Collaborative Research: Uranium series dating of pteropods to generate absolute ages of marine sediments

The layers of ocean sediment hold a record of the planet reaching back hundreds of millions of years. To read that record, scientists need to know the age of each layer. Radiocarbon dating works well over the last 50,000 years, but there is no direct way to measure the age of ocean sediment past that point. Scientists estimate ages by matching patterns across different records. The relatively high error of this matching leads to uncertainty. It complicates interpretation of cause and effect and understanding whether past changes occur simultaneously in the ocean and on land. This project will build a clock that can date sediment further back in time to resolve those questions. The researchers will use the shells of pteropods, tiny sea snails that live near the ocean surface and sink to the seafloor when they die. Their shells take in natural uranium from seawater, which decays to thorium at a known rate. By measuring the amounts of uranium and thorium, the team can find the age of a shell. The researchers will apply this method to decipher how the South Asian monsoon, which brings rain to billions of people, has shifted in the geologic past. The pteropod dating method could be used on an additional 2,000 sediment cores already stored in U.S. research collections. The project maintains laboratory analytical capability in measuring uranium isotopes and supports training of graduate students. Public outreach events will teach children and families about isotopes and natural radioactivity. Marine sediment preserves a continuous record of Earth history, but absolute dating of this sediment is extremely difficult. Radiocarbon dating is effective only over the past 50,000 years; older intervals are dated indirectly by tuning proxy records to reference curves and/or magnetostratigraphy and biostratigraphy methods. This project develops and applies high-precision uranium-series (U-series) dating of aragonitic pteropod shells, which contain high uranium concentrations and exhibit closed-system behavior that make them well suited to direct radiometric dating. Pilot data from core top pteropods yield uranium and thorium (U-Th) ages in close agreement with independent radiocarbon ages, establishing the viability of the technique. The research pursues two objectives. First, it optimizes and validates a pteropod U-series dating technique. The team will test whether U-series ages are consistent across the different pteropod genera that make up mixed sediment assemblages. The researchers will evaluate closed-system fidelity during burial by comparing paired U-Th and radiocarbon dates downcore and develop open-system corrections for alpha-recoil redistribution if needed. Second, the researchers will apply the validated method at International Ocean Discovery Program Site U1467 in the Maldives to assess the timing and orbital phasing of South Asian monsoon intensity relative to terrestrial records. Analytical work combines microfossil picking, cleaning, and imaging, followed by isotope-dilution measurement of U and Th isotopes via multicollector plasma mass spectrometry. The resulting absolute chronologies can serve as benchmarks for dating the thousands of additional sediment cores held in U.S. repositories where pteropod preservation is favorable. 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.