features, the numerous open holes, and the sediment-free opening 

 of one chimney indicate that carbonate deposition is presently 

 active in this area. 



3. We propose that the carbonate structures are formed 

 through aragonite, Mg-calcite, and dolomite precipitation from 

 methane and dissolved carbonate-rich fluids that emanate from 

 these structures. An open plumbing network is initially 

 created and maintained by the deposition of the carbonate within 

 and on top of the terrigenous substrata that comprise the 

 seafloor. In some structures, the plumbing tubes are eventually 

 cemented with carbonate causing the venting fluids to seek 

 alternate routes of expulsion around the structure. The ultimate 

 source zones for these fluids are most likely located within the 

 Oligocene to Miocene accretionary prism underlying the outer 

 continental margin. 



ACKNOWLEDGMENTS 



We gratefully acknowledge the financial support of NOAA ' s 

 Office of Undersea Research in providing the submersible Mermaid 

 with its support vessel Aloha and ROV Recon-IV in this study. 

 The research support for this study was provided by the Oregon 

 State University Sea Grant Program. 



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