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the Cl profile of the samples, 3 mm/year, the hydrogen in the surface pore fluid
must have equilibrated with H
2
O at 70-400 mbsf 20-140 ky ago and then migrated
up through the sediment column for 20-140 ky. It may seem strange that isotopic
equilibrium has not been achieved in contemporary sediments after 20-140 ky,
because the isotopic exchange by sulfate reduction bacteria requires about 40 days
(Campbell et al.
2009
).
We were afraid the gases equilibrated with coexistent pore water during its
sampling and storage. Our sampling procedure, however, conducted extracting the
gases from pore waters within a day of storage in a refrigerator after sampling
the sediment from the seafloor. Isotopic equilibrium is reached slowly in low-
temperature conditions or without catalysis (Koehler et al.
2000
). The gas extraction
interrupted the contact between H
2
and H
2
O and the involvement of microbes living
in sediment, and it is unlikely that the hydrogen and water isotopically reached
equilibrium at a room temperature.
As stated above, the observed dD
H2
cannot be explained by isotopic equilibrium
with H
2
O at reasonable temperature, and it would be the net value of hydrogen that
is produced through fermentation occurring throughout the core as suggested by the
NH
4
+
profile (Eq. 7), and consumed by sulfate and carbonate reduction above
2 mbsf (Eqs. 9 and 8, respectively). The dD
H2
values show a heavier shift to −745‰
by sulfate and carbonate reduction, as microbes preferentially utilize
1
H during
their metabolism (Landmeyer et al.
2000
). The explanation of the dD
H2
distribution
is consistent with that of the hydrogen distribution described above.
We succeeded in retrieving dissolved gases from pore water and measured the
concentrations and isotopic ratios of trace gases. The reactive components of these
gases provide useful information about early diagenesis in surface sediment, which
involves biochemical reactions. In the next section, the conservative components of
these gases provide information on their origin and pathways, which is one of the
important geochemical subjects.
4.4
Gas Chemistry of Nonreactive Components
Figure
4g
shows
4
He-richer fluids distribute at deeper sediments, suggesting
4
He-
rich fluids diffuse upward into the surface sediment. Figure
5
shows a correlation
diagram between the observed
3
He/
4
He and
4
He/
20
Ne ratios. The
3
He/
4
He ratios are
normalized to
R
atm
, the atmospheric
3
He/
4
He ratio of 1.393 × 10
−6
(Davidson and
Emerson
1990
). The samples in the diagram lie in a mixing field of three end-
members: primordial He derived from the mantle beneath the Nankai accretionary
prism, radiogenic He produced from U and Th in crustal rocks, and atmospheric He
dissolved in seawater (ASW). This distribution suggests that He in our sample is
well explained by simple mixing of the three sources (Sano and Wakita
1985
). We
can correct for atmospheric He using the
r
value (Eq. 2) (Craig et al.
1978
):
(
3
He /
4
He)
=
[
(
3
He /
4
He
)
−
(
3
He
/
4
He)
×
r
]/ 1
(
−
r
)
,
(11)
cor
obs
a
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