Abstract:

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 begin 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.

Key words: redox sensitive trace metal, Mo-U covariation, redox, paleohydrology