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sampling greatly reduced the effect of degassing. Air contamination was assessed
using noble gases and was negligible. Profiles of isotopic compositions of CH 4
and SCO 2 were strongly consistent between fluid and gas samples, suggesting
that the vacuum extraction procedure was successful. Distribution of dD H2 values
was consistent with control of hydrogen by fermentation and oxidation of sulfate
and carbonate. The relationship of 3 He/ 4 He and 4 He/ 20 Ne ratios suggested that He
composition can be explained by simple mixing of primordial mantle He, radio-
genic crustal He, and atmospheric He. When corrected for the atmospheric He
component, the helium in the samples is almost entirely of radiogenic origin. The
samples represent fluids discharged through a splay fault from the plate boundary,
but the contribution of mantle He was very low. It is clear that crustal He perva-
sively mixed into the fluids from sediment and crust surrounding the pathway of
the discharging fluid.
Keywords Seepage • Pore fluid • Nankai Trough • Oomine Ridge • Gas chemistry
• Hydrogen • Helium
1
Introduction
Pore fluids in surface sediments of the deep seafloor originate from seawater prior
to burial. Early diagenesis takes place in the surface sediments, causing reactive
components to migrate to the pore fluids from sediments. Particularly at seepage
areas, advective fluids bear chemical components with different compositions
from those of the pore fluids in the surface sediments. We investigated the distribu-
tion of chemical components in the surface sediments from a seepage site at
Oomine Ridge in the Nankai accretionary prism and inferred the origin and path-
way of the advective fluids.
H 2 , one of the most reactive of these components, plays an important role in
microbial diagenesis in anoxic sediments (e.g., ZoBell 1947 ). However, quantifying
its concentration is difficult because of its great reducing power and high diffusiv-
ity. With conventional procedures, hydrogen in sediment samples escapes through
degassing and air contamination occurs during sampling and storage. Moreover,
hydrogen must be reliably sampled to acquire information from its isotopic compo-
sition. We succeeded in extracting hydrogen and other gases dissolved in pore
fluids by squeezing sediments with the newly designed vacuum extraction.
We used chemical and isotopic compositions of CH 4 and H 2 in pore water to
consider the origin, diagenesis, and transport of pore fluids, and used compositions
of noble gases in pore water to investigate the origin and nature of the pathway of
seeping fluid. In this way, we demonstrated that gas chemistry of extracted pore
water, combined with conventional pore-water chemistry, sheds light on the reduc-
tive/oxidative conditions of the in situ environment and its transition with increas-
ing depth to provide information on in situ geochemical processes of biochemical
and tectonic relevance.
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