129I/I ratios and halogen concentrations in pore water of Hydrate Ridge and their relevance for the origin of gas hydrates; a progress report

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doi: 10.2973/odp.proc.sr.204.107.2006
Author(s): Fehn, Udo; Lu, Zunli; Tomaru, Hitoshi
Ocean Drilling Program, Leg 204, Shipboard Scientific Party, College Station, TX
Author Affiliation(s): Primary:
University of Rochester, Department of Earth and Environmental Sciences, Rochester, NY, United States
Other:
Christian-Albrechts-Universität zu Kiel, Federal Republic of Germany
Joint Oceanographic Institutions, United States
University of Texas at Austin, United States
University of Leicester, United Kingdom
Eastern Kentucky University, United States
U. S. Geological Survey, United States
Volume Title: Proceedings of the Ocean Drilling Program; scientific results; drilling gas hydrates on Hydrate Ridge, Cascadia continental margin; covering Leg 204 of the cruises of the drilling vessel JOIDES Resolution; Victoria, British Columbia, Canada, to Victoria, British Columbia, Canada; Sites 1244-1252; 7 July-2 September 2002
Volume Author(s): Tréhu, Anne M.; Bohrmann, Gerhard; Torres, Marta E.; Rack, Frank R.; Bangs, Nathan L.; Barr, Samantha R.; Borowski, Walter S.; Claypool, George E.; Collett, Timothy S.; Delwiche, Mark E.; Dickens, Gerald R.; Goldberg, David S.; Gràcia, Eulàlia; Guèrin, Gilles; Holland, Melanie; Johnson, Joel E.; Lee, Young-Joo; Liu, Char-Shine; Long, Philip E.; Milkov, Alexei V.; Riedel, Michael; Schultheiss, Peter; Su Xin; Teichert, Barbara; Tomaru, Hitoshi; Vanneste, Maarten; Watanabe, Mahito; Weinberger, Jill L.; Boetius, Antje; Brockman, Fred J.; Deyhle, Annette; Fehn, Udo; Flemings, Peter B.; Girguis, Peter R.; Heesemann, Martin; Joye, Samantha B.; Lorenson, Thomas D.; Mills, Christopher T.; Musgrave, Robert J.; Popa, Radu; Ussler, Bill; Wilkes, Heinz; Winckler, Gisela; Winters, William J.
Source: Proceedings of the Ocean Drilling Program; scientific results; drilling gas hydrates on Hydrate Ridge, Cascadia continental margin; covering Leg 204 of the cruises of the drilling vessel JOIDES Resolution; Victoria, British Columbia, Canada, to Victoria, British Columbia, Canada; Sites 1244-1252; 7 July-2 September 2002, Anne M. Tréhu, Gerhard Bohrmann, Marta E. Torres, Frank R. Rack, Nathan L. Bangs, Samantha R. Barr, Walter S. Borowski, George E. Claypool, Timothy S. Collett, Mark E. Delwiche, Gerald R. Dickens, David S. Goldberg, Eulàlia Gràcia, Gilles Guèrin, Melanie Holland, Joel E. Johnson, Young-Joo Lee, Char-Shine Liu, Philip E. Long, Alexei V. Milkov, Michael Riedel, Peter Schultheiss, Su Xin, Barbara Teichert, Hitoshi Tomaru, Maarten Vanneste, Mahito Watanabe, Jill L. Weinberger, Antje Boetius, Fred J. Brockman, Annette Deyhle, Udo Fehn, Peter B. Flemings, Peter R. Girguis, Martin Heesemann, Samantha B. Joye, Thomas D. Lorenson, Christopher T. Mills, Robert J. Musgrave, Radu Popa, Bill Ussler, Heinz Wilkes, Gisela Winckler and William J. Winters; Ocean Drilling Program, Leg 204, Shipboard Scientific Party, College Station, TX. Proceedings of the Ocean Drilling Program, Scientific Results (CD ROM), Vol.204, 25p. Publisher: Texas A&M University, Ocean Drilling Program, College Station, TX, United States. ISSN: 1096-2514
Note: In English. Data report; available only on CD-ROM in PDF format and on the Web in PDF or HTML. 43 refs.CD-ROM format, ISSN 1096-2514; WWW format, ISSN 1096-7451; illus., incl. sect., 2 tables, geol. sketch maps
Summary: We report iodine and bromine concentrations in a total of 256 pore water samples collected from all nine sites of Ocean Drilling Program Leg 204, Hydrate Ridge. In a subset of these samples, we also determined iodine ages in the fluids using the cosmogenic isotope 129I (T1/2 = 15.7 Ma). The presence of this cosmogenic isotope, combined with the strong association of iodine with methane, allows the identification of the organic source material responsible for iodine and methane in gas hydrates. In all cores, iodine concentrations were found to increase strongly with depth from values close to that of seawater (0.0004 mM) to concentrations >0.5 mM. Several of the cores taken from the northwest flank of the southern summit show a pronounced maximum in iodine concentrations at depths between 100 and 150 meters below seafloor in the layer just above the bottom-simulating reflector. This maximum is especially visible at Site 1245, where concentrations reach values as high as 2.3 mM, but maxima are absent in the cores taken from the slope basin sites (Sites 1251 and 1252). Bromine concentrations follow similar trends, but enrichment factors for Br are only 4-8 times that of seawater (i.e., considerably lower than those for iodine). Iodine concentrations are sufficient to allow isotope determinations by accelerator mass spectrometry in individual pore water samples collected onboard (∼5 mL). We report 129I/I ratios in a few samples from each core and a more complete profile for one flank site (Site 1245). All 129I/I ratios are below the marine input ratio (Ri = 1500×10-15). The lowest values found at most sites are between 150 and 250×10-15, which correspond to minimum ages between 40 and 55 Ma, respectively. These ages rule out derivation of most of the iodine (and, by association, of methane) from the sediments hosting the gas hydrates or from currently subducting sediments. The iodine maximum at Site 1245 is accompanied by an increase in 129I/I ratios, suggesting the presence of an additional source with an age younger than 10 Ma; there is indication that younger sources also contribute at other sites, but data coverage is not yet sufficient to allow a definitive identification of sources there. Likely sources for the older component are formations of early Eocene age close to the backstop in the overriding wedge, whereas the younger sources might be found in recent sediments underlying the current locations of the gas hydrates.
Year of Publication: 2005
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 07 Marine Geology and Oceanography; Cascadia subduction zone; Concentration; East Pacific; Gas hydrates; Halogens; Hydrate Ridge; I-129; Iodine; Isotopes; Leg 204; Marine sediments; North Pacific; Northeast Pacific; Ocean Drilling Program; Pacific Ocean; Pore water; Radioactive isotopes; Sediments
Coordinates: N443400 N443500 W1250400 W1250900
Record ID: 2007035102
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