Redox-sensitive trace metals as paleoredox proxies: A review and analysis of data from modern sediments

Redox-sensitive trace metals have been used extensively as geochemical proxies to infer the redox-status of marine sediments at the time of their deposition, and by extension, the concentration of oxygen in the overlying water and atmosphere. However, to reliably apply these paleoredox proxies in ancient sediment samples we must calibrate their geochemical behavior in modern sedimentary environments with known redox-status. Here we report a new compilation and analysis of modern trace-metal enrichment data for a range of marine depositional environments, including euxinic basins, continental margin upwelling settings, and normal oxic settings. The enrichments (i.e., concentrations normalized to aluminum content) of vanadium, uranium, molybdenum and rhenium (the VUMoRe database) in the various depositional categories were analyzed using receiver operating characteristic (ROC) curve analysis to systematically identify threshold values that differentiate the various settings. The enrichment of both Mo (> 5 mu g g(-1)/%) and V (> 23 mu g g(-1)/%), but with V not exceeding 46 mu g g(-1)/%, provides strong evidence for a euxinic basin-type depositional environment. Furthermore, the enrichment of V (> 46 mu g g(-1)/%), U (> 5 mu g g(-1)/%) and Mo (> 5 mu g g(-1)/%) is strong evidence for sediments depositing within the anoxic core of perennial OMZ environments, whereas the enrichment of U (> 1 mu g g(-1)/%) coinciding with a low enrichment of V (< 23 mu g g(-1)/%) and Mo (< 5 mu g g(-1)/%) is strong evidence of sediment deposition in the oxic water beneath the core of a perennial OMZ environment. The new method we describe here for determining enrichment thresholds of trace metal-based redox proxies will support the reliable classification of ancient depositional environments and the ongoing development of trace metals as paleoredox proxies.