Oxygen and sulfur isotopes in sulfate in modern euxinic systems with implications for evaluating the extent of euxinia in ancient oceans
Oxygen and sulfur isotopes in sulfate in modern euxinic systems with implications for evaluating the extent of euxinia in ancient oceans
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DOI：
10.1016/j.gca.2017.02.020
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发表时间：
2017-05-15
影响因子：
5
通讯作者：
Johnston, David T.
中科院分区：
文献类型：
--
作者：
Gomes, Maya L.;Johnston, David T.
在古代海洋中，由水柱中硫化物的存在来定义的正辛条件是很常见的。然而，水柱中硫化物的存在如何影响硫化物氧化和硫酸盐还原速率之间的平衡尚不清楚，硫化物氧化和硫酸盐还原速率在调节海洋-大气系统的净氧化还原状态方面起着主要作用。由于硫化物在溶液中扩散到氧化带的速度比在沉积物中扩散得更快，因此正辛烷可能会导致更高的硫化物氧化速率。或者，在丁香酸环境中，适当氧化剂的总体利用率较低，可以抑制硫化物氧化。在这里，我们通过模拟从飓风过后的富氧状态到现代的富氧状态硫酸盐的浓度和硫、氧同位素组成的演变，来限制沿海池塘中四个正辛酸水柱中硫化物氧化和硫酸盐还原的速率。与深度相关的一维水柱硫酸盐地球化学模型的结果表明，与现代富氧海洋相比，富辛体系中还原后再被氧化的硫酸盐比例较低(0.11-0.42)。这意味着，由于氧化剂的限制和物理运输，在丁香体系中硫化物的再氧化速率很低。正辛酸系统中低分数硫化物的再氧化对于探索海洋和大气中的氧水平如何在地球历史上发生变化具有重要的意义。我们使用海洋硫酸盐同位素盒模型来探索在正辛酸体系中低再氧化速率如何影响海洋硫酸盐硫和氧同位素记录。模拟结果表明，海相硫酸盐硫和氧同位素组成在大兴岩扩张过程中增加，其模式与海相硫酸盐岩储层的其他同位素变化不同。因此，由于硫化物氧化速率相对硫酸盐还原速率较低，海洋硫酸盐硫氧同位素盒模型可以应用于古代同位素记录，以评估古代海洋中正己烷的扩张。(C)2017爱思唯尔有限公司。保留所有权利。
Euxinic conditions, which are defined by the presence of sulfide in the water column, were common in ancient oceans. However, it is not clear how the presence of sulfide in the water column affects the balance between rates of sulfide oxidation and sulfate reduction, which plays a major role in regulating the net redox state of the ocean-atmosphere system. Euxinia could lead to higher rates of sulfide oxidation because sulfide may diffuse more rapidly into the oxic zone in solution than in sediment. Alternatively, sulfide oxidation could be inhibited by low overall availability of suitable oxidants in euxinic settings. Here, we constrain rates of sulfide oxidation versus sulfate reduction in four euxinic water columns in coastal ponds by modeling the evolution of the concentration and sulfur and oxygen isotope compositions of sulfate from post-hurricane, welloxygenated conditions to modern, euxinic conditions. The results of the one-dimensional, depth-dependent models of water column sulfate geochemistry indicate that the fraction of sulfate reduced that is subsequently reoxidized is low (0.11-0.42) in euxinic systems relative to the modern well-oxygenated ocean (0.75-0.90). This implies that sulfide reoxidation rates are low in euxinic systems because of oxidant limitation and physical transport. Low fractional sulfide reoxidation in euxinic systems has important implications for exploring how oxygen levels in the ocean and atmosphere have changed through Earth history. We use a marine sulfate isotope box model to explore how low reoxidation rates in euxinic systems affect marine sulfate sulfur and oxygen isotope records. Model results indicate that marine sulfate sulfur and oxygen isotope compositions increase during the expansion of euxinia with patterns that are distinct from other isotopic changes to the marine sulfate reservoir. Thus, marine sulfate sulfur and oxygen isotope box models can be applied to ancient isotope records in order to evaluate the expansion of euxinia in ancient oceans because sulfide oxidation rates are low relative to sulfate reduction rates. (C) 2017 Elsevier Ltd. All rights reserved.