Application of arsenic in barite as a redox indicator for suboxic/anoxic redox condition
Application of arsenic in barite as a redox indicator for suboxic/anoxic redox condition
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DOI：
10.1016/j.chemgeo.2016.10.016
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发表时间：
2016-12
期刊：
影响因子：
3.9
通讯作者：
K. Tokunaga;T. Uruga;K. Nitta;Y. Terada;O. Sekizawa;S. Kawagucci;Y. Takahashi
中科院分区：
文献类型：
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作者：
K. Tokunaga;T. Uruga;K. Nitta;Y. Terada;O. Sekizawa;S. Kawagucci;Y. Takahashi
氧化还原条件是控制水环境中各种元素行为的重要因素。矿物中对氧化还原反应敏感的微量元素描述了水中的氧化还原条件，并提供了有关过去环境变化的信息，除非它在沉积物埋藏过程中被溶解或重结晶。本研究探讨了重晶石中砷/砷比值(As(V)/As(III))作为一种新的地球化学指标在古环境重建中的应用。实验室实验表明，As(III)和As(V)均可分别与As(III)和As(V)共沉淀重晶石形成重晶石，在氧化还原反应达到平衡的情况下，可或多或少地保留共存水中As(V)/As(III)比值的信息。在此基础上，对玉马川温泉和冲绳热液喷口天然重晶石中As(V)/As(III)比值进行了显微X射线荧光和显微X射线吸收精细结构分析，以(A)评价重晶石中As(V)/As(III)比值作为自然体系氧化还原指示剂的可靠性，以及(B)评价水中重晶石的沉积氧化还原条件，无论是沉积在As(V)/As(III)氧化还原边界以下或以上，还是亚氧/缺氧边界。我们以前的研究表明，重晶石-硒氧阴离子体系也可以作为氧-亚氧界面的替代，因为重晶石中的硒/亚硒酸盐[Se(VI)/Se(IV)]的比值主要与水中的比值相关。由于Se(VI)/Se(IV)的氧化还原界面比As(V)/As(III)界面具有更高的氧化还原电势，这两个系统的结合可以将氧化还原条件扩展到更大的区域，这可以通过对单个重晶石颗粒的分析来确定。因此，一个重晶石颗粒可以提供关于水中As(V)/As(III)和Se(VI)/Se(IV)比值的信息，以及重晶石是在氧化、亚缺氧还是缺氧氧化还原环境中沉淀出来的。
Redox condition is an important factor that controls the behavior of various elements in an aquatic environment. Incorporation of redox-sensitive trace elements in minerals describes redox conditions in water and provides information about past environmental changes, unless it is dissolved or recrystallized during sediment burial. In the present study, we explore the application of arsenate/arsenite ratios (As(V)/As(III)) in barite as a new geochemical proxy for paleo-environmental reconstruction. Laboratory experiments showed that both As(III) and As(V) could be incorporated into barite as As(III)- and As(V)-coprecipitated barite, respectively, which could more or less retain the information of the As(V)/As(III) ratio in coexistent water, if under equilibrium in terms of the redox reactions. Furthermore, the As(V)/As(III) ratios in natural barite collected in (i) Tamagawa Hot Spring and (ii) Okinawa hydrothermal vent were determined by micro-X-ray fluorescence and micro-X-ray absorption fine structure analysis to (a) evaluate the reliability of the As(V)/As(III) ratio in barite as a redox indicator in natural system and (b) estimate the depositional redox condition in water whether barite precipitated below or above the redox boundary of As(V)/As(III), or the suboxic/anoxic boundary. Our previous study showed that barite–selenium oxyanion system could also be used as a proxy for the oxic-suboxic boundary because the selenate/selenite [Se(VI)/Se(IV)] ratio in barite is primarily correlated with the ratio in water. Given that the redox boundary of the Se(VI)/Se(IV) ratio has higher redox potential than the As(V)/As(III) boundary, the combination of these two systems can extend the redox condition, which can be specified by the analysis of one single barite grain to a wide region. Thus, one barite particle can provide information on the As(V)/As(III) and Se(VI)/Se(IV) ratios in water, and whether barite precipitated under oxic, suboxic, or anoxic redox environments.