Table 2. These redox reactions can also result in the
production or consumption of acid, depending on
the suite of reactants available. The addition of acid
should be buffered by the dissolution of carbonate
solids, if they are present, and removal of acid may
promote precipitation of carbonates. For example,
as shown in Table 2B, utilization of oxygen as a
terminal electron acceptor is quite effective in producing acid, and should favor the dissolution of any
carbonate minerals in the oxygenated zone. If
manganese or iron serves as the terminal electron
acceptor, acid is consumed, and carbonate may
precipitate from the pore waters. If sulfate serves as
the terminal electron acceptor, and no iron oxides
are available to permit iron sulfides to precipitate,
pore waters are acidified and may dissolve carbonates. A similar behavior occurs if iron can be leached from silicates and exchanged for dissolved
magnesium. However, if iron oxyhydroxides are
present, they dissolve and favor carbonate precipitation. This example illustrates the importance of
iron availability in helping regulate the pH of marine pore waters. The behavior of nitrogen (oxidized
to nitric acid if oxygen is present, and consuming a
proton if it is released as ammonia in the absence of
oxygen) also contributes to pH buffering. Many
other solid phases respond to these pH changes,
changes that are largely influenced by the oxidation
of organic matter and the behavior of carbonate and
iron-bearing minerals.
Another interesting effect is the co-precipitation of
some trace constituents with phases created by cycles
of a more abundant substance. For example, as ferrous iron diffuses upward, other trace metals or
phosphate may co-precipitate with the oxyhydroxides when oxygen or nitrate is encountered. These
constituents may re-dissolve as the ferric oxyhydroxides are buried more deeply.
Reversible Adsorption and Desorption
Pore waters are in intimate contact with solid surfaces, and may exchange solutes reversibly on short
timescales. Consequently, a change in pore water
concentration is accompanied by an additional
change in the adsorbed inventory. If a solute is
strongly adsorbed, its transport will be dominated by
movement of the particulate phase, rather than by
migration through the dissolved phase. The ionic
speciation of the solute is often critical in determining the degree of adsorption. Adsorption and
desorption equilibria are temperature dependent,
and are responsible for some of the artifacts noted
earlier.
Diffusion
The random motion of solutes in pore waters results
in a net transport from regions of high concentration
to regions of low concentration, as described by
Fick’s laws for diffusion. This is the principal
mechanism for transport over short distance scales,
and the rate of diffusion depends on the solute in
question, the porosity of the sediment, the temperature, and to some extent on the ensemble of other
ions present and their concentration gradients. This
last effect results from the speciation of the ion and
cross-coupling among ion gradients that produces
electrical potentials. A rough estimate of the relationship between length- and time-scales for which
diffusion is effective can be obtained from using
Fick’s Second Law to evaluate the time for a diffusive
front to migrate from a perturbation introduced in a
one-dimensional system. The time is given by the
relationship t ¼ x
2 /(2D), where t is the time for response at a distance x from the perturbation in a
sediment with a diffusivity of D. In deep-sea sediments, many solutes have diffusivities in the range of
5 Â 10
À6 cm
2 s
À1
, so the diffusive front migrates
approximately 1 cm in 1 day, but requires 30 years to
migrate 1 m and 0.3 million years to migrate 100 m.
Advection
It is most convenient to define pore water spatial
coordinates relative to the sediment–water interface.
If sediments are accumulating and contain some
water, pore waters must be moving relative to this
interface. In addition, flow may be driven by strong
heating at depth, or other externally imposed forcing. These directed flows are considered advection
and may be an important transport mechanism. The
relative importance of advection in transporting a
solute can be evaluated by considering the dimensionless Peclet number, D/UL, where D represents the
diffusivity through sediments, U is the flow velocity,
and L is the length scale over which transport must
be accomplished. If this number is much larger than
one, advection can be ignored, and transport is
dominated by diffusion.
