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

海洋环境中微量金属的主要汇之一是缺氧的开放海洋环境，它形成于高产的大陆边缘，全球微生物硫酸盐还原的约70%发生在那里。由此产生的硫化物条件与海洋沉积物中痕量金属通过还原、吸附和进入还原硫池的硫化物相的固定和固定密切相关。硫池的形成主要由硫铁矿等硫化物和硫化有机质组成，是还原和氧化（中间）硫种复杂循环的结果。虽然微量金属的富集机制主要与黄铁矿的形成有关，但硫动力学，特别是硫化有机质与微量金属积累的相关性仍然是一个谜，尽管它可能是解释缺氧开阔海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且对氧化还原敏感的微量金属，在含氧海水中表现保守，并具有吸附Mn和fe氧化物的亲和力。然而，在硫化氢条件下，Mo在硫化氢存在下，其地球化学行为迅速改变，呈颗粒反应性，并在沉积物中富集，这使其成为追踪硫化氢条件的有价值的代用物。相比之下，即使硫化物浓度升高，W仍可溶解，其在上升流条件下的缺氧沉积物中的富集机制仍不清楚。然而，海洋沉积物的W浓度和同位素特征成为有价值的氧化还原指标，补充了已建立的Mo同位素氧化还原指标。鉴于缺氧开放海洋环境对Mo和W的全球质量平衡的重要性，本研究旨在弥补主要的知识空白，并改进基于Mo和W的氧化还原代理的应用。为此目的，将对纳米比亚大陆架沿岸不同氧化还原体系的沉积物进行调查。纳米比亚陆架受Benguela上升流系统控制，该系统驱动地表水的初级生产，导致氧气利用率提高，并形成缺氧的底部水，覆盖在富含有机物的陆架沉积物上。该计划旨在研究这些表层沉积物中的硫形态和氧化还原动力学，涵盖从缺氧-硫化物到全氧的氧化还原条件。这将通过顺序提取和不同硫种的稳定同位素分析来实现。这些结果将与Mo和W同位素分析相结合，以了解这些氧化还原敏感痕量金属的富集机制。总体目标是更详细地限制缺氧开阔海洋环境中Mo和w基氧化还原指标，重点关注硫化有机质。

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.