slower and involves the physical mixing of both
solids and water by organismal activities. Accurately
quantifying the effect of irrigation has posed a significant modeling challenge, and for convenience it is
usually introduced into equations as a parameter
called non-local transport. Irrigation effect are
commonly observed in estuarine, shelf, and slope
locations overlain by oxygenated bottom water. The
importance of irrigation in promoting benthic exchange depends on the density of burrows, the depth
to which they penetrate, how frequently they are
flushed, and the length scale over which diagenetic
reactions for the solute of interest take place. Effect
of Irrigation on pore water profiles have not been
detected in sediments overlain by bottom waters low
in oxygen, due to the exclusion of macrofauna, or in
deep-sea settings where macrofauna are less abundant than in margin settings. Irrigation effects are
greatest in the upper 10 cm, but pore water profiles
in some deep basins of the California Borderland
indicate observable effects to nearly 2 m.
Boundary Conditions
Pore water profiles are dependent on boundary
conditions. An upper boundary condition is imposed
by the solute concentration in the overlying water,
although complications exist if the solute is very reactive. Above the sediment–water interface is a diffusive sublayer that may be several hundred
micrometers thick in the deep sea, and thinner in
more energetic environments. A significant concentration gradient may exist through this zone if the
scale length characterizing the solute profile in
the sediment is small enough to approach this distance. It can be more difficult to identify a lower
boundary condition. Sometimes the solute will approach a constant value if its reactions cease at
depth. In other cases, the solute may reach a constant
value dictated by solubility constraints or by its
disappearance.
Interpretation of Pore Water Profiles
Some examples of pore water profiles are illustrated
in Figure 3, drawn schematically to illustrate the
range of behavior that may be observed when different factors are important. Several assumptions
have been made in drawing these profiles. One is that
they represent steady-state, relative to the sediment–
water interface. A second is that the diffusivity is not
depth-dependent. A third is that any reactions go to
zero at infinite depth. A fourth is that advection has
been ignored. The shape of a profile is a clue to interpret what factors are important and their depth
dependence, but the shape may not have a unique
interpretation, particularly if these assumptions are
not valid. It is also important to remember that
concentration gradients of solutes adjust until
transport is equal to the net reactions occurring. If a
solute is involved in competing reactions, such as
dissolution of one phase and precipitation of a less
soluble phase, the net reaction could be zero and no
concentration gradient would be created.
See also
Authigenic Deposits. Calcium Carbonates.
Further Reading
Aller RC (1988) Benthic fauna and biogeochemical
processes in marine sediments: The role of burrow
structure. In: Blackburn TH and Sorensen J (eds.)
Nitrogen Cycling in Coastal Marine Environments,
pp. 301--338. New York: John Wiley.
Berner RA (1974) Kinetic models for early diagenesis of
nitrogen, sulfur, phosphorus, and silicon. In: Goldberg
ED (ed.) The Sea, vol. 5, pp. 427--450. New York: John
Wiley.
Berner RA (1980) Early Diagenesis. Princeton: Princeton
Press.
Boudreau BP (1997) Diagenetic Models and Their
Implementation. Berlin: Springer-Verlag.
Boudreau BP and Jorgensen BB (2000) The Benthic
Boundary Layer: Transport Processes and Biogeochemistry. New York: Oxford University Press.
Boudreau BP (2000) The mathematics of early diagenesis:
from worms to waves. Reviews of Geophysics 38:
389--416.
Burdige DJ (1993) The biogeochemistry of manganese and
iron reduction in marine sediments. Earth Science
Reviews 35: 249--284.
Fanning KA and Manheim FT (eds.) (1982) The Dynamic
Environment of the Ocean Floor. Lexington, MA: DC
Heath and Co.
Froelich PN, Klinkhammer GP, Bender ML, et al.
(1979) Early oxidation of organic matter in pelagic
sediments of the eastern equatorial Atlantic: Suboxic
diagenesis. Geochimica Cosmochimica Acta 43:
1075--1090.
Lerman A (1977) Migrational processes and chemical
reactions in interstitial waters. In: Goldberg E, McCave
I, O’Brien J, and Steele J (eds.) The Sea, vol. 6,
pp. 695--738. New York: John Wiley.
Li Y-H and Gregory S (1974) Diffusion of ions in sea water
and deep-sea sediments. Geochimica Cosmochimica
Acta 38: 703--714.
