Dynamic interactions between Fe, Mn, S and C cycles in the Okinawa Trough seep sediments

Abstract

The semi-enclosed Okinawa Trough hosts hydrothermal vents and cold seep systems. The authigenic carbonate formation in the Okinawa Trough is thought to be associated with the anaerobic oxidation of methane (AOM) driven by Fe/Mn oxide reduction due to hydrothermal metal supply. However, in seep sediments near hydrothermal vents, Fe, Mn, S and C cycles and their diagenetic interactions are disconnected. Herein, a comprehensive suite of pore water solute concentrations and isotope ratios of four gravity cores (GC02, GC04, GC05, GC07) and three drilling cores (QZ02-C1, QZ03, QZ04) were analyzed to examine the diagenetic processes in the Okinawa Trough seep sediments. The accumulation of total dissolved Fe (DFe) and total dissolved Mn (DMn) in pore water suggested that metal oxide reduction occurred commonly in the sediments. In the Manganous zones, dissimilatory Mn reduction generally increased closer to the hydrothermal field, indicating that hydrothermal Mn supplies promoted organic matter mineralization. In the Sulfidic zones of Sites QZ03 and QZ04, the DMn and HS⁻ contents reached peaks synchronously revealed inorganic Mn reduction coupled with HS⁻ oxidation. At 21–25 m below the seafloor (mbsf) of Site QZ02-C1 and at 0.5 and 1.1 mbsf of Site GC02, the accumulation of DFe in pore water, accompanied by the negative offset in δ¹³C_DIC, suggested Fe-AOM or a Fe-derived cryptic S cycle in the Sulfidic zones. In the Methanic zone of Site QZ03, high Mg²⁺ consumption with DFe and DMn enrichment and effectively neutralized pore water implied that Fe/Mn oxide reduction could promote dolomite and siderite precipitation. However, the upward migration of deep CO₂ prevented precipitation. The results revealed that hydrothermal metal is crucial in the diagenetic processes in methane seep sediments, e.g., it promotes organic matter degradation, enhances benthic filtering of methane by promoting Fe-rich carbonate precipitation, and alters the geochemical cycles of Mg²⁺, HS⁻, total alkalinity, pH, dissolved inorganic carbon, and CH₄. Overall, this research helps understand Fe, Mn, S and C cycles in back-arc basins and can be important for the CH₄-rich but $S{O}_4^{2-}$-depleted Archaean ocean.