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首页> 外文期刊>Precambrian Research >Positive cerium anomalies imply pre-GOE redox stratification and manganese oxidation in Paleoproterozoic shallow marine environments
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Positive cerium anomalies imply pre-GOE redox stratification and manganese oxidation in Paleoproterozoic shallow marine environments

机译：阳性铈异常意味着古普罗佐浅海洋环境前的GOE氧化锰分层和锰氧化

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摘要

The Paleoproterozoic Koegas Subgroup (Transvaal Supergroup, South Africa) was deposited in the immediate prelude to the Great Oxidation Event (GOE), and can therefore shed light on oceanic paleoredox conditions just before atmospheric oxidation. Manganese enrichments of similar to 16 wt% in diagenetic kutnahorite horizons suggest that Mn2+ oxidation occurred, either by free O-2 or by an ancient photosystem. Iron and molybdenum isotope trends also support the existence of a Mn(4+ )oxide sediment flux, suggesting that the Koegas basin may have been redox stratified. Evidence from detrital and authigenic pyrite with mass-independently fractionated sulfur isotopes, however, suggests that the atmosphere was devoid of oxygen. To resolve this contradiction, this paper presents new constraints on pathways of Mn2+ oxidation from field, petrographic, stable isotope, and rare earth element and yttrium (REYSN) analysis of stromatolitic carbonates from the upper Koegas Subgroup. Ferroan dolostones and limestones preserve marine REYSN arrays with positive Ce-SN anomalies. These differences are explained by a redox stratified basin, whereby Mn2+ and CO3+ are oxidized at a redoxcline and Ce is adsorped onto sinking Mn oxide particles. Mn oxide particles and a negative Ce anomaly from the oxidized upper water column are transferred into carbonates accumulating above the redoxcline. Diagenetic fluids later reduce the Mn oxides to kutnahorite. Below the redoxcline, reduction of Mn oxides enriches carbonates in Mn and a positive Ce anomaly. This contribution adds evidence for the development of oxygen oases and redox-stratified basins before the GOE. Redox stratification was best developed during transgressions. During regressions, a deltaic system prograded into the Koegas Basin. High sedimentation rates likely allowed for preservation of detrital pyrite only in the deltaic sandstones, thus explaining the contradictory geochemical evidence. No previously unknown ancient photosystem of Mn oxidation is required to explain Mn oxidation.
机译：古普罗佐科西卡斯亚群（Transvaal Supergroup，South Africa）被存入了大氧化事件（GOE）的直接前果，因此在大气氧化之前可以脱落在海洋古罗索条件下。锰富集在成岩kutnohorite视野中类似于16wt％，表明MN2 +氧化发生，无论是免费的O-2还是古代光系统。铁和钼同位素趋势也支持存在Mn（4 +）氧化物沉积物通量，表明Koegas盆地可能已氧化还原分层。然而，来自毒品和Aheathenic硫铁矿的证据具有大规模分级的硫同位素，表明气氛缺乏氧气。为了解决这一矛盾，本文为来自上部Koegas亚组的脱氨酸碳酸盐碳酸盐盐的Mn2 +氧化途径对Mn2 +氧化的途径提出了新的限制。 Ferroan Dolostones和Limestones用阳性CE-SN异常保留海洋Reysn阵列。这些差异由氧化还原分层盆地解释，其中Mn2 +和CO 3 +在红星霉素中氧化，Ce被浸泡在沉没的Mn氧化物颗粒上。将Mn氧化物颗粒和来自氧化上水柱的负CE异常转移到碳酸盐中，其积累在红星霉素上方。成岩流体后来将Mn氧化物减少到Kutnahorite中。低于红星素，还原Mn氧化物富集Mn和阳性Ce异常中的碳酸盐。这一贡献增加了在GOE之前开发氧气和氧化还原分层盆地的证据。雷威分层在违规期间最好开发。在回归期间，将一个促进的曲纳斯盆地中的红细系统。高沉的沉降率可能允许在Deltaic Sandstones中保存滴乳，因此解释了矛盾的地球化学证据。不需要以前未知的Mn氧化古代光纤以解释Mn氧化。

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