Methane seepage has been extensively observed in various continental margin settings. It has profound effects on the marine redox environment and the molybdenum (Mo) cycles in marine sediments. Therefore, there has been much recent attention on the redox-sensitive behavior of Mo in methane seepage environments. However, the characteristics of the Mo isotope composition in the cold-seep system remain poorly understood. In this study, we performed geochemical analyses, including Mo content and isotope composition, on sediment samples (core QDN-MS6) from the “Haima” cold-seep deposit area in the South China Sea. The analysis reveals a significant concentration of authigenic pyrite in the mid-section of QDN-MS6 core (373–403 cm). Moreover, the δS value in this interval is notably elevated with high total sulfur/total organic carbon ratio. Additionally, the sediments in the mid-section exhibits substantial enrichment in Mo (enrichment factors of Mo ranging from 5.29 to 39.32). This implies that the sediments in the mid-section are influenced by sulfate-driven anaerobic oxidation of methane. Most notably, the sediments in the mid-section displayed distinct low δMo values (with an average of −0.7‰). After careful consideration, we ruled out the influence of organic matter, an oxic environment, a weakly sulfidic environment, and incomplete removal of thiopolybdate as contributing factors. Based on δFe-Fe/Al ratios, (Mo–U) enrichment factors, and As enrichment factors, we propose that the “benthic Fe-Mn redox shuttle process” is the primary cause of the observed light δMo signatures in sediments. This newly identified mechanism sheds light on Mo isotope cycling in methane seepage environments and enhances our understanding of the Mo isotope cycling process.

中文翻译：

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底栖铁锰氧化还原穿梭过程控制的甲烷渗流沉积物中的同位素轻钼


在各种大陆边缘环境中都广泛观察到了甲烷渗漏。它对海洋氧化还原环境和海洋沉积物中的钼（Mo）循环具有深远的影响。因此，近年来，Mo在甲烷渗流环境中的氧化还原敏感行为引起了人们的广泛关注。然而，人们对冷泉系统中钼同位素组成的特征仍然知之甚少。在本研究中，我们对南海“海马”冷泉矿床区的沉积物样品（岩心QDN-MS6）进行了地球化学分析，包括Mo含量和同位素组成。分析显示，QDN-MS6 岩心中部 (373–403 cm) 存在显着浓度的自生黄铁矿。此外，当总硫/总有机碳比较高时，该区间的δS值显着升高。此外，中段沉积物显示出Mo的显着富集（Mo的富集因子范围为5.29至39.32）。这意味着中段的沉积物受到硫酸盐驱动的甲烷厌氧氧化的影响。最值得注意的是，中段沉积物显示出明显的低δMo值（平均值为-0.7‰）。经过仔细考虑，我们排除了有机物、好氧环境、弱硫化环境以及硫代聚硼酸盐去除不完全等因素的影响。基于δFe-Fe/Al比、(Mo-U)富集因子和As富集因子，我们提出“底栖Fe-Mn氧化还原穿梭过程”是沉积物中观测到的光δMo特征的主要原因。这一新发现的机制揭示了甲烷渗流环境中的钼同位素循环，并增强了我们对钼同位素循环过程的理解。