characterize most of these sites (Fig. 5; Suess et al . 1985; 

 Kulm et al . 1986; Ritger et al. 1987 ). Methane concentrations 

 generally range from 180 to 420 nl/liter with maxima of 1300 nl/1 

 in the free water at 1 m above the seafloor, which are 3 to >10 

 times higher than those found in the ambient waters on the 

 adjacent abyssal plain (Kulm et al . 1986). The expelled fluids 

 are derived from the pore waters that migrate upward through the 

 sediments of the accretionary prism beneath the vents. The 

 clastic sediments of Astoria Fan, which were accreted to the 

 continental slope during late Pleistocene time, are the ultimate 

 source of the methane-enriched fluids (Fig. 1). 



Carbonate slabs and crusts are found a few centimeters 

 beneath the sediment surface at each vent site or completely 

 cover the sediments, forming large carbonate surfaces. Isolated, 

 conical carbonate chimneys rise from 1- 2 m above these 

 sediment-covered vent sites (Fig. 5). One isolated carbonate 

 chimney was located above a sharp-crested ridge on the second 

 thrust ridge landward of the marginal ridge (Fig 4). Numerous 

 cavities, grooves and flutes, which have smoothly rounded edges, 

 give the chimneys a 'sculptured' appearance (Ritger et al. 

 1987). Each chimney is covered with numerous organisms, 

 consisting of corals and sponges. The two slope chimneys contain 

 magnesian calcite, dolomite and aragonite with minor amounts of 

 detrital clays, silt and sand (Ritger et al . 1987). 



At each vent site, where the authigenic carbonates are 

 forming, methane-enriched waters were being expelled from the 

 underlying accretionary prism (Ritger et al. 1987). According 

 to the stable carbon isotope data the carbonates are derived from 

 a reservoir that is extremely depleted in carbon 13. Values of 

 kl3C of all carbonate samples collected on the margin range from 

 -34.9 °/oo to -66.7 °/oo PDB; this indicates a methane-derived 

 carbon source (Ritger et al . 1987). The two slope chimneys 

 analyzed have a kl3C value of -32.5 and -32.6 °/oo and kl80 

 values of +5.4 to +6.2 °/oo, which indicate that the carbonates 

 are marine and formed close to equilibrium with the ambient 

 bottom water temperature. This confirms other evidence that the 

 temperature of the venting fluid is only slightly elevated, if at 

 all different, from that of the bottom water. 



Continental Shelf 



The chimneys recovered on the outer continental shelf in 

 1985 range in height from 1 to 2 m (Table 1; Figs 6,7; Schroeder 

 et al. 1987). The conical chimneys (Fig. 6, chimneys 1,3) are 

 very similar in shape and size to those found on the lower slope. 

 Each chimney has a hollow center, or vertical cavity, along with 

 at least one large cavity in the side wall; numerous smaller 

 tubes penetrate the walls. Chimney 2 is unique because of its 

 cylindrical shape; it contains more small tubes and cavities than 

 the conical ones. A secondary tube that runs parallel to the 



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