The I/Ca paleo-oxygenation proxy in planktonic foraminifera: A multispecies core-top calibration

Iodine-to-calcium ratios (I/Ca) in planktonic foraminifera have been used for the reconstruction of upper ocean oxygenation in the geologic past, particularly for detecting oxygen-deficient zones (ODZs). We examine the response of I/Ca in various planktonic foraminifera species from core-top sediment samples to changes in iodate and the minimum oxygen concentration in the uppermost 500 m of the water column ([O₂]_min). Published data are concentrated in the Atlantic Ocean and in areas with either very high or very low oxygen concentrations. Our study supplements those studies using samples from the GEOTRACES GP16 transect through the southern lobe of the eastern tropical Pacific oxygen deficient zone and areas in the western equatorial Pacific and Atlantic Ocean characterized by intermediate oxygen concentrations ([O₂]_min of 100–170 µmol/kg). We use Mg/Ca values measured alongside I/Ca to determine apparent calcification depth for foraminifera in the tropical Pacific. We find that I/Ca values in foraminifera species from different water depth habitats vary predictably based on patterns in iodate concentration spatially and with depth in the upper water column. In well-oxygenated areas of the ocean, I/Ca values are relatively high, with the highest values in deep subsurface foraminifera species that lived furthest from the mixed layer where primary productivity converts iodate to iodide and organic iodine. In ODZs, I/Ca values are relatively low and this pattern reverses, with the lowest I/Ca values in deep subsurface species living near or within the ODZ where iodate is used as an electron acceptor during oxidation of sinking organic matter. The difference between deep subsurface and surface I/Ca values can therefore provide another line of evidence for identifying the core of an ODZ in paleoceanographic records. This study suggests that the large scatter in the I/Ca data reflects the complexity of the relationship between the distribution of iodate and dissolved oxygen through the water column and spatially, and the depths at which foraminifera calcify with respect to that distribution. We employ a linear correlation between I/Ca and [O₂]_min as the most parsimonious explanation for this complex system. Across the eastern tropical Pacific transect, I/Ca responds to small oxygenation changes on the order of 10s of µmol/kg, suggesting that I/Ca may be able to detect small relative changes in oxygenation of this magnitude in consecutive downcore samples. We note, however, that interpretation of such changes is strengthened by the use of additional paleo-oxygenation proxies.