Geoscience Reference
In-Depth Information
10.1 Early diagenesis
the sea floor watching the sediment sinking beneath their feet (
Fig. 10.1
) and draw up the
balance sheet of matter for one species, say sulfate, between two levels at depth
z
one
and the sulfate concentration
in the water is
C
(
z
). In the stationary state, i.e. after “some” time, we can write that the
difference between the influx of sulfate at
z
less the outflux at
z
ϕ
z
is equal to the
quantity of sulfate destroyed by biological activity. For our observer, the advective flux of
sulfate is equal to
+
is the sedimentation rate, corrected where necessary for
compaction, which progressively expels water from the sediment. Likewise, the diffusive
flux of sulfate is
ϕv
C
(
z
), where
v
d
z
, where
D
is the diffusion coefficient of sulfate in salt water.
We can then write the diagenesis equation, standardizing it to a unit area:
−
ϕ
D
d
C
(
z
)
/
D
d
C
d
z
D
d
C
d
z
ϕv
−
ϕ
z
−
ϕv
−
ϕ
z
+
z
=−
C
C
P
(
z
)
z
(10.8)
where
P
(
z
) is the rate of destruction of sulfate by microbial activity. The rate
P
(
z
) will be
expressed in a suitable form, probably by a first-order kinetic with an appropriate stoichio-
to the diagenetic equation are usually combinations of exponentials. By examining
(10.8)
above, we can infer that the concentration of sulfate in pore water, like the abundance of
organic particles, must decrease with depth:
Fig. 10.2
shows the concentrations of sulfate
ion in the interstitial water of sediments collected from the Saanich River fjord (west coast
of Canada). It could be shown that
(10.8)
implies exponential decrease in sulfates from the
initial sulfate concentration of seawater, which
Fig. 10.2
confirms. The reduced sulfur is in
Seawater
sediment interface
−
0
v
z
z
+
Δ
z
Sediment
Depth,
z
Figure 10.1
The diagenesis reference frame. Sediment sinks beneath the ocean floor at the rate
v
.
