Proxies for Paleo-Oxygenation: A Downcore Comparison Between Benthic Foraminiferal Surface Porosity and I/Ca

Benthic foraminiferal surface porosity (the mean percentage of surface area covered by pores; higher porosity: lower oxygenation) and iodine to calcium ratio (I/Ca, higher I/Ca: higher oxygenation) are both promising paleoceanographic proxies that will advance through testing in down-core studies. Here we report the first downcore comparison (similar to 45 kyr) of these proxies for a core from the southern Brazilian margin (26 degrees 40 degrees 22' S, 46 degrees 26 degrees 46' W, 475 m water depth). Both proxies are most sensitive to low-O-2 conditions (<50 mu mol/kg), and not well-constrained at higher O-2 concentrations. Porosity values are generally low (<15%) and I/Ca ranges between similar to 4 and similar to 6 mu mol/mol throughout the core. The two proxies are overall consistent, suggesting that bottom-water oxygen concentrations at the site remained above 50 mu mol/kg during the last 45 kyr. Several non-O-2 factors (e.g., iodate reduction rates, water mass mixing, temperature, foraminiferal shell robustness) could influence the proxies and require further investigation.