HR: 1340h
AN: GC33A-0940    [Abstracts]
TI: Time scale of iodine cycling in gas hydrate systems at active margins: implications for the organic carbon cycle
AU: * Fehn, U
EM: fehn@earth.rochester.edu
AF: University of Rochester, 227 Hutchison Hall, Dept. of Earth and Env. Sci., Rochester, NY 14627, United States
AU: Lu, Z
EM: luzunli@earth.rochester.edu
AF: University of Rochester, 227 Hutchison Hall, Dept. of Earth and Env. Sci., Rochester, NY 14627, United States
AB: Marine gas hydrates in active margins constitute a large reservoir of organically derived carbon whose origin is, however, not well understood. Iodine is found at highly elevated levels in pore waters associated with gas hydrates, and can be used as tracer for the origin of methane, due to its strongly biophilic behavior and the presence of a long-lived cosmogenic isotope (¹²⁹I; T_1/2 = 15.7 My). We applied the iodine isotopic system to determine ages of organic sources for iodine in hydrate fields of the Pacific margin, such as Cascadia Margin, Nankai Trough, Peru Slope, and Costa Rica Margin. In all of the sites, iodine ages in excess of 30 Ma were found, demonstrating that a major part of the iodine is not derived from local sources. In most cases, the ages are also older than the subducting marine sediments, suggesting derivation from sources in the upper plate of the subduction zones. Isotope mixing diagrams demonstrate also the presence of one or more younger sources (< 10 Ma) at all of these margins. The complex fluid flows migrating through the wedge to pick up iodine released from organic matter can be considered as a sampling of sources with various ages. In contrast to the endmember ages, the mixed isotope signature in most of the samples should represent the averaged time since the burial of a suite of organic source materials. Using a compiled database of ~250 iodine dates, the age distribution summarizing all of the margins can be calculated by combining the age density normalized to the number of samples at each margin. The Gaussian fit to this distribution results in an age with highest probability of 30 Ma. We suggest that, on average, the iodine and associated organic carbon currently returning to the oceanic reservoir was buried in the marginal wedges ~30 My ago. Given the wide occurrence of hydrates at these margins, our results indicate that the forearc area serves as an important mechanism for organic carbon cycling operating at tectonic time scales.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3002 Continental shelf and slope processes (4219)
DE: 3004 Gas and hydrate systems
DE: 3036 Ocean drilling
SC: Global Environmental Change [GC]
MN: 2007 Fall Meeting