76
3
Quantification of Early Diagenesis: Dissolved Constituents in Marine Pore Water
be reproduced in the pore water fraction on
account of diffusion processes. After a definite
time of non-stationary conditions, which depends on the depth of the profile under study
and the required accuracy of the measurement,
the concentration of the ocean floor water prevails again, stationary and constant within the
entire profile. The details and the problems concerning stationary and non-stationary conditions in pore water will be thoroughly discussed
in Section 3.2.
Figure 3.2b shows the concentration profile of
a substance which is consumed by early diagenetic reactions in the upper layers of the sediment. For example, the profiles of dissolved oxygen often look like this (cf. Chap. 6). Assuming
steady-state conditions (cf. Sect. 3.2), the
concentration gradient near the sediment surface
will display its highest increases, because whatever is consumed in the deeper layers will arrive
there by means of diffusion. With increasing
depths the concentration gradient and the diffusive material transport will decline, until, at a
particular depth, concentration, gradient, and
diffusion simultaneously approach zero. This
sediment depth, referred to as the penetration
depth of a substance, often represents a steadystate condition composed of the diffusive
reinforcement from above and the depletion of
the substance in the sediment by the reactions of
early diagenesis.
In principle, the situation depicted in Figure
3.2c is quite the same. Here, the process is just
limited to one reactive layer. With an example of
oxygen as the dissolved substance, this could
be a layer containing easily degradable organic
matter, or the sedimentary depth in which
oxygen encounters and reacts with a reductive
solute species coming from below (e.g. Mn
2+
or
Fe
2+
). At any rate, the space above the reactive
layer is characterized by a constant gradient in
the concentration profile and thus by a diffusive transport which is everywhere the same.
Within the reactive layer the dissolved substance is brought to zero concentration by
depletion.
The reverse case, which is in principle quite
similar, is demonstrated in Figure 3.2d and e.
Here, a substance is released anywhere into the
layers near the sediment surface (Fig. 3.2d),
whereas it might be released into the pore water
only in one particular layer (Fig. 3.2e). The solute
could be, for example, silica that often displays
such concentration profiles on account of the
dissolution of sedimentary opal.
The concentration profile shown in Figure
3.2f is much more complex than the others in
Figure 3.2 and only comprehensible if the
interactions of several processes taking place at
various depths below the sediment surface are
regarded. Such a profile is characteristic, for
example, of the concentrations of divalent manganese in marine pore water (see also Figure 3.1,
Figure 3.8, and Chapter 11). Manganese oxide
functions as an electron acceptor and reacts
with organic matter in reactive layer 2, whereupon chemically reduced divalent manganese
dissolves. A small percentage diffuses downwards (flat gradient) and precipitates as manganese carbonate or manganese sulfide in reactive
layer 3. The major proportion diffuses upwards
(much steeper gradient), encounters dissolved
oxygen from the sediment surface once again in
reactive layer 1, is reoxidized and precipitates
back to manganese oxide.
The described relations can be summarized by
stating a few rules for reading and understanding
of pore water concentration profiles:
• Diffusive material fluxes always occur in the
form of concentration gradients; concentration gradients always represent diffusive
material fluxes.
• Reactions occurring in pore water in most
cases constitute changes in the concentration gradient; changes in the concentration gradient always represent reactions
occurring in pore water.
• A concave-shaped alteration in the concentration gradient profile (cf. Fig. 3.2b, c)
signifies the depletion of a substance from
pore water; conversely, a convex-shaped
concentration gradient profile (cf. Fig. 3.2d,
e) always depicts the release of a substance
into the pore water.
However, the following also needs to be
observed:
• If a substance is involved in two reactions
taking place at the same depth, and is
released into the pore water fraction by one
reaction and then withdrawn again by the
other, neither of the participating reactions
will become evident in the pore-water
concentration profile.
3
Quantification of Early Diagenesis: Dissolved Constituents in Marine Pore Water
be reproduced in the pore water fraction on
account of diffusion processes. After a definite
time of non-stationary conditions, which depends on the depth of the profile under study
and the required accuracy of the measurement,
the concentration of the ocean floor water prevails again, stationary and constant within the
entire profile. The details and the problems concerning stationary and non-stationary conditions in pore water will be thoroughly discussed
in Section 3.2.
Figure 3.2b shows the concentration profile of
a substance which is consumed by early diagenetic reactions in the upper layers of the sediment. For example, the profiles of dissolved oxygen often look like this (cf. Chap. 6). Assuming
steady-state conditions (cf. Sect. 3.2), the
concentration gradient near the sediment surface
will display its highest increases, because whatever is consumed in the deeper layers will arrive
there by means of diffusion. With increasing
depths the concentration gradient and the diffusive material transport will decline, until, at a
particular depth, concentration, gradient, and
diffusion simultaneously approach zero. This
sediment depth, referred to as the penetration
depth of a substance, often represents a steadystate condition composed of the diffusive
reinforcement from above and the depletion of
the substance in the sediment by the reactions of
early diagenesis.
In principle, the situation depicted in Figure
3.2c is quite the same. Here, the process is just
limited to one reactive layer. With an example of
oxygen as the dissolved substance, this could
be a layer containing easily degradable organic
matter, or the sedimentary depth in which
oxygen encounters and reacts with a reductive
solute species coming from below (e.g. Mn
2+
or
Fe
2+
). At any rate, the space above the reactive
layer is characterized by a constant gradient in
the concentration profile and thus by a diffusive transport which is everywhere the same.
Within the reactive layer the dissolved substance is brought to zero concentration by
depletion.
The reverse case, which is in principle quite
similar, is demonstrated in Figure 3.2d and e.
Here, a substance is released anywhere into the
layers near the sediment surface (Fig. 3.2d),
whereas it might be released into the pore water
only in one particular layer (Fig. 3.2e). The solute
could be, for example, silica that often displays
such concentration profiles on account of the
dissolution of sedimentary opal.
The concentration profile shown in Figure
3.2f is much more complex than the others in
Figure 3.2 and only comprehensible if the
interactions of several processes taking place at
various depths below the sediment surface are
regarded. Such a profile is characteristic, for
example, of the concentrations of divalent manganese in marine pore water (see also Figure 3.1,
Figure 3.8, and Chapter 11). Manganese oxide
functions as an electron acceptor and reacts
with organic matter in reactive layer 2, whereupon chemically reduced divalent manganese
dissolves. A small percentage diffuses downwards (flat gradient) and precipitates as manganese carbonate or manganese sulfide in reactive
layer 3. The major proportion diffuses upwards
(much steeper gradient), encounters dissolved
oxygen from the sediment surface once again in
reactive layer 1, is reoxidized and precipitates
back to manganese oxide.
The described relations can be summarized by
stating a few rules for reading and understanding
of pore water concentration profiles:
• Diffusive material fluxes always occur in the
form of concentration gradients; concentration gradients always represent diffusive
material fluxes.
• Reactions occurring in pore water in most
cases constitute changes in the concentration gradient; changes in the concentration gradient always represent reactions
occurring in pore water.
• A concave-shaped alteration in the concentration gradient profile (cf. Fig. 3.2b, c)
signifies the depletion of a substance from
pore water; conversely, a convex-shaped
concentration gradient profile (cf. Fig. 3.2d,
e) always depicts the release of a substance
into the pore water.
However, the following also needs to be
observed:
• If a substance is involved in two reactions
taking place at the same depth, and is
released into the pore water fraction by one
reaction and then withdrawn again by the
other, neither of the participating reactions
will become evident in the pore-water
concentration profile.