Title: Geochemistry of hydrothermal vent fluids from the northern Juan de Fuca Ridge
Abstract: The presence of aqueous organic compounds derived from sedimentary organic matter has the potential to influence a range of chemical processes in hydrothermal vent environments.For example, hydrothermal alteration experiments indicate that alteration of organic-rich sediments leads to up to an order of magnitude more metals in solution than alteration of organic-poor basalt.This result is in contrast to traditional models for the evolution of vent fluids at sediment-covered mid-ocean ridge axis environments, and indicates the fundamental importance of including the effects of organic compounds in models of crustal alteration processes.However, in order to rigorously constrain their role in crustal alteration processes, quantitative information on the abundances and distributions of organic compounds in hydrothermal vent fluids is required.This thesis was undertaken to provide quantitative information on the distributions and stable carbon isotopic compositions of several low-molecular weight organic compounds (Ci-C4 alkanes, CZ-C3 alkenes, benzene and toluene) in fluids collected in July, 2000, at three sites on the northern Juan de Fuca Ridge: the Dead Dog and ODP Mound fields, which are located at Middle Valley, and the Main Endeavour Field, located on the Endeavour segment.At Middle Valley, the ridge axis is covered by up to 1.5 km of hemipelagic sediment containing up to 0.5 wt.% organic carbon.The Main Endeavour Field (MEF) is located approximately 70 km south of Middle Valley in a sediment-free ridge-crest environment, but previously measured high concentrations of NH3 and isotopically light C~ relative to other bare-rock sites suggest that the chemical composition of these fluids is affected by sub-seafloor alteration of sedimentary material (LILLEY et aL., 1993).Differences in the absolute and relative concentrations of NH3 and organic compounds and the stable carbon isotopic compositions of the Ci-C3 organic compounds suggest that the three fields represent a continuum in terms of the extent of secondary alteration of the aqueous organic compounds, with the Dead Dog fluids the least altered, the MEF fluids the most altered and ODP Mound fluids in an intermediate state.At the two Middle Valley sites, the greater extent of alteration in the ODP Mound fluids as compared to the Dead Dog fluids is due either to higher temperatures in the subsurface reaction zone, or a greater residence time of the fluids at high temperatures.Higher reaction zone Program, little did I know that my doctoral research would take me to the bottom of the ocean in DSV Alvin.As an advisor, Jeff was able to strike the perfect balance between knowing when to let me run with an idea, and knowing when to step in and save me from myself.I have benefited immeasurably from his expectations and high scientific ideals, and thank him for his support and guidance.I would also like to thank my commttee, Meg Tivey, Tim Eglinton and Tina Voelker, for their support.Each of them brought a unique scientific perspective to this research, and this thesis has benefited greatly from their insights.I would also like to thank Meg in particular for being a wonderful role model as to how one actually balances the demands of scientific and family life-and for being right about the timing of key events in January, 1999.Thank you to Lary Ball for his assistance with the ICP-MS and to Leah Houghton and Carl Johnson for their assistance with the IRM-GCMS analysis.I would also like to thank Robert Zierenberg, of the University of California at Davis, for assistance with the sulfur isotope measurements and for the insightful discussions of the Middle Valley system.I would also like to acknowledge the captain and crew of the RI Atlantis and the pilots of DSV Alvin.Their efforts and expertise combined to provide us with a highly successful cruise in July, 2000.Tom McCollom, Peter Saccocia, Karen Hurley, Halle Marbet and Dane Percy provided invaluable assistance in the collection and processing of the fluid samples at sea.Thank you to my fa