Evaluating the importance of sulfurized organic matter for the sedimentary cycling of molybdenum and tungsten in upwelling regimes: a case study on the Namibian shelf

海洋环境中痕量金属的主要汇之一是沿高生产力大陆边缘形成的缺氧开阔海洋环境，全球约70%的微生物硫酸盐还原发生在那里。由此产生的硫化条件与海洋沉积物中痕量金属通过还原、吸附和并入还原硫库的硫化物相而被固定和固定密切相关。该硫气藏的形成主要由黄铁矿等铁硫化物和硫化有机质组成，是还原和氧化(中间)硫物种复杂循环的结果。虽然痕量金属的富集机制主要与黄铁矿的形成有关，但硫动力学，特别是硫化有机物与痕量金属聚集的相关性仍然是个谜，尽管这可能是解释缺氧远洋环境中某些氧化还原敏感元素的大范围浓度和同位素组成的关键。这包括钼(Mo)和钨(W)，它们都是与生物相关的痕量金属，对氧化还原敏感，在含氧海水中表现保守，并具有吸附在锰和铁氧化物上的亲和力。然而，在硫化条件下，在硫化氢的存在下，钼迅速改变了其地球化学行为，成为颗粒反应并在沉积物中富集，这使得它成为示踪硫化条件的有价值的替代物。相反，即使在硫化物浓度升高的情况下，W仍然是可溶的，其在上升流地区缺氧沉积物中的富集性机制仍然受到很少的限制。然而，海洋沉积物的W浓度和同位素特征作为有价值的氧化还原替代物出现，补充了已建立的Mo同位素氧化还原替代物。鉴于海洋缺氧环境对钼和钨的全球物质平衡的重要性，这项研究旨在填补主要的知识空白，并改进基于钼和钨的氧化还原试剂的应用。为此，将调查纳米比亚陆架不同氧化还原制度的沉积物。纳米比亚陆架受本格拉上升流系统控制，该系统驱动地表水的初级生产，从而提高了对氧气的利用，并在富含有机物质的陆架沉积物上形成了缺氧底水。该计划将研究这些表层沉积物中硫的形态和氧化还原动力学，涵盖从缺氧-硫化到完全氧化的氧化还原条件。这将通过顺序提取和对不同硫物种的稳定同位素分析来实现。这些结果将与钼和钨同位素分析相结合，以了解这些氧化还原敏感的痕量金属的富集机理。总体目标是更详细地限制在缺氧的开阔海洋环境中使用的钼和钨氧化还原代用品，重点放在硫化有机物质上。

One of the major sinks of trace metals in the marine environment are anoxic open-marine settings, which form along highly productive continental margins, where about 70% of global microbial sulfate reduction takes place. Resulting sulfidic conditions are highly relevant for the immobilization and fixation of trace metals in marine sediments via reduction, adsorption and incorporation into sulfidic phases of the reduced sulfur pool. The formation of this sulfur pool, which is mainly composed of iron-sulfides, e.g. pyrite, and sulfurized organic matter, is the result of a complex cycling of reduced and oxidized (intermediate) sulfur species. While enrichment mechanisms of trace metals are mainly connected to pyrite formation, the relevance of sulfur dynamics and in particular of sulfurized organic matter for trace metal accumulation remains enigmatic, although it is potetially the key to explain wide ranges of concentrations and isotope composition of certain redox-sensitive elements in anoxic open-marine settings. This includes molybdenum (Mo) and tungsten (W), which are both bio-relevant and redox-sensitive trace metals, behave conservative in oxic seawater and have an affinity to adsorb onto Mn- and Fe-oxides. However, under sulfidic conditions, at the presence of hydrogen sulfide, Mo rapidly changes its geochemical behavior, becomes particle-reactive and enriched in sediments, which makes it a valuable proxy to trace sulfidic conditions. In contrast, W remains soluble even at elevated sulfide concentrations and its enrichment mechanisms in anoxic sediments of upwelling regimes are still poorly constrained. Nevertheless, W concentrations and isotope signatures of marine sediments emerge as valuable redox proxies, complementing the established Mo isotope redox proxy. Given the importance of anoxic open-marine settings for the global mass balance of Mo and, presumably, W, this study aims to close major knowledge gaps and to improve the application of Mo- and W-based redox proxies.For this purpose, sediments from different redox regimes along the Namibian shelf will be investigated. The Namibian shelf is controlled by the Benguela upwelling system, which drives primary production in the surface water, resulting in enhanced oxygen utilization and the formation of anoxic bottom water overlying the organic-rich shelf sediments. The plan is to investigate sulfur speciation and redox dynamics in these surface sediments, covering redox conditions from anoxic-sulfidic to fully oxic. This will be achieved by sequential extraction and stable isotope analyses of different sulfur species. These results will be coupled with Mo and W isotope analyses in order to gain an understanding of enrichment mechanisms of these redox-sensitive trace metals. The overall aim is to constrain Mo- and W-based redox proxies in anoxic open-marine settings in more detail, focussing on sulfurized organic matter.