INFLUENCE OF GLOBAL SEA LEVEL ON THE PROTEROZOIC OCEANIC MOLYBDENUM INVENTORY

Molybdenum is a critically important micronutrient, whose presence in the global ocean may have played a major role in the early Earth’s evolutionary history (Anbar and Knoll, 2002). Molybdenum is delivered to the ocean under oxic weathering conditions as stable and unreactive MoO₄^2– and is removed by association with Mn-oxides and, with much greater efficiency, by association with organic mater and authigenic sulfide minerals in sulfidic environments. The concentration of molybdenum in the oceans thus reflects a complex function of global redox and the hydrologic conditions that influence the areal extent of euxinic sedimentation.

Recent compilation of Proterozoic [Mo] (Scott et al., 2008) broadly reflects the evolution of global oceanic redox, with [Mo] of euxinic shales rising to >50 ppm only after the Great Oxidation Event, and reaching 100-300 ppm only after terminal Neoproterozoic oceanic oxygenation. By contrast, sparse data from the late Mesoproterozoic (1300-1000 Ma) suggests substantially depleted oceanic [Mo] despite evidence for increased oceanic oxygenation (cf. Kah et al., 2004).

Here we present data from marine shale of the ~1.1 Ga Atar and El Mreiti groups, Mauritania. Epicratonic environments of the El Mreiti Group reflect development of pore-water euxinia under an intermittently oxygenated water column, and record [Mo] <9 ppm. By contrast, deeper-water pericratonic environments of the Atar Group reflect persistent water column euxinia, yet record [Mo] typically <1 ppm. We suggest that globally high sea level in the non-glaciated late Mesoproterozoic may have led directly to the development of a critically depleted oceanic Mo reservoir. Within extensive epeiric seas, broad expanses of the seafloor would have been conducive to development of pore water euxinia, ultimately resulting in the systematic drawdown of [Mo] prior to its delivery to open ocean euxinic environments. In this scenario, even small-scale changes in sea level would enhance or restrict the oxic delivery of molybdenum to the open ocean, resulting in an oceanic molybdenum reservoir that was highly sensitive to local geographic and hydrologic conditions.