seafloor, but the extent of its subsurface penetration could not 

 be determined from the video observation only. 



Other carbonate edifices rise up to 0.6 meter above the 

 seafloor and many have a constricted stem just above the 

 sediment-water interface and a bulbous thicker structure above it 

 with numerous intertwined tube sections which appear to be 

 conduits for venting fluids. (Using these criteria, we were able 

 to differentiate normal sedimentary boulders and rock fragments 

 on the seafloor from those constructional carbonate features 

 related to fluid vents.) The surfaces of the carbonate edifices 

 are covered with numerous holes, borings and sharp protrusions 

 which harbor benthic organisms, such as chitons, worms and 

 solitary and encrusting corals. 



One of these carbonate edifices was recovered intact in an 

 otter trawl (Fig. 9). This structure (weighing 200 lbs) showed a 

 clear demarkation line separating a chalky, whitish upper section 

 covered with knobs and irregular protrusions to a sulf ide-stained 

 black lower section having essentially the same surface features. 

 Iron oxide stains near the color boundary indicate that it marks 

 the oxic-reducing transition which, in the area of investigation, 

 roughly coincides with the sediment-water interface. This 

 indicates that the lower portion of the structure was buried in 

 the sediment; the buried section is reminiscent of the 

 cylindrical chimneys recovered by the fishing vessel Kodiak . 

 However, in the upper portion of the edifice there is a profusion 

 of cemented tubes which are exceedingly complex in shape, but 

 they clearly indicate a plugged (cemented) plumbing system. 

 Several generations of interior and exterior cement can be 

 recognized, notably a white fibrous carbonate cement which fills 

 the last remaining voids within the tubes (Fig. 10). Another 

 generation of cement is a 5-mm thick botryoidal cement with amber 

 colored laminae which covers the exterior of the tubes and the 

 interstices between adjoining tube segments (Fig. 11). 



The bulk of the chimney as well as the walls of the 

 plumbing system consist of aragonite and/or Mg-calcite cemented 

 glauconite sand. The insoluble fraction of two representative 

 samples ranges from 29 to 68 wt.-%, and consists of well-sorted 

 glauconitic grains (0.063 to 0.25 mm in diameter) and some small 

 fraction of clays (Table 2). The dark glauconitic groundmass 

 generally consists of aragonite and Mg-calcite or just aragonite. 

 The gray cement is usually Mg-calcite while the white cement is 

 aragonite . 



The stable isotopic composition of the irregular carbonate 

 edifice shows that samples 1 through 5 taken from this chimney 

 structure (Fig. 9) are highly depleted in carbon-13 and range 

 from 5 13C -48.27 to -53.31 PDB (Table 3). The oxygen values are 

 positive, ranging from 6 180 +4.19 to +4.84 PDB (Table 3). The 

 highly depleted carbon-13 values indicate a methane and CO2- 



165 



