Behaviour of chromium isotopes in the eastern sub-tropical Atlantic Oxygen Minimum Zone
Behaviour of chromium isotopes in the eastern sub-tropical Atlantic Oxygen Minimum Zone
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DOI：
10.1016/j.gca.2018.03.004
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发表时间：
2018-09
影响因子：
5
通讯作者：
H. Goring-Harford;J. Klar;C. Pearce;D. Connelly;E. Achterberg;R. James
中科院分区：
文献类型：
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作者：
H. Goring-Harford;J. Klar;C. Pearce;D. Connelly;E. Achterberg;R. James
为了验证在古代自生海洋沉积物中使用铬同位素特征来重建过去的大气和海洋氧合水平，需要对现代海洋中铬（Cr）同位素组成的变异性进行限制。本文研究了亚热带大西洋东部陆架、斜坡和开阔海域的溶解Cr浓度（CrT，其中CrT= Cr（VI） + Cr(III)）和Cr同位素（δ53Cr）数据。虽然OMZ核心的溶解氧浓度低至44-90 μmol kg−1，但没有证据表明Cr（VI）被去除。但海水δ53Cr变化幅度较大，变化范围为1.08 ~ 1.72‰。货架CrTconcentrations略低(2.21 ±0.07 nmol 公斤−1)比在开放海域的水深(0到160之间 m, 2.48 ±0.07 nmol 公斤−1)。陆架水域的δ53Cr值也较高（1.41 ± 0.14‰，低于160 m的开阔海域δ53Cr值为1.18 ± 0.05‰）。这与Cr（VI）部分还原为Cr（III）相一致，随后同位素轻Cr（III）被去除到生物颗粒上。我们还提供了从陆架沉积物中输入相对同位素重Cr的证据。中深水团（AAIW和NADW） δ53Cr值的变化范围相当有限（1.19 ± 0.09‰），有机质再矿化或Cr（III）再氧化的Cr输入似乎很小。因此，深海自生海相沉积具有真实记录海水δ53Cr的潜力，而陆架沉积物的海水δ53Cr档案必须谨慎解释。
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−1in 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 CrTconcentrations 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.