78
3
Quantification of Early Diagenesis: Dissolved Constituents in Marine Pore Water
A given concentration in the pore water of a
pelagic sediment, several meters below the
sediment surface, can be measured today, next
month, next year, and after 10 years. Within the
margins of reasonable analytical precision, we will
always measure just about the same value and
rightfully declare the situation to be steady-state
(That there are also exceptions to the rule may be
concluded from the Examples 2 and 3 described in
Section 15.3.2). At the same time, pore water
concentrations in sedimentary surface areas near
the same pelagic sediment could be subject to
considerable seasonal variation (such as residual
deposits of algal bloom periods) and thus be
classified as being in a non-steady state.
A calculated example for pore water concentrations in a non-steady state condition is shown
in Figure 3.3. Details concerning the calculation
procedure will not be discussed here. The conceptual model employed will be described in
Section 3.2.4, a suitable computer model is
described in Chapter 15. A typical diffusion
coefficient characteristic of young marine
sediments and a characteristic porosity coefficient were used in the calculation.
In the calculated example shown in Figure 3.3
the assumption was made that a constant concentration of ‘1’ has been prevalent for a long time in
the pore water of the young sediment and in the
bottom water above it. Then, the bottom water
underwent a momentary change to yield a concentration of ‘9’. By means of diffusion, the new
concentration gradually spreads into the pore
water. After 2 hours it reaches a depth of less
than 1 cm, after 48 hours, respectively, a depth of
about 3 to 4 cm below the sediment surface. If the
concentration of ‘9’ remains constant long
enough in the bottom water above the sediment,
this concentration will theoretically penetrate into
an infinitely great depth. To what extent, and into
what depths of pore water, these concentrations
are to be assigned to steady state or non-steady
states can only be determined in each particular
case, having its own concentration in the bottom
water over a given period of time. At any rate, the
allocation of one or the other state can only be
done separately for each system, each parameter,
and each time interval. Calculations as shown in
Figure 3.3 are likely to produce valuable preliminary concepts, for evaluating real measured pore
water profiles.
In the next example, the influence of seasonal
variation in the bottom water lying above the
sediment will be examined, as well as the resulting
non-steady states in the pore water fraction. To
this end, the following boundary conditions are
selected: A substance concentration of ‘1’ is supposed to be prevalent in the bottom water over
one half year, afterwards the concentration changes to ‘10’ for one half year, then it changes back
to a value of ‘1’, and so on, continually changing.
Figure 3.4 shows the result of such an oscillatory
situation after several years. The curve denoted
‘a’ demonstrates the situation in which the
bottom water concentration was ‘1’ after half a
year; the curve denoted ‘b’ reflects the situation
in which the bottom water concentration was ‘10’
after half a year. It is evident that essential effects
of such changes can only be observed down to a
depth of less than 0.2 m below the sediment
surface. In this model calculation, the effects of
Fig. 3.4 Calculated non-steady state concentration profiles in the pore water of a young sediment. The calculation assumes that concentrations of ‘1’ and ‘10’ were
prevailing alternately in the bottom water over the sediment, each over a period of half a year. After several
years, the curve a. reflects the situation of concentration ‘1’ at the end of one half year, whereas curve b.
reflects the situation of concentration ‘10’ at the end of
one half year. The calculation of such non-steady state
concentration profiles can be performed, for example,
with the aid of the model programs EXPLICIT or
CoTReM (cf. Chapter 15).
3
Quantification of Early Diagenesis: Dissolved Constituents in Marine Pore Water
A given concentration in the pore water of a
pelagic sediment, several meters below the
sediment surface, can be measured today, next
month, next year, and after 10 years. Within the
margins of reasonable analytical precision, we will
always measure just about the same value and
rightfully declare the situation to be steady-state
(That there are also exceptions to the rule may be
concluded from the Examples 2 and 3 described in
Section 15.3.2). At the same time, pore water
concentrations in sedimentary surface areas near
the same pelagic sediment could be subject to
considerable seasonal variation (such as residual
deposits of algal bloom periods) and thus be
classified as being in a non-steady state.
A calculated example for pore water concentrations in a non-steady state condition is shown
in Figure 3.3. Details concerning the calculation
procedure will not be discussed here. The conceptual model employed will be described in
Section 3.2.4, a suitable computer model is
described in Chapter 15. A typical diffusion
coefficient characteristic of young marine
sediments and a characteristic porosity coefficient were used in the calculation.
In the calculated example shown in Figure 3.3
the assumption was made that a constant concentration of ‘1’ has been prevalent for a long time in
the pore water of the young sediment and in the
bottom water above it. Then, the bottom water
underwent a momentary change to yield a concentration of ‘9’. By means of diffusion, the new
concentration gradually spreads into the pore
water. After 2 hours it reaches a depth of less
than 1 cm, after 48 hours, respectively, a depth of
about 3 to 4 cm below the sediment surface. If the
concentration of ‘9’ remains constant long
enough in the bottom water above the sediment,
this concentration will theoretically penetrate into
an infinitely great depth. To what extent, and into
what depths of pore water, these concentrations
are to be assigned to steady state or non-steady
states can only be determined in each particular
case, having its own concentration in the bottom
water over a given period of time. At any rate, the
allocation of one or the other state can only be
done separately for each system, each parameter,
and each time interval. Calculations as shown in
Figure 3.3 are likely to produce valuable preliminary concepts, for evaluating real measured pore
water profiles.
In the next example, the influence of seasonal
variation in the bottom water lying above the
sediment will be examined, as well as the resulting
non-steady states in the pore water fraction. To
this end, the following boundary conditions are
selected: A substance concentration of ‘1’ is supposed to be prevalent in the bottom water over
one half year, afterwards the concentration changes to ‘10’ for one half year, then it changes back
to a value of ‘1’, and so on, continually changing.
Figure 3.4 shows the result of such an oscillatory
situation after several years. The curve denoted
‘a’ demonstrates the situation in which the
bottom water concentration was ‘1’ after half a
year; the curve denoted ‘b’ reflects the situation
in which the bottom water concentration was ‘10’
after half a year. It is evident that essential effects
of such changes can only be observed down to a
depth of less than 0.2 m below the sediment
surface. In this model calculation, the effects of
Fig. 3.4 Calculated non-steady state concentration profiles in the pore water of a young sediment. The calculation assumes that concentrations of ‘1’ and ‘10’ were
prevailing alternately in the bottom water over the sediment, each over a period of half a year. After several
years, the curve a. reflects the situation of concentration ‘1’ at the end of one half year, whereas curve b.
reflects the situation of concentration ‘10’ at the end of
one half year. The calculation of such non-steady state
concentration profiles can be performed, for example,
with the aid of the model programs EXPLICIT or
CoTReM (cf. Chapter 15).