113
transfer, i.e. seen in the long run, are integrated
into a permanent cycle. From the publications of
many authors (e.g. Aller 1988, 1990, 1994; Dicke
1986) it can be concluded that this layer,
essentially influenced by bioturbation, generally
reaches 5 to 10 cm, rarely even up to 15 cm, below
the sediment surface. Only when a sediment
particle has reached a greater depth in the course
of continued sedimentation, will the ‘recycling’
process become a rare event until it ultimately
comes to a final standstill.
This continued recycling of the sediment’s
solid phase is of great importance for a number of
geobiochemical processes. By the action of this
cycle, the electron donor (organic matter) and the
electron acceptors (e.g. iron and manganese
oxides) of the biogeochemical redox processes in
the sediment are continually procured from the
sediment surface. The results obtained by Fossing
and Jørgensen (1990) can only be understood
when considerably more sulfate is chemically
reduced in the upper layers of the sediment than
is accounted for by the sedimentation rate of
directly imported organic matter. The re-oxidation
of once released sulfide back to sulfate, which is
also of importance in this regard, can only
proceed when the oxides of iron and manganese,
the electron acceptors, are continually supplied
from the surface in an oxidized state, and return
there in a chemically reduced form. Such a cycle
has been described for iron and manganese, for
instance, by Aller (1990); Van Cappellen and Wang
(1996) as well as Haese (1997).
Bioirrigation
The process in which living organisms in the
sediment actively transport bottom water through
their habitats is known as bioirrigation. In this
process, oxygen-rich water is usually pumped into
the sediment, and water with less oxygen is
pumped out. Figure 3.26 which is derived from the
publication by Glud et al. (1994) demonstrates an
oxygen profile measured in situ with the aid of a
microelectrode. The electrode recorded a normal
profile reaching about 30 mm below the sediment
surface. Beyond, from about 30 mm to 80 mm
below the sediment surface, the oxygen
concentration was practically zero, whereas an
open cavity was detected between 80 and 90 mm
flooded with oxygen-rich bottom water.
In the publications submitted by Archer and
Devol (1992), a comparative study on the purely
diffusive oxygen flux (based on measurements
with microelectrodes) and the total oxygen flux
(based on incubations) was conducted for the
shelf and the continental slope off the State of
Washington. In a similar study, Glud et al. (1994)
investigated the continental slope off Angola and
Namibia. It was shown that the flux induced by
bioirrigation was several times larger than the
diffusive flux. However, such high fluxes only
occur in densely populated sediments, mostly on
the shelf. For deep sea conditions the total
oxygen uptake was only slightly higher than the
diffusive oxygen uptake of the sediment.
Advection of Pore Water
Except for bioirrigation, advection within the pore
water fraction may only result from pressure
gradients. Advection is sometimes referred to as
convection, yet the terms are used rather synonymously. There are, in principle, three different
potential causes leading to such pressure gradients, and thus to an advective flux:
• Sediment compaction and a resulting flow of
water towards the sediment surface.
• Seafloor areas with warmer currents and a
resulting upward-directed water flow, that
corresponds with water flows directed downwards at other locations.
• Currents in bottom water that induce pressure
differences at luv and lee sides of uneven
patches on the sedimentary surface.
The first case can be quite easily assessed
quantitatively on the basis of porosity measurements: one might imagine a fresh and water-rich
deposited sediment column and observe the compaction as a further descent of the solid phase
relative to the water that remains at the same
place. Hence, the upwards directed advective flux
results as the movement of water relative to the
further sinking sediment. If the sediment exhibits
a water content of approximately 0.9 at its
surface, and even a value of 0.5 in several meters
depth, and provided that the boundary between
sediment and bottom water moves upwards due
to the accumulation of new sediment, then it will
inevitably follow that the compaction induces an
advective flux which will not exceed values similar
to the rate of sedimentation. As for deep sea
3.6
Influence of Bioturbation, Bioirrigation, and Advection
transfer, i.e. seen in the long run, are integrated
into a permanent cycle. From the publications of
many authors (e.g. Aller 1988, 1990, 1994; Dicke
1986) it can be concluded that this layer,
essentially influenced by bioturbation, generally
reaches 5 to 10 cm, rarely even up to 15 cm, below
the sediment surface. Only when a sediment
particle has reached a greater depth in the course
of continued sedimentation, will the ‘recycling’
process become a rare event until it ultimately
comes to a final standstill.
