serve as an analog for near future climate conditions. Here we use sub-annual δ¹⁸O analyses of Verrill Island oyster fossils to estimate sea surface temperatures (SST) and seasonality. We also evaluate the suitability of this species for paleoenvironmental reconstruction through comparison with coeval data from other mollusks.

Eight Dendostrea frons oyster shells collected from Verrill Island were sampled along their growth axes for δ¹⁸O analysis. We find that, in some cases, the oysters record seasonal variability in temperature and/or water δ¹⁸O. As a preliminary temperature calculation, we assumed a constant δ¹⁸O water value of +1.0‰, approximating modern δ¹⁸O values. Six oysters seem to reflect average growth temperatures and two preserve apparent seasonal temperature curves. The δ¹⁸O results range from -1.11 to 0.39‰, roughly corresponding to a temperature change of ~7°C. The limited δ¹⁸O range in some shells may be attributable to rapid growth. We report a mean δ¹⁸O value of -0.51 ± 0.17‰, corresponding to a temperature of ~23°C. These results are broadly similar to modern SST in Bermuda (~20 to ~28°C) and are on the warmer end of previous local Last Interglacial SST estimates (~8 to ~27°C). However, the data will be refined via ongoing clumped isotope analyses that will better define local water δ¹⁸O conditions during shell growth. These data enable more robust interspecies comparison with other records from Bermuda and will contribute to a broader understanding of climate and North Atlantic circulation during the Last Interglacial.