defining the transition between source and sink regions requires location of inflection points in the
pore water profile. If the pore water profile is in
steady-state, material balance calculations can be
carried out for solutes if the transport mechanisms
are known. This usually involves fitting a reactiontransport model that defines the depth dependence of
the net reaction-kinetics. Transport processes are
discussed below, and may include molecular diffusion, advection, and macrofaunal irrigation. The
impact of reactions occurring at depths beyond the
range of sampling may also be evident in pore water
profiles.
Interpretation of the Sedimentary Record
Are changes in solid phase profiles with depth due to
diagenetic reactions during burial, or due to temporal variations in the composition or rain rate of
solid phase inputs? Pore water profiles provide a way
to evaluate the contemporary rates of reactions (assuming they are in steady-state) and predict the effect
of diagenetic reactions on solid phase profiles. Solid
phase changes that exceed the diagenetic effects derived from modeling pore water profiles must reflect
nonsteady-state behavior in the input of solid phases
to the sediment column.
Estimation of Benthic Exchange Rates
Sediments are a sink or source for many solutes in
the water column. Thus, they can play an important
role in regulating the composition of the overlying
waters. This approach provides information that can
be compared to direct measurements of benthic
fluxes.
Recovery of Deep Pore Water that may be Fossil
Water
The trapping of pore fluids as sediments are buried
may potentially preserve fluid from a time when
ocean composition differed from the present, such as
the last glacial period when salinity should have been
greater than at present and the isotopic composition
of water should have been heavier. However, pore
water is an open system, and diffusion facilitates the
re-equilibration between fossil pore water and bottom waters. Consequently, relict signals may be difficult to detect, even in the absence of any influence
of diagenetic reactions.
Sampling Techniques
Initial studies of pore waters utilized retrieval of cores,
sectioning them into intervals, and centrifugation to
separate pore waters from the associated solids. This
approach works well for many solutes in sediments
with high porosity, as long as appropriate precautions are taken to minimize artifacts (changes in
composition attributable to recovering and processing samples). During sample processing, it is often
critical to regulate temperature and eliminate contact
between reducing sediments and oxygen, depending
on the solute of interest. For studies related to nearsurface diagenesis, it is essential to obtain cores that
have undisturbed interfaces and with bottom water
still in contact with the sediment. To extract water
from low porosity sediments, or minimize contact
with gas phases, squeezing techniques have been
developed. Several kinds of squeezing devices have
been utilized, but all rely on compressing sediments
while permitting water to escape through a filtration
assembly.
To avoid artifacts associated with retrieving cores
from the deep sea, devices have been developed to
carry out filtration in situ. These devices avoid the
pressure- and temperature-dependent perturbations
associated with core retrieval, but have their own
logistical difficulties in deployment to minimize
leakage and obtain accurate sampling resolution.
One strategy drives a probe called a harpoon into
sediments; openings at various distances along the
harpoon shaft permit water to be drawn through
filters into sample reservoirs. Another device collects
cores, seals the bottom, and squeezes water by
driving a piston and filter pack down onto the core;
sequential aliquots of water are collected and assumed to represent water from progressively deeper
intervals. Other strategies have relied on inserting
probes that contain water that may communicate
with pore water through a dialysis membrane. These
devices, named ‘peepers’, require several days to
equilibrate with pore waters and must be initially
filled with a solution that will not significantly contaminate the surrounding sediment with exotic
solutes.
Several of the artifacts noted above may be avoided through the use of in situ electrodes that can be
inserted directly into sediment and measure activities
of various solutes. Systems to measure oxygen and
pH are often used. Very recently, new electrodes to
measure pCO 2 , sulfide, iron, and manganese have
been developed. By using microelectrodes, gradients
over short distances can be resolved.
Finally, some tools have been developed to retrieve
pressurized cores and extract pore fluids onboard
ships at in situ pressures . These tools are particularly
important where high quantities of methane are
found, either dissolved in pore fluid or as a gas
hydrate.
