BIOLOGY AND ECOLOGY OF THE OREGON CONTINENTAL SHELF 

 EDGE ASSOCIATED WITH THE ACCRETIONARY PRISM 



Andrew G. Carey, Jr., David L. Stein, Gary L. Taghon, 



and Anne E. DeBevoise 



College of Oceanography 



Oregon State University 



Corvallis, OR 97331-5503 



ABSTRACT 



As part of an exploratory study of the geology and biology 

 of the upper Oregon subduction zone, we undertook submersible re- 

 connaissance dives on the northern continental shelf edge and 

 upper slope. Our objectives were: (1) to ascertain the 

 structure of the benthic assemblages present, and (2) to study 

 assemblages associated with possible active vents. Although 

 direct observations and photography demonstrated that abundant 

 calcium carbonate slabs and chimneys occur on Nehalem Bank, there 

 are now no active subduction-driven vents. Presumably, methane 

 from extensive past venting geochemically formed the calcium 

 carbonate edifices. The vent field appears to be extinct; 

 however, hard deposits produced by venting have changed the 

 nature of the animal community in the area by providing substrate 

 for hard bottom organisms. In addition to a typical outer 

 continental shelf fine sand-mud community, a hard substrate 

 epifaunal community is also present. 



INTRODUCTION 



In plate convergence zones where oceanic crust is being 

 subducted, methane is expelled from faults and permeable zones 

 within accreted sediments (Kulm et al., 1986; Boulegue et al . 

 1987). In 1984 investigators from Oregon State University disco- 

 vered vents and faunal communities associated with the subduction 

 zone on the lower continental slope at depths of 2000 to 2400 m 

 off central Oregon ( Suess et al. 1985; Kulm et al. 1986). During 

 dives with the research submersible Alvin , methane-enriched 

 bottom waters (182-428 ml/1) were sampled. These sites contain 

 methane-derived authigenic carbonate slabs and chimneys, and the 

 surrounding sandstone rocks contain carbonate cement clearly 

 derived from methane (Kulm et al. 1986, Ritger et al. 1987). 



The 1985 discovery of calcium carbonate chimneys at 247 m 

 depth on the northern Oregon shelf edge by commercial fishermen 

 aboard F/V Kodiak and other vessels suggested that fluids are 

 expelled along the entire width of the accretionary complex of 

 the subduction zone ( Schroeder et al . 1987; Kulm et al. submit- 

 ted, this volume; Fig. 1). This conclusion is supported by 

 collections from earlier benthic trawls and dredges on the Oregon 



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