GEOCHEMICAL EVIDENCE FOR THE RESTRICTION OF OXIC DEPOSITIONAL ENVIRONMENTS IN THE PROTEROZOIC OCEAN

Cycling of molybdenum on the Earth's surface is intimately related to the redox state of the ocean and atmosphere. Oxidative weathering of continental crust and riverine transport delivers Mo to the ocean where it is compartmentalized into oxic, suboxic and euxinic environments, representing low, intermediate and high rates of Mo removal, respectively. The size of the oceanic reservoir is thus dependent on the weathering flux and the relative importance of these three sinks. This relationship suggests that the evolution of the redox state of the Earth's atmosphere and ocean from an anoxic Archean to an oxic Phanerozoic must have been accompanied by a related evolution of the oceanic Mo reservoir.

In euxinic environments, the magnitude of Mo enrichment and the strength of Mo/TOC coupling track the concentration of Mo in the water column. Where dissolved Mo becomes limited, as in the modern Black Sea, the magnitude of enrichment decreases, and Mo is decoupled from organic matter. Our survey of Proterozoic euxinic black shales demonstrates a similar relationship between Mo concentration and TOC decoupling, consistent with a reduced oceanic Mo reservoir. Numerical modeling of Mo sources and sinks suggests that a reduced oceanic reservoir does not require global euxinia but rather a more general restriction of oxic depostion, a result consistent with the common occurrence of suboxic shales in the Proterozoic. Similarly, the strong Mo enrichment and coupling of Mo/TOC observed in late Neoproterozoic black shales requires a dramatic increase in the size of the oceanic reservoir, reflecting expansion of oxic environments throughout the deep ocean.