HR: 16:00h
AN: OS52D-01    [PDF]
TI: Constraints on Gas Hydrate Dynamics at the Southern Summit of Hydrate Ridge Based on Dissolved Chloride Data
AU: Torres, M E
EM: mtorres@coas.oregonstate.edu
AF: Oregon State University, COAS, Corvallis, OR 97331-5503 United States
AU: Wallmann, K
EM: kwallmann@geomar.de
AF: GEOMAR, 1-3 Wischhofstrasse, Kiel, D-24148 Germany
AU: * Trehu, A M
EM: trehu@coas.oregonstate.edu
AF: Oregon State University, COAS, Corvallis, OR 97331-5503 United States
AU: Bohrmann, G
EM: gbohrmann@uni-bremen.de
AF: University of Bremen, Research Center Ocean Margins, Bremen, D-28334 Germany
AU: Borowski, W S
EM: w.borowski@eku.edu
AF: Eastern Kentucky University, Department of Earth Sciences, Richmond, KY 40475-3102 United States
AU: Tomaru, H
EM: tomaru@gbs.eps.s.u-tokyo.ac.jp
AF: University of Tokyo, Department of Earth and Planetary Science, Tokyo, 113-0033 Japan
AB: At the summit of Hydrate Ridge (ODP Sites 1249 and 1250), pore fluids are highly enriched in dissolved chloride (up to 1360 mM) in a zone that extends from the near the sediment surface (~1 mbsf) to depths of 20-30mbsf. The presence of this brine indicates that gas hydrate formation here must be very recent and rapid. Below 30 mbsf, high chloride brines give way to chloride values near seawater concentrations with even lower chloride anomalies superimposed on baseline values. Fresher chloride anomalies represent intervals where gas hydrate has dissociated during sediment recovery. The low chloride values observed below 30 mbsf, and similar anomalies observed at sites drilled in a nearby basin and on the flanks of Hydrate Ridge, can be used to estimate the gas hydrate concentration in these sediments. The observed chloride enrichment at Site 1249 can be reproduced by a one dimensional, non-steady state, transport-reaction model only if methane transport occurs in the gas phase; i.e. the observed chloride enrichment cannot be generated exclusively from transport of methane dissolved in the pore fluids. Model best fits to measured chloride values require an enhanced rate of methane transfer from the gas phase in near-surface sediments, possibly due to enhanced bubble formation. Transport of methane as free gas within the GHSZ is consistent with geophysical observations at this site. The gas hydrate currently present at the Hydrate Ridge summit could have been generated in less than 1,000 years if we allow for enhanced transfer rates of methane gas near the seafloor, and ~5,000 years if the gas dissolution rate remains constant with depth. Such high values highlight the dynamic nature of this system. In comparison, gas hydrate present in the deeper sediments of Site 1249, at other sites on Hydrate Ridge, and other locations where no positive chloride anomalies are observed, must be significantly older than 40,000 years.
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1724 Ocean sciences
DE: 4825 Geochemistry
DE: 4842 Modeling
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting