Manganese fluxes to the oceans

Abstract

Application of a simple model describing regional variations in the contents of manganese and associated minor metals in deep-sea sediments suggests that solid manganese phases are being removed from the <0.5 μm fraction of seawater at ∼1–7 · 10 12g yr −1 in excess of the rate of stream-supplied manganese. This flux is consistent with: (1) the relative rates of sediment accumulation in the Atlantic and Pacific Oceans; (2) the contrast between the oceanic residence time of manganese calculated from stream-supply data (14 · 10 3 yr) and from the flux of manganese precipitating in marine sediments or as manganese nodules (0.38–2.4 · 10 3 yr); (3) the surplus mass of manganese revealed by geochemical balance calculations (22.9 · 10 2g). On this basis excess manganese is accumulating in deep-sea sediments at 0.2–2.0 · 10 −6 g cm −2 yr −1. Manganese supplied to the upper layers of marine sediments by diagenesis has been evaluated with the aid of vertical advection—diffusion—reaction models. The calculated diagenetic flux of manganese at the sediment surface in a near-shore environment is in agreement with the known accretion rate of manganese deposits (1.7 · 10 −2 g cm −2 10 −3 yr −1) and the regionally variable flux over the area assessed is consistent with the presence or absence of manganese nodules at or near the water-sediment interface. The diagenetic flux at the surface of deep-sea sediments has been calculated at 0.7 · 10 −4 g cm −2 10 −3 yr −1 when the upper, oxic, zone of the sediment is ∼20 cm thick. A limiting factor on the in situ production flux of dissolved manganese in deep-sea sediments appears to be the availability of reducing agents for manganese dissolution rather than the rate of downward transport of manganese-rich sediment to a reaction boundary where dissolution takes place. Various estimates of the rate of upward-migrating manganese suggest that manganese precipitates in the oxic zone with a rate constant of ∼10 −7 sec −1 with the result that diagenetic processes cannot supply the flux of excess manganese through more than ∼0.25 m of oxic sediment. However, estimates of the flux of manganese to the oceans by submarine volcanic processes (0.79–1.1 · 10 12g yr −1) are similar to the surplus mass of manganese detected by geochemical balance calculations (0.7 · 10 12g yr −1). If submarine hydrothermal solutions provide only 10% of this excess then their computed discharge rate (39 g cm −2 yr −1) and residence time in the upper layer of oceanic crust (130,000 yr) agree well with these parameters for continental thermal springs.