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Abstract Faunal analog reconstructions suggest that Last Interglacial (MIS 5e) sea surface temperatures were cooler around Bermuda and in the Caribbean than modern climate. Here we describe new and revised clumped isotope measurements ofCittarium picafossil shells supporting previous findings of cooler than modern temperatures in Bermuda during the Last Interglacial. We resolve temperature and δ¹⁸O_wdifferences between two closely located and apparently coeval sites described in Winkelstern et al. (2017),https://doi.org/10.1002/2016pa003014through reprocessing raw isotopic data with the updated Brand/IUPAC parameters. New subannual‐resolution clumped isotope data reveal large variations in δ¹⁸O_wout of phase with seasonal temperature changes (i.e., lower δ¹⁸O_wvalues in winter). Supported by modern δ¹⁸O_wmeasurements identifying similar processes occurring today, we suggest past variations in coastal δ¹⁸O_wwere driven by seasonally variable freshwater discharge from a subterranean aquifer beneath the island. Taken together, our results emphasize the importance of δ¹⁸O_win controlling carbonate δ¹⁸O, and suggest that typical assumptions of constant δ¹⁸O_wshould be made cautiously in nearshore settings and can contribute to less accurate reconstructions of paleotemperature. 

Since the mid-1970s, groundwater resources in Bermuda have been explored to supplement growing potable water demand on the island. Much of this work has focused on modeling the shape and size of freshwater lenses beneath the island’s surface, mainly the Devonshire Lens. Less attention has been paid to how these freshwater lenses interact with surrounding coastal seawater, a process that may grow in importance as sea levels rise. Due to isotopic differences between aquifer water and seawater, these interactions can be tracked using the oxygen isotopic composition of water (δ¹⁸O_w) samples collected from coastal and subterranean areas. A pilot study found more temporal variation in coastal seawater δ¹⁸O_walong Bermuda’s South Shore (the section of the coast closest to the Devonshire Lens) compared to elsewhere around the island and suggested that freshwater was discharging into coastal seawater from the Devonshire Lens in significant quantities. However, this study was limited by its small dataset so could not quantify the full spatial and temporal variability of δ¹⁸O_win this area. Here, we present salinity and δ¹⁸O_wmeasurements from seawater samples collected around Bermuda and in wells tapping the Devonshire Lens on timescales ranging from hourly to monthly to better visualize the dynamic interaction between coastal seawater and aquifer-sourced freshwater. We find tight correlation between salinity and δ¹⁸O_win well waters, indicating a simple linear mixing relationship between seawater and aquifer water in the subsurface. We confirm previous findings of larger variability in δ¹⁸O_walong the South Shore compared to elsewhere and relate observed changes to tidal height on hourly to monthly timescales. Surprisingly, South Shore seawater salinity does not vary in accordance with δ¹⁸O_w, implying additional mechanisms, such as the addition of salt spray, must be acting to mute salinity changes. These findings also demonstrate the potential in using δ¹⁸O_wto study submarine groundwater discharge, as salinity measurements alone did not detect as much variability. As sea levels rise and interactions between ocean and aquifer waters change, coastal and well water δ¹⁸O_wmeasurements may be helpful in tracking these processes, and in particular, changes in aquifer size. 

The islands of Bermuda preserve carbonates from several glacial and interglacial intervals with demonstrated potential for reconstructing past North Atlantic climate. Here, we describe new clumped and conventional stable isotope data from Dendostrea (oyster) shells collected from a Last Interglacial / Marine Isotope Stage 5e (MIS 5e) deposit. Interpretation of these and past data is supported by new amino acid racemization age dating results from nine localities around Bermuda. We find that the fossil oyster population on Verrill Island (within the present Great Sound of Bermuda) records MIS 5e temperatures and water δ18O values that are similar to modern. These data contrast with the much cooler temperatures and lighter reconstructed water δ18O values reconstructed for sites on the southern shore. This contrast may in part be due to timing, with the Verrill Island deposit plausibly representing an earlier and warmer portion of MIS 5e. The data also reflect meaningful, highly local differences in environment, with modern Great Sound shells perhaps living in partially restricted waters buffered from cooler and groundwater-influenced conditions along the South Shore. The mobility of Cittarium pica marine snails used in previous work likely also introduces exaggerated variability in those cases. Critically, incorporation of clumped isotope data across multiple sites and genera enables an understanding of Bermudian MIS 5e climate that would be meaningfully different if given data from only one site. We seek to illustrate both the complexities and potential of working with clumped isotope paleoclimate data from coastal deposits.