Planktonic foraminifera Iodine/Calcium ratio: is it a proxy for dissolved oxygen in the ocean?

Abstract. Direct observations indicate a declining trend in ocean oxygen concentrations, which is not quantitatively captured by models. The complexity of oxygenation variability, linked to both physical and biochemical parameters, must be investigated across different climate contexts. Foraminiferal iodine-to-calcium (I/Ca) ratios have emerged as a proxy for subsurface oxygen concentrations although its capacity for quantitative reconstructions remains to be elucidated. We provide a new database, including the first results from the Mediterranean Sea and new samples in the Arabian Sea together with the parameters of biochemistry (nutrient concentration, pH, chlorophyll, oxygen, net primary productivity), physical and geographical (temperature, salinity, latitude, distance from the coast, water depths) and diagenesis potential (depth in core, sediment age) to better understand the proxy behaviour. Considering the strong spatiotemporal variability in dissolved oxygen in subsurface ocean, we propose to use statistically robust 25^th percentile of oxygen concentration (p25 [O₂]) instead of minimum concentration in the upper 500 m in the water column. Our results affirm that oxygen concentration is the primary driver of foraminiferal I/Ca and we propose a new calibration equation of foraminiferal I/Ca against p25 [O₂]. Based on a new database, we identify a complex relationship between p25 [O₂] and iodine speciation, which is one of the main sources of scatter. A comparison with synthetic calcite reveals that planktonic foraminiferal tests can incorporate either more or less iodate at high p25 [O₂] than abiotic calcite, probably due to “vital effects”. The Mediterranean Sea samples present a wide range from low to high I/Ca (0.6 to 6.9 µmol/mol) in well-oxygenated water, which cannot be explained solely by authigenic calcite precipitation. Our results highlight the complex behaviour of the I/Ca proxy, while reinforcing its semi-quantitative reconstruction in palaeoceanography.