Manganese fluxes in the benthic boundary layer1

Abstract

Diagenetic modeling and mass-balance calculations were applied to box-core and sediment-trap data from three stations at 300-400-m depth in the Laurentian Trough to estimate downward and upward fluxes of manganese across the sediment-water interface, fluxes across the redox boundary in the sediment, rates of dissolution and precipitation of manganese, and rates of manganese accumulation.At all stations the cycling of manganese between the oxidizing and the reducing zone of the sediment was quantitatively more important than the cycling between the sediment and the water column. The redox boundary was the site of the largest fluxes. Downward fluxes across this boundary (0.45, 1.23, and 13.9 mmol mm2 d-l) were 3-50 times the rates of sedimentation or accumulation of manganese. The production of dissolved manganese turns over the inventory of particulate manganese in the sediment surface layer in 43-207 days. A small proportion of the dissolved manganese produced (13-29%) escapes the sediment, reprecipitates in the water column and, in part, returns to the sediment. Increased rates of bioturbation increase the rate of internal manganese cycling more than they do the rate of cycling across the sediment surface.

Coastal hypoxia and sediment biogeochemistry

“…As oxygen concentrations in the bottom water decrease, the thin oxygenated surface layer of sediment decreases in thickness with the result that manganese (II) and iron (II) diffusing from more reduced layers at depth are less efficiently oxidized in oxygenated surface layers. Consequently, a large proportion of dissolved manganese and iron can escape to bottom waters (Sundby and Silverberg, 1985;Konovalov et al, 2007;Pakhornova et al, 2007). Less re-oxidation, hence less precipitation of manganese and iron oxides, lowers the recycling efficiency and thus the contribution of metal oxides to organic matter degradation.…”

“…5). This recycling efficiency primarily depends on bottom-water oxygen levels (Wijsman et al, 2001) and rates of bioturbation (Sundby and Silverberg, 1985;Canfield et al, 1993;Thamdrup, 2000). Iron effluxes are maximal under hypoxic conditions because iron is trapped at lower oxygen levels and efficiently recycled and retained within sediments at higher oxygen levels (Pakhomova et al, 2007).…”

“…Particle displacement has many consequences for sediment biogeochemistry: e.g. labile organic matter is transported downward and diluted into a larger pool of refractory carbon and metal oxides delivered to or formed in the surface layer may be transported downward to more reduced layers (Sundby and Silverberg, 1985;Thamdrup et al, 1994;Ferro et al, 2003;Burdige, 2006). Conversely, reduced products such as iron sulphide (pyrite) may be mixed from subsurface to surface layers where they are then oxidized by oxygen, nitrate or other oxidants (Berner and Westrich, 1985;Aller and Rude, 1988;.…”

“…The manganese and iron mobilized from hypoxic sediments are often transported laterally to either more oxic settings where oxidation, precipitation and thus settling occurs or to fully anoxic settings where iron can be trapped by reaction with dissolved sulphide (Sundby and Silverberg, 1985;Wijsman et al, 2001;Severmann et al, 2008). This shuttle of iron and manganese from hypoxic to oxic or anoxic settings results not only in iron and manganese depletion of hypoxic sediments, but also in manganese enrichment in oxic settings (Sundby and Silverberg, 1985) and iron enrichment in anoxic settings (Wijsman et al, 2001;Anderson and Raiswell, 2004;Severmann et al, 2008). Phosphate and trace elements such as arsenic that are intimately linked to iron oxides are consequently also sensitive to bottom-water oxygen levels.…”

Dissimilatory Fe(III) and Mn(IV) Reduction

“…A better technique for estimating in situ rates of Fe(III) and Mn(IV) reduction might be to follow the fate of radiolabeled Fe(III) or Mn(IV) added as a tracer to undisturbed sediment cores (193 In environments in which the zones of Fe(III) and Mn(IV) reduction are broader and better defined, diagenetic modeling based on geochemical profiles of solid and dissolved iron and manganese species (3,29,50,139,163,271,304) has the potential to provide accurate estimates of the rates of Fe(III) and Mn(IV) reduction.…”