Behaviour of chromium isotopes in the eastern sub-tropical Atlantic Oxygen Minimum Zone

Heather J Goring-Harford,J.K Klar,Christopher R Pearce,Douglas P Connelly,Eric P Achterberg,Rachael H James

doi:10.1016/j.gca.2018.03.004

Abstract

Constraints on the variability of chromium (Cr) isotopic compositions in the modern ocean are required to validate the use of Cr isotopic signatures in ancient authigenic marine sediments for reconstructing past levels of atmospheric and ocean oxygenation. This study presents dissolved Cr concentrations (CrT, where CrT = Cr(VI) + Cr(III)) and Cr isotope data (δ53Cr) for shelf, slope and open ocean waters within the oxygen minimum zone (OMZ) of the eastern sub-tropical Atlantic Ocean. Although dissolved oxygen concentrations were as low as 44–90 μmol kg−1 in the core of the OMZ, there was no evidence for removal of Cr(VI). Nonetheless, there was significant variability in seawater δ53Cr, with values ranging from 1.08 to 1.72‰. Shelf CrT concentrations were slightly lower (2.21 ± 0.07 nmol kg−1) than in open ocean waters at the same water depth (between 0 and 160 m, 2.48 ± 0.07 nmol kg−1). The shelf waters also had higher δ53Cr values (1.41 ± 0.14‰ compared to 1.18 ± 0.05‰ for open ocean waters shallower than 160 m). This is consistent with partial reduction of Cr(VI) to Cr(III), with subsequent removal of isotopically light Cr(III) onto biogenic particles. We also provide evidence for input of relatively isotopically heavy Cr from sediments on the shelf. Intermediate and deep water masses (AAIW and NADW) show a rather limited range of δ53Cr values (1.19 ± 0.09‰) and inputs of Cr from remineralisation of organic material or re-oxidation of Cr(III) appear to be minimal. Authigenic marine precipitates deposited in deep water in the open ocean therefore have the potential to faithfully record seawater δ53Cr, whereas archives of seawater δ53Cr derived from shelf sediments must be interpreted with caution.

Highlights

Analyses of the chromium (Cr) isotopic composition of authigenic marine sediments are being used to reconstruct the Cr isotopic composition of ancient seawater and, in Harford).turn, to provide constraints on the evolution of atmospheric oxygen and the redox history of the oceans (Crowe et al, 2013; Frei et al, 2009; Planavsky et al, 2014)
We show that Cr(VI) is unlikely to be reduced under dysoxic conditions, but that CrT and d53Cr are affected by biological processes and inputs from sediments on the shelf whereas deep waters are relatively unaffected by internal Cr cycling
Considered by themselves, there is no correlation (R2 = 0.12) between lnCrT and d53Cr for seawater samples from the eastern tropical Atlantic Ocean; the range of d53Cr is relatively large and the range of CrT is relatively low compared to seawater samples collected from other parts of the ocean

Summary

Introduction

Turn, to provide constraints on the evolution of atmospheric oxygen and the redox history of the oceans (Crowe et al, 2013; Frei et al, 2009; Planavsky et al, 2014). This is because Cr has two stable valence states at Earth surface conditions, Cr(III) and Cr(VI), and mass dependent fractionation of the four stable Cr isotopes (50Cr, 52Cr, 53Cr and 54Cr) occurs during oxidation and reduction reactions Chromium isotope ratios are expressed in delta notation relative to the international reference material

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Discussion

Conclusion