112
3
Quantification of Early Diagenesis: Dissolved Constituents in Marine Pore Water
restricts the use of dialysis-peepers to shallow
water environments where diving is possible, and
thus makes it especially useful in tidal areas. It
should be pointed out that dialysis does not
simply result in the sampling of pore water under
in situ conditions. Only those aquatic species that
are small enough to pass through the membrane
can find their way through the peephole into the
cell. The species distribution is therefore dependent on the nature of the membrane used. A
review of different techniques is given by
Davison et al. (2000). More relevant to the study
of deep sea sediments are in situ techniques
using the diffusive equilibration in thin films
(DET) and diffusive gradients in thin films (DGT)
(Davison et al. 1991; Davison and Zhang 1994;
Davison et al. 1997; Davison et al. 2000).
Figure 3.25 shows concentration-depth
profiles for Mn and Fe through the sediment/water
interface with a depth resolution which would be
impossible to obtain using any other technique.
Measurements such as these have resulted in
meaningful measurements of reaction rates within
the uppermost layers of young sediments and of
diffusive fluxes of various metals through the
sediment/water interface for the first time.
However, the preparation of the thin films, and
their handling under field conditions is still quite
troublesome and much work needs to be undertaken to further develop these techniques.
Nonetheless, this seems to be a most promising
approach for obtaining reliable in situ determinations of elements in sediment pore waters.
3.6
Influence of Bioturbation,
Bioirrigation, and Advection
For the pure geochemist it would be most convenient if this chapter would close at this point.
The concomitance of diffusive transport and
reactions determinable on the basis of gradient
alterations - although they are rather numerous -
results in a sufficiently sophisticated system of
pore water and sediment, quite well understood
and calculable. For deeper zones below the sediment surface, beyond approximately 0.5 to 1 m, a
description of the system in terms of diffusion and
reactions probably seems to be quite acceptable,
with a certain degree of accuracy obtained. The
closer the zones under study are located to the
sediment surface, the more inhomogeneities and
non-steady state conditions become of quantitative importance. These are mainly determined by
the activity of organisms living in the sediment.
Depending on whether the effect on the sediment or the pore water is considered, this phenomenon is referred to as bioturbation or bioirrigation, respectively. Advection (sometimes also
called convection) is the term used for the motion
of water coupled to a pressure gradient which
partly overlaps with bioirrigation, when permeabilities are influenced by the habitats of the organisms. At the end of this section, model concepts
will be outlined that allow for a quantitative
description of this heterogeneous, yet very reactive domain of the sediment.
Bioturbation
Bioturbation refers to the spatial rearrangement of
the sediment’s solid phase by diverse organisms,
at least temporarily living in the sediment. This
process implies that all sedimentary particles in
the upper layers, which are inhabited by
macroorganisms, are subject to a continual
Fig. 3.26 In situ measurement of an oxygen profile in a
water depth of 3100 m, off the mouth of the Congo River.
The microelectrode detected an open cavity flooded with
oxygen-rich bottom water in a depth between 80 mm and
90 mm below the sediment surface (after Glud et al. 1994).
3
Quantification of Early Diagenesis: Dissolved Constituents in Marine Pore Water
restricts the use of dialysis-peepers to shallow
water environments where diving is possible, and
thus makes it especially useful in tidal areas. It
should be pointed out that dialysis does not
simply result in the sampling of pore water under
in situ conditions. Only those aquatic species that
are small enough to pass through the membrane
can find their way through the peephole into the
cell. The species distribution is therefore dependent on the nature of the membrane used. A
review of different techniques is given by
Davison et al. (2000). More relevant to the study
of deep sea sediments are in situ techniques
using the diffusive equilibration in thin films
(DET) and diffusive gradients in thin films (DGT)
(Davison et al. 1991; Davison and Zhang 1994;
Davison et al. 1997; Davison et al. 2000).
Figure 3.25 shows concentration-depth
profiles for Mn and Fe through the sediment/water
interface with a depth resolution which would be
impossible to obtain using any other technique.
Measurements such as these have resulted in
meaningful measurements of reaction rates within
the uppermost layers of young sediments and of
diffusive fluxes of various metals through the
sediment/water interface for the first time.
However, the preparation of the thin films, and
their handling under field conditions is still quite
troublesome and much work needs to be undertaken to further develop these techniques.
Nonetheless, this seems to be a most promising
approach for obtaining reliable in situ determinations of elements in sediment pore waters.
3.6
Influence of Bioturbation,
Bioirrigation, and Advection
For the pure geochemist it would be most convenient if this chapter would close at this point.
The concomitance of diffusive transport and
reactions determinable on the basis of gradient
alterations - although they are rather numerous -
results in a sufficiently sophisticated system of
pore water and sediment, quite well understood
and calculable. For deeper zones below the sediment surface, beyond approximately 0.5 to 1 m, a
description of the system in terms of diffusion and
reactions probably seems to be quite acceptable,
with a certain degree of accuracy obtained. The
closer the zones under study are located to the
sediment surface, the more inhomogeneities and
non-steady state conditions become of quantitative importance. These are mainly determined by
the activity of organisms living in the sediment.
Depending on whether the effect on the sediment or the pore water is considered, this phenomenon is referred to as bioturbation or bioirrigation, respectively. Advection (sometimes also
called convection) is the term used for the motion
of water coupled to a pressure gradient which
partly overlaps with bioirrigation, when permeabilities are influenced by the habitats of the organisms. At the end of this section, model concepts
will be outlined that allow for a quantitative
description of this heterogeneous, yet very reactive domain of the sediment.
Bioturbation
Bioturbation refers to the spatial rearrangement of
the sediment’s solid phase by diverse organisms,
at least temporarily living in the sediment. This
process implies that all sedimentary particles in
the upper layers, which are inhabited by
macroorganisms, are subject to a continual
Fig. 3.26 In situ measurement of an oxygen profile in a
water depth of 3100 m, off the mouth of the Congo River.
The microelectrode detected an open cavity flooded with
oxygen-rich bottom water in a depth between 80 mm and
90 mm below the sediment surface (after Glud et al. 1994).