Gas Chemistry of Pore Fluids from Oomine Ridge on the Nankai Accretionary Prism - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 260

year, showing seepage to be a localized phenomenon. The estimated rate is low
enough for the pore water to be dominantly controlled by diffusion rather than
advection in sediment.
3. As a benefit of gas extraction, dD H2 values were successfully determined. Above
2 mbsf, hydrogen was oxidized by sulfate and carbonate, and the heavier shift
to −745‰ could reflect oxidation by microbes (Eqs. 8 and 9), as 1 H is preferen-
tially utilized by methanogens for carbonate reduction.
4. The strong correlation between the 3 He/ 4 He and 4 He/ 20 Ne ratios suggests that the
He composition can be explained by a simple mixing of primordial He, radiogenic
He, and atmospheric He. Correction for atmospheric He shows the predominant
contribution of radiogenic He. Although the samples were collected from a
seepage field in the discharge zone of fluid through a splay fault from the plate
boundary, the crustal He component from sediment and crust surrounding the
pathway of the discharging fluid was pervasively mixed in the fluids.
Acknowledgments We thank the NSS operation team (University of Tokyo), K. Ikehara (AIST),
M. Murayama (Kochi University), M. Komure (Kyushu University), K. Fujino (Nihon University),
W. Tanikawa (Kyoto University), S. Suganuma (Kochi University), and Marine Works Japan for
their support of sampling, and we thank the captain and crew of R/V Kaiyo during the KY04-11
cruise for their continued dedication.
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