Petrographical and Geochemical Signatures Linked to Fe/Mn Reduction in Subsurface Marine Sediments from the Hydrate-Bearing Area, Dongsha, the South China Sea

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.

Metal-Driven Anaerobic Oxidation of Methane as an Important Methane Sink in Methanic Cold Seep Sediments

“…A large number of findings have since been emerging about the biogeochemistry of cold seep carbonates, benthos, and sediments in the Haima cold seep ( 30 – 33 ). Massive amounts of terrigenous metal oxides are supplied into the continental slope of the South China Sea from rivers ( 34 ). Consequently, iron/manganese-containing minerals are part of the major components in the sediments of this region with high-flux methane seeps, rendering it a natural laboratory to investigate the role of metal oxides in the methane cycle.…”

Metal-driven anaerobic oxidation of methane as an important methane sink in methanic cold seep sediments

“…A large number of findings have since been emerging about the biogeochemistry of cold-seep carbonates, benthos and sediments in Haima cold seep [29][30][31][32] . Massive amounts of terrigenous metal oxides are supplied into the continental slope of the South China Sea from rivers 33 . Consequently, iron/manganese-containing minerals are part of the major components in the sediments of this region with high-flux methane seeps, rendering it be such a natural laboratory to investigate the role of metal oxides in methane cycle.…”