Abstract

Abstract. The interaction between fluid seepage, bottom water redox, and chemosynthetic communities was studied at cold seeps across one of the world's largest oxygen minimum zones (OMZ) located at the Makran convergent continental margin. Push cores were obtained from seeps within and below the core-OMZ with a remotely operated vehicle. Extracted sediment pore water was analyzed for sulfide and sulfate concentrations. Depending on oxygen availability in the bottom water, seeps were either colonized by microbial mats or by mats and macrofauna. The latter, including ampharetid polychaetes and vesicomyid clams, occurred in distinct benthic habitats, which were arranged in a concentric fashion around gas orifices. At most sites colonized by microbial mats, hydrogen sulfide was exported into the bottom water. Where macrofauna was widely abundant, hydrogen sulfide was retained within the sediment. Numerical modeling of pore water profiles was performed in order to assess rates of fluid advection and bioirrigation. While the magnitude of upward fluid flow decreased from 11 cm yr−1 to <1 cm yr−1 and the sulfate/methane transition (SMT) deepened with increasing distance from the central gas orifice, the fluxes of sulfate into the SMT did not significantly differ (6.6–9.3 mol m−2 yr−1). Depth-integrated rates of bioirrigation increased from 120 cm yr−1 in the central habitat, characterized by microbial mats and sparse macrofauna, to 297 cm yr−1 in the habitat of large and few small vesicomyid clams. These results reveal that chemosynthetic macrofauna inhabiting the outer seep habitats below the core-OMZ efficiently bioirrigate and thus transport sulfate down into the upper 10 to 15 cm of the sediment. In this way the animals deal with the lower upward flux of methane in outer habitats by stimulating rates of anaerobic oxidation of methane (AOM) with sulfate high enough to provide hydrogen sulfide for chemosynthesis. Through bioirrigation, macrofauna engineer their geochemical environment and fuel upward sulfide flux via AOM. Furthermore, due to the introduction of oxygenated bottom water into the sediment via bioirrigation, the depth of the sulfide sink gradually deepens towards outer habitats. We therefore suggest that – in addition to the oxygen levels in the water column, which determine whether macrofaunal communities can develop or not – it is the depth of the SMT and thus of sulfide production that determines which chemosynthetic communities are able to exploit the sulfide at depth. We hypothesize that large vesicomyid clams, by efficiently expanding the sulfate zone down into the sediment, could cut off smaller or less mobile organisms, as e.g. small clams and sulfur bacteria, from the sulfide source.

Highlights

  • Cold seep sites at the seafloor are created typically by the focused upward migration of both dissolved and/or gaseous hydrocarbons

  • Sulfate concentrations at the two backg4r5ound sites (Fig. 3) barely decreased over the sampled sediment interval and represent the non-seep pore water situation in the area, which is governed by diffusion and neither affected by fluid seepage nor shallow hydrocarbon oxidation

  • In the surface sediment layer of orange microbial mat habitats at the core-oxygen minimum zones (OMZ) sites, we found sulfate concentrations well below measured ambient bottom water values (∼31 mM)

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Summary

Introduction

Cold seep sites at the seafloor are created typically by the focused upward migration of both dissolved and/or gaseous hydrocarbons. Numerous studies have demonstrated that the flux of methane and sulfide towards the sediment/water interface determines the microbial and faunal community composition at cold seeps Temperature and salinity were determined during our cruise with a Seabird 911+ CTD equipped with an SBE 43 (Seabird Electronics) oxygen sensor, and temperature and salinity probes. These data were used as input parameters for geochemical modeling and flux calculations

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