南部非洲Witwatersrand Mozaan层序的含碳酸锰泥岩作为细菌锰呼吸作用和中古生海洋游离分子氧可用性的证据

IF 1.2 4区 地球科学 Q2 GEOLOGY
A. Smith, N. Beukes, J. M. Cochrane, J. Gutzmer
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引用次数: 2

摘要

南部非洲的中太古宙(2.96至2.91 Ga)Witwatersrand Mozaan序列包含多个单元,这些单元表明在大氧化事件发生约5亿年前,海洋水柱中存在游离分子氧。Mozaan群和West Rand群的Thalu和Brixton组的岩石地层相关性现在为含氧水柱提供了进一步的证据。这两个地层包含多个含碳酸锰泥岩层。本研究记录了这些地层及其地层学、矿物学、岩石学、全岩地球化学以及来自深层岩芯的稳定碳和氧同位素。含碳酸锰矿层出现在向上粗化单元的底部,表明在海平面上升期间沉积,随后是回归。泥岩在含锰碳酸盐结核周围表现出尖锐的接触和压实,表明后者早成岩生长。结核由菱锰矿岩芯和钾铁矿边缘组成,或由钾铁矿岩芯和铁白云石边缘组成,表明锰浓度从岩芯到边缘降低。相对于周围的泥岩,碳酸盐结核的锰含量明显富集。归一化为页岩的结核中的稀土元素和钇含量显示出重稀土元素过轻稀土元素富集以及铕和钇的正异常,表明沉淀来自混合海洋热液。结核碳酸盐的碳同位素和氧同位素相对于Pee Dee Belemnite分别贫13C(-12.2至-21.30/00)和18O(-14.9至-23.90/00)。结果表明,铁和锰的主要沉积方式是由铁和锰氧化细菌促进的沉淀。铁和锰来源于远端高温热液羽流中溶解的Fe2+和Mn2+。所研究单元的地层位置表明,沉积发生在中大陆架到外大陆架上,在那里,铁的氧化和沉积进行得足够远,环境可以转变为锰的氧化和沉淀。然后,这些沉淀物与有机碳反应形成菱锰矿,该菱锰矿非常局部地成核以生长结核。由于可用的锰全部被还原并结合到碳酸盐中,Fe3+氢氧化物也被任何过量的有机碳还原,并在生长的后期结合到结核边缘中。所提出的结核形成模型的主要含义是,在架子的水柱中可以获得游离分子氧,使氧化锰的微需氧化学石自养生物发挥作用。氧气浓度超过约5μM。这种浓度,以及所研究单元的沉积环境,都在先前研究的范围和参数内,这些研究表明了太古宙海洋“氧绿洲”的条件。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Manganese carbonate-bearing mudstone of the Witwatersrand-Mozaan succession in southern Africa as evidence for bacterial manganese respiration and availability of free molecular oxygen in Mesoarchaean oceans
The Mesoarchaean (2.96 to 2.91 Ga) Witwatersrand-Mozaan succession of southern Africa contains multiple units that show evidence for the presence of free molecular oxygen in oceanic water columns approximately 500 million years prior to the Great Oxidation Event. The lithostratgraphically correlatable Thalu and Brixton formations of the Mozaan and West Rand groups, respectively, now yield further evidence for an oxygen-containing water column. The two formations contain multiple beds of manganese carbonate-bearing mudstone. This study documents these beds and their stratigraphy, mineralogy, petrography, whole rock geochemistry and stable carbon and oxygen isotopes from deep-level drill cores. The manganese carbonate-bearing beds occur towards the base of upward-coarsening units, indicating deposition during higher sea levels, followed by regressions. The mudstones show sharp contacts and compaction around manganiferous carbonate concretions, suggesting early diagenetic growth of the latter. The concretions are composed of either rhodochrosite cores and kutnahorite rims, or kutnahorite cores and ankerite rims, illustrating a decrease in manganese concentration from core to rim. Relative to the surrounding mudstone, the carbonate concretions are markedly enriched in manganese. The rare earth element and yttrium contents in the concretions, normalised to shale, show heavy over light rare earth element enrichment and positive europium and yttrium anomalies, indicating precipitation from mixed marine-hydrothermal water. The carbon and oxygen isotopes of concretionary carbonates are depleted in 13C (-12.2 to -21.30/00) and 18O (-14.9 to -23.90/00) relative to Pee Dee Belemnite, respectively. It is concluded that the main mode of deposition for iron and manganese was by precipitation likely facilitated by iron- and manganese-oxidising bacteria. The iron and manganese were sourced as dissolved Fe2+ and Mn2+ from a distal, high-temperature hydrothermal plume. The stratigraphic position of the studied units suggests that deposition occurred on the middle to outer shelf where iron oxidation and deposition proceeded far enough so that the environment could transition to manganese oxidation and deposition. These precipitates then reacted with organic carbon to form rhodochrosite that nucleated very locally to grow concretions. As the available manganese was all reduced and incorporated into the carbonates, Fe3+-oxyhydroxides were also reduced by any excess organic carbon and incorporated into the concretion rims during later stages of growth. The main implication of the proposed model for concretion formation is that free molecular oxygen was available in the water column of the shelf for manganese-oxidising microaerophyllic chemolithoautotrophs to function. The oxygen concentration was in excess of approximately 5 μM. This concentration, along with the depositional setting of the studied units, falls within the ranges and parameters of previous studies indicating conditions in “oxygen oases” of Archaean oceans.
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来源期刊
CiteScore
3.60
自引率
5.60%
发文量
18
审稿时长
>12 weeks
期刊介绍: The South African Journal of Geology publishes scientific papers, notes, stratigraphic descriptions and discussions in the broadly defined fields of geoscience that are related directly or indirectly to the geology of Africa. Contributions relevant to former supercontinental entities such as Gondwana and Rodinia are also welcome as are topical studies on any geoscience-related discipline. Review papers are welcome as long as they represent original, new syntheses. Special issues are also encouraged but terms for these must be negotiated with the Editors.
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