Chromium isotopic composition of core‐top planktonic foraminifera

The chromium isotope system (53Cr/52Cr expressed as δ53Cr relative to NIST SRM 979) is potentially a powerful proxy for the redox state of the ocean–atmosphere system, but a lack of temporally continuous, well‐calibrated archives has limited its application to date. Marine carbonates could potentially serve as a common and continuous Cr isotope archive. Here, we present the first evaluation of planktonic foraminiferal calcite as an archive of seawater δ53Cr. We show that single foraminiferal species from globally distributed core tops yielded variable δ53Cr, ranging from 0.1‰ to 2.5‰. These values do not match with the existing measurements of seawater δ53Cr. Further, within a single core‐top, species with similar water column distributions (i.e., depth habitats) yielded variable δ53Cr values. In addition, mixed layer and thermocline species do not consistently exhibit decreasing trends in δ53Cr as expected based on current understanding of Cr cycling in the ocean. These observations suggest that either seawater δ53Cr is more heterogeneous than previously thought or that there is significant and species‐dependent Cr isotope fractionation during foraminiferal calcification. Given that the δ53Cr variability is comparable to that observed in geological samples throughout Earth's history, interpreting planktonic foraminiferal δ53Cr without calibrating modern foraminifera further, and without additional seawater measurements, would lead to erroneous conclusions. Our core‐top survey clearly indicates that planktonic foraminifera are not a straightforward δ53Cr archive and should not be used to study marine redox evolution without additional study. It likewise cautions against the use of δ53Cr in bulk carbonate or other biogenic archives pending further work on vital effects and the geographic heterogeneity of the Cr isotope composition of seawater.