Coupled molybdenum, iron and uranium stable isotopes as oceanic paleoredox proxies during the Paleoproterozoic Shunga Event
Coupled molybdenum, iron and uranium stable isotopes as oceanic paleoredox proxies during the Paleoproterozoic Shunga Event
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DOI：
10.1016/j.chemgeo.2013.08.003
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发表时间：
2013-12-20
期刊：
影响因子：
3.9
通讯作者：
Cheron, Sandrine
中科院分区：
文献类型：
--
作者：
Asael, Dan;Tissot, Francois L. H.;Cheron, Sandrine
古元古代是地球大气和海洋氧化还原演化的重要时期。在这里，我们提出了一个多代理研究的钼，铀和铁同位素与铁形态的黑色页岩和粉砂岩从上Zaonega形成的Onega盆地在卡累利阿。本文通过对2.05Ga顺加事件和Lomagundi碳同位素漂移事件的研究，进一步揭示了顺加事件的海洋氧化还原环境（缺氧和硫化物）和该部分的下部经历了变质作用，通过黄铁矿改变磁黄铁矿。在这一事件中，该系统是封闭的铁，但不是硫。Mo和U同位素组成（经碎屑输入校正）不受变质作用和S损失的影响，并且在整个剖面中相当均匀。Fe同位素组成在剖面的下部异常重（高达δ Fe-56（IRMM-14）= 0.83ppm），并向上部变轻，这也显示出显著的[Mo]和[U]富集。我们认为，这种模式反映了相对于水柱的氧化还原结构的沉积位点的位置的变化。上半部分沉积在一个本地富氧部分的盆地，其中H2S的可用性是最高的，去除钼和U是更有效的和沉淀黄铁矿捕获相对非分馏溶解铁。换句话说，有利于铁的定量吸收。与此相反，较低的间隔沉积在较低的边缘的缺氧楔H2S的可用性较低，和去除钼和U的效率较低。在这一部分的水柱中的黄铁矿沉淀反映了一个更分馏溶解的铁水库，由于更持久，非定量的铁吸收，因为低硫化物条件下的低效率的黄铁矿形成和更大的访问大型海洋池的铁。对U同位素信号进行了碎屑贡献校正，得到了与河流输入相似的组成，并表明共沉淀成碳酸盐是U去除的主要过程，我们估计同期海洋的Mo同位素组成为三角洲Mo-98（SW）= 0.85 +/-0.21%。这是最低的值尚未报告的元古宙海洋，这表明海洋钼循环占主导地位的缺氧和缺氧汇与可忽略不计的钼去除到好氧环境。最近的研究表明，在Lomagundi事件期间，海洋大气中的氧气水平急剧增加，随后发生了戏剧性的崩溃。我们的研究结果从黑色页岩的2.05 Ga Shunga事件是一致的后Lomagundi减少生物圈氧水平。(C)2013年爱思唯尔B。V.保留所有权利。
The Paleoproterozoic Era was a time of remarkable importance in the redox evolution of Earth's atmosphere and oceans. Here, we present a multi-proxy study of Mo, U and Fe isotopes together with Fe speciation of black shales and siltstones from the upper Zaonega Formation of the Onega Basin in Karelia. We attempt to better understand oceanic redox conditions during the 2.05 Ga Shunga Event as the next step following the Great Oxidation Event (GOE) and the Lomagundi carbon isotope excursion Event.A cautious examination of the Fe speciation data shows that the studied section was deposited under dominantly euxinic conditions (anoxic and sulfidic) and that the lower part of the section experienced metamorphism through which pyrite was altered to pyrrhotite. During this episode, the system was closed with respect to Fe but not sulfur. The Mo and U isotopic compositions (corrected for detrital input) were not affected by the metamorphism and loss of S and are fairly uniform throughout the entire section. The Fe isotope compositions are exceptionally heavy in the lower part of the section (up to delta Fe-56(IRMM-14) = 0.83 parts per thousand) and become lighter towards the upper intervals, which also show significant [Mo] and [U] enrichments. We suggest that this pattern reflects changes in the position of the deposition site relative to the redox structure of the water column. The upper part was deposited within a locally euxinic portion of the basin where H2S availability was highest, removal of Mo and U was more efficient and precipitated pyrite captured relatively non-fractionated dissolved Fe. In other words, quantitative uptake of Fe was favored. In contrast, the lower interval was deposited on the lower margin of a euxinic wedge where H2S availability was lower, and removal of Mo and U was less efficient. Pyrite precipitation in this part of the water column reflected a more fractionated dissolved Fe reservoir due to more protracted, non-quantitative Fe uptake because of less efficient pyrite formation under lower sulfide conditions and greater access to the large oceanic pool of Fe. The U isotopic signal was corrected for detrital contribution giving compositions similar to the riverine input and suggesting that co-precipitation into carbonates was the main process of U removal at this time.We estimate the Mo isotope composition of the contemporaneous ocean to be delta Mo-98(SW) = 0.85 +/- 0.21%. This is the lowest value yet reported for the Proterozoic ocean, suggesting that the oceanic Mo cycle was dominated by euxinic and anoxic sinks with negligible Mo removal into oxic environments. Recent studies have proposed a sharp increase in ocean-atmosphere oxygen levels during the Lomagundi Event followed by a dramatic crash. Our results from black shales of the 2.05 Ga Shunga Event are consistent with a post-Lomagundi decrease in biospheric oxygen levels. (C) 2013 Elsevier B. V. All rights reserved.