OCE-PRF: Constraints on sea-level change over four glacial cycles through uranium-series dating of submerged Bahamian cave deposits

In recent geologic history, the Earth’s climate has fluctuated between ice ages and warm periods such as the present. These ice age cycles occur on roughly 100,000-year timescales and are marked by 100 meter-scale changes in global sea level in response to the growth and retreat of large continental ice sheets. The timing, duration, and magnitude of ice age sea-level fluctuations is largely controlled by changes in Earth’s surface temperature due to variations in the Earth’s orbit around the sun and the amount of planet-warming greenhouse gases in the atmosphere.

Our understanding of the complex interactions between Earth’s orbit and greenhouse gases that drive sea-level change remains incomplete. Coastal cave deposits called speleothems provide rare, but valuable constraints on past sea-level change. Speleothems provide an upper limit on the position of the ocean surface, as they form from cavern dripwaters and cannot grow when submerged below sea level and can be precisely dated using U-Th dating or geochronology.

For this project, the researcher will produce a novel, speleothem-based sea level record extending back ~400,000-years, covering the last four glacial cycles. This project will improve our understanding of the fundamental physical processes that drive sea-level change during ice age cycles and, by extension, the sea-level rise in a warming world. This is a timely topic, as “what are the rates, mechanisms, impacts and geographic variability of sea level change?” has been highlighted as one of the eight priority science questions by the National Academy of Sciences’ 2015-2025 Decadal Survey of Ocean Sciences.

The project will leverage an extraordinary collection of speleothems recovered from presently-submerged Bahamian coastal caves (blue holes) during the late 1980s/early 1990s at depths ranging from 4 to 77 m below present sea level. Such records are exceedingly rare, requiring many years of expensive and risky fieldwork and coordination with experience cave divers.

In pursuing the project, the PI will have access to (1) the entire Bahamas collection (~117 speleothems), many of which remain undated, (2) state-of-the-art U-series analytical facilities and (3) cutting-edge microanalytical facilities available at University of Minnesota – Twin Cities (e.g., micro computed tomography [CT], electron probe microanalysis [EPMA]) to assess the physical, chemical and petrologic characteristics of growth hiatus boundaries, which is crucial for determining the precise timing of cave inundation due to sea-level rise. Finally, the results will be evaluated using GIA modelling to place the data in the broader context of global mean sea level.

Taken together, this approach will greatly enhance the quality of sea-level information that can be extracted from speleothem-based archives, providing critical constraints on the timing and duration of late Pleistocene sea level change.

Results will be useful to the broader Ocean Sciences community to produce robust sea-level projections to help policymakers, stakeholders and the general public in coastal communities adapt to future sea-level rise. Potential avenues for disseminating project results to the general public include outreach efforts such as the Skype a Scientist program, where the PI has an active record of participation.

The PI will also partner with NOAA’s Science on a Sphere program to develop data visualizations that effectively convey key concepts of late Pleistocene sea-level change to a non-specialist audience. The PI is also committed to broadening participation in the Ocean Sciences and will partner with the hosting institution to foster an inclusive work environment and also encourage increased STEM participation from historically underrepresented groups through ongoing K-12 outreach initiatives.

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.