CHEMICAL PROCESSES IN ESTUARINE
SEDIMENTS
W. R. Martin, Woods Hole Oceanographic Institution,
Woods Hole, MA, USA
& 2009 Elsevier Ltd. All rights reserved.
Introduction
The physical and chemical environments of estuarine
sediments vary over wide ranges. The sediments may
lie beneath nearly fresh water or may have salinities
near that of the open ocean. They may be intertidal
or always under several meters of water. They may
be covered by grasses, by algal or bacterial mats, or
may be free of surface biological cover. They may be
permeable sands, affected by groundwater flow, or
may be impermeable muds. Nonetheless, common
features determine their chemical environments.
They exist in productive, coastal waters and therefore experience large fluxes of particulate organic
matter. They experience large inputs of terrestrial
material that supply Fe and Mn oxides. Most estuarine sediments lie underneath oxic water columns,
and support abundant macrofauna that both mix
sedimentary particles and irrigate the seabed. Common features such as oxic bottom water, abundant
supplies of organic matter, Fe and Mn oxides, a
supply of sulfur from seawater (except for freshwater
end members of estuarine systems), and active macrofauna produce the set of biogeochemical processes
that determines the chemical environment and its
effects on carbon, nutrient, and contaminant cycling
in the coastal ocean.
The upper sediment column in estuarine and coastal
environments can be regarded as a slowly stirred reactor to which substrates are added at the top and
mixed downward. Transport processes, both in the
dissolved and solid phases, distribute reactants within
the reactor, while at the same time biogeochemical
processes alter the particles and the pore waters surrounding them. This combination of transport and
reaction drives solute exchanges between the sediments and overlying waters that are important to
coastal carbon, nutrient, and contaminant cycles. It
transforms the ‘reactive’ component of sedimentary
particles so that the accumulating sediments differ in
important ways from the particles that fall through
the water column to the seafloor. These processes are
outlined in Figure 1.
Because the rates of chemical reactions in this
system are faster than mixing rates, the reactor is not
chemically uniform; rather, roughly speaking, it has a
layered structure. In all but a few locations, there is a
thin, oxic layer at the sediment surface. As particles
pass through this layer and through the underlying
anoxic sediments, they are altered by heterotrophic
respiration: organic matter is gradually oxidized, and
a series of oxidants – O 2 , N(V) in NO 3
À , Mn(IV) and
Fe(III) in particulate oxides, and S(VI) in SO 4
2À – are
consumed. Some of the energy originally stored in
organic matter remains in the reduced products of
these reactions: Fe(II), and S(–II), and, to some extent,
Mn(II). This energy is available to chemolithotrophic
bacteria, and the result is rapid internal cycling between oxidized and reduced forms of Mn, Fe, and S.
Reduced Mn, Fe, and S that escape bacterial oxidation may be oxidized abiotically by O 2 in the surface oxic layer. The result of these processes is
sediments with an oxic ‘cap’, in which precipitation
of newly formed Fe and Mn oxides may limit benthic/
water column exchange of many solutes, and an
underlying anoxic layer in which rapid cycling
between oxidized and reduced forms limits the rate
of burial of products of heterotrophic respiration.
The chemical processes defining this system are
oxidation–reduction reactions, including ‘heterotrophic respiration’, by which organic matter is
oxidized, ‘chemolithotrophic respiration’, most importantly oxidizing Fe(II) and S(–II), and ‘abiotic
oxidation’; and ‘authigenic mineral formation’, which
converts dissolved products of these reactions to solid
phases. The stirring within the sedimentary reactor,
which is vital to its operation, is accomplished by
solute diffusion and particle mixing, accomplished
predominantly by the feeding and burrowing activities of sedimentary fauna.
In the following paragraphs, oxidation/reduction
reactions in sediments and authigenic mineral formation are discussed. The effects of these reactions
on the sedimentary environment are illustrated using
profiles of solutes in sedimentary pore waters at a
near-shore location. Then, there is a brief discussion
of the role that sedimentary chemical processes play
in nutrient cycles and the cycling of anthropogenic
contaminants.
