Collaborative Research: Advanced calibration of a carbonate ion proxy

Evidence from ice cores in Antarctica has shown that the carbon dioxide (CO2) content of the atmosphere has changed systematically with Earth’s climate over the last 800,000-years, with lower atmospheric CO2 concentrations in cold (glacial) intervals. Because the oceans must maintain a balance between alkalinity supplied by continental weathering, and alkalinity removed by calcium carbonate burial, at least over long time periods, a reduction in the burial of calcium carbonate on continental shelves would induce an increase in the carbonate ion concentration of seawater to preserve more calcium carbonate in deep sea sediments.

If seawater had a greater carbonate ion concentration than today, then it would absorb more CO2 from the atmosphere. However, the exact mechanism(s) responsible for this CO2 variability remain(s) to be determined. To address this key knowledge gap, the investigators seek to test a method for estimating the carbonate ion concentration of seawater in the past.

Existing methods for estimating carbonate ion concentration are labor-intensive and expensive. This proposal aims to test a method that would be much faster and much less expensive. The scientific goal is to combine estimates of carbonate ion concentration from marine sediments with the other geologic evidence to determine the relative importance of biological processes and of changes in sea level as factors that contributed to lower atmospheric CO2 concentration during glacial intervals.

Broader impacts activities include training, mentoring, and the involving undergraduate students in research, community outreach events, and workforce development.

The investigators plan to advance the calibration of an existing proxy for calcium carbonate (CaCO3) dissolution, and to determine if the proxy also provides reliable estimates of bottom water carbonate ion concentration. Calcium carbonate dissolution in the deep sea is sensitive to changes in bottom water undersaturation (expressed as Delta [CO32-]), so a proxy method for CaCO3 dissolution (Globorotalia menardii Fragmentation Index – MFI), if rigorously calibrated, should provide a measure of bottom water Delta [CO32-].

The MFI method is rapid, inexpensive, only requires a microscope, and is suited to the involvement of undergraduate students in research on the global carbon cycle. The immediate goal is to determine if the MFI method can be used to provide a reliable measure of Delta [CO32-] through a 3-pronged calibration effort with previously collected samples: (1) Determine the MFI across a range of water depths in the Eastern Equatorial Pacific, where Delta [CO32-] and the rate of CaCO3 dissolution have been measured independently; (2) conduct a global survey, including the Eastern Equatorial Pacific, to compare MFI against climatological Delta [CO32-] to assess potential regional variability in the relationship between Delta [CO32-] and MFI; and (3) determine the MFI from marine sediments from the Central Equatorial Pacific, covering the last 160,000-years, in two cores where Delta [CO32-] has already been measured using B/Ca ratios in epibenthic foraminifera.

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.