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Positive cerium anomalies imply pre-GOE redox stratification and manganese oxidation in Paleoproterozoic shallow marine environments
Precambrian Research ( IF 3.8 ) Pub Date : 2020-07-01 , DOI: 10.1016/j.precamres.2020.105767
Matthew R. Warke , Harald Strauss , Stefan Schröder

Abstract 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 ~16 wt% in diagenetic kutnahorite horizons suggest that Mn2+ oxidation occurred, either by free O2 or by an ancient photosystem. Iron and molybdenum isotope trends also support the existence of a Mn4+ 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 CeSN anomalies. These differences are explained by a redox stratified basin, whereby Mn2+ and Ce3+ 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.

中文翻译:

正铈异常意味着古元古代浅层海洋环境中GOE前氧化还原分层和锰氧化

摘要 古元古代 Koegas 亚群(南非德兰士瓦超群)是在大氧化事件 (GOE) 的前奏中沉积的,因此可以揭示大气氧化之前的海洋古氧化还原条件。成岩 kutnahorite 层中约 16 wt% 的锰富集表明发生了 Mn2+ 氧化,无论是通过游离 O2 还是通过古老的光系统。铁和钼同位素趋势也支持 Mn4+ 氧化物沉积物通量的存在,表明 Koegas 盆地可能已被氧化还原分层。然而,来自碎屑和自生黄铁矿具有与质量无关的分馏硫同位素的证据表明大气中没有氧气。为解决这一矛盾,本文从野外、岩相、来自上 Koegas 亚群的叠层石碳酸盐的稳定同位素以及稀土元素和钇 (REYSN) 分析。铁质白云岩和石灰岩保存了具有正 CeSN 异常的海洋 REYSN 阵列。这些差异可以通过氧化还原分层盆地来解释,其中 Mn2+ 和 Ce3+ 在氧化还原线中被氧化,而 Ce 被吸附到下沉的 Mn 氧化物颗粒上。来自氧化的上部水柱的 Mn 氧化物颗粒和 Ce 负异常被转移到在氧化还原线上方积累的碳酸盐中。成岩流体后来将锰氧化物还原为钾锌矿。在氧化还原线以下,Mn 氧化物的还原使 Mn 中的碳酸盐富集,并且 Ce 异常。这一贡献为 GOE 之前氧绿洲和氧化还原层状盆地的发展提供了证据。氧化还原分层在海侵期间最好地发展。在回归过程中,三角洲系统进入 Koegas 盆地。高沉积速率可能只允许在三角洲砂岩中保存碎屑黄铁矿,从而解释了相互矛盾的地球化学证据。不需要以前未知的古代锰氧化光系统来解释锰氧化。
更新日期:2020-07-01
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