Halogen and 129-I systematics in gas hydrate fields at the northern Cascadia margin (IODP Expedition 311): insights from numerical modeling

Lu, Zunli, Hensen, Christian, Fehn, Udo and Wallmann, Klaus (2008) Halogen and 129-I systematics in gas hydrate fields at the northern Cascadia margin (IODP Expedition 311): insights from numerical modeling Geochemistry Geophysics Geosystems, 9 (10). Q10006. DOI 10.1029/2008GC002156.

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We measured halogen concentrations and I-129/I ratios in five drilling sites of Integrated Ocean Drilling Program Expedition 311 (offshore Vancouver Island, Canada) in order to identify potential sources of fluids and methane in gas hydrate fields. Iodine is dominated by organic decomposition and transports with fluids in reducing environments and the presence of the cosmogenic radioisotope I-129 (T-1/2 = 15.7 Ma) allows the age determination of organic sources for iodine. Here we report halogen concentrations in 135 pore water samples, I concentrations in 48 sediment samples, and I-129/I ratios measured in a subset of 20 pore water samples. Most I-129/I ratios fall into a range around 500 x 10(-15), corresponding to a minimum age of 25 Ma and the lowest ratio of 188 x 10(-15) (T-min = 47 Ma) was observed at 208 m below sea floor (mbsf) in Site 1326. These ages are considerably older than that of the local sediments in the gas hydrate fields and that of the subducting sediments on the Juan de Fuca plate, indicating that old, accreted sediments in the accretionary wedge contribute a significant amount of iodide and, by association, of methane to the gas hydrate occurrences. A geochemical transport-reaction model was applied to simulate the advection of deeply sourced fluids and the release of iodide, bromide, and ammonia in the host sediments due to organic matter degradation. The model was first tested with data from two well studied areas, Ocean Drilling Program Site 1230 (Peru margin) and Site 1245 (Hydrate Ridge). The model results for the Expedition 311 sites indicate that the in situ release of young iodine is relatively minor in comparison to the contribution of migrating fluids, carrying large amounts of old iodine from deep sources. The comparison between the sites demonstrates that the total organic content has a strong effect on the rate of in situ iodine release and that lateral flows along fractures can have a significant influence on pore water chemistry, especially at the Cascadia margin. The iodine results indicate that mobilization and transport of methane from sources in the upper plate of active margins is an important process which can also play a substantial role in the formation of gas hydrate fields.

Document Type: Article
Keywords: halogen, 129-I/I ratio, gas hydrate, IODP Expedition 311, numerical modeling, gas and hydrate systems, Subduction zone processes, geochemical modeling
Research affiliation: OceanRep > SFB 574 > B1
OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
OceanRep > SFB 574
OceanRep > The Future Ocean - Cluster of Excellence
OceanRep > SFB 574 > B5
Refereed: Yes
DOI etc.: 10.1029/2008GC002156
ISSN: 1525-2027
Projects: Future Ocean, IODP
Contribution Number:
SFB 574158
Date Deposited: 03 Dec 2008 16:52
Last Modified: 07 Mar 2018 14:40
URI: http://eprints.uni-kiel.de/id/eprint/6465

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