Trace element cycling in a subterranean estuary: Part 2. Geochemistry of the pore water
Trace element cycling in a subterranean estuary: Part 2. Geochemistry of the pore water
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DOI：
10.1016/j.gca.2005.10.019
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发表时间：
2006-02-15
影响因子：
5
通讯作者：
Sholkovitz, ER
中科院分区：
文献类型：
--
作者：
Charette, MA;Sholkovitz, ER
海底地下水排放(SGD)是进入海洋的溶解元素的一个重要来源，但人们对控制其从沙质沿海含水层中流动的化学反应知之甚少。从SGD到沿海海洋的元素净通量取决于地下水-海水混合区的生物地球化学反应，最近被称为“地下河口”。本文是关于瓦科伊特湾沿海含水层/地下河口生物地球化学的两篇系列文章的第二篇。第一篇论文从Charette等人岩芯的沉积物组成角度论述了Fe、Mn、P、BA、U和Th的生物地球化学。[Charette，M.A.，Sholkovitz，E.R.，Hansell，C.M.，2005。地下河口中的微量元素循环：第一部分：可渗透沉积物的地球化学。地平线。宇宙人。学报，69,2095-2109]。本文利用地下河口孔隙水资料和海湾表层水资料，对河口化学和沿海含水层中铁、锰、铀、钡、锶的化学成岩作用进行了较为详细的研究。2002年7月期间，从湾头采集了9条高分辨率的孔隙水(地下水)剖面。在地下河口的盐度过渡带，存在着非保守的Ba、Sr添加。然而，锶的释放程度明显小于其碱土邻居巴。孔隙水中的Ba浓度接近3000 nm，而海湾表层水的浓度为25-50 nm；孔隙水的锶盐度分布表明，地下河口的锶含量增加了26%。在中低盐度条件下，海水阳离子与河流悬浮粘土矿物颗粒中的Ba离子发生离子交换作用，从而将溶解的Ba2+释放到表层河口混合区。结果表明，氧化锰的还原溶解以及盐度的变化也可能是地下河口孔隙水中保持高浓度钡的一个重要过程。相比之下，地下河口的孔隙水U显著减少，这是由于SGD通过减少可渗透沉积物而驱动海水循环的结果。这一发现得到了海湾表层水中铀浓度的支持，与邻近的沿海水域相比，表层水中的铀显著减少。使用SGD的全球估计，我们计算出地下河口U的去除量为20×10(6)Mol Uy(-1)，这与海洋的其他主要U汇的数量级相同。我们的结果表明，有必要重新审视和重新评估可能受到与SGD相关的过程影响的要素的海洋预算。(C)2005 Elsevier Inc.保留所有权利。
Submarine groundwater discharge (SGD) is an important source of dissolved elements to the ocean, yet little is known regarding the chemical reactions that control their flux from sandy coastal aquifers. The net flux of elements from SGD to the coastal ocean is dependent on biogeochemical reactions in the groundwater-seawater mixing zone, recently termed the "subterranean estuary." This paper is the second in a two part series on the biogeochemistry of the Waquoit Bay coastal aquifer/subterranean estuary. The first paper addressed the biogeochemistry of Fe, Mn, P, Ba, U, and Th from the perspective of the sediment composition of cores Charette et al. [Charette, M.A., Sholkovitz, E.R., Hansell, C.M., 2005. Trace element cycling in a subterranean estuary: Part 1. Geochemistry of the permeable sediments. Geochim. Cosmochim. Acta, 69, 2095-2109]. This paper uses pore water data from the subterranean estuary, along with Bay surface water data, to establish a more detailed view into the estuarine chemistry and the chemical diagenesis of Fe, Mn, U, Ba and Sr in coastal aquifers. Nine high-resolution pore water (groundwater) profiles were collected from the head of the Bay during July 2002. There were non-conservative additions of both Ba and Sr in the salinity transition zone of the subterranean estuary. However, the extent of Sr release was significantly less than that of its alkaline earth neighbor Ba. Pore water Ba concentrations approached 3000 nM compared with 25-50 nM in the surface waters of the Bay; the pore water Sr-salinity distribution suggests a 26% elevation in the amount of Sr added to the subterranean estuary. The release of dissolved Ba to the mixing zone of surface estuaries is frequently attributed to an ion-exchange process whereby seawater cations react with Ba from river suspended clay mineral particles at low to intermediate salinity. Results presented here suggest that reductive dissolution of Mn oxides, in conjunction with changes in salinity, may also be an important process in maintaining high concentrations of Ba in the pore water of subterranean estuaries. In contrast, pore water U was significantly depleted in the subterranean estuary, a result of SGD-driven circulation of seawater through reducing permeable sediments. This finding is supported by surface water concentrations of U in the Bay, which were significantly depleted in U compared with adjacent coastal waters. Using a global estimate of SGD, we calculate U removal in subterranean estuaries at 20 x 10(6) Mol U y(-1), which is the same order of magnitude as the other major U sinks for the ocean. Our results suggest a need to revisit and reevaluate the oceanic budgets for elements that are likely influenced by SGD-associated processes. (c) 2005 Elsevier Inc. All rights reserved.