A comparative study of iron and manganese diagenesis in continental slope and deep sea basin sediments off Uruguay (SW Atlantic)

Abstract

A comparative study of iron and manganese diagenesis in continental slope and deep sea basin sediments off Uruguay (SW Atlantic) Abstract Pore water and solid phase from surface sediments of the continental slope off Uruguay and from the Argentine Basin (southwestern Atlantic) were investigated geochemically to ascribe characteristic early diagenetic reactions of iron and manganese. Solid-phase iron speciation was determined by extractions as well as by Mössbauer spectroscopy. Both methods showed good agreement ( < 6% deviation) for total-Fe speciation. The proportion of easy reducible iron oxyhydroxide relative to total-Fe oxides decreased from the continental slope to the deep sea which is attributed to an increase in crystallinity during transport as well as to a general decrease of iron mobilization. The product of iron reoxidation is Fe oxyhydroxide which made up less than 5% of total Fe. In addition to this fraction, a proportion of smectite bound iron was found to be redox reactive. This fraction made up to 10% of total Fe in sediments of the Argentine Basin and was quantitatively extracted by 1 N HCl. The redox reactive Fe(+II) fraction of smectite was almost completely reoxidized within 24 h under air atmosphere and may therefore considerably contribute to iron redox cycling if bioturbation occurs. In the case of the slope sediments we found concurrent iron and manganese release to pore water. It is not clear whether this is caused by dissimilatory iron and manganese reduction at the same depth or dissimilatory iron reduction alone inducing Mn(+IV) reduction by (abiotic) reaction with released Fe 2+ . The Argentine Basin sediment showed a significant manganese solid-phase enrichment above the denitrification depth despite the absence of a distinct pore-water gradient of Mn. This implies a recent termination of manganese mobilization and thus a non-steady-state situation with respect to sedimentation or to organic carbon burial rate.

Palaeoclimatic changes in the NW Iberian ­Peninsula during the last 3000 years inferred from ­diagenetic ­proxies in the Ría de Muros sedimentary record

“…Similar processes have been described in other Galician Rías (León et al 2004, Álvarez-Iglesias & Rubio 2008) and in similar coastal environments with high concentrations of Fe and Mn and high input of labile organic matter (Sørensen & Jørgensen 1987, Hines et al 1991, Canfield et al 1993, Haese et al 2000. Fig.…”

A Mössbauer spectroscopic study of the iron redox transition in eastern Mediterranean sediments

“…In addition to iron oxides, bacteria can directly couple the reduction of Fe(III) bound in smectites to organic matter oxidation (Kostka et al, 1999). The tan-green color transition in pelagic sediments is indicative of clay-bound Fe(II)/Fe(III) transition (Lyle, 1983;König et al, 1997;Haese et al, 2000). Structural Fe(II) in phyllosilicates can be oxidized by nitrate, as shown for Peru margin sediments (König et al, 1999).…”

Modeling biogeochemical processes in sediments from the Rhône River prodelta area (NW Mediterranean Sea)

“…The most reactive manganese and iron oxide fractions (amorphous oxides) were extracted with an ascorbate reagent (Kostka and Luther, 1994;Anschutz et al, 1998Anschutz et al, , 2000Haese et al, 2000). This reagent consisted of a deaerated solution of 50 g of sodium citrate and 50 g of sodium bicarbonate in 1 l of deionized water to which 20 g of ascorbic acid was slowly added to a final pH of 8.…”