Iron and manganese shuttle has no effect on sedimentary thallium and vanadium isotope signatures in Black Sea sediments
Iron and manganese shuttle has no effect on sedimentary thallium and vanadium isotope signatures in Black Sea sediments
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铁和锰穿梭对黑海沉积物中的沉积铊和钒同位素特征没有影响
DOI：
10.1016/j.gca.2021.11.010
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发表时间：
2021
影响因子：
5
通讯作者：
J. Owens
中科院分区：
文献类型：
--
作者：
Xinming Chen;Siqi Li;S. Newby;T. Lyons;Fei Wu;J. Owens
富有机沉积物中的铊和钒同位素最近被用来通过追踪局部或全球锰氧化物的埋藏来重建海洋氧合作用。铁锰穿梭是一种众所周知的机制，它可以将在轻度氧化浅水环境下形成的铁和锰氧化相相关的元素横向和垂直输送到正辛质(硫化物水柱)深水。铁、锰穿梭强烈地影响了在缺氧和正辛酸环境下堆积的沉积物的铁、钼同位素组成。与Mo类似，Tl和V也富含在大洋铁锰结壳和结核中，这带来了现代海水中Tl和V的最大同位素偏移量。因此，铁和锰穿梭的机制可能(但目前不受限制)影响现代和古代记录中富有机沉积物中的Tl和V同位素组成(ε205Tl和δ51V)。我们使用了已知为Fe穿梭保留了丰度和沉积同位素特征的黑海沉积物。这是由于铁从氧化程度较高的边缘运移到深厚的正己质盆地，并相应地在下伏沉积物中富集。这些样品被用来测试在好氧、亚氧和正己烷水柱下沉积的沉积物中潜在的铁和锰对ε205Tl和δ51V的穿梭效应。来自三种沉积环境的自生ε205Tl难以区分(-2.7g±0.03，-2.4g±0.50和-2.4g±0.30)，这与黑海表层海水(-2.2g±0.30)的误差范围内。相比之下，含氧和亚缺氧沉积物的δ-51V值相似(-1.06m±0.33‰和-1.04m±0.30‰)，明显低于正辛质沉积物(-0.57m±0.06‰)。黑海含氧、亚缺氧和正辛酸水柱下沉积的沉积物几乎没有永久埋藏的锰氧化物，因此捕获了表层海水的ε205Tl。这一发现与表层海水和河流输入之间难以区分的ε205Tl一致。沉积δ51V似乎主要受沉积环境的氧化还原状态控制，几乎不受Fe或Mn穿梭的影响。黑海ε205Tl和δ51V缺乏Fe和Mn穿梭效应，可能是由于该盆地氧化还原层结水柱深度2000m处的浅(∼100m)化学跃层和很慢的深水更新率所致。氧化物中的铁和锰不会被输送到沉积物中，而是以硫化铁和还原的锰矿物相的形式溶解并在水柱中被捕获。我们的结果表明，至少在极端的水柱氧化还原层结的情况下，铁和锰的穿梭传输机制可能不会对Tl和V同位素产生强烈的影响。需要进一步的工作来限制同位素信号，使用其他现代的，通过推断，不同的古代环境来提供额外的穿梭机制。
Thallium and V isotopes in organic-rich sediments have recently been explored for reconstructing marine oxygenation by tracking the burial of Mn oxides either locally or globally. The ‘Fe and Mn shuttle’ is a well known mechanism that can transport elements associated with Fe and Mn oxide phases formed under mildly oxidizing shallow water environments both laterally and vertically to euxinic (sulfidic water column) deep waters. The Fe and Mn shuttle has been demonstrated to strongly affect Fe and Mo isotope compositions of sediments accumulating beneath anoxic and euxinic settings. Similar to Mo, Tl and V are also enriched in oceanic ferromanganese crust and nodules, which impart the largest isotopic offsets from modern seawater for Tl and V. Thus, the mechanism of an Fe and Mn shuttle has the potential to, but currently unconstrained, affect Tl and V isotopic compositions (ε205Tl and δ51V) in organic-rich sediments in the modern and ancient records.We have used Black Sea sediments that are known to preserve enrichment and sedimentary isotope signatures for an Fe shuttle. This is due to an Fe transport from the more oxic margin to the deep euxinic basin and corresponding enrichment in the underlying sediments. These samples were used to test for a potential Fe and Mn shuttle effect on ε205Tl and δ51V in sediments deposited under oxic, suboxic, and euxinic water columns. Authigenic ε205Tl from all three depositional settings is indistinguishable (–2.7 ± 0.3, –2.4 ± 0.5, and –2.4 ± 0.3), which is within error of the Black Sea surface seawater (–2.2 ± 0.3). In contrast, oxic and suboxic sediment have similar δ51V values (–1.06 ± 0.33‰ and –1.04 ± 0.30‰), which are significantly more negative than euxinic sediments (–0.57 ± 0.06‰).All the sediments deposited under oxic, suboxic, and euxinic water columns in the Black Sea capture ε205Tl of the surface seawater because there is almost no permanent burial of Mn oxides in this basin. This finding is consistent with the indistinguishable ε205Tl between the surface seawater and the riverine input. Sedimentary δ51V appears to be controlled predominantly by the redox state of the depositional environments with little effect from an Fe or Mn shuttle. The lack of Fe and Mn shuttle effect on ε205Tl and δ51V in the Black Sea is likely due to the shallow (∼100 m) chemocline in the strongly, 2000-m deep redox-stratified water column of this basin and very slow deep-water renewal rate. Iron and Mn in oxides are not delivered to the sediments, instead, they are dissolved and captured in the water column as iron sulfide and reduced Mn mineral phases. Our results suggest that Fe and Mn shuttle transport mechanisms may not strongly affect Tl and V isotopes, at least in extreme cases of water column redox stratification. Further work is required to constrain the isotopic signals using other modern and, by inference, diverse ancient settings for additional shuttling mechanisms.