Mo-U共变作为古沉积环境氧化还原条件分析的重要指标——进展、问题与展望

Mo-U Covariation as An Important Proxy for Sedimentary Environment Redox Conditions—Progress,Problems and Prospects

由于氧化还原敏感金属元素(RSM)在氧化水体中化学性质保守,而在缺氧水体中还原活化并向沉积物中转移,因而黑色页岩中的RSM富集度被广泛用于古沉积环境和水体化学条件的研究。近年来不仅建立了判定海水氧化还原(redox)状态的定量指标体系,而且成功地将RSM用于盆地古水文条件重建和大气氧化水平分析,特别是Mo-U富集机理及其共变应用研究最为迅速。沉积物的Mo-U富集度主要受海水Mo-U浓度以及水体氧化还原状态和盆地水文特征的控制。U在Fe(Ⅲ)-Fe(Ⅱ)还原带活化并向沉积物中转移,而Mo仅当海水中含有H2S时才向沉积物富集,因而可据Mo和U富集度的差别区分水体缺氧与硫化特征。沉积物中Mo/TOC比可大致反映沉积水体中的Mo浓度,故可用于判定同时代不同古地理条件下沉积水体的局限程度。利用Mo-U的差异性富集机制还可区分弱氧化-缺氧-硫化水体特征,并判断是否存在Mo的颗粒传输。作者对Mo-U共变用于古沉积环境的氧化还原条件分析的进展、问题进行评述,并对可能的发展方向提出展望。

The enrichment of redox sensitive trace metals （RSM）in organic-rich black shales has been widely used to track the evolution of palaeoenvironments and water chemistry due to the contrasting behavior in oxic （conservative and soluble）and anoxic（euxinic）（active and insoluble）conditions.Recently,great progress on RSM study has been made in terms of quantitative index of seawater redox state,reconstruction of ancient basin paleohydrology,evaluation of atmospheric oxygen level,and particularly the mechanism and application of Mo-U covariation.The enrichment of Mo-U in sediments is mainly controlled by their concentration in seawater, redox state of seawater,and basin paleohydrology.Activating and transferring of U from water to sediments be-gin at Fe（Ⅲ）-Fe（Ⅱ）reduction boundary （suboxic zone）,while Mo is enriched in sediments only when soluable sulfides exist in the water mass,thus this discrepancy of Mo-U behavior can be used to differentiate seawater anoxia or euxinia.Mo/TOC ratio in sediments is another important parameter that has been used widely to determine the degree of deepwater restriction in several Phanerozoic sedimentary basins because it scales on the first-order with aqueous Mo concentration.Based on different enrichment mechanisms of Mo and U,we can also distinguish suboxic-anoxic-euxinic conditions of water mass,and determine whether there is particulate shuttle of Mo.In this paper,we reviewed the progress and potential problems in redox analysis using Mo-U covariation in ancient sediments,and put forward some tentative suggestions for future study.