This continued recycling of the sediment’s
solid phase is of great importance for a number of
geobiochemical processes. By the action of this
cycle, the electron donor (organic matter) and the
electron acceptors (e.g. iron and manganese
oxides) of the biogeochemical redox processes in
the sediment are continually procured from the
sediment surface. The results obtained by Fossing
and Jørgensen (1990) can only be understood
when considerably more sulfate is chemically
reduced in the upper layers of the sediment than
is accounted for by the sedimentation rate of
directly imported organic matter. The re-oxidation
of once released sulfide back to sulfate, which is
also of importance in this regard, can only
proceed when the oxides of iron and manganese,
the electron acceptors, are continually supplied
from the surface in an oxidized state, and return
there in a chemically reduced form. Such a cycle
has been described for iron and manganese, for
instance, by Aller (1990); Van Cappellen and Wang
(1996) as well as Haese (1997).
Bioirrigation
The process in which living organisms in the
sediment actively transport bottom water through
their habitats is known as bioirrigation. In this
process, oxygen-rich water is usually pumped into
the sediment, and water with less oxygen is
pumped out. Figure 3.26 which is derived from the
publication by Glud et al. (1994) demonstrates an
oxygen profile measured in situ with the aid of a
microelectrode. The electrode recorded a normal
profile reaching about 30 mm below the sediment
surface. Beyond, from about 30 mm to 80 mm
below the sediment surface, the oxygen
concentration was practically zero, whereas an
open cavity was detected between 80 and 90 mm
flooded with oxygen-rich bottom water.
In the publications submitted by Archer and
Devol (1992), a comparative study on the purely
diffusive oxygen flux (based on measurements
with microelectrodes) and the total oxygen flux
(based on incubations) was conducted for the
shelf and the continental slope off the State of
Washington. In a similar study, Glud et al. (1994)
investigated the continental slope off Angola and
Namibia. It was shown that the flux induced by
bioirrigation was several times larger than the
diffusive flux. However, such high fluxes only
occur in densely populated sediments, mostly on
the shelf. For deep sea conditions the total
oxygen uptake was only slightly higher than the
diffusive oxygen uptake of the sediment.
Advection of Pore Water
Except for bioirrigation, advection within the pore
water fraction may only result from pressure
gradients. Advection is sometimes referred to as
convection, yet the terms are used rather synonymously. There are, in principle, three different
potential causes leading to such pressure gradients, and thus to an advective flux:
• Sediment compaction and a resulting flow of
water towards the sediment surface.
• Seafloor areas with warmer currents and a
resulting upward-directed water flow, that
corresponds with water flows directed downwards at other locations.
• Currents in bottom water that induce pressure
differences at luv and lee sides of uneven
patches on the sedimentary surface.
The first case can be quite easily assessed
quantitatively on the basis of porosity measurements: one might imagine a fresh and water-rich
deposited sediment column and observe the compaction as a further descent of the solid phase
relative to the water that remains at the same
place. Hence, the upwards directed advective flux
results as the movement of water relative to the
further sinking sediment. If the sediment exhibits
a water content of approximately 0.9 at its
surface, and even a value of 0.5 in several meters
depth, and provided that the boundary between
sediment and bottom water moves upwards due
to the accumulation of new sediment, then it will
inevitably follow that the compaction induces an
advective flux which will not exceed values similar
to the rate of sedimentation. As for deep sea
3.6
Influence of Bioturbation, Bioirrigation, and Advection