Chromium isotopic composition of core‐top planktonic foraminifera

• Published in: Geobiology
• Main Authors: Wang, X. L., Planavsky, N. J., Hull, P. M., Tripati, A. E., Zou, H. J., Elder, L., Henehan, M.
• Other Authors: Agouron Institute
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2016
• Subjects: Planktonic foraminifera
• Online Access: http://dx.doi.org/10.1111/gbi.12198; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fgbi.12198; https://onlinelibrary.wiley.com/doi/pdf/10.1111/gbi.12198

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