Uranium and molybdenum isotope evidence for an episode of widespread ocean oxygenation during the late Ediacaran Period
Uranium and molybdenum isotope evidence for an episode of widespread ocean oxygenation during the late Ediacaran Period
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DOI：
10.1016/j.gca.2015.02.025
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发表时间：
2015-05-01
影响因子：
5
通讯作者：
Anbar, Ariel D.
中科院分区：
文献类型：
--
作者：
Kendall, Brian;Komiya, Tsuyoshi;Anbar, Ariel D.
为了更好地估计埃迪卡拉纪晚期海洋氧合程度，我们测量了华南陡山坨组上部IV段富缺氧（缺氧和硫化物）有机质泥岩（ORM）的U和Mo同位素组成。大多数样品的平均U-238增量为0.24 +/- 0.16 ppm（2SD;相对于标准CRM 145），这略高于限制黑海的铀-238平均增量0.02 +/-0.12ppm（深水单元I）的富氧沉积物，并类似于一个模拟三角洲的U-238值为0.2千分之一的开放海洋富氧沉积物在现代良好的含氧海洋。由于与U-235相比，U-238优先被去除到富氧沉积物中，因此扩大的海洋缺氧将使U-238相对于U-235消耗海水，最终导致具有低三角洲U-238的ORM沉积。因此，第IV段ORM的高U-238三角洲表明，在约2000年，普遍存在广泛的海洋氧化作用。从强富氧底层沃茨（[H2S]（aq）> 11 μ M）沉积的沉积物的Mo同位素组成要么直接记录了全球海水Mo同位素组成（如果从深层沃茨中Mo的去除是定量的），要么代表海水的最小值（如果Mo的去除不是定量的）。在第四段顶部附近，Mo-98三角洲接近现代海水值2.34 +/- 0.10 ppm。高三角洲钼-98点广泛的海洋氧化，因为优先去除同位素轻钼沉积物发生在更大程度上在O-2丰富的O-2缺乏的海洋环境相比。然而，大多数成员IV ORM的Δ Mo-98值接近千分之0（相对于标准NIST SRM 3134 =千分之0.25），表明广泛缺氧。低三角洲Mo-98与第四成员ORM的高Mo浓度不一致，这表明在含氧良好的海洋中存在大量海水Mo库存，以及高三角洲U-238。因此，我们建议，大多数成员IV ORM的低三角洲Mo-98分馏同期海水。驱动这种同位素分馏的可能机制包括：（1）溶解的硫化物不足以定量硫钼酸盐形成并捕获沉积物中类似海水的三角洲Mo-98特征，或（2）通过颗粒状Fe-将同位素轻Mo输送到沉积物从富氧ORM获得的Mo同位素数据汇编表明，有短暂的广泛海洋氧合事件，在新元古代晚期和古生代早期，因此，第四成员并不包括不可逆转的深海氧化作用。相反，复杂的海洋氧化还原变化可能标志着从O-2缺乏的元古代海洋到广泛含氧的晚古生代海洋的过渡。(C)2015爱思唯尔有限公司版权所有。
To improve estimates of the extent of ocean oxygenation during the late Ediacaran Period, we measured the U and Mo isotope compositions of euxinic (anoxic and sulfidic) organic-rich mudrocks (ORM) of Member IV, upper Doushantuo Formation, South China. The average delta U-238 of most samples is 0.24 +/- 0.16 parts per thousand (2SD; relative to standard CRM145), which is slightly higher than the average delta U-238 of 0.02 +/- 0.12 parts per thousand for restricted Black Sea (deep-water Unit I) euxinic sediments and is similar to a modeled delta U-238 value of 0.2 parts per thousand for open ocean euxinic sediments in the modern well-oxygenated oceans. Because U-238 is preferentially removed to euxinic sediments compared to U-235, expanded ocean anoxia will deplete seawater of U-238 relative to U-235, ultimately leading to deposition of ORM with low delta U-238. Hence, the high delta U-238 of Member IV ORM points to a common occurrence of extensive ocean oxygenation ca. 560 to 551 Myr ago.The Mo isotope composition of sediments deposited from strongly euxinic bottom waters ([H2S](aq) > 11 mu M) either directly records the global seawater Mo isotope composition (if Mo removal from deep waters is quantitative) or represents a minimum value for seawater (if Mo removal is not quantitative). Near the top of Member IV, delta Mo-98 approaches the modern seawater value of 2.34 +/- 0.10 parts per thousand. High delta Mo-98 points to widespread ocean oxygenation because the preferential removal of isotopically light Mo to sediments occurs to a greater extent in O-2-rich compared to O-2-deficient marine environments. However, the delta Mo-98 value for most Member IV ORM is near 0 parts per thousand(relative to standard NIST SRM 3134 = 0.25 parts per thousand), suggesting extensive anoxia. The low delta Mo-98 is at odds with the high Mo concentrations of Member IV ORM, which suggest a large seawater Mo inventory in well-oxygenated oceans, and the high delta U-238. Hence, we propose that the low delta Mo-98 of most Member IV ORM was fractionated from contemporaneous seawater. Possible mechanisms driving this isotope fractionation include: (1) inadequate dissolved sulfide for quantitative thiomolybdate formation and capture of a seawater-like delta Mo-98 signature in sediments or (2) delivery of isotopically light Mo to sediments via a particulate Fe-Mn oxyhydroxide shuttle.A compilation of Mo isotope data from euxinic ORM suggests that there were transient episodes of extensive ocean oxygenation that break up intervals of less oxygenated oceans during late Neoproterozoic and early Paleozoic time. Hence, Member IV does not capture irreversible deep ocean oxygenation. Instead, complex ocean redox variations likely marked the transition from O-2-deficient Proterozoic oceans to widely oxygenated later Phanerozoic oceans. (C) 2015 Elsevier Ltd. All rights reserved.