Luther GW III, Reimers CE, Nuzzio DB, and Lovalvo D
(1999) In situ deployment of voltammetric,
potentiometric, and amperometric microelectrodes
388 PORE WATER CHEMISTRY
solids and water by organismal activities. Accurately
quantifying the effect of irrigation has posed a significant modeling challenge, and for convenience it is
usually introduced into equations as a parameter
called non-local transport. Irrigation effect are
commonly observed in estuarine, shelf, and slope
locations overlain by oxygenated bottom water. The
importance of irrigation in promoting benthic exchange depends on the density of burrows, the depth
to which they penetrate, how frequently they are
flushed, and the length scale over which diagenetic
reactions for the solute of interest take place. Effect
of Irrigation on pore water profiles have not been
detected in sediments overlain by bottom waters low
in oxygen, due to the exclusion of macrofauna, or in
deep-sea settings where macrofauna are less abundant than in margin settings. Irrigation effects are
greatest in the upper 10 cm, but pore water profiles
in some deep basins of the California Borderland
indicate observable effects to nearly 2 m.
Boundary Conditions
Pore water profiles are dependent on boundary
conditions. An upper boundary condition is imposed
by the solute concentration in the overlying water,
although complications exist if the solute is very reactive. Above the sediment–water interface is a diffusive sublayer that may be several hundred
micrometers thick in the deep sea, and thinner in
more energetic environments. A significant concentration gradient may exist through this zone if the
scale length characterizing the solute profile in
the sediment is small enough to approach this distance. It can be more difficult to identify a lower
boundary condition. Sometimes the solute will approach a constant value if its reactions cease at
depth. In other cases, the solute may reach a constant
value dictated by solubility constraints or by its
disappearance.
Interpretation of Pore Water Profiles
Some examples of pore water profiles are illustrated
in Figure 3, drawn schematically to illustrate the
range of behavior that may be observed when different factors are important. Several assumptions
have been made in drawing these profiles. One is that
they represent steady-state, relative to the sediment–
water interface. A second is that the diffusivity is not
depth-dependent. A third is that any reactions go to
zero at infinite depth. A fourth is that advection has
been ignored. The shape of a profile is a clue to interpret what factors are important and their depth
dependence, but the shape may not have a unique
interpretation, particularly if these assumptions are
not valid. It is also important to remember that
concentration gradients of solutes adjust until
transport is equal to the net reactions occurring. If a
solute is involved in competing reactions, such as
dissolution of one phase and precipitation of a less
soluble phase, the net reaction could be zero and no
concentration gradient would be created.
See also
Authigenic Deposits. Calcium Carbonates.
Further Reading
Aller RC (1988) Benthic fauna and biogeochemical
processes in marine sediments: The role of burrow
structure. In: Blackburn TH and Sorensen J (eds.)
Nitrogen Cycling in Coastal Marine Environments,
pp. 301--338. New York: John Wiley.
Berner RA (1974) Kinetic models for early diagenesis of
nitrogen, sulfur, phosphorus, and silicon. In: Goldberg
ED (ed.) The Sea, vol. 5, pp. 427--450. New York: John
Wiley.
Berner RA (1980) Early Diagenesis. Princeton: Princeton
Press.
Boudreau BP (1997) Diagenetic Models and Their
Implementation. Berlin: Springer-Verlag.
Boudreau BP and Jorgensen BB (2000) The Benthic
Boundary Layer: Transport Processes and Biogeochemistry. New York: Oxford University Press.
Boudreau BP (2000) The mathematics of early diagenesis:
from worms to waves. Reviews of Geophysics 38:
389--416.
Burdige DJ (1993) The biogeochemistry of manganese and
iron reduction in marine sediments. Earth Science
Reviews 35: 249--284.
Fanning KA and Manheim FT (eds.) (1982) The Dynamic
Environment of the Ocean Floor. Lexington, MA: DC
Heath and Co.
Froelich PN, Klinkhammer GP, Bender ML, et al.
(1979) Early oxidation of organic matter in pelagic
sediments of the eastern equatorial Atlantic: Suboxic
diagenesis. Geochimica Cosmochimica Acta 43:
1075--1090.
Lerman A (1977) Migrational processes and chemical
reactions in interstitial waters. In: Goldberg E, McCave
I, O’Brien J, and Steele J (eds.) The Sea, vol. 6,
pp. 695--738. New York: John Wiley.
Li Y-H and Gregory S (1974) Diffusion of ions in sea water
and deep-sea sediments. Geochimica Cosmochimica
Acta 38: 703--714.
Luther GW III, Reimers CE, Nuzzio DB, and Lovalvo D
(1999) In situ deployment of voltammetric,
potentiometric, and amperometric microelectrodes
388 PORE WATER CHEMISTRY