Implication From Mineralogical and Geochemical Characteristics of Authigenic Micronodules in the Haima Cold Seeps for Understanding the Manganese Geochemistry in Active Seepage

Abstract

The multi‐valent manganese is ubiquitous in various marine environments and is involved in many biogeochemical processes with electron transfers between different oxidation states. The involvement of Mn (IV) in the anaerobic oxidation of methane (AOM) in cold seeps has long been proposed by experimental simulation and was suggested to serve as an important sink of methane in the paleo‐oceanic environment. Nonetheless, metal‐dependent AOM processes still lack explicit geological evidence. Meanwhile, research on micronodules gained momentum in recent years, but the authigenic manganese micronodules in active cold seeps were yet not well studied. In this study, Mn‐rich sediments were discovered at the active Haima cold seeps area in the South China Sea, from which Mn‐micronodules were then extracted. These micronodules are irregular or subspheroidal with rough and porous surfaces and are mainly composed of 7 Å and 10 Å phyllomanganates. They are enriched in Mn, and deficient in Fe, Co, Ni, Cu, and REYs. Characteristic microbial‐like mineralization structures are also observed. Based on these mineralogical/geochemical features and the abnormally high growth rate (calculated by the cobalt chronometer algorithm), these authigenic manganese micronodules were probably formed rapidly via biomineralization and chemical precipitation and were supplied with sufficient Mn by the metal‐rich seepage fluids. The occurrence of these micronodules demonstrates manganese bacterial activities, and indirectly reflects metal‐dependent AOM.