It is important to note that this article is not a
comprehensive review, but instead focuses on the
range of chemical reactions occurring in coastal
445
SEDIMENTS
W. R. Martin, Woods Hole Oceanographic Institution,
Woods Hole, MA, USA
& 2009 Elsevier Ltd. All rights reserved.
Introduction
The physical and chemical environments of estuarine
sediments vary over wide ranges. The sediments may
lie beneath nearly fresh water or may have salinities
near that of the open ocean. They may be intertidal
or always under several meters of water. They may
be covered by grasses, by algal or bacterial mats, or
may be free of surface biological cover. They may be
permeable sands, affected by groundwater flow, or
may be impermeable muds. Nonetheless, common
features determine their chemical environments.
They exist in productive, coastal waters and therefore experience large fluxes of particulate organic
matter. They experience large inputs of terrestrial
material that supply Fe and Mn oxides. Most estuarine sediments lie underneath oxic water columns,
and support abundant macrofauna that both mix
sedimentary particles and irrigate the seabed. Common features such as oxic bottom water, abundant
supplies of organic matter, Fe and Mn oxides, a
supply of sulfur from seawater (except for freshwater
end members of estuarine systems), and active macrofauna produce the set of biogeochemical processes
that determines the chemical environment and its
effects on carbon, nutrient, and contaminant cycling
in the coastal ocean.
The upper sediment column in estuarine and coastal
environments can be regarded as a slowly stirred reactor to which substrates are added at the top and
mixed downward. Transport processes, both in the
dissolved and solid phases, distribute reactants within
the reactor, while at the same time biogeochemical
processes alter the particles and the pore waters surrounding them. This combination of transport and
reaction drives solute exchanges between the sediments and overlying waters that are important to
coastal carbon, nutrient, and contaminant cycles. It
transforms the ‘reactive’ component of sedimentary
particles so that the accumulating sediments differ in
important ways from the particles that fall through
the water column to the seafloor. These processes are
outlined in Figure 1.
Because the rates of chemical reactions in this
system are faster than mixing rates, the reactor is not
chemically uniform; rather, roughly speaking, it has a
layered structure. In all but a few locations, there is a
thin, oxic layer at the sediment surface. As particles
pass through this layer and through the underlying
anoxic sediments, they are altered by heterotrophic
respiration: organic matter is gradually oxidized, and
a series of oxidants – O 2 , N(V) in NO 3
À , Mn(IV) and
Fe(III) in particulate oxides, and S(VI) in SO 4
2À – are
consumed. Some of the energy originally stored in
organic matter remains in the reduced products of
these reactions: Fe(II), and S(–II), and, to some extent,
Mn(II). This energy is available to chemolithotrophic
bacteria, and the result is rapid internal cycling between oxidized and reduced forms of Mn, Fe, and S.
Reduced Mn, Fe, and S that escape bacterial oxidation may be oxidized abiotically by O 2 in the surface oxic layer. The result of these processes is
sediments with an oxic ‘cap’, in which precipitation
of newly formed Fe and Mn oxides may limit benthic/
water column exchange of many solutes, and an
underlying anoxic layer in which rapid cycling
between oxidized and reduced forms limits the rate
of burial of products of heterotrophic respiration.
The chemical processes defining this system are
oxidation–reduction reactions, including ‘heterotrophic respiration’, by which organic matter is
oxidized, ‘chemolithotrophic respiration’, most importantly oxidizing Fe(II) and S(–II), and ‘abiotic
oxidation’; and ‘authigenic mineral formation’, which
converts dissolved products of these reactions to solid
phases. The stirring within the sedimentary reactor,
which is vital to its operation, is accomplished by
solute diffusion and particle mixing, accomplished
predominantly by the feeding and burrowing activities of sedimentary fauna.
In the following paragraphs, oxidation/reduction
reactions in sediments and authigenic mineral formation are discussed. The effects of these reactions
on the sedimentary environment are illustrated using
profiles of solutes in sedimentary pore waters at a
near-shore location. Then, there is a brief discussion
of the role that sedimentary chemical processes play
in nutrient cycles and the cycling of anthropogenic
contaminants.
It is important to note that this article is not a
comprehensive review, but instead focuses on the
range of chemical reactions occurring in coastal
445