Chromium isotopic composition of core-top planktonic foraminifera

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• 作者：X. L. Wang ; N. J. Planavsky ; P. M. Hull ; A. E. Tripati ; H. J. Zou ; L. Elder and M. Henehan
• 刊名：Geobiology
• 出版年：2017
• 出版时间：January 2017
• 年：2017
• 卷：15
• 期：1
• 页码：51-64
• 全文大小：1490K
• ISSN：1472-4669

文摘

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