Oxidative scavenging of thallium by birnessite: explanation for thallium enrichment and stable isotope fractionation in marine ferromanganese precipitates
Oxidative scavenging of thallium by birnessite: explanation for thallium enrichment and stable isotope fractionation in marine ferromanganese precipitates
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DOI：
10.1016/j.gca.2012.01.036
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发表时间：
2012-05
影响因子：
5
通讯作者：
C. Peacock;E. Moon
中科院分区：
文献类型：
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作者：
C. Peacock;E. Moon
在海洋铁锰结壳中记录的铊稳定同位素作为过去海洋和气候条件的示踪剂显示出很大的前景。解释最近的Tl稳定同位素时间序列数据的关键是详细的，分子水平的理解Tl清除铁锰结壳矿物和Tl稳定同位素分馏发生在吸收。为此，我们确定的机制，铊吸附在地壳中的主要铁锰矿物，即六方水钠锰矿，钙锰矿和水铁矿，使用XAS。我们恭维我们的数据与微焦点XAS的铊富水成铁锰结壳。我们表明，Tl（I）被氧化为Tl（III）在六方水钠锰矿的吸附过程中，但不是在钙钛矿，三斜水钠锰矿和水铁矿的吸附过程中。Tl（III）在六方水钠锰矿表面形成内球络合物，位于叶锰片中的八面体空位处。我们表明，在水钠锰矿的还原溶解过程中，Tl（I）氧化为Tl（III）在化学上是不利的;并提出Tl（I）的氧化是由Tl（III）表面络合物的形成驱动的。最近的理论计算预测了大的平衡稳定同位素分馏之间的Tl（I）和Tl（III），导致的Tl（III）的物种，是丰富的重205 Tl同位素。鉴于这项工作，我们提出了一个分子吸附-氧化-分馏机制，提供了一个统一的解释最近观察到的地球化学行为的铊在海洋富铁锰沉积物。在这种机制中，六方水钠锰矿的比例决定了Tl氧化的程度，从而控制了Tl富集和同位素分馏的程度。这项工作是第一个提供了一个分子解释的趋势，微量元素富集和稳定同位素组成的地质矿床。我们的分子吸附-氧化-分馏机制将最终有助于解释Tl信号在海洋沉积档案，提供新的约束过去的海洋和气候变化。此外，我们的机制也应该有助于解释其他氧化还原敏感元素在富铁锰海洋沉积物，也可能被用作古海洋和古气候代理的组成关系。
Tl stable isotopes recorded in marine ferromanganese crusts show great promise as a tracer of past marine and climatic conditions. Key to interpreting recent Tl stable isotope time-series data is a detailed, molecular-level understanding of Tl scavenging by ferromanganese crust minerals and Tl stable isotope fractionation occurring during uptake. To this end, we determine the mechanism of Tl sorption to the primary ferromanganese minerals in crusts, namely hexagonal birnessite, todorokite and ferrihydrite, using XAS. We compliment our data with micro-focus XAS of a Tl-enriched hydrogenetic ferromanganese crust. We show that Tl(I) is oxidised to Tl(III) during sorption to hexagonal birnessite, but not during sorption to todorokite, triclinic birnessite and ferrihydrite. Tl(III) forms an inner-sphere complex at the hexagonal birnessite surface, located at vacant octahedral sites in the phyllomanganate sheets. We show that oxidation of Tl(I) to Tl(III) during reductive dissolution of birnessite is thermodynamically unfavourable; and propose that oxidation of Tl(I) is driven by the formation of the Tl(III) surface complex. Recent theoretical calculations predict a large equilibrium stable isotope fractionation between Tl(I) and Tl(III), leading to Tl(III) species that are enriched in the heavy205Tl isotope. In light of this work, we propose a molecular sorption–oxidation–fractionation mechanism that provides a unifying explanation for the recently observed geochemical behaviour of Tl in marine ferromanganese-rich sediments. In this mechanism, the proportion of hexagonal birnessite dictates the extent of Tl oxidation, which controls the extent of Tl enrichment and isotope fractionation. This work is among the first to provide a molecular explanation for reported trends in trace element enrichments and stable isotope compositions in geologic deposits. Our molecular sorption–oxidation–fractionation mechanism will ultimately help interpret Tl signals in marine sedimentary archives to provide new constraints on past oceanic and climatic change. In addition, our mechanism should also help explain compositional relationships of other redox-sensitive elements in ferromanganese-rich marine sediments that might also be used as paleoceanographic and paleoclimate proxies.