A deep dive into the planktic foraminiferal I/Ca in global core-tops

The carbonate iodine-to‑calcium (I/Ca) ratio has been adopted as a redox proxy for reconstructing the upper ocean oxygenation. The I/Ca threshold indicative of low oxygen conditions has not been quantitatively constrained due to insufficient core-top data. Here we compile and statistically analyze published core-top planktic foraminiferal I/Ca data, along with benthic I/Ca and water column data. The minimum p -values obtained from serial t -tests are used to identify threshold values in all datasets. The newly compiled planktic core-top dataset identifies three I/Ca thresholds: 0.85, 2.3, and 5 μmol/mol, with the potential of semi-quantitatively indicating minimum oxygen concentrations of approximately 50, 90, and 140 μmol/kg. The 2.3 μmol/mol threshold is the most reliable value separating low vs. high regional ocean oxygen conditions, consistent with previous studies and theoretically calculated values indicative of seawater iodate reduction. The potential of 0.85 and 5 μmol/mol threshold values in paleo-reconstruction work needs to be further tested in future studies. This expanded core-top I/Ca dataset further supports the validity of I/Ca proxy applications in ancient carbonate formed at the vast majority of continental shelf locations. We also demonstrate that linear regressions between I/Ca and modern seawater oxygen are unlikely to produce useful paleo-estimates. This study highlights the importance of measuring planktic I/Ca in deep-dwelling species (and in multiple species wherever possible), especially when applying this proxy at locations featuring a deep oxycline or near the edge of the areal extent of an oxygen minimum zone. We also synthesize caveats and important open questions about the carbonate I/Ca proxy.