(Table 1) Summary of sampling points described and main petrographic, morphological and geochemical characteristics

Targeted sampling on the Dolgovskoy Mound (northern Shatsky Ridge) revealed the presence of spectacular laterally extensive and differently shaped authigenic carbonates. The sampling stations were selected based on sidescan sonar and profiler images that show patchy backscatter and irregular and discontinuous reflections in the near subsurface. The interpretation of acoustic data from the top part of the mound supports the seafloor observations and the sampling that revealed the presence of a complex subsurface plumbing system characterized by carbonates and gas. The crusts sampled consist of carbonate cemented layered hemipelagic sedimentary Unit 1 associated with several centimetres thick microbial mats. Three different carbonate morphologies were observed: (a) tabular slabs, (b) subsurface cavernous carbonates consisting of void chambers up to 20 cm**3 in size and (c) chimney and tubular conduits vertically oriented or forming a subhorizontal network in the subsurface. The methanogenic origin of the carbonates is established based on visual observations of fluids seepage structures, 13C depletion of the carbonates (d13C varying between -36.7 per mil and -27.4 per mil), and by thin carbonate layers present within the thick microbial mats. Laboratory experiments with a Hele-Shaw cell were conducted in order to simulate the gas seepage through contrasting grain size media present on the seafloor. Combined petrography, visual observations and sandbox simulations allowed a characterization of the dynamics and the structures of the plumbing system in the near subsurface. Based on sample observations and the experiments, three observed morphologies of authigenic carbonates are interpreted, respectively, as (a) Darcian porous flow through the finely laminated clayey/coccolith-rich layers, (b) gas accumulation chambers at sites where significant fluid escape was impeded by thicker clayey layers forming the laminated Unit1 and (c) focussed vertical fluid venting and subhorizontal migration of overpressured fluids released from (b). The Hele-Shaw cell experiments represent a promising tool for investigating shallow fluid flow pathways in marine systems.

(Table 1) U and Th isotope analyses of pyrite and anhydrite of ODP Site 158-957

Hydrothermal circulation at oceanic spreading ridges causes sea water to penetrate to depths of 2 to 3 km in the oceanic crust where it is heated to ~400 °C before venting at spectacular 'black smokers'. These hydrothermal systems exert a strong influence on ocean chemistry (Edmond et al., 1979, doi:10.1016/0012-821X(79)90061-X), yet their structure, longevity and magnitude remain largely unresolved (Elderfield and Schultz., 1996, doi:10.1146/annurev.earth.24.1.191). The active Transatlantic Geotraverse (TAG) deposit, at 26° N on the Mid-Atlantic Ridge, is one of the largest, oldest and most intensively studied of the massive sulphide mounds that accumulate beneath black-smoker fields. Here we report ages of sulphides and anhydrites from the recently drilled (Humphris et al., 1995, doi:10.1038/377713a0) TAG substrate structures -determined from 234U-230Th systematics analysed by thermal ionization mass spectrometry. The new precise ages combined with existing data (Lalou et al., 1993, doi:10.1029/92JB01898; 1998, doi:10.2973/odp.proc.sr.158.214.1998) show that the oldest material (11,000 to 37,000 years old) forms a layer across the centre of the deposit with younger material (2,300-7,800 years old) both above and below. This stratigraphy confirms that much of the sulphide and anhydrite are precipitated within the mound by mixing of entrained sea water with hydrothermal fluid (James and Elderfield, 1996, doi:10.1130/0091-7613(1996)024<1147:COOFFA>2.3.CO;2). The age distribution is consistent with episodic activity of the hydrothermal system recurring at intervals of up to 2,000 years.