Rare earth element association with foraminifera
Rare earth element association with foraminifera
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DOI：
10.1016/j.gca.2012.07.009
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发表时间：
2012-10
影响因子：
5
通讯作者：
N. L. Roberts;A. Piotrowski;H. Elderfield;T. Eglinton;M. Lomas
中科院分区：
文献类型：
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作者：
N. L. Roberts;A. Piotrowski;H. Elderfield;T. Eglinton;M. Lomas
新同位素作为一种古海洋学工具正被广泛用于恢复水团的来源和流向。使用未经化学清洗的浮游有孔虫的新方法已证明是避免深海古海洋学信号受到碎屑物质污染的一种有前途的手段。然而，从底部沃茨的Nd同位素信号变得与浮游有孔虫，壳内的稀土元素（REE）浓度的空间分布，并在不断变化的氧化还原条件下的稀土离子的可能的流动性的确切机制，还没有得到充分的研究。在这里，我们提供了从西北大西洋的浮游生物拖、沉积物陷阱和沉积物核心中采集的浮游有孔虫混合物种的稀土元素浓度和Nd同位素数据。我们使用了多种地球化学技术来评估如何，在何处以及何时稀土元素成为与浮游有孔虫，因为它们通过水柱解决，居住在表面和被埋在沉积物中。从浮游生物丝束和沉积物陷阱之间的200和2938米的水深有孔虫壳的分析表明，只有约20%，其相关的Nd是生物纳入方解石结构。剩余的80%与自生金属氧化物和有机物有关，它们在水柱中形成，并与碳酸盐结构无关。这些有机相和自生相的再矿化将离子释放回溶液中，并产生新的结合位点，使Nd同位素比率与环境海水进行部分平衡，因为有孔虫穿过水柱。从沉积物岩心化石有孔虫壳的分析表明，其稀土元素浓度增加了10倍，在沉积物-水界面，并获得了底层水的同位素签名。稀土离子在方解石内层之间的吸附和络合作用对有孔虫体内稀土含量的升高有重要作用。在此富集阶段，最可能的稀土离子来源是来自底层沃茨和与底层沃茨处于化学平衡的氧化物相的矿化。由于浮游有孔虫埋藏在沉积物-水界面之下，氧化还原敏感的离子浓度在壳内根据孔隙水氧浓度进行调节。被动氧化还原敏感的离子（例如RE 3+离子）的浓度也在一定程度上由该过程控制。我们推断，（a）Nd同位素签名的底层水保存在浮游有孔虫和（B），它依赖于有限的流动性的颗粒反应性RE 3+离子，在某些环境中的微米尺度沉淀的MnCO 3的帮助。这一研究表明，可能存在着底水Nd同位素特征不被浮游有孔虫保存的沉积环境。在使用钻孔记录解释古海洋学变化之前，必须进行验证其他岩心位置的测试。
Neodymium isotopes are becoming widely used as a palaeoceanographic tool for reconstructing the source and flow direction of water masses. A new method using planktonic foraminifera which have not been chemically cleaned has proven to be a promising means of avoiding contamination of the deep ocean palaeoceanographic signal by detrital material. However, the exact mechanism by which the Nd isotope signal from bottom waters becomes associated with planktonic foraminifera, the spatial distribution of rare earth element (REE) concentrations within the shell, and the possible mobility of REE ions during changing redox conditions, have not been fully investigated. Here we present REE concentration and Nd isotope data from mixed species of planktonic foraminifera taken from plankton tows, sediment traps and a sediment core from the NW Atlantic. We used multiple geochemical techniques to evaluate how, where and when REEs become associated with planktonic foraminifera as they settle through the water column, reside at the surface and are buried in the sediment. Analyses of foraminifera shells from plankton tows and sediment traps between 200 and 2938m water depth indicate that only ∼20% of their associated Nd is biogenically incorporated into the calcite structure. The remaining 80% is associated with authigenic metal oxides and organic matter, which form in the water column, and remain extraneous to the carbonate structure. Remineralisation of these organic and authigenic phases releases ions back into solution and creates new binding sites, allowing the Nd isotope ratio to undergo partial equilibration with the ambient seawater, as the foraminifera fall through the water column. Analyses of fossil foraminifera shells from sediment cores show that their REE concentrations increase by up to 10-fold at the sediment–water interface, and acquire an isotopic signature of bottom water. Adsorption and complexation of REE3+ions between the inner layers of calcite contributes significantly to elevated REE concentrations in foraminifera. The most likely source of REE ions at this stage of enrichment is from bottom waters and from the remineralisation of oxide phases which are in chemical equilibrium with the bottom waters. As planktonic foraminifera are buried below the sediment–water interface redox-sensitive ion concentrations are adjusted within the shells depending on the pore-water oxygen concentration. The concentration of ions which are passively redox sensitive, such as REE3+ions, is also controlled to some extent by this process. We infer that (a) the Nd isotope signature of bottom water is preserved in planktonic foraminifera and (b) that it relies on the limited mobility of particle reactive REE3+ions, aided in some environments by micron-scale precipitation of MnCO3. This study indicates that there may be sedimentary environments under which the bottom water Nd isotope signature is not preserved by planktonic foraminifera. Tests to validate other core sites must be carried out before downcore records can be used to interpret palaeoceanographic changes.