Natural variations of δ30Si ratios during progressive basalt weathering, Hawaiian Islands
Natural variations of δ30Si ratios during progressive basalt weathering, Hawaiian Islands
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DOI：
10.1016/j.gca.2005.05.008
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发表时间：
2005-10
影响因子：
5
通讯作者：
K. Ziegler;O. Chadwick;M. Brzezinski;E. Kelly
中科院分区：
文献类型：
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作者：
K. Ziegler;O. Chadwick;M. Brzezinski;E. Kelly
硅的稳定同位素可以用来追踪硅从其大陆来源移动到海洋沉积物中的沉淀物的生物地球化学路径。沿途，硅被并入粘土矿物中，被植物吸收，形成植物蛋白石，并渗入河流，河流是主要的陆路到海洋的管道。与火成岩相比，排出大陆的水富含重硅同位素，但控制分馏的机制尚未阐明。我们研究了沿夏威夷群岛400万年玄武岩土壤年代序列的硅同位素分馏。利用这些样品的自然背景，结合实验室实验，我们论证了风化系统中溶解硅的同位素组成是由岩石崩解、粘土矿物新合成和硅生物循环的综合作用决定的。风化作用优先将28Si释放到溶液中，而次生矿物形成优先将28Si从溶液中带走。在潮湿的环境中，淋洗的土壤失去了大量的可溶性硅，从而造成整个土壤系统净损失30Si。随着土壤的发育和新生粘土矿物中硅含量的增加，δ30SiBulk土壤值逐渐向更负的值变化；玄武岩δ30Si值约为−0.5Si值，但老土壤的‰30Si值最高可达δ2.5Si值。随着风化的进行，固体和溶解δ30Si值之间的差值基本保持不变，因此，来自较老土壤的土壤水具有更负的δ30Si成分。在夏威夷土壤的上层，这种由风化驱动的δ30Si漂移是通过添加未风化的原生矿物来改变的，携带δ30Si值约为−0.5‰，并通过植物进行硅的生物循环，在植硅体中产生负的δ30Si值，在来自上层的土壤溶液中产生正的δ30Si值。由于这些近表层中溶解硅的浓度很高，河流的δ30Si值比根据下层风化状态预测的要多。当结合公布的来自世界各地大河流的δ30Si值时，我们发现夏威夷的结果指向输送到海洋的硅同位素的风化控制，从而表明了一种重要的大陆-海洋联系，值得进一步研究。
Silicon stable isotopes can be used to trace the biogeochemical pathways of Si as it moves from its continental sources to its sink in ocean sediments. Along the way, Si is incorporated into clay minerals, taken up by plants where it forms plant opal, and leached into rivers, the major land-to-ocean conduit. Compared to igneous rocks, the waters that drain continents are enriched in heavy Si isotopes, but the mechanisms that control fractionation have not been elucidated. We studied Si isotope fractionation along a 4 million yr basaltic soil chronosequence on the Hawaiian Islands. Using the natural context of these samples in combination with laboratory experiments, we demonstrate that the isotopic composition of dissolved Si in weathering systems is determined by the combined effects of rock disintegration, clay mineral neosynthesis, and Si biocycling. Weathering preferentially releases28Si into solution, whereas secondary mineral formation preferentially removes28Si from solution. In humid environments, leached soils have lost large amounts of this soluble Si, thus creating a net loss of30Si from the entire soil system. As soils develop and greater fractions of Si reside in neoformed clay minerals, δ30Sibulk soilvalues change progressively toward more negative values; basalt δ30Si values are about −0.5‰, but older soils have δ30Si values up to −2.5‰. The difference between the solid and solution δ30Si values remains more or less constant with progressive weathering, and therefore, soil water from older soils has a more negative δ30Si composition. In the upper horizons of the Hawaiian soils, this weathering-driven δ30Si shift is modified by the addition of unweathered primary minerals via dust, carrying δ30Si values of about −0.5‰, and by biocycling of Si via plants, producing negative δ30Si values in phytoliths and positive δ30Si values in soil solutions derived from upper horizons. Due to the high concentrations of dissolved Si in these near-surface layers, rivers have more positive δ30Si values than predicted based on the weathering status of the lower horizons. When combined with published δ30Si values from large rivers worldwide, we find that the results from Hawaii point to weathering control of Si isotopes delivered to the oceans, and thus, to an important continent-ocean linkage that warrants further investigation.