Ocean anoxia and the concentrations of molybdenum and vanadium in seawater

Abstract

Molybdenum and vanadium are relatively unreactive in seawater and are preferentially concentrated in sediments overlain by anoxic or near-anoxic waters. We investigate the geochemistry of these metals by interpreting measurements from anoxic basins and sediment porewaters. The concentrations of Mo and V in the water column of anoxic basins are usually lower than in oxic seawater because of uptake into highly anoxic sediments. The mechanism of removal from solution appears to be diffusion across the sediment-water interface and immobilization within the sediments. Measurements from open ocean porewaters imply fluxes of vanadium from sediments that are greater than 10 times the dissolved and particulate river inflow, which is inconsistent with the distribution of vanadium in deep waters of the ocean and previous geochemical studies; this inconsistency shows that early diagenesis of this element is poorly understood. We estimate that the area of ocean sediment at present overlain by anoxic and near-anoxic water is accumulating 25 ± 15% and 8 ± 5% of the river influx of Mo and V, respectively. This fraction is 17 ± 10% for uranium, which has been shown to share some of the redox behavior of Mo and V. A case is made that changes in the concentrations of these elements in the ocean should be sensitive indicators of the portion of the ocean bottom overlain by anoxic seawater through geologic time.