Determination of the stable iron isotopic composition of sequentially leached iron phases in marine sediments
Determination of the stable iron isotopic composition of sequentially leached iron phases in marine sediments
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DOI：
10.1016/j.chemgeo.2015.12.003
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发表时间：
2016-02-10
期刊：
影响因子：
3.9
通讯作者：
Staubwasser, Michael
中科院分区：
文献类型：
--
作者：
Henkel, Susann;Kasten, Sabine;Staubwasser, Michael
活性铁（氧）氧化物矿物优先进行早期的氧化还原循环，从而产生溶解的Fe(II), Fe（II）在颗粒表面吸附，并形成自生的Fe矿物。沉积物中铁的分配传统上是通过应用顺序萃取来研究的，目标是操作确定的铁相。在这里，我们通过开发delta Fe-56分析的样品处理方案来补充现有的顺序浸出方法，我们随后使用该方案研究现代海洋沉积物中与异化铁还原相关的铁相特定分馏。碳酸铁用醋酸盐萃取，易还原性氧化物（如水合铁和绢铁石）用盐酸羟胺萃取，可还原性氧化物（如针铁矿和赤铁矿）用柠檬酸二硫铵萃取，磁铁矿用草酸铵萃取。随后，样品被反复氧化，加热，并通过铁沉淀和柱层析纯化。该方法应用于从北海（Helgoland岛以南）收集的表层沉积物。醋酸盐溶出部分（以菱铁矿和铁白云石为目标）表现出明显的下核三角洲Fe-56趋势。该铁池在靠近沉积物-水界面的Fe-56中耗尽最多，与孔隙水Fe（II）的趋势相似。我们将这一池解释为表面还原的铁（II），而不是菱铁矿或铁矿石，它们与氧化物表面进行电子和原子交换。仅使用0.5 M HCl或na -二硫代石的普通提取可能无法解决这种趋势，因为它们从同位素不同的池中溶解铁，导致混合信号。例如，钠二亚硝酸盐浸出的目标是可还原的铁氧化物的总量，它们的同位素指纹可能不同。因此，顺序提取铁同位素方案的发展为铁在各种环境条件下的行为的详细研究提供了新的机会。(C) 2015 Elsevier B.V.版权所有
Reactive iron (oxyhyclr)oxide minerals preferentially undergo early cliagenetic redox cycling which can result in the production of dissolved Fe(II), the adsorption of Fe(II) onto particle surfaces, and the formation of authigenic Fe minerals. The partitioning of iron in sediments has traditionally been studied by applying sequential extractions that target operationally-defined iron phases. Here, we complement an existing sequential leaching method by developing a sample processing protocol for delta Fe-56 analysis, which we subsequently use to study Fe phasespecific fractionation related to dissimilatory iron reduction in a modem marine sediment. Carbonate-Fe was extracted by acetate, easily reducible oxides (e.g. ferrihydrite and lepiclocrocite) by hyclroxylamine-HCl, reducible oxides (e.g. goethite and hematite) by dithionite-citrate, and magnetite by ammonium oxalate. Subsequently, the samples were repeatedly oxidized, heated and purified via Fe precipitation and column chromatography. The method was applied to surface sediments collected from the North Sea, south of the island of Helgoland. The acetate-soluble fraction (targeting siderite and ankerite) showed a pronounced downcore delta Fe-56 trend. This iron pool was most depleted in Fe-56 close to the sediment-water interface, similar to trends observed for porewater Fe(II). We interpret this pool as surface-reduced Fe(II), rather than siderite or ankerite, that was open to electron and atom exchange with the oxide surface. Common extractions using 0.5 M HCl or Na-dithionite alone may not resolve such trends, as they dissolve iron from isotopically distinct pools leading to a mixed signal. Na-dithionite leaching alone, for example, targets the sum of reducible Fe oxides that potentially differ in their isotopic fingerprint. Hence, the development of a sequential extraction Fe isotope protocol provides a new opportunity for detailed study of the behavior of iron in a wide range of environmental settings. (C) 2015 Elsevier B.V. All rights reserved.