Chromium isotopic composition of core‐top planktonic foraminifera. Issue 1 (8th July 2016)

• Record Type: Journal Article
• Authors: Wang, X. L.; Planavsky, N. J.; Hull, P. M.; Tripati, A. E.; Zou, H. J.; Elder, L.; Henehan, M.
• Is Part Of: Geobiology. Volume 15:Issue 1(2017)
• Page Start: 51
• Page End: 64
• Publication Date: 2016-07-08
• Subjects: Geobiology -- Periodicals; Biogeochemistry -- Periodicals; Ecology -- Periodicals; 551
• DOI: 10.1111/gbi.12198
• Languages: English
• ISSNs: 1472-4677

Abstract

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.