Iron sulfide formation in young and rapidly-deposited permeable sands at the land-sea transition zone.
Iron sulfide formation in young and rapidly-deposited permeable sands at the land-sea transition zone.
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DOI：
10.1016/j.scitotenv.2018.08.278
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发表时间：
2019-02
期刊：
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通讯作者：
Stephan L. Seibert;M. Böttcher;F. Schubert;T. Pollmann;L. Giani;S. Tsukamoto;M. Frechen;H. Freund;Hannelore Waska;H. Simon;T. Holt;J. Greskowiak;G. Massmann
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文献类型：
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作者：
Stephan L. Seibert;M. Böttcher;F. Schubert;T. Pollmann;L. Giani;S. Tsukamoto;M. Frechen;H. Freund;Hannelore Waska;H. Simon;T. Holt;J. Greskowiak;G. Massmann
在沿海地区的海陆过渡带中，常有有机贫质、渗透性好的石英砂。然而，碳、硫和铁的生物地球化学循环在这里并没有得到很好的研究。因此，这项工作的目的是提高我们对这些突出的沿海沉积物中的化学过程的理解。为此目的，在德国斯皮克鲁奥岛的沙丘基地收集了10 m的岩芯。此外，还从一口多层井中采集了为期一年的地下水样本，以记录季节性水化学变化。方法包括分析地球化学(总碳、总无机碳、活性铁、总硫、还原无机硫)和水化学参数(场参数、主要离子、DOC和DOM的分子组成)，以及稳定硫同位素(δ34S-硫酸盐、-硫化物、-还原无机硫)。此外，还应用了光激发发光(OSL)测年技术。结果表明，岩心沉积物非常年轻(<；500a)，沉积速度很快。它们的特征是有机碳(<；0.1dw.)、活性铁(~10 mmoL/kg)和硫化铁(<；3 mmol/kg)含量非常低。地下水盐度在顶端岩心沉积物中较低，在一年中的大部分时间都在深层增加。然而，采样点受到(季节性)不同盐度的影响，这可能与生物地球化学循环有关。例如，在淹没事件期间，海水中不稳定的DOM的渗透除驱动沉积有机质外，还驱动微生物呼吸。氧和硝酸盐是近地表地下水有机质分解的主要电子受体，而硫酸盐还原仅限于较低的微咸水沉积物。在这里，自生黄铁矿的形成是基于对溶解硫化物的检测，完整的黄铁矿框体，以及与溶解硫化物和固体硫化物的稳定硫同位素特征相匹配的。我们的结论是，极低的有机碳含量限制了低有机渗透性石英砂中黄铁矿的形成。
Organic-poor, permeable quartz sands are often present at land-sea transition zones in coastal regions. Yet, the biogeochemical cycles of carbon, sulfur, and iron are not well studied here. The aim of this work was, therefore, to improve our understanding regarding the chemical processes in these prominent coastal sediments. A 10 m core was collected at a dune base of the barrier island Spiekeroog, Germany, for this purpose. Additionally, groundwater was sampled from a multi-level well for one year to record seasonal hydrochemical variations. Methods included the analyses of geochemical (total carbon, total inorganic carbon, reactive iron, total sulfur, reduced inorganic sulfur) and hydrochemical parameters (field parameters, major ions, DOC, and molecular compositions of DOM), as well as stable sulfur isotopes (δ34S-sulfate, -sulfide, -total reduced inorganic sulfur). Moreover, optically stimulated luminescence (OSL) dating was applied. Results show that the core sediments are very young (<500 a) and were rapidly deposited. They are characterized by remarkably low contents of organic carbon (<0.1% dw.), reactive iron (~10 mmol/kg), and iron sulfides (<3 mmol/kg). Groundwater salinities were low in the top core sediments and increased at depth during most times of the year. However, the sampling site is subject to (seasonally) varying salinities, which could be linked to the biogeochemical cycles. For instance, the infiltration of seawater-derived labile DOM during inundation events drives microbial respiration besides sedimentary organic matter. Oxygen and nitrate were the dominant electron acceptors for the decomposition of organic matter in near-surface groundwater, while sulfate reduction was constrained to the lower brackish sediments. Here, authigenic pyrite formation was inferred based on the detection of dissolved sulfide, intact pyrite framboids, and matching stable sulfur isotope signatures of dissolved and solid sulfides. We concluded that the extremely low organic carbon contents limit pyrite formation in the organic-poor, permeable quartz sands.