Geochemical stratigraphy, sedimentology, and Mo isotope systematics of the ca. 2.58-2.50 Ga-old Transvaal Supergroup carbonate platform, South Africa
Geochemical stratigraphy, sedimentology, and Mo isotope systematics of the ca. 2.58-2.50 Ga-old Transvaal Supergroup carbonate platform, South Africa
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DOI：
10.1016/j.precamres.2015.04.014
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发表时间：
2015-09-01
影响因子：
3.8
通讯作者：
Taubald, Heinrich
中科院分区：
文献类型：
--
作者：
Eroglu, Suemeyya;Schoenberg, Ronny;Taubald, Heinrich
南非新太古代-古元古代德兰士瓦超群包含保存完好的Campbelland-Malmani碳酸盐叠层石台地，它是在2.40-2.32 Ga大氧化事件（GOE）前不久的浅海中沉积的。该平台由交替的叠层石碳酸盐岩和泥岩组成，是（同位素）地球化学制图的一个突出候选者，用于调查在GOE之前的浅水沃茨中积累的极少量游离氧的出现。沉积档案中的钼同位素被广泛用作现代和古代环境中氧化还原变化的替代物，最近的证据表明，氧合钼（MoO 42-）直接从海水转移到无机碳酸盐中，分馏可以忽略不计，本研究分析了中国南海北部古近系碳酸盐岩和泥岩样品的主量元素、微量元素和钼同位素组成，KMF-5岩心。地球化学指标，如铁和锰的浓度和铁锰丰度比揭示了一些地球化学指标的保存，尽管广泛的硅化和硅化的平台。重三角洲硅化碳酸盐的Si-30值在0.53和2.35千分之一点Si沉淀从地表水在早期成岩作用，而不是后来的热液叠印。这一评估得到了整个沉积序列中硅石（燧石）层撕裂结构的频繁观察的支持。整个Malmani-Campbellrand平台的整个岩石样品的Mo-98三角洲值范围在千分之-0.82和+1.40之间，与Griqualand西部地区更深斜坡碳酸盐的报告值相似，但变化与岩性或沉积水深无关。三角洲Mo-98值的这些大的变化表明钼的氧化还原循环，从而在当时的大气-海洋系统中存在游离氧，与早期对坎贝尔兰碳酸盐和页岩的Mo同位素研究一致。然而，在手标本尺度上也发现了碳酸盐岩的δ Mo-98值在+0.40至+0.87 ‰之间的类似范围，这表明该碳酸盐岩台地的Mo同位素地层学存在样本偏差的危险。以前未发表的吸附实验的结果表明，钼库存Malmani-Campbellrand碳酸盐岩不仅是从海水中的初级吸附的影响，但在更大程度上由次生过程在早期成岩作用，这也影响了钼同位素组成的样品在局部范围内。我们的研究结果表明，Mo的浓度和同位素组成在古老的叠层石碳酸盐岩受到氧化还原变化的微生物垫和软沉积物在早期成岩作用和后来的石化，因此不能用来定量重建的自由大气中的氧气量或其波动通过地球的历史。尽管如此，我们解释我们的重钼同位素签名碳酸盐作为新太古代海水的最低值，并加强假设，自由大气氧建立了一个沉重的海洋钼水库在那个时候。(C)2015爱思唯尔B. V.保留所有权利。
The Neoarchean-Paleoproterozoic Transvaal Supergroup in South Africa contains the well-preserved stromatolitic Campbellrand-Malmani carbonate platform, which was deposited in shallow seawater shortly before the 2.40-2.32 Ga Great Oxidation Event (GOE). This platform is composed of alternating stromatolitic carbonates and mudstones and is a prominent candidate for (isotope-) geochemical mapping to investigate the appearance of very small amounts of free oxygen that accumulated in shallow waters preceding the GOE. Mo isotopes in sedimentary archives are widely used as a proxy for redox-changes in modern and ancient environments and recent evidence suggests that oxy-molybdate (MoO42-) is directly transferred from ocean water to inorganic carbonates with negligible fractionation, thus reflecting oceanic Mo isotope signatures.In this study we analyzed major and trace element compositions as well as Mo isotopic compositions of carbonate and mudstone samples from the KMF-5 drill core. Geochemical indicators, such as Fe and Mn concentrations and Fe-to-Mn abundance ratios reveal the preservation of some geochemical indicators despite the widespread silicification and dolomitization of the platform. Heavy delta Si-30 values of silicified carbonates between 0.53 and 2.35 parts per thousand point to Si precipitation from surface water during early diagenesis rather than to a later hydrothermal overprint. This assessment is supported by the frequent observation of rip-up structures of silica (chert) layers within the entire sedimentary succession. The delta Mo-98 values of whole rock samples throughout the Malmani-Campbellrand platform range between -0.82 and +1.40 parts per thousand, similar to values reported for deeper slope carbonates from the Griqualand West area, but variations are independent from lithology or depositional water depth. These large variations in delta Mo-98 values indicate molybdenum redox cycling and thus the presence of free oxygen in the atmosphere-ocean system at that time, in agreement with earlier Mo isotopic studies on Campbellrand carbonates and shales. A similar range in delta Mo-98 values for carbonates between +0.40 and +0.87 parts per thousand, however, was also found on the hand specimen scale, indicating the danger of a sample bias on the Mo isotopic stratigraphy of this carbonate platform. Results of previously unpublished adsorption experiments of Mo on CaCO3 clearly indicate that the Mo inventory of Malmani-Campbellrand carbonates was not only influenced by primary adsorption from seawater, but to a much larger degree by secondary processes during early diagenesis, which also affected the Mo isotopic composition of the samples on a local scale. Our results indicate that Mo concentrations and isotopic compositions in ancient stromatolitic carbonates were subject to redox changes within microbial mats and within the soft sediment during early diagenesis and later lithification, and as such cannot be used to quantitatively reconstruct the amount of free atmospheric oxygen or its fluctuations through Earth's history. Nevertheless, we interpret our heavy Mo isotopic signatures from carbonates as a minimum value for Neoarchean seawater and reinforce the assumption that free atmospheric oxygen built up a heavy oceanic Mo reservoir at that time. (C) 2015 Elsevier B.V. All rights reserved.