EAR-Climate: Estimating Seawater Boron Isotope Ratios from Halite Evaporites

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This research will improve estimates of ocean pH and atmospheric carbon dioxide concentrations ~36 million years ago by resolving how the composition of boron in seawater has changed over time. The composition of boron in seawater needs to be known in order use the small amounts of boron in fossil shells to determine the pH of the water they grew in and the carbon dioxide that would be in equilibrium with that water. This project will analyze rock salt deposits where boron is present in small amounts of evaporated seawater that are captured within the salt crystals, as well as within minerals. By analyzing the boron in these salt deposits and determining the composition of boron in ancient seawater, this research will allow the boron in fossil shells to be translated into more accurate estimates for atmospheric carbon dioxide, which will improve understanding about the connection between climate and carbon dioxide. The project will support two early-career women scientists and one under-represented minority scientist, and the researchers will share their scientific expertise on these topics with a high-school educational outreach program on the South Side of Chicago.The project will investigate the behavior of boron in halite evaporite formations to determine the phases that host boron (e.g. fluid inclusions or trace minerals disseminated in the halite) and constrain the stable isotopic composition of boron in ancient seawater. By measuring major element compositions across several halite subsamples bearing fluid inclusions, mixing relationships between the various mineral and fluid components that contribute boron to bulk dissolved halite samples will be established, which will allow for calculation of the endmember composition of the fluid inclusions and the boron isotopic composition of ancient seawater. Experimentally precipitated halites, natural salt cores, and ultimately a ~36 million year old marine halite sample from Spain will be studied in this project. Stable isotope analysis of boron by inductively coupled plasma mass spectrometry will be established in the geochemical facilities at the University of Chicago. In addition to providing an independent constraint on ocean pH and the carbonate system, this research will also determine the distribution coefficient of boron in gypsum and resolve the long-term evolution of the boron geochemical cycle.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.