Geoscience Reference
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Concentrations of H
2
in our pore water samples were low above 2 mbsf and
reached a peak of about 500 mmol/kg at 2.5 mbsf (Fig.
4c
). Hydrogen is an intermediate
product during fermentation (Conrad
1999
). As mentioned above in the interpretation
of the ammonium profile, fermentation occurred at all depths in the core sediment and
can be considered the origin of the hydrogen in pore water (Dolfing
1988
). Sulfate
reducers take up H
2
in the sulfate reduction zone above 1.5 mbsf:
4H
+ +→ +
SO
2
−
H
+
HS
−
4H O
(9)
2
4
2
Below 2 mbsf, in the methane production zone, H
2
would be utilized by methano-
gens through carbonate reduction as shown in Eq. 8. Sulfate reducers utilize hydrogen
more effectively than methanogens, leading to the peak of H
2
concentration (Lovley and
Goodwin
1988
). Distribution of dissolved H
2
in the pore water was controlled by pro-
duction via fermentation and consumption by oxidants such as sulfate and carbonate.
As for the isotopic composition of hydrogen, dD
H2
values were around −745‰
above 2 mbsf and as light as −755‰ at 2.5 mbsf (Fig.
4f
). dD
H2
in natural environ-
ments is considered to be related to isotopic equilibrium with ambient H
2
O at in situ
temperature, promoted by microbial metabolisms (Romanek et al.
2003
; Valentine
et al.
2004
). Given this tendency, dD
H2
can be calculated as follows (Horibe and
Craig
1995
):
( ) ( )
2
D / H
/ D / H
=+× +×
1.05 2.01 10 /
5
T
2
2.06 10 /
9
T
4
HO
H
2
14
6
+×
1.80 10 /
T
,
(10)
where
T
is temperature (K). The observed dD
H2
value has been shown to be useful
for estimating the equilibrium temperature, which has strong correlations with
measured temperatures of hydrothermal samples (Proskurowski et al.
2006
;
Kawagucci et al.
2010
). If the observed dD
H2
were controlled by equilibrium
between H
2
and H
2
O, the observed value of −742‰ above 2 mbsf corresponds to
23°C, and the value of −755‰ at 2.5 mbsf corresponds to 16°C using Eq. 10.
The temperature of the bottom seawater was recorded as 1.674 ± 0.004°C by the
temperature sensor attached to the NSS during core sampling. Heat flow values were
previously reported to be 120 mW/m
2
at the outside of bacterial mats and 200 mW/
m
2
at the inside of the mats on Oomine Ridge (Goto et al.
2003
). These values are
much higher than the reference value of about 50 mW/m
2
for the outer ridge of the
Nankai accretionary prism. Such a high heat flow would result from seepage. Seepage
must be highly variable from place to place to yield a heat-flow range from 50 to 200
mW/m
2
. The thermal conductivity of the sediment sample, measured onboard, was
around 1 W/m/K. Taking this value into account, the corresponding temperature gra-
dient is 50-200 K/km. Given such a thermal gradient in this area, the in situ tempera-
ture several meters below the seafloor can be estimated to be about 2°C, which is not
consistent with the estimated temperatures of 16°C and 23°C using dD
H2
values.
The estimated temperature using dD
H2
values suggests that hydrogen reached
equilibrium with water at depths below the seafloor ranging from 70 to 400 m,
given the thermal gradient in the sediment. At the advection rate we estimated from
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