The unusual chemistry of molybdenum (Mo) makes this trace element interesting to both geochemists and biochemists. Geochemically, Mo is relatively unreactive in oxygenated, aqueous solutions, and hence is a nominally conservative element in the oceans. In fact, Mo is removed so slowly from seawater that it is the most abundant transition metal in the oceans despite being a ppm-level constituent of the crust. In contrast, Mo is readily removed from solution in anoxic-sulfidic (“euxinic”) settings, so that Mo enrichments in sediments are considered diagnostic of reducing depositional conditions. Few elements possess such bimodal redox behavior at the Earth’s surface.