From the Hå kon Mosby Mud Volcano (HMMV) on the southwest Barents Sea shelf, gas and fluids are expelled by active mud volcanism. We studied the mass transfer phenomena and microbial conversions in the surface layers using in situ microsensor measurements and on retrieved cores. The HMMV consists of three concentric habitats: a central area with gray mud, a surrounding area covered by white mats of big sulfide oxidizing filamentous bacteria (Beggiatoa), and a peripheral area colonized by symbiontic tube worms (Pogonophora). A fourth habitat comprised gray microbial mats near gas seeps. The differences between these four methane-fueled habitats are best explained by different transport rates of sulfate into the sediments and porewater upflow rates. The upflow velocities were estimated by two independent methods at 3-6 m yr 21 in the central area and 0.3-1 m yr 21 in Beggiatoa mats. In the central area no sulfide was found, indicating that the rapidly rising sulfate-free fluids caused sulfate limitation that inhibited anaerobic oxidation of methane (AOM). Under Beggiatoa mats a steep sulfide peak was found at 2 to 3 cm below the seafloor (bsf), most likely due to AOM. All sulfide was oxidized anaerobically, possibly through nitrate reduction by Beggiatoa. The Beggiatoa mats were dominated by a single filamentous morphotype with a diameter of 10 mm and abundant sulfur inclusions. A high diversity of sulfide oxidizer morphotypes was observed in a grayish microbial mat near gas vents, where aerobic sulfide oxidation was important. The sediments colonized by Pogonophora were influenced by bioventilation, allowing sulfate penetration and AOM to 70 cm bsf. The HMMV is a unique and diverse ecosystem, the structure and functioning of which is mainly controlled by pore-water flow.Interest in anaerobic oxidation of methane (AOM) and its linkage to sulfate reduction was strongly stimulated by the recent discovery of the microorganisms involved (Boetius et al. 2000;Michaelis et al. 2002;Orphan et al. 2002). Evidence was presented that consortia of methanotrophic archaea and sulfate-reducing bacteria are responsible for the process. These microorganisms were found in high abundance in methane-rich sediments above gas hydrates and various types of cold seeps. The microbial conversion of methane and sulfate to CO 2 and sulfide in surface sediments is usually accompanied by sulfide oxidation (SO) by free-living and symbiotic bacteria.1 Corresponding author (dbeer@mpi-bremen.de).

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SUMMARY New geophysical data have become available from shipborne and satellite measurements allowing a re‐evaluation of the largely unknown junction of the Arctic spreading centre and the northeastern Siberian continental margin where the transpolar mid‐ocean Gakkel Ridge abuts against the continental slope of the Laptev Sea. Based on multichannel seismic reflection and gravity data, this sediment‐covered spreading axis can be traced to the continental rise where it is cut‐off by a transcurrent fault. Further continuation of the extensional axis into the continental slope can be attributed to two asymmetric grabens, which terminate against the prominent Khatanga–Lomonosov Fracture. Remnants of hydrothermal fauna and high heat‐flow values of approximately 100 mW m−2 documented around these grabens in the up‐slope area are typical for an oceanic spreading axis. Thus we consider these grabens to be morphotectonic termination of the global Atlantic–Arctic spreading system with plate motions shifting to the Khatanga–Lomonosov Fracture. The high heat flow and the distribution of earthquake epicentres allow us to assume that the present‐day divergent plate tectonic boundary passes from the Gakkel Ridge to the eastern part of Laptev Sea with an offset of initial rifting along the Bel'kov–Svyatoi Nos Rift to the projected prolongation of the buried spreading axis by 140–150 km.

On the basis of heat-flow measurements, seismic mapping, and sediment pore-water analysis, we demonstrate widespread and efficient ventilation of the 18-22 Ma oceanic crust of the northeast equatorial Pacific Ocean. Recharge and discharge appear to be associated with basement outcrops, including seamounts and north-south-trending faults, along which sediment cover thins out and volcanic rocks are exposed. Low-temperature hydrothermal circulation through the volcanic crust leads to the reduction of heat flow through overlying sediments, with measured heat-flow values that are well below those expected from conductive cooling curves for lithosphere of this age. Typically, dissolved pore-water oxygen decreases from the sediment surface downward, reaching minimum values at mid-depth and rising again in the lower part of the cores investigated, clearly indicating oxygen-rich seawater circulation through the oceanic crust underneath the sediments. If the residence time of the circulating fluids in the upper crust is short or the fluid flux is large, oxic conditions may be preserved, and oxygen can diffuse upwards into the sediments. This process, leading to widespread oxic conditions in the near-basement sediments, may cause the oxidation of residual reduced material stored in the deeper sediments, resulting in downward fluxes of the reaction products into the basement and from there back into the oceans. Considering the widespread existence of this type of off-axis ventilation, the net effect of the resulting return flow of reaction products on biogeochemical cycles and element fluxes (e.g., carbon and nitrogen) may be very large.

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