or in the aerated vadose zone of the sandy beach. One
major pathway of microbial decay, for instance, involves
the breakdown of organic molecules and their oxidation
by sulfate-reducing bacteria, which utilize the sulfate
exogenically in the environment as the energy source for
the decomposition, and in the process, organic matter is
removed from the sandy beach environment.
Bacterial reduction of sulfate to sulfide is responsible
for the oxidation of organic matter buried in sediments
(Lovley and Phillips, 1986; Machel, 2001) that concomitantly results in pigmentation of light-colored sediments
to grey or black. The sulfate ion is common in seawater,
sediment, or in waters rich with decaying organic material,
and sulfate-reducing bacteria are common in anaerobic
environments where they utilize the sulfate ion as an electron donor, aiding in the degradation of organic materials.
Sulfate reduction is the dominant terminal step in the
biomediated mineralization processes of sulfate-rich sediments where the sulfate reducers inhibit the methanogens
by competing for common substrates. This sulfate reduction is quantitatively important in the overall oxidation
of organic matter (Barton and Fauque, 2009).
Various minerals can be precipitated by biomediation,
the best known being iron sulfide (pyrite) and calcite. If
Mn, Cu, and Zn are present in the environment, they also
can produce sulfides. Generally, Fe is the most common
transition metal cation in natural environments, so Fe
sulfide (as pyrite) is the most common mineral. Precipitates of pyrite are commonly framboidal (framboids being
small clusters of pyrite crystals resembling a raspberry
<1 mm in size, but ranging from 0.5 to 40 mm in size;
Wilkin et al., 1996; Sawlowlicz, 1993; Schieber, 2002).
While organic-matter-rich sediments inherently tend to
be black or dark grey, the fine-grained precipitated iron
sulfide disseminated throughout the sediment as a result
of bacterial decay of organic material similarly renders
sediments to various shades of grey to black. Calcite can
be precipitated in association with microbial activity, particularly in wet parts of a beach (e.g., that associated with
cyanobacterial mats; Kremer et al., 2008).
Microbial changes in the sediments leading to pyrite
precipitation and sediment pigmentation carry with them
hydrochemical changes in pH and redox conditions (i.e.,
Eh). Groundwaters under beaches often are anoxic and
with the biomediated transformation taking place that
result in the formation of sulfides; the groundwaters can
become acidic and markedly oxygen depleted. From
a generally alkaline state for seawater, the pH may
decrease to 6.5 or 6.0 in the pore water of beach sediments.
The Eh may be negative, with any decrease in Eh generally being related to the decrease in the dissolved oxygen
in pore waters (Zobell, 1946; Haraguchi, 2012). Weakly
acidic groundwater dissolves shell that is buried in beach
sand such that shells lose their luster, appear corroded,
or may be completely dissolved away. (In this context,
the microbial activities that have been subsumed under
“biological processes” grade into “chemical processes”
in that the acidity of the groundwaters has increased as
a result of biological activity, and this affects carbonate
mineral solubility).
Ionic chemistry is also affected by microbial activity.
The extent that sulfate and phosphate are microbially utilized in the environment, determining the depletion of sulfate ion and affecting phosphate concentrations, is an
example of how biological (microbiological) processes
affect ionic concentrations (e.g., Jansson, 1987; Lovley,
1991) and, vice versa, how the resulting ionic chemistry
dictates development of precipitates (Berner and Raiswell,
1984). Such microbial activity also changes sulfide concentrations and, with precipitation of sulfide, changes the
(transition) metal concentration in waters of species such
as Fe and Mn.
In terms of chemical processes, estuaries in general,
with their variety of environments ranging from deltas,
shallow water sand platforms, tidal mud flats and sand
flats, saltmarsh and mangroves, subaqueous shoals to
deep water mud beds, among others, manifest a diversity
of chemical processes and a variety of authigenic and diagenetic minerals, particularly where there is interaction
between microbiota, anoxic sediments, muddy sediments,
and different hydrochemical fields (cf., Cook, 1973; Cook
and Mayo, 1980; Pye, 1984; Pye et al., 1990; Rasmussen
et al., 1998; Hedges and Keil, 1999; Pirrie et al., 2000;
Aller, 2004; Bush et al., 2004; Michalopoulos and Aller,
2004; Byrne et al., 2011). Sandy beaches and gravelly
beaches, with their well-drained and more aerobic conditions and limited grain composition, however, represent
the relatively low diversity end of the spectrum of chemical processes that occur in estuaries and present a more
limited range of possible chemical processes and products.
The chemical processes on estuarine beaches are dissolution, precipitation of minerals (authigenesis),
biomediated mineral precipitation, diagenesis of minerals,
diagenetic structure development, sediment pigmentation
(e.g., pyrite mottling), the effects of freshwater throughflow, and the oxidation of organic matter. The products
of precipitation, often resulting in color mottling, in
cemented laminae, or in development of nodules, commonly occur as diagenetic overprints on a primary sediment (i.e., either laminated, burrowed, or thoroughly
bioturbated).
Note that in the context of precipitation of minerals, the
concepts of authigenetic minerals and diagenetic minerals
can overlap. The broad definition of authigenesis is of
a mineral generated in situ. These would include mineral
precipitates deposited on the estuary floor. At the mineral
level for the process of mineral precipitation and/or alteration, the broad definition of diagenesis is the mineralogical alteration of one mineral to another. At the larger
scale, e.g., at the sediment level where minerals are crystallizing in the sediment pore spaces lithifying the sediment, the cementing agents are considered by many
authors to be diagenetic, but since they are crystallizing
in situ, they are also considered by other authors to be
authigenic. In this contribution, minerals precipitated from
estuarine waters, regardless of whether they are open
68
BEACH PROCESSES
major pathway of microbial decay, for instance, involves
the breakdown of organic molecules and their oxidation
by sulfate-reducing bacteria, which utilize the sulfate
exogenically in the environment as the energy source for
the decomposition, and in the process, organic matter is
removed from the sandy beach environment.
