冷泉渗漏对海洋沉积物氧化还原环境地球化学识别的影响--以南海东北部F站位活动冷泉为例

The impact of cold seepage on geochemical indices for redox conditions of marine sediments―Site F active seep site in the northeastern South China Sea

氧化还原敏感元素(Mo、U、V、Re、Ni、Co、Cr)已被广泛用于判别沉积环境的氧化还原状态,以及计算上覆水体和大气层中的氧气浓度。然而海底冷泉活动由于微生物作用形成的硫化氢可以导致这些元素指示的氧化还原信号发生变化和模糊,进而影响氧化还原状态判别的有效性。文章通过对南海活动冷泉F站位海底3根插管沉积物的氧化还原敏感元素地球化学特征分析,发现冷泉活动区海底沉积物相比正常海底环境普遍表现为较高的Mo含量,并指示孔隙水为硫化环境,表明甲烷厌氧氧化作用形成的硫化氢对沉积物中Mo富集具有促进作用。冷泉沉积物的U/Th、V/Cr和Ni/Co显示底层海水具有较高的氧浓度,与正常海底特征相一致。但是V/(V+Ni)>0.7指示了沉积物形成时的环境缺氧,可能与陆源碎屑中较低的Ni含量有关。冷泉沉积物Re/Mo比值接近现代海水值,与现代海洋硫化盆地的特征类似,指示了冷泉附近海洋硫化分层特征。因此冷泉活动区海底沉积物的Re和Mo容易受到甲烷渗漏的影响,不能可靠指示真实的氧化还原状态。

Redox-sensitive elements(Mo,U,V,Re,Ni,Co,Cr)have been widely used as geochemical indicators to infer the redox states of marine sediments at deposition,as well as oxygen concentrations in overlying water and atmosphere.However,the sulfidation environment in pore water formed by cold seepage due to microbial activity can result in alterations and ambiguities of redox signals indicated by these elements,which may challenge the effectiveness of the reconstructed redox state.In this paper,the contents of redox-sensitive elements of three push core sediments at the active seep site F of the South China Sea were studied.Compared with the oxic sediments,the seep sediments generally show higher Mo content,indicating the fixed Mo by hydrogen sulfide from the anaerobic oxidation of methane.U/Th,V/Cr,and Ni/Co indicate that the seep sediments are formed in the bottom water with high oxygen concentration,which is consistent with the measured results.However,V/(V+Ni)>0.7 indicates anoxic conditions,which may be related to the lower Ni content in terrestrial debris.The Re/Mo ratio is similar to the modern seawater value,indicating a euxinic environment.The above analysis shows that Re and Mo in cold seep sediments are easily affected by methane seepage and possibly not used as geochemical indices for redox conditions in a methane-rich environment.