Opposing authigenic controls on the isotopic signature of dissolved iron in hydrothermal plumes
Opposing authigenic controls on the isotopic signature of dissolved iron in hydrothermal plumes
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DOI：
10.1016/j.gca.2016.12.022
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发表时间：
2017-04
影响因子：
5
通讯作者：
A.J.M. Lough;J. Klar;W. Homoky;S. Comer-Warner;J. Milton;D. Connelly;R. James;R. Mills
中科院分区：
文献类型：
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作者：
A.J.M. Lough;J. Klar;W. Homoky;S. Comer-Warner;J. Milton;D. Connelly;R. James;R. Mills
铁是海洋中一种稀缺但必不可少的微量营养素，限制了表层海洋许多区域的初级生产力。向海洋内部供应铁的机制和速率仍然缺乏了解和量化。如果这些边界通量具有明显的特征，不同铁库的铁同位素比值可能被用来追踪全球铁生物地球化学循环的源和汇。海底热液喷口将富含金属的流体从大洋中脊释放到深海。如果了解对深海羽流扩散过程中铁同位素分馏的当地控制，铁同位素比率有可能被用来追踪热液溶解铁向海洋的输入。在这项研究中，我们使用总可溶铁(TDFe)和溶解铁(DFE)采样程序评估了南大洋热液羽流中Fe同位素的行为。结果表明，热液羽流中的δ56Fe值(δ56dFe)在早期热液羽流扩散过程中发生了剧烈的变化，变化范围为−2.39±0.05‰到−0.13±0.06‰(2SD)。TdFe(δ56TDFe)的同位素组成始终大于DFE值，范围为−0.31g±0.03‰~0.78g±0.05g‰，与氢氧化铁沉淀相一致。热液羽流中存在的DFE包括稳定的DFE物种，有可能被输送到深海。我们估计，从羽流中输出的稳定的DFE的δ56Fe将为−0.28±0.17‰。此外，我们还发现，在浮力羽流中沉淀的自生硫化铁和氢氧化铁矿物的比例对进入中性浮力羽流的溶解Fe的同位素组成起相反的控制作用。我们发现，在其他地方现代喷口遗址报道的自生条件下，这种控制产生了可变的溶解Fe同位素特征，因此在根据海洋沉积物记录重建过去的热液作用时，应该考虑到这一点。
Iron is a scarce but essential micronutrient in the oceans that limits primary productivity in many regions of the surface ocean. The mechanisms and rates of Fe supply to the ocean interior are still poorly understood and quantified. Iron isotope ratios of different Fe pools can potentially be used to trace sources and sinks of the global Fe biogeochemical cycle if these boundary fluxes have distinct signatures. Seafloor hydrothermal vents emit metal rich fluids from mid-ocean ridges into the deep ocean. Iron isotope ratios have the potential to be used to trace the input of hydrothermal dissolved iron to the oceans if the local controls on the fractionation of Fe isotopes during plume dispersal in the deep ocean are understood. In this study we assess the behaviour of Fe isotopes in a Southern Ocean hydrothermal plume using a sampling program of Total Dissolvable Fe (TDFe), and dissolved Fe (dFe). We demonstrate that δ56Fe values of dFe (δ56dFe) within the hydrothermal plume change dramatically during early plume dispersal, ranging from −2.39 ± 0.05‰ to −0.13 ± 0.06‰ (2 SD). The isotopic composition of TDFe (δ56TDFe) was consistently heavier than dFe values, ranging from −0.31 ± 0.03‰ to 0.78 ± 0.05‰, consistent with Fe oxyhydroxide precipitation as the plume samples age. The dFe present in the hydrothermal plume includes stabilised dFe species with potential to be transported to the deep ocean. We estimate that stable dFe exported from the plume will have a δ56Fe of −0.28 ± 0.17‰. Further, we show that the proportion of authigenic iron-sulfide and iron-oxyhydroxide minerals precipitating in the buoyant plume exert opposing controls on the resultant isotope composition of dissolved Fe passed into the neutrally buoyant plume. We show that such controls yield variable dissolved Fe isotope signatures under the authigenic conditions reported from modern vent sites elsewhere, and so ought to be considered during iron isotope reconstructions of past hydrothermalism from ocean sediment records.