PORE WATER CHEMISTRY
D. Hammond, University of Southern California,
Los Angeles, CA, USA
Copyright & 2001 Elsevier Ltd.
Introduction
As marine sediments are deposited, they trap sea
water in the pore space between grains. This pore
water (sometimes called interstitial water) may represent more than 90% of the volume of the bulk
sediment in fine-grained deposits near the sediment–
water interface. The volume fraction of the bulk
sediment that is water is called the porosity. Porosity
usually decreases rapidly with increasing depth
through the uppermost sediments, with the profile
depending on lithology and accumulation rate. Ultimately, porosity may decrease to only 5–20% as the
sediment becomes lithified. As sediments are compacted during burial, the pore fluid is squeezed upward, traveling through fractures, burrows, or
perhaps the sediment pore space itself.
As sediment is buried, its composition may be
modified by chemical reactions, a process called
chemical diagenesis. Pore water provides a medium
that permits a solute to migrate from a site where it is
produced to another site where it may be removed.
For example, in organic-rich sediments, pyrite (FeS 2 )
is a common end product of diagenesis. While the
intermediate steps in pyrite formation are not fully
understood, they involve sulfate reduction to sulfide
at sites where reactive organic matter is found, and
reduction of insoluble ferric oxides to form soluble
ferrous iron at sites of iron-bearing minerals. Iron
sulfides have very low solubility, and their deposition
is usually localized at one of the two sites. If sulfide is
released faster than ferrous iron, the sulfide diffuses
from its site of production on an organic-rich particle
to the mineral containing iron, where it forms insoluble iron sulfides. This can produce pyrite overgrowths on iron-bearing minerals. Alternatively, if
the iron is more readily released, it diffuses to form
iron sulfides near the sites of organic particles such as
shells or localized pockets of organic substrate.
Because diagenesis may alter only a small fraction
of the solid phases, its impact may be difficult to detect from studies of solid phases alone. Pore water
chemistry is much more sensitive to such changes. For
example, in a sediment of 80% porosity, dissolution
of 0.1 weight percent CaCO 3 from the solid phase
(near the detection limit measurable in solid phases)
would make a change of 6 mmol kg
À1 in the concentration of dissolved calcium. This change in pore
water would be easily detectable because it results in a
concentration 60% greater than that in the starting
sea water. Of course, this calculation assumes that the
pore water acts as a closed system, which is generally
not the case as noted below. However, this example
illustrates that pore water chemistry is more sensitive
than solid phase chemistry to diagenesis.
Studies of pore waters have become a standard
tool for understanding the biogeochemical processes
that influence sediments, and considerable efforts
have been invested during the past several decades to
develop techniques to collect samples, evaluate
whether vertical profiles exhibit artifacts introduced
during collection and handling, and develop approaches to model the observed profiles and obtain
quantitative estimates of reaction kinetics and stoichiometry. Usually, modeling approaches assume
steady-state behavior, but when time-dependent
constraints can be established, nonsteady-state approaches can be applied.
Reasons to Study Pore Water
Composition
The study of pore waters can reveal many processes
that are important in regulating the biogeochemical
cycles of the ocean and in evaluating the impact of
diagenesis on the sedimentary record recorded in
solid phases. Some applications of pore water studies
are given below.
Calculation of Mineral Stability
Thermodynamic calculations can be carried out to
determine which solid phases should be dissolving,
precipitating, or in equilibrium with the pore fluid
chemistry. While these calculations do not guarantee
the presence of minerals that are at or above saturation, or the absence of minerals that are undersaturated, they are a very useful indicator of whether
it may be worthwhile to search for minerals that
could be present in only trace abundance.
Identification of Sites of Reaction and Reaction
Stoichiometry
Maxima in pore water profiles indicate localized inputs, and minima define sinks for solutes. However,
381
D. Hammond, University of Southern California,
Los Angeles, CA, USA
Copyright & 2001 Elsevier Ltd.
Introduction
As marine sediments are deposited, they trap sea
water in the pore space between grains. This pore
water (sometimes called interstitial water) may represent more than 90% of the volume of the bulk
sediment in fine-grained deposits near the sediment–
water interface. The volume fraction of the bulk
sediment that is water is called the porosity. Porosity
usually decreases rapidly with increasing depth
through the uppermost sediments, with the profile
depending on lithology and accumulation rate. Ultimately, porosity may decrease to only 5–20% as the
sediment becomes lithified. As sediments are compacted during burial, the pore fluid is squeezed upward, traveling through fractures, burrows, or
perhaps the sediment pore space itself.
As sediment is buried, its composition may be
modified by chemical reactions, a process called
chemical diagenesis. Pore water provides a medium
that permits a solute to migrate from a site where it is
produced to another site where it may be removed.
For example, in organic-rich sediments, pyrite (FeS 2 )
is a common end product of diagenesis. While the
intermediate steps in pyrite formation are not fully
understood, they involve sulfate reduction to sulfide
at sites where reactive organic matter is found, and
reduction of insoluble ferric oxides to form soluble
ferrous iron at sites of iron-bearing minerals. Iron
sulfides have very low solubility, and their deposition
is usually localized at one of the two sites. If sulfide is
released faster than ferrous iron, the sulfide diffuses
from its site of production on an organic-rich particle
to the mineral containing iron, where it forms insoluble iron sulfides. This can produce pyrite overgrowths on iron-bearing minerals. Alternatively, if
the iron is more readily released, it diffuses to form
iron sulfides near the sites of organic particles such as
shells or localized pockets of organic substrate.
Because diagenesis may alter only a small fraction
of the solid phases, its impact may be difficult to detect from studies of solid phases alone. Pore water
chemistry is much more sensitive to such changes. For
example, in a sediment of 80% porosity, dissolution
of 0.1 weight percent CaCO 3 from the solid phase
(near the detection limit measurable in solid phases)
would make a change of 6 mmol kg
À1 in the concentration of dissolved calcium. This change in pore
water would be easily detectable because it results in a
concentration 60% greater than that in the starting
sea water. Of course, this calculation assumes that the
pore water acts as a closed system, which is generally
not the case as noted below. However, this example
illustrates that pore water chemistry is more sensitive
than solid phase chemistry to diagenesis.
Studies of pore waters have become a standard
tool for understanding the biogeochemical processes
that influence sediments, and considerable efforts
have been invested during the past several decades to
develop techniques to collect samples, evaluate
whether vertical profiles exhibit artifacts introduced
during collection and handling, and develop approaches to model the observed profiles and obtain
quantitative estimates of reaction kinetics and stoichiometry. Usually, modeling approaches assume
steady-state behavior, but when time-dependent
constraints can be established, nonsteady-state approaches can be applied.
Reasons to Study Pore Water
Composition
The study of pore waters can reveal many processes
that are important in regulating the biogeochemical
cycles of the ocean and in evaluating the impact of
diagenesis on the sedimentary record recorded in
solid phases. Some applications of pore water studies
are given below.
Calculation of Mineral Stability
Thermodynamic calculations can be carried out to
determine which solid phases should be dissolving,
precipitating, or in equilibrium with the pore fluid
chemistry. While these calculations do not guarantee
the presence of minerals that are at or above saturation, or the absence of minerals that are undersaturated, they are a very useful indicator of whether
it may be worthwhile to search for minerals that
could be present in only trace abundance.
Identification of Sites of Reaction and Reaction
Stoichiometry
Maxima in pore water profiles indicate localized inputs, and minima define sinks for solutes. However,
381