Bacterial reduction of sulfate to sulfide is responsible
for the oxidation of organic matter buried in sediments
(Lovley and Phillips, 1986; Machel, 2001) that concomitantly results in pigmentation of light-colored sediments
to grey or black. The sulfate ion is common in seawater,
sediment, or in waters rich with decaying organic material,
and sulfate-reducing bacteria are common in anaerobic
environments where they utilize the sulfate ion as an electron donor, aiding in the degradation of organic materials.
Sulfate reduction is the dominant terminal step in the
biomediated mineralization processes of sulfate-rich sediments where the sulfate reducers inhibit the methanogens
by competing for common substrates. This sulfate reduction is quantitatively important in the overall oxidation
of organic matter (Barton and Fauque, 2009).
Various minerals can be precipitated by biomediation,
the best known being iron sulfide (pyrite) and calcite. If
Mn, Cu, and Zn are present in the environment, they also
can produce sulfides. Generally, Fe is the most common
transition metal cation in natural environments, so Fe
sulfide (as pyrite) is the most common mineral. Precipitates of pyrite are commonly framboidal (framboids being
small clusters of pyrite crystals resembling a raspberry
<1 mm in size, but ranging from 0.5 to 40 mm in size;
Wilkin et al., 1996; Sawlowlicz, 1993; Schieber, 2002).
While organic-matter-rich sediments inherently tend to
be black or dark grey, the fine-grained precipitated iron
sulfide disseminated throughout the sediment as a result
of bacterial decay of organic material similarly renders
sediments to various shades of grey to black. Calcite can
be precipitated in association with microbial activity, particularly in wet parts of a beach (e.g., that associated with
cyanobacterial mats; Kremer et al., 2008).
Microbial changes in the sediments leading to pyrite
precipitation and sediment pigmentation carry with them
hydrochemical changes in pH and redox conditions (i.e.,
Eh). Groundwaters under beaches often are anoxic and
with the biomediated transformation taking place that
result in the formation of sulfides; the groundwaters can
become acidic and markedly oxygen depleted. From
a generally alkaline state for seawater, the pH may
decrease to 6.5 or 6.0 in the pore water of beach sediments.
The Eh may be negative, with any decrease in Eh generally being related to the decrease in the dissolved oxygen
in pore waters (Zobell, 1946; Haraguchi, 2012). Weakly
acidic groundwater dissolves shell that is buried in beach
sand such that shells lose their luster, appear corroded,
or may be completely dissolved away. (In this context,
the microbial activities that have been subsumed under
“biological processes” grade into “chemical processes”
in that the acidity of the groundwaters has increased as
a result of biological activity, and this affects carbonate
mineral solubility).
Ionic chemistry is also affected by microbial activity.
The extent that sulfate and phosphate are microbially utilized in the environment, determining the depletion of sulfate ion and affecting phosphate concentrations, is an
example of how biological (microbiological) processes
affect ionic concentrations (e.g., Jansson, 1987; Lovley,
1991) and, vice versa, how the resulting ionic chemistry
dictates development of precipitates (Berner and Raiswell,
1984). Such microbial activity also changes sulfide concentrations and, with precipitation of sulfide, changes the
(transition) metal concentration in waters of species such
as Fe and Mn.
In terms of chemical processes, estuaries in general,
with their variety of environments ranging from deltas,
shallow water sand platforms, tidal mud flats and sand
flats, saltmarsh and mangroves, subaqueous shoals to
deep water mud beds, among others, manifest a diversity
of chemical processes and a variety of authigenic and diagenetic minerals, particularly where there is interaction
between microbiota, anoxic sediments, muddy sediments,
and different hydrochemical fields (cf., Cook, 1973; Cook
and Mayo, 1980; Pye, 1984; Pye et al., 1990; Rasmussen
et al., 1998; Hedges and Keil, 1999; Pirrie et al., 2000;
Aller, 2004; Bush et al., 2004; Michalopoulos and Aller,
2004; Byrne et al., 2011). Sandy beaches and gravelly
beaches, with their well-drained and more aerobic conditions and limited grain composition, however, represent
the relatively low diversity end of the spectrum of chemical processes that occur in estuaries and present a more
limited range of possible chemical processes and products.
The chemical processes on estuarine beaches are dissolution, precipitation of minerals (authigenesis),
biomediated mineral precipitation, diagenesis of minerals,
diagenetic structure development, sediment pigmentation
(e.g., pyrite mottling), the effects of freshwater throughflow, and the oxidation of organic matter. The products
of precipitation, often resulting in color mottling, in
cemented laminae, or in development of nodules, commonly occur as diagenetic overprints on a primary sediment (i.e., either laminated, burrowed, or thoroughly
bioturbated).
Note that in the context of precipitation of minerals, the
concepts of authigenetic minerals and diagenetic minerals
can overlap. The broad definition of authigenesis is of
a mineral generated in situ. These would include mineral
precipitates deposited on the estuary floor. At the mineral
level for the process of mineral precipitation and/or alteration, the broad definition of diagenesis is the mineralogical alteration of one mineral to another. At the larger
scale, e.g., at the sediment level where minerals are crystallizing in the sediment pore spaces lithifying the sediment, the cementing agents are considered by many
authors to be diagenetic, but since they are crystallizing
in situ, they are also considered by other authors to be
authigenic. In this contribution, minerals precipitated from
estuarine waters, regardless of whether they are open
68
BEACH PROCESSES