Geoscience Reference
In-Depth Information
4.2
Pore-Water Chemistry
Figure
3g
shows that chloride ion (Cl
−
) concentrations at the seepage site on
Oomine Ridge decreased with depth, as mentioned by Toki et al. (
2004
). Because
chloride is a nonreactive component, the Cl profile cannot be explained by the
normal interaction of seawater and sediment in hemipelagic environments, instead
suggesting low-Cl fluid moving upward from deeper in the sediment. The pore
water at this site was influenced by seeping fluid characterized as Cl-depleted (Toki
et al.
2004
). Likewise, stable-isotope profiles of pore water (d
18
O and dD) showed
depletion with depth (Fig.
3h, i
), suggesting that the seeping fluid was d
18
O- and
dD-depleted. These results are consistent with a previous study in this area (Toki
et al.
2004
), which proposed ground water input as the cause of the chemical and
isotopic features.
The convex-upward Cl concentration profile observed in the Oomine Ridge core
can be fitted with a steady-state kinetic model (Martens and Berner
1977
) incorpo-
rating diffusion and pore-water velocity:
2
2
DCxvCx
∂ ∂ +∂ ∂=
/
/
0,
(3)
s
where
x
= depth below the sediment-water interface measured positively downward (cm).
C
= Cl concentration in pore water (mmol/kg);
D
s
= whole-sediment diffusion coefficient for Cl, 160 cm
2
/year (Li and Gregory
1974
); and
v
= advection rate (cm/year).
This yields the solution
CCC xC
=− −
(
)(1 exp(-
α
))
+
,
(4)
0
0
where
C
=
C
0
at
x
= 0 and
C
→
C
∞
as
x
→ ∞, a =
v
/
D
s
(cm
-1
).
The core data are well fitted using a of 3 × 10
−3
cm
−1
,
C
0
of 560 mmol/kg, and
C
∞
of 490 mmol/kg. The a value corresponds to an advection rate of 3 mm/year.
A previous estimate of the rate based on a 30-cm core from Oomine Ridge was
0.4-2.0 m/year and referred to seepage as a localized phenomenon (Toki et al.
2004
). The advection rate determined in this study is considerably lower than the
previously reported values and supports the inhomogeneity of seepage. In this case,
the nearly linear gradient of the Cl profile shows that the transport of pore water is
dominantly controlled by diffusion and very weak advection in the sediment
column.
Sulfate (SO
4
2−
) concentrations rapidly decreased to zero within 2 m of the sedi-
ment surface and remained there below 2 m (Fig.
3f
), indicating either that sulfate
reduction occurred in the surface sediments or that sulfate-free fluid flowing
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