Manganese oxide shuttling in pre-GOE oceans – evidence from molybdenum and iron isotopes
Manganese oxide shuttling in pre-GOE oceans – evidence from molybdenum and iron isotopes
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DOI：
10.1016/j.epsl.2016.07.013
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发表时间：
2016-10
影响因子：
5.3
通讯作者：
F. Kurzweil;M. Wille;Niklas Gantert;N. Beukes;R. Schoenberg
中科院分区：
文献类型：
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作者：
F. Kurzweil;M. Wille;Niklas Gantert;N. Beukes;R. Schoenberg
富氧浅海环境的局部出现被认为明显早于大气氧化作用，发生在大约24亿年前的大氧化事件（GOE）期间。然而，这种“氧气绿洲”对氧化还原敏感元素的迁移性、分布和同位素组成的潜在影响仍然知之甚少。在这里，我们提供了来自南非Koegas亚群浅海碳酸盐和硅酸盐铁地层的新的钼和铁同位素数据，证实了在GOE之前当地海洋氧化还原分层。Mn浓度与δ 98 Mo和δ 56 Fe值均呈负相关，这表明颗粒锰在古元古代海洋中Mo和Fe的循环中起着重要作用。根据这些趋势，我们认为孔隙水中的钼酸盐是通过(1)高δ 98 Mo的海水钼酸盐的扩散输运和(2)在Mn氧化物溶解过程中吸附的低δ 98 Mo钼酸盐的再释放来补充的。在Mn趋化斜带附近，轻Mo的相对贡献最大，且Mn氧化物通量最大，导致沉积物中Mn浓度与δ 98 Mo值呈负相关。δ 56 Fe值与Mn浓度呈负相关，可能与Mn氧化物氧化Fe （II）过程中Fe同位素分馏有关，导致上层水柱δ 56 Fe值较低，接近Mn趋化斜带。我们认为，这些信号在古元古代沉积物中的保存意味着垂直延伸的趋化线的存在，其梯度更平滑，可能是低大气氧浓度的结果。此外，我们认为，铁（II）的非生物氧化的锰氧化物粒子穿梭可能促进了沉积的Koegas铁层。
The local occurrence of oxygen-rich shallow marine water environments has been suggested to significantly predate atmospheric oxygenation, which occurred during the Great Oxidation Event (GOE) ca. 2.4 billion years ago. However, the potential influence of such ‘oxygen oases’ on the mobility, distribution and isotopic composition of redox sensitive elements remains poorly understood. Here, we provide new molybdenum and iron isotopic data from shallow marine carbonate and silicate iron formations of the Koegas Subgroup, South Africa, that confirm local ocean redox stratification prior to the GOE. Mn concentrations correlate negatively with both δ 98 Mo and δ 56 Fe values, which highlights the substantial role of particulate manganese for the cycling of Mo and Fe in the Paleoproterozoic oceans. Based on these trends we propose that pore water molybdate was recharged (1) by the diffusional transport of seawater molybdate with high δ 98 Mo and (2) by the re-liberation of adsorbed molybdate with low δ 98 Mo during Mn oxide dissolution within the sediment. The relative contribution of isotopically light Mo is highest close to a Mn chemocline, where the flux of Mn oxides is largest, causing the negative correlation of Mn concentrations and δ 98 Mo values in the Koegas sediments. The negative correlation between δ 56 Fe values and Mn concentrations is likely related to Fe isotope fractionation during Fe (II) oxidation by Mn oxides, resulting in lower δ 56 Fe values in the uppermost water column close to a Mn chemocline. We argue that the preservation of these signals within Paleoproterozoic sediments implies the existence of vertically extended chemoclines with a smoother gradient, probably as a result of low atmospheric oxygen concentrations. Furthermore, we suggest that abiotic oxidation of Fe (II) by a Mn oxide particle shuttle might have promoted the deposition of the Koegas iron formations.