Uranium isotopes as a proxy for primary depositional redox conditions in organic-rich marine systems
Uranium isotopes as a proxy for primary depositional redox conditions in organic-rich marine systems
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DOI：
10.1016/j.epsl.2019.115878
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发表时间：
2020
影响因子：
5.3
通讯作者：
Michelle L. Abshire;S. Romaniello;Amy Kuzminov;J. Cofrancesco;S. Severmann;N. Riedinger
中科院分区：
文献类型：
--
作者：
Michelle L. Abshire;S. Romaniello;Amy Kuzminov;J. Cofrancesco;S. Severmann;N. Riedinger
在海洋沉积物中，自生铀（U）富集和U同位素组成是解释氧化还原条件变化的重要工具，然而，它们作为古代用品的使用需要对沉积物富集或贫化的主导过程有全面的了解。这项研究的重点是从纳米比亚大陆边缘，高生产力的结果在扩大的氧气最小区（OMZ）的有机丰富的表层沉积物的U含量和238 U/235 U的比例。所调查的岩心取样点分别位于陆棚、陆棚坡折和陆坡上，在陆棚坡上，底层水的氧化还原条件分别从缺氧到亚氧再到好氧。虽然所有岩心都具有相对高的总有机碳（TOC）含量（高达12重量%），每个位置显示唯一的U到TOC关系。陆架沉积物表现出公平的U和TOC之间的相关性，而陆架坡折带和斜坡沉积物显示出明显的U和TOC的脱钩。在纳米比亚大陆边，富含颗粒的霞状层将富含有机物的沉积物从OMZ内通过含氧水输送到斜坡上再沉积。由于U对氧化还原条件变化的敏感性，这种横向运动导致还原U相通过氧化释放回水柱，同时将部分矿化的有机碳输送到斜坡。迁移过程中U的氧化并没有改变再沉积物中238 U/235 U的平均原始同位素特征，高TOC、低U含量和高δ 238 U值的组合可能成为确定古OMZ边界的有用工具。
In marine sediments, authigenic uranium (U) enrichments and U isotope compositions are important tools for interpreting changes in redox conditions, however, their use as paleoproxies requires a comprehensive understanding of the dominant processes that contribute to sediments becoming enriched or depleted. This study focuses on the U content and 238 U/235 U ratio of organic-rich surface sediments from the Namibian continental margin, where high productivity results in an expanded oxygen minimum zone (OMZ). The investigated core sample sites are located on the shelf, shelf break, and slope where bottom water redox conditions vary from anoxic to suboxic to oxic, respectively. While all cores have relatively high total organic carbon (TOC) contents (up to 12 wt.%), each location displays a unique U to TOC relationship. Shelf sediment exhibit a fair correlation between U and TOC, while the shelf break and slope sediments show a pronounced decoupling of U and TOC. On the Namibia continental margin, particle-rich nepheloid layers transport organic-rich deposits from within the OMZ, through oxic water, to be redeposited on the slope. Due to the sensitivity of U to changes in redox conditions, this lateral movement results in the release of the reduced U phases back into the water column through oxidation while transporting the partially remineralized organic carbon to the slope. Oxidation of U during transport does not alter the average primary 238 U/235 U isotopic signature in redeposited sediment, and the combination of high TOC, low U content and high δ 238 U values may become a useful tool for the identification of the boundaries of ancient OMZs.