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).

Submarine groundwater discharge (SGD) from subseafloor aquifers, through muddy sediments, was studied in Eckernförde Bay (western Baltic Sea). The fluid discharge was clearly traced by 222 Rn enrichment in the water column and by the chloride profiles in pore water. At several sites, a considerable decrease in chloride, to levels less than 10% of bottom-water concentrations, was observed within the upper few centimeters of sediment. Studies at 196 sites revealed that Ͼ22% of the seafloor of the bay area was affected by freshwater admixture and active fluid venting. A maximal discharge rate of Ͼ9 L m Ϫ2 d Ϫ1 was computed by modeling pore water profiles. Based on pore water data, the freshwater flow from subseafloor aquifers to Eckernförde Bay was estimated to range from 4 ϫ 10 6 to 57 ϫ 10 6 m 3 yr Ϫ1 . Therefore, 0.3-4.1% of the water volume of the bay is replaced each year. Owing to negligible surface runoff by rivers, SGD is a significant pathway within the hydrological cycle of this coastal zone. High-resolution bathymetric data and side-scan sonar surveys of pockmarks, depressions up to 300 m long, were obtained by using an autonomous underwater vehicle. Steep edges, with depths increasing by more than 2 m within 8-10 m in lateral directions, equivalent to slopes with an angle of as much as 11Њ, were observed. The formation of pockmarks within muddy sediments is suggested to be caused by the interaction between sediment fluidization and bottom currents. Fluid discharge from glacial coastal sediments covered by mud deposits is probably a widespread, but easily overlooked, pathway affecting the cycle of methane and dissolved constituents to coastal waters of the Baltic Sea.For coastal areas, Sonrel (1868) reported the discharge of freshwater from submarine springs and speculated on their use and risks for sailors. Since then a few studies have considered the importance of fluid discharge from sediments for nutrient budgets of coastal environments, formation of offshore plankton blooms, hydrological cycles, or the release of trace elements and gases such as radon from the seafloor (Johannes 1980;Valiela et al. 1990;Moore 1996;Cable et al. 1997;Laroche et al. 1997). For the South Atlantic Bight, on the eastern coast of the United States, Moore (1996) deduced that the quantity of submarine groundwater discharge (SGD; here taken as fresh plus salt water) represented 40% of the river input into the study area. Although these figures probably overestimate the contribution of submarine seepage 1 Corresponding author (mschlueter@awi-bremerhaven.de). AcknowledgmentsWe thank the captains and crews of RVs Littorina, A. v. Humbold, and Alkor for their support during several cruises. We are grateful to W. Lemke and J. Harff from the Institute of Baltic Research (IOW, Warnemünde) for providing the vibro corer system. Jayne Wolf-Welling is gratefully acknowledged for improving the English text. We thank DeBeers Marines/Maridan for conducting the AUV dives. Application of GIS was supported by Dr. A. Schäf-er. The pap...

The past decades have seen remarkable changes in the Arctic, a hotspot for climate change. Nevertheless, impacts of such changes on the biogeochemical cycles and Arctic marine ecosystems are still largely unknown. During cruises to the deep-sea observatory HAUSGARTEN in July 2007 and 2008, we investigated the biogeochemical recycling of organic matter in Arctic margin sediments by performing shipboard measurements of oxygen profiles, bacterial activities and biogenic sediment compounds (pigment, protein, organic carbon, and phospholipid contents). Additional in situ oxygen profiles were performed at two sites. This study aims at characterizing benthic mineralization activity along local bathymetric and latitudinal transects. The spatial coverage of this study is unique since it focuses on the transition from shelf to Deep Ocean, and from close to the ice edge to more open waters. Biogeochemical recycling across the continental margin showed a classical bathymetric pattern with overall low fluxes except for the deepest station located in the Molloy Hole (5500 m), a seafloor depression acting as an organic matter depot center. A gradient in benthic mineralization rates arises along the latitudinal transect with clearly higher values at the southern stations (average diffusive oxygen uptake of 0.49 ± 0.18 mmol O2 m-2 d-1) compared to the northern sites (0.22 ± 0.09 mmol O2 m-2 d-1). The benthic mineralization activity at the HAUSGARTEN observatory thus increases southward and appears to reflect the amount of organic matter reaching the seafloor rather than its lability. Although organic matter content and potential bacterial activity clearly follow this gradient, sediment pigments and phospholipids exhibit no increase with latitude whereas satellite images of surface ocean chlorophyll a indicate local seasonal patterns of primary production. Our results suggest that predicted increases in primary production in the Arctic Ocean could induce a larger export of more refractory organic matter due to the longer production season and the extension of the ice-free zone.

The transfer of organic carbon from surface waters to the seafloor was calculated for the northern North Atlantic east of Greenland. This calculation is based on an empirically derived relationship between the rain rate of remineralizable organic carbon, derived by in situ O2 profiles, water depth, and primary production. The reliability of this attempt is supported by the good correspondence of calculated rain rates with an independent data set of particle trap studies and shipboard measurements of O2 profiles. For water depths of > 500 m the total seafloor remineralization rate is 2.7 × 106 tC yr−1 for the northern North Atlantic. Low and nearly similar average rain rates of 0.60 and 0.65 gC m−2 yr−1 have been derived for the deep basins of the Norwegian and Greenland Seas. Therefore, 1.7–1.8% of the primary production is transferred to the seafloor of the basins. A considerably higher average flux of 3.8 gC m−2 yr−1 was calculated for the Iceland Plateau, where ∼3.3% of primary produced organic carbon reaches the seafloor. The sediments of the Iceland Plateau receive 1.0 × 106 tC yr−1 or ∼37% of the organic carbon rain rate to the seafloor derived for the entire northern North Atlantic. The transfer of primary produced organic carbon below water depths of 500 and 1000 m suggests that 10.3 × 106 tC yr−1 and 4.5 × 106 tC yr−1are exported from surface waters. This is 2‐4.4% of the organic carbon produced in the photic zone of the northern North Atlantic east of Greenland.

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