382 PORE WATER CHEMISTRY
pore water profile. If the pore water profile is in
steady-state, material balance calculations can be
carried out for solutes if the transport mechanisms
are known. This usually involves fitting a reactiontransport model that defines the depth dependence of
the net reaction-kinetics. Transport processes are
discussed below, and may include molecular diffusion, advection, and macrofaunal irrigation. The
impact of reactions occurring at depths beyond the
range of sampling may also be evident in pore water
profiles.
Interpretation of the Sedimentary Record
Are changes in solid phase profiles with depth due to
diagenetic reactions during burial, or due to temporal variations in the composition or rain rate of
solid phase inputs? Pore water profiles provide a way
to evaluate the contemporary rates of reactions (assuming they are in steady-state) and predict the effect
of diagenetic reactions on solid phase profiles. Solid
phase changes that exceed the diagenetic effects derived from modeling pore water profiles must reflect
nonsteady-state behavior in the input of solid phases
to the sediment column.
Estimation of Benthic Exchange Rates
Sediments are a sink or source for many solutes in
the water column. Thus, they can play an important
role in regulating the composition of the overlying
waters. This approach provides information that can
be compared to direct measurements of benthic
fluxes.
Recovery of Deep Pore Water that may be Fossil
Water
The trapping of pore fluids as sediments are buried
may potentially preserve fluid from a time when
ocean composition differed from the present, such as
the last glacial period when salinity should have been
greater than at present and the isotopic composition
of water should have been heavier. However, pore
water is an open system, and diffusion facilitates the
re-equilibration between fossil pore water and bottom waters. Consequently, relict signals may be difficult to detect, even in the absence of any influence
of diagenetic reactions.
Sampling Techniques
Initial studies of pore waters utilized retrieval of cores,
sectioning them into intervals, and centrifugation to
separate pore waters from the associated solids. This
approach works well for many solutes in sediments
with high porosity, as long as appropriate precautions are taken to minimize artifacts (changes in
composition attributable to recovering and processing samples). During sample processing, it is often
critical to regulate temperature and eliminate contact
between reducing sediments and oxygen, depending
on the solute of interest. For studies related to nearsurface diagenesis, it is essential to obtain cores that
have undisturbed interfaces and with bottom water
still in contact with the sediment. To extract water
from low porosity sediments, or minimize contact
with gas phases, squeezing techniques have been
developed. Several kinds of squeezing devices have
been utilized, but all rely on compressing sediments
while permitting water to escape through a filtration
assembly.
To avoid artifacts associated with retrieving cores
from the deep sea, devices have been developed to
carry out filtration in situ. These devices avoid the
pressure- and temperature-dependent perturbations
associated with core retrieval, but have their own
logistical difficulties in deployment to minimize
leakage and obtain accurate sampling resolution.
One strategy drives a probe called a harpoon into
sediments; openings at various distances along the
harpoon shaft permit water to be drawn through
filters into sample reservoirs. Another device collects
cores, seals the bottom, and squeezes water by
driving a piston and filter pack down onto the core;
sequential aliquots of water are collected and assumed to represent water from progressively deeper
intervals. Other strategies have relied on inserting
probes that contain water that may communicate
with pore water through a dialysis membrane. These
devices, named ‘peepers’, require several days to
equilibrate with pore waters and must be initially
filled with a solution that will not significantly contaminate the surrounding sediment with exotic
solutes.
Several of the artifacts noted above may be avoided through the use of in situ electrodes that can be
inserted directly into sediment and measure activities
of various solutes. Systems to measure oxygen and
pH are often used. Very recently, new electrodes to
measure pCO 2 , sulfide, iron, and manganese have
been developed. By using microelectrodes, gradients
over short distances can be resolved.
Finally, some tools have been developed to retrieve
pressurized cores and extract pore fluids onboard
ships at in situ pressures . These tools are particularly
important where high quantities of methane are
found, either dissolved in pore fluid or as a gas
hydrate.
382 PORE WATER CHEMISTRY