Abstract
Fe and Mn oxides and (oxy)-hydroxides are the most abundant solid-phase electron acceptors in marine sediments, and dissimilatory Fe/Mn reduction usually links with the anaerobic oxidation of methane (AOM) and organic matter oxidation (OMO) in sediments. In this study, we report the results from subsurface marine sediments in the Dongsha hydrate-bearing area in the South China Sea. The petrological and geochemical signatures show that the Fe/Mn reduction mediated by AOM and OMO might occur in sediments above the sulfate-methane transition zone. X-ray diffraction and scanning electron microscopy analyses of sediments indicate that Fe(III)/Mn(IV)-oxides and authigenic carbonate minerals coexisted in the Fe/Mn reduction zone. The lower δ13C values of dissolved inorganic carbon, coupled with an evident increase in total inorganic carbon contents and a decrease in Ca2+ and Mg2+ concentrations indicate the onset of AOM in this zone, and the greater variation of PO43− and NH4+ concentrations in pore water suggests the higher OMO rates in subsurface sediments. Geochemical and mineralogical analyses suggest that the previously buried Fe(III)/Mn(IV) oxides might be activated and lead to the onset of Fe/Mn reduction induced by AOM and OMO. These findings may extend our understanding of the biogeochemical processes involved in Fe/Mn reduction in continental shelves with abundant methane, organic matter, and terrigenous metal oxides.
Highlights
Iron (Fe) and manganese (Mn) oxides and-hydroxides are the most abundant solid-phase electron acceptors in marine sediments [1]
The petrographical and geochemical signatures indicate the coexistence between metal-driven
The profiles of the methane and sulfate concentrations in pore water suggest that the current sulfate-methane transition zone (SMTZ) depth in this core is around 600 cm
Summary
Iron (Fe) and manganese (Mn) oxides and (oxy)-hydroxides are the most abundant solid-phase electron acceptors in marine sediments [1]. Minerals 2019, 9, 624 a sequence of reduction processes involving oxygen, nitrite, nitrate, high-valence manganese (Mn4+ ). Iron (Fe3+ ) oxides, and sulfate above the sulfate-methane transition zone (SMTZ) in sediments [3,4,5,6]. Metal reduction, coupled with OMO takes place mostly in suboxic zones [5,7]. In sediments from the Skagerrak, Panama Basin, Black Sea, and the northern Barents Sea, Mn reduction was a dominant process coupled with anaerobic carbon oxidation at a depth of 10 cm in the upper sediments below the free oxygen zone [8,9,10,11,12]. Reyes et al summarized that the bacterial community had been shown to link with the ongoing Fe reduction at the top 30 cm depth in freshwater and marine sediments [13,14,15,16,17]
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