Chromium isotopic composition of core‐top planktonic foraminifera

Abstract

The chromium isotope system (⁵³ Cr/⁵² Cr expressed as δ⁵³ Cr 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 δ⁵³ Cr. We show that single foraminiferal species from globally distributed core tops yielded variable δ⁵³ Cr, ranging from 0.1‰ to 2.5‰. These values do not match with the existing measurements of seawater δ⁵³ Cr. Further, within a single core-top, species with similar water column distributions (i.e., depth habitats) yielded variable δ⁵³ Cr values. In addition, mixed layer and thermocline species do not consistently exhibit decreasing trends in δ⁵³ Cr as expected based on current understanding of Cr cycling in the ocean. These observations suggest that either seawater δ⁵³ Cr is more heterogeneous than previously thought or that there is significant and species-dependent Cr isotope fractionation during foraminiferal calcification. Given that the δ⁵³ Cr variability is comparable to that observed in geological samples throughout Earth's history, interpreting planktonic foraminiferal δ⁵³ Cr 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 δ⁵³ Cr archive and should not be used to study marine redox evolution without additional study. It likewise cautions against the use of δ⁵³ Cr in bulk carbonate or other biogenic archives pending further work on vital effects and the geographic heterogeneity of the Cr isotope composition of seawater.