The behaviour of molybdenum and manganese during early sediment diagenesis — offshore Baja California, Mexico

One oceanic and three hemipelagic box cores, taken off Baja California, reveal a surficial enrichment of Mn and Mo. Detailed investigation of the geochemical changes with depth in these cores indicates a loss of these elements during suboxic diagenesis. The loss of Mn differs from that of Mo. An assessment of the oxidation state (MnO x ) of Mn with depth reveals, in all cores, a lower oxidation state in the surficial 3 cm compared with the sediment below. We interpret this as being due to hausmannite (Mn 3O 4) precipitation within surface sediments. Within the redoxcline of the hemipelagic sediments, MnO x shows a gradual decrease from ∼ 1.85–1.90 at the surface to ∼ 1.00 at depth, which appears to be caused by partial dissolution of Mn 4+ oxyhydroxides and sorption of Mn 2+ onto remaining oxides. A mixed MnCa carbonate was observed at the base of two cores. The distribution of Mo with depth shows a more accentuated decrease in concentration at shallower depth compared with Mn. These trends suggest a reduction of Mo(VI) on or in Mn oxyhydroxides during suboxic diagenesis, with no attendant uptake of Mo 3+ onto remaining oxides. Surface, hausmannite-containing oxides, also show lower Mo Mn ratios. This implies that the content of Mn 4+ oxides, not total Mn, is responsible for MoO 4 2− uptake in the sediment. These features are also revealed from the association of Mo and Mn in other sediments and ferromanganese nodules. The sediment geochemistry of Mo may indicate short-term variations in pore water chemistry, or the influence of Mn diagenesis on Mo-containing shales or ferromanganese nodule formation.