Barium isotope signatures of barite–fluid ion exchange in Equatorial Pacific sediments
Barium isotope signatures of barite–fluid ion exchange in Equatorial Pacific sediments
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DOI：
10.1016/j.epsl.2023.118150
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发表时间：
2023-06
影响因子：
5.3
通讯作者：
J. Middleton;A. Paytan;M. Auro;M. Saito;T. Horner
中科院分区：
文献类型：
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作者：
J. Middleton;A. Paytan;M. Auro;M. Saito;T. Horner
矿物重晶石 (BaSO 4) 中的钡 (Ba) 同位素组成正在成为远洋 Ba、碳和硫循环的强大示踪剂。然而，识别和限制可能改变BaSO 4 中Ba（δ 138 Ba）主要同位素组成的过程至关重要，特别是在早期成岩作用期间。为此，我们分析了赤道太平洋沉积物中孔隙水和 BaSO 4 的 Ba 同位素组成，并使用这些相同的 BaSO 4 进行了一系列实验室实验，以评估 Ba 同位素变化率。我们发现沉积物BaSO 4 的Ba同位素组成≈+ 0.1‰，相对于周围孔隙水抵消了≈− 0.16‰。使用同位素标记海水的实验表明，即使在化学平衡下，Ba 离子也会通过耦合的 BaSO 4 溶解-沉淀在固相和液相之间进行广泛而快速的转移。使用BaSO 4 沉淀和溶解过程中Ba同位素分馏的已发表值，我们计算出在同位素平衡时，共置的BaSO 4 和孔隙水应表现出− 0.17‰的Ba同位素偏移，类似于在赤道太平洋沉积物中观察到的偏移。总而言之，现场数据、实验室实验和计算表明，赤道太平洋沉积物中发生了离子交换，并且该过程也驱动了所观察到的远洋 BaSO 4 和孔隙水之间的 Ba 同位素偏移。这一发现意味着离子交换可能会改变沉积物 BaSO 4 中 Ba 的同位素组成，尽管改变的程度取决于固相中 Ba 的比例。更广泛地说，这项研究提供了一个例子，说明离子交换介导的过程如何在海洋地球化学中广泛存在，以及这些过程可能影响除 Ba 和 BaSO 4 之外的其他金属和矿物。
The isotope composition of barium (Ba) in the mineral barite (BaSO 4) is emerging as a powerful tracer of the pelagic Ba, carbon, and sulfur cycles. However, it is critical to identify and constrain processes that may alter the primary isotope composition of Ba, δ 138 Ba, in BaSO 4, particularly during early diagenesis. To this end, we analyzed the Ba isotope composition of porewaters and co-located BaSO 4 in sediments from the Equatorial Pacific and performed a series of laboratory experiments with these same BaSO 4 to assess rates of Ba isotope alteration. We find that sedimentary BaSO 4 exhibit Ba isotope compositions≈+ 0.1‰ and are offset by≈− 0.16‰ relative to ambient porewaters. Experiments using isotope-labeled seawater show extensive and rapid transfer of Ba ions between the solid and fluid phase through coupled BaSO 4 dissolution-precipitation, even at chemical equilibrium. Using published values for Ba isotope fractionation during BaSO 4 precipitation and dissolution, we calculate that co-located BaSO 4 and porewaters should exhibit Ba isotope offset of− 0.17‰ at isotopic equilibrium, similar to the offsets observed in Equatorial Pacific sediments. Altogether, the field data, laboratory experiments, and calculations indicate that ion exchange occurs in Equatorial Pacific sediments and that this process also drives the observed Ba isotope offsets between pelagic BaSO 4 and porewaters. This finding implies that ion exchange may alter the isotope composition of Ba in sedimentary BaSO 4, though the degree of alteration will depend on the proportion of Ba held in the solid phase. More broadly, this study provides an example of how ion-exchange-mediated processes are widespread in marine geochemistry and that these processes likely affect other metals and minerals beyond Ba and BaSO 4.