Diagenesis of Fe and S in Amazon inner shelf muds: apparent dominance of Fe reduction and implications for the genesis of ironstones

It is commonly observed that SO₄^2- reduction dominates the anaerobic decomposition of organic matter and determines redox properties in organic-rich shelf environments. Some areas of inner shelf muds near the mouth of the Amazon River are apparent exceptions to such a generalization. These deposits are sufficiently rich in organic carbon (0.6 ± 0.1%) to support NH₄⁺ production rates in the upper 0-20 cm of 0.4-6 mmol m^-2 day^-1 (average of 3.0 ± 0.3), comparable to reactive sediments on other shelves. Anoxic conditions usually occur near the sediment-water interface, as evidenced by the absence of NO₃^- and the presence of dissolved Mn and Fe a few centimeters below the surface. In contrast to other similarly reactive deposits, Fe-reduction zones in Amazon shelf muds (and presumably other comparable environments) are often vertically extensive (>∼1 m) and characterized by high concentrations of dissolved Fe (0.3-0.7 mM) and elevated levels of HCO₃^- (12-14 meq l^-1). These extensive Fe-reduction zones occur with or without evidence of significant SO₄^2- depletion (surface values ∼20-28 mM) in essentially unbioturbated deposits. Total solid phase S seldom exceeds 0.1% weight (often <0.03%), indicating little net S precipitation in the upper ∼2 m. This produces an initial deposit with a fresh water S/C diagenetic signature. Physical reworking of the sea floor periodically regenerates oxidized Fe and Mn compounds. Solid phase oxidant regeneration superimposed upon a relatively fixed C input produces a high reactive Fe/C ratio (Fe average = 5.3 ± 0.8%; range 4->7) and allows Fe and, to a lesser extent, Mn reduction to dominate the sediment redox properties. Pore waters are supersaturated with respect to a variety of authigenic minerals including siderite and vivianite, often typical of nonsulfidic sediments and low SO₄^2- environments. Authigenic siderite was also observed in SEM photographs. These Fe²⁺-generating deposits are interbedded with or lie up-current from Fe-coated quartz and 'glauconitic' sands (superficial oolites?), implying Fe precipitation on sand nuclei during mud/sand mixing or lateral export and segregation of Fe colloids from disturbed muds. The overall facies relations and authigenic mineral suites are, in many ways, similar to Paleozoic oolitic ironstones and may represent an incipient or actual analogue.