bearing source for the precipitating fluids that created this 

 carbonate structure. 



Field Observations and Sampling: Central Oregon Shelf 



Two submersible dives and two ROV deployments were 

 conducted in the area around the northern end of Heceta Bank (see 

 Fig. 1, symbol HB ) where clams known to harbor chemosynthetic 

 bacteria were recovered during earlier trawling expeditions by 

 benthic biologists (A.G. Carey) at Oregon State University. No 

 carbonate structures were encountered in this area. The seafloor 

 was covered with sediments to the north of the bank and benthic 

 animals, especially sea pens, were quite abundant here. Several 

 dives made on Heceta Bank in another submersible program did not 

 reveal any carbonate structures. Therefore, we conclude that 

 this area of the Oregon shelf probably does not contain any 

 obvious carbonate structures which might be related to fluid 

 venting. 



DISCUSSION 



Comparison of Chimney Structures 



From the ROV observations on the northern Oregon shelf we 

 know that the cylindrical carbonate chimney observed from the ROV 

 on the seafloor in 1987 is virtually identical in shape and 

 diameter to the cylindrical chimney which was recovered in a 

 trawl net from the outer continental shelf in 1985 (see Table 1). 

 The large sediment-free opening (15 to 20 cm) that was observed 

 within the chimney suggests that fluid venting is actively 

 occurring through this structure. A previous study shows that 

 all chimneys dredged from this area possess an internal plumbing 

 system consisting of large cavities as well as small tubes 

 ( Schroeder et al . 1987). This plumbing network allows the 

 methane-enriched and CO2- charged fluids, which are apparently 

 derived from permeable sand zones or fault zones within the 

 underlying strata, to flow through the chimneys and precipitate 

 the authigenic carbonate that forms the chimney walls. The new 

 chimney structure appears to be embedded within the sedimentary 

 rocks of the seafloor to some unknown depth. In this case, the 

 flow probably emanates from a hole in the top of the chimney and 

 carbonate precipitation occurs uniformally over the external 

 portions of the structure that protrude from the seafloor. From 

 the distribution of open tubes within the cylindrical chimney 

 obtained in the trawl (Fig. 7), it is clear that the 

 precipitation patterns may also produce a rather complex and 

 elaborately intertwined plumbing system (Schroeder et al . 1987). 

 While the high seas at the surface and the lack of an acoustic 

 tracking system on the ROV, precluded us from scanning the 

 opening of the embedded chimney for internal tubes, we believe 

 that it was technically feasible to accomplish such a task. 



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