KEYNOTE: MO-TOC COVARIATION IN MODERN ANOXIC MARINE ENVIRONMENTS: IMPLICATIONS FOR ANALYSIS OF PALEOREDOX AND -HYDROGRAPHIC CONDITIONS

Sedimentary Mo concentrations, [Mo]_s, have been widely used as a proxy for benthic redox potential owing to generally strong enrichment in organic-rich marine facies deposited under anoxic conditions.  A detailed analysis of [Mo]_s-TOC covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that (1) [Mo]_s is not related in a simple manner to benthic redox potential, for which it can serve as only a rough indicator, and (2) patterns of [Mo]_s-TOC covariation can provide paleohydrologic information, e.g., regarding the degree of restriction of the subchemoclinal watermass and temporal changes thereof related to deepwater renewal.  These inferences are based on data from four anoxic silled basins (the Black Sea, Framvaren Fjord, Cariaco Basin, and Saanich Inlet) and one upwelling zone (the SW African Shelf).  The silled basins represent a spectrum of degrees of deepwater restriction, as reflected in their residence times (ranging from <10 y for Saanich Inlet to ~2000 y for the Black Sea) and aqueous Mo concentrations ([Mo]_aq, ranging from 80-100% of the seawater concentration for Saanich Inlet to 3-5% for the Black Sea).  Restriction-related differences in water chemistry are recorded in the sediment as variation in the quantity of Mo accumulated per unit organic carbon, [Mo]_s/TOC, which ranges from ~45±5 for Saanich Inlet to ~4.5±1 for the Black Sea.  This reflects control of [Mo]_s by [Mo]_aq, the latter being depleted in silled basins through sedimentation without adequate resupply through deepwater renewal.  Stratigraphic variation in [Mo]_s/TOC ratios for a single environment can provide information about paleohydrologic events.  For example, deepening of the Framvaren Fjord sill at ~1853 A.D. is recorded in the sediment as an initial increase in [Mo]_s and TOC concentrations, followed by a general trend toward higher TOC and lower [Mo]_s as sustained deepwater stagnation depleted dissolved O₂ through respiration and [Mo]_aq through sedimentation.  Consequently, at timescales associated with deepwater renewal in silled basins, [Mo]_s may be positively correlated with benthic redox potential—antithetic to the prevailing paradigm.