Irrigation
In coastal and slope sediments, organisms create
burrows that act as conduits through which bottom
waters may move . Flow through a burrow may be
driven by active pumping by the organism, or by
hydrodynamic effects created by the flow of bottom
waters past the burrow orifice. Thus, the presence of
burrows creates a complex geometry for the effective
shape of the sediment–water interface and for the
386 PORE WATER CHEMISTRY
production or consumption of acid, depending on
the suite of reactants available. The addition of acid
should be buffered by the dissolution of carbonate
solids, if they are present, and removal of acid may
promote precipitation of carbonates. For example,
as shown in Table 2B, utilization of oxygen as a
terminal electron acceptor is quite effective in producing acid, and should favor the dissolution of any
carbonate minerals in the oxygenated zone. If
manganese or iron serves as the terminal electron
acceptor, acid is consumed, and carbonate may
precipitate from the pore waters. If sulfate serves as
the terminal electron acceptor, and no iron oxides
are available to permit iron sulfides to precipitate,
pore waters are acidified and may dissolve carbonates. A similar behavior occurs if iron can be leached from silicates and exchanged for dissolved
magnesium. However, if iron oxyhydroxides are
present, they dissolve and favor carbonate precipitation. This example illustrates the importance of
iron availability in helping regulate the pH of marine pore waters. The behavior of nitrogen (oxidized
to nitric acid if oxygen is present, and consuming a
proton if it is released as ammonia in the absence of
oxygen) also contributes to pH buffering. Many
other solid phases respond to these pH changes,
changes that are largely influenced by the oxidation
of organic matter and the behavior of carbonate and
iron-bearing minerals.
Another interesting effect is the co-precipitation of
some trace constituents with phases created by cycles
of a more abundant substance. For example, as ferrous iron diffuses upward, other trace metals or
phosphate may co-precipitate with the oxyhydroxides when oxygen or nitrate is encountered. These
constituents may re-dissolve as the ferric oxyhydroxides are buried more deeply.
Reversible Adsorption and Desorption
Pore waters are in intimate contact with solid surfaces, and may exchange solutes reversibly on short
timescales. Consequently, a change in pore water
concentration is accompanied by an additional
change in the adsorbed inventory. If a solute is
strongly adsorbed, its transport will be dominated by
movement of the particulate phase, rather than by
migration through the dissolved phase. The ionic
speciation of the solute is often critical in determining the degree of adsorption. Adsorption and
desorption equilibria are temperature dependent,
and are responsible for some of the artifacts noted
earlier.
Diffusion
The random motion of solutes in pore waters results
in a net transport from regions of high concentration
to regions of low concentration, as described by
Fick’s laws for diffusion. This is the principal
mechanism for transport over short distance scales,
and the rate of diffusion depends on the solute in
question, the porosity of the sediment, the temperature, and to some extent on the ensemble of other
ions present and their concentration gradients. This
last effect results from the speciation of the ion and
cross-coupling among ion gradients that produces
electrical potentials. A rough estimate of the relationship between length- and time-scales for which
diffusion is effective can be obtained from using
Fick’s Second Law to evaluate the time for a diffusive
front to migrate from a perturbation introduced in a
one-dimensional system. The time is given by the
relationship t ¼ x
2 /(2D), where t is the time for response at a distance x from the perturbation in a
sediment with a diffusivity of D. In deep-sea sediments, many solutes have diffusivities in the range of
5 Â 10
À6 cm
2 s
À1
, so the diffusive front migrates
approximately 1 cm in 1 day, but requires 30 years to
migrate 1 m and 0.3 million years to migrate 100 m.
Advection
It is most convenient to define pore water spatial
coordinates relative to the sediment–water interface.
If sediments are accumulating and contain some
water, pore waters must be moving relative to this
interface. In addition, flow may be driven by strong
heating at depth, or other externally imposed forcing. These directed flows are considered advection
and may be an important transport mechanism. The
relative importance of advection in transporting a
solute can be evaluated by considering the dimensionless Peclet number, D/UL, where D represents the
diffusivity through sediments, U is the flow velocity,
and L is the length scale over which transport must
be accomplished. If this number is much larger than
one, advection can be ignored, and transport is
dominated by diffusion.
Irrigation
In coastal and slope sediments, organisms create
burrows that act as conduits through which bottom
waters may move . Flow through a burrow may be
driven by active pumping by the organism, or by
hydrodynamic effects created by the flow of bottom
waters past the burrow orifice. Thus, the presence of
burrows creates a complex geometry for the effective
shape of the sediment–water interface and for the
386 PORE WATER CHEMISTRY