This Project Grant award from the National Science Foundation's Geosciences Program (CFDA 47.050) aims to advance the calibration of an existing proxy for calcium carbonate (CaCO3) dissolution and determine if it can reliably estimate bottom water carbonate ion concentration. The $314,367 award to Grand Valley State University, granted on February 15, 2025, will fund a 3-pronged calibration effort to: (1) measure the Globorotalia menardii fragmentation index (MFI) across a range of water depths in the Eastern Equatorial Pacific where carbonate ion concentration has been independently measured; (2) conduct a global survey to compare MFI against climatological carbonate ion concentrations; and (3) determine the MFI from marine sediments in the Central Equatorial Pacific covering the last 160,000 years, where carbonate ion concentrations have been measured using B/Ca ratios. The goal is to develop a rapid, inexpensive proxy method that can provide reliable estimates of past carbonate ion concentrations and contribute to understanding the mechanisms behind historical atmospheric CO2 variability. No sub-awards are planned under this grant.