Down the thermodynamic ladder: A comparative study of marine redox gradients across diverse sedimentary environments

The thermodynamic succession of electron acceptors used in microbial oxidation of organic matter and associated redox products lead to the vertical chemical zonation in diffusion-controlled marine sediments. The structure of this gradient is a key player in the recycling of organic carbon and nutrients in the marine environment. However, little attention has been given to the advection-dominated sedimentary systems where the co-existence of different reduced and oxidized chemical species is expected. In this study, performing multi-analyte Au/Hg voltammetric microelectrode profiling on cores from diverse sediment depositional settings in the Mediterranean Sea (lagoon, coastal marine, submarine canyon and shallow-water vent sediments), it is shown that there exists an excess of reducing species (H2S, Mn2+, Fe2+ and FeSaq) in the advective shallow vent sediments compared to similar upper sediment sections of diffusion-controlled environments. The ex-situ upward diffusive flux of reduced species accounted for at least 26 percent of oxygen consumption in the shallow water vent sediments, and this figure is likely higher when in-situ advective fluxes and the oxidation of other known vent-derived reduced components are taken into account. In contrast, the upward fluxes of reduced species do not significantly contribute to oxygen consumption in the studied lagoon, coastal or submarine canyon sediments, where organic matter oxidation by oxygen likely dominates. A major implication of these findings is that the reduced substrate advection in shallow vents leads to a potential for chemoautotrophy in an otherwise oligotrophic marine system, whereas in diffusion-controlled sediments the vertical input of organic matter and its heterotrophic use is more important.