Paleoclimate and Hydrology of Bermuda from the Last Interglacial to Today Using Oxygen and Clumped Isotope Geochemistry

Abstract

The Last Interglacial (LIG), also known as Marine Isotope Stage 5e (MIS 5e), was an interval with climate as warm or slightly warmer than today. Global mean sea surface temperatures were reconstructed to be ~2°C warmer than present, accompanied by a ~6-9m increase in the sea level. At the regional scale, this warming was non-uniform. The North Atlantic/Greenland/Iceland/Nordic Seas were reconstructed to be warmer than today, whereas the Central Atlantic/Caribbean Seas were reconstructed to be cooler than modern. Situated at the boundary between these two regions, Bermuda can provide a unique record of Last Interglacial climate. In this dissertation, I conduct and optimize isotopic analyses of modern waters and fossil invertebrates from Bermuda to reconstruct modern hydrology and Last Interglacial climate, respectively. First, I determine the most accurate approach for estimating seasonality by comparing multiple isotopic sampling techniques. Validation of these techniques in the modern, where true seasonality and annual mean temperatures are known, improves our ability to accurately apply these methods toward paleoseasonality reconstruction. This study showed high resolution paleoseasonality reconstruction of temperature and δ18Ow is possible with multiple isotopic sampling techniques, especially when a sampling and analysis framework is chosen that balances resolution and growth rate, a finding which can be extrapolated to other species and time periods. Second, I evaluate modern variability in salinity and the oxygen isotopic composition of water (δ18Ow) across various types of modern water samples collected from Bermuda. Well water samples tapping Bermuda’s main freshwater aquifer show the aquifer has changed shape over recent decades and demonstrate linear mixing of seawater and freshwater underground. Seawater samples from the South Shore show variability in δ18Ow on the order of ~2.4‰ on monthly timescales and up to 1.4‰ on hourly timescales. We propose that this significantly greater variability in δ18Ow compared to seawater samples from elsewhere around the island is the result of variable freshwater discharge from the underground aquifer. This is the first detailed study of Bermudan hydrology using δ18Ow. Studies like this set an important benchmark for long-term sustainable extraction of freshwater on the island. Lastly, MIS 5e seasonality in temperature and δ18Ow were reconstructed via oxygen and clumped isotope measurements of multiple fossil invertebrate species collected from both the Great Sound and South Shore. Combined with previously published data, our new data captures a decreasing trend in temperature and δ18Ow through MIS 5e, from close-to-modern conditions to cooler or more depleted conditions relative to today. Previous paleoceanographic studies have regarded MIS 5e as a single time slice, classified as either warmer or cooler than today for a given location. These findings illustrate that MIS 5e-aged outcrops like those in Bermuda can’t all be viewed as representing the same moment in time, but rather subsample a ~10-15 kyr interval of time during which local climate changed measurably. The reconstructed trends in temperature and δ18Ow have implications for ice melt histories and past ocean circulation patterns. Together, this dissertation demonstrates effective application of the novel clumped isotope methods to estimate temperature seasonality, documents the current state of freshwater-saltwater interaction in modern Bermudan waters, and emphasizes that the interval of time known as MIS 5e contains changes in climate that should not be treated as a single moment in time.