Interaction between hydrocarbon seepage, chemosynthetic communities, and bottom water redox at cold seeps of the Makran accretionary prism: insights from habitat-specific pore water sampling and modeling

摘要

The interaction between fluid seepage, bottom water redox, andchemosynthetic communities was studied at cold seeps across one of theworld's largest oxygen minimum zones (OMZ) located at the Makran convergentcontinental margin. Push cores were obtained from seeps within and below thecore-OMZ with a remotely operated vehicle. Extracted sediment pore water wasanalyzed for sulfide and sulfate concentrations. Depending on oxygen availabilityin the bottom water, seeps were either colonized by microbial mats or bymats and macrofauna. The latter, including ampharetid polychaetes andvesicomyid clams, occurred in distinct benthic habitats, which were arrangedin a concentric fashion around gas orifices. At most sites colonized bymicrobial mats, hydrogen sulfide was exported into the bottom water. Wheremacrofauna was widely abundant, hydrogen sulfide was retained within thesediment.Numerical modeling of pore water profiles was performed in order to assessrates of fluid advection and bioirrigation. While the magnitude of upwardfluid flow decreased from 11 cm yr?1 to <1 cm yr?1 and thesulfate/methane transition (SMT) deepened with increasing distance from thecentral gas orifice, the fluxes of sulfate into the SMT did notsignificantly differ (6.6–9.3 mol m?2 yr?1). Depth-integratedrates of bioirrigation increased from 120 cm yr?1 in the centralhabitat, characterized by microbial mats and sparse macrofauna, to 297 cm yr?1in the habitat of large and few small vesicomyid clams. Theseresults reveal that chemosynthetic macrofauna inhabiting the outer seephabitats below the core-OMZ efficiently bioirrigate and thus transportsulfate down into the upper 10 to 15 cm of the sediment. In this way the animalsdeal with the lower upward flux of methane in outer habitats by stimulatingrates of anaerobic oxidation of methane (AOM) with sulfate high enough toprovide hydrogen sulfide for chemosynthesis. Through bioirrigation,macrofauna engineer their geochemical environment and fuel upward sulfideflux via AOM. Furthermore, due to the introduction of oxygenated bottomwater into the sediment via bioirrigation, the depth of the sulfide sinkgradually deepens towards outer habitats. We therefore suggest that – inaddition to the oxygen levels in the water column, which determine whethermacrofaunal communities can develop or not – it is the depth of the SMT andthus of sulfide production that determines which chemosynthetic communitiesare able to exploit the sulfide at depth. We hypothesize that largevesicomyid clams, by efficiently expanding the sulfate zone down into the sediment,could cut off smaller or less mobile organisms, as e.g. small clams andsulfur bacteria, from the sulfide source.