The formation of phosphatic minerals (probably as
carbonate fluorapatite) as a result of early diagenetic
phosphatization of calcareous or clayey minerals in highintertidal or supratidal sediments in estuarine environments by phosphate-rich pore waters where phosphate
has been transported to the estuary by rivers has been
recorded by Cook and Mayo (1980) in the Broad Sound
estuary in Queensland.
Fine-grained pyrite embedded in the wood of mangrove
stumps in anoxic coastal sediments of North Western
Australia, when exposed to oxidizing conditions, is chemically altered to goethite and hematite (Semeniuk, 1980).
Diagenetic marcasite overgrown on earlier pyrite has been
reported from stranded organic-matter-rich Holocene
estuarine mud in eastern Australia (Bush et al., 2004) –
here diagenetic pyrite occurs throughout the marine to
freshwater-brackish sediments, but marcasite only occurs
in the upper part of the estuarine sequence (which comprises sediments accumulated under freshwater-brackish
conditions), forming overgrowths on earlier-formed pyrite
crystals.
Synsedimentary diagenetic crystallographic overgrowths of pyrite were recorded by Wilkin and Barnes
(1997a), as mentioned earlier, in the estuary of the
Pettaquamscutt River. Framboids of pyrite that have
settled on the sediment surface and have been shallowly
buried are infilled and crystallographically overgrown,
locally to have euhedral overgrowths as a result of nucleation and continued pyrite growth on framboids originally
formed above the sediment-water interface.
As an example of synsedimentary diagenesis, Clark
and Lutz (1980) reported that living molluscs in a tidal
marsh are being pyritized (not by framboids but by
euhedral pyrite crystals), indicating that pyritization can
commence in a shell even before the death of the organism. In one shell Clark and Lutz (1980) found pyrite
replacing aragonite in parts of its outer shell layer, especially around fractures, borings, and concentrations of
organic matrix.
Summary
In estuarine environments there are five types of mineralization (Figure 1). The most prevalent is the transformation by biochemical breakdown, including by
microorganisms, of organic molecules of plants and animals to inorganic molecules such as PO 4
3À , SO 4
2À , S
2À ,
NH 4
+ , NH 3 , NO 3
À
, and NO 2
À and the oxidation of
organic matter. With these processes, free molecules and
ions are available for plant uptake or for further chemical
processing. Mineralization specifically by sulfate reduction, resulting in the widespread oxidation of organic
matter in estuarine sediments and in precipitation of
fine-grained pyrite as a mineral product, quantitatively is
also an important process. Estuarine environments accumulate organic matter in their sediments, and sulfatereducing bacteria in anaerobic environments utilizing
sulfate ions to aid in the oxidation and degradation of this
organic material produce sulfide which, in combining with
any available Fe, produces iron sulfides, including FeS 2 .
As such, pyrite as fine-grained framboids, formed through
biomediation of sulfate-reducing bacteria, is a common
and widespread product in estuarine sedimentary deposits.
There is a range of other minerals that form in estuarine
environments. Many are biominerals formed by organisms as skeletal structures for support, defense, tissue
strengthening, feeding apparatus, or navigation; these
include calcite, aragonite, bone, and magnetite. Upon
death of the organism, these biominerals can be incorporated into the sediments of estuaries. In this regard, this
aspect of biomineralization contributes to the sediment
accumulation in estuaries, from gravel-sized particles to
mud-sized particles.
Some of the minerals in the estuarine environment are
direct inorganic precipitates from estuarine waters and
accumulate on the sediment surface or within sediment
interstices, the latter potentially cementing estuarine sediments. Some of the more common minerals include calcite, aragonite, gypsum, apatite, and pyrite. A range of
other minerals are diagenetic, formed as alteration products of earlier-formed minerals; these include goethite
and hematite as products of the oxidation of pyrite, crystallographic overgrowths of framboidal pyrite, marcasite
formed by alteration of pyrite, dolomite formed by alteration of calcite/aragonite, phosphatization of carbonate
minerals or clay minerals, and the pyritization of molluscan carbonate.
The various minerals, their diagenetic alterations, and
biominerals often are specific to sub-environments in the
estuary. For instance, mineral precipitates can be environment specific in their occurrence: freshwater seepage is
the specialized location for shoreline primary carbonate
precipitation and for diagenetic formation of dolomite;
hypersaline tidal flats and tidal zones are the specialized
location for carbonate precipitation as calcite, aragonite,
and gypsum; dark gray sandy and muddy sediments,
abundant with fine-grained pyrite framboids, are ubiquitous in estuaries wherever there is accumulating organic
matter and anaerobic conditions; and as such, this form
of pyrite mineralization is widespread in estuaries. In
terms of biominerals, leaving aside the factor that biogenic
skeletal materials can be fragmented and transported,
often environment-specific biotic assemblages result in
skeletal suites with either diagnostic shell types and their
fragments (Semeniuk et al., 2000) and possibly mineralogy, or a specific proportion of foraminiferal tests and diatom frustules (Revets, 2000), or vertebrate skeletal parts,
all with their geochemical/mineralogical signatures
related to sub-environments in the estuary.
Interesting minerals that form in anoxic estuarine
environments are pyrite framboids and, while they have
been much studied, the details of their formation are still
in debate (Wilkin and Barnes, 1997b). At one extreme
they are viewed to be wholly biogenic, i.e., pyritized bacteria (Schneiderhohn, 1923; Love, 1957), or pyritized
organic spherical globules or gaseous vacuoles
MINERALIZATION
443
carbonate fluorapatite) as a result of early diagenetic
phosphatization of calcareous or clayey minerals in highintertidal or supratidal sediments in estuarine environments by phosphate-rich pore waters where phosphate
has been transported to the estuary by rivers has been
recorded by Cook and Mayo (1980) in the Broad Sound
estuary in Queensland.
Fine-grained pyrite embedded in the wood of mangrove
stumps in anoxic coastal sediments of North Western
Australia, when exposed to oxidizing conditions, is chemically altered to goethite and hematite (Semeniuk, 1980).
Diagenetic marcasite overgrown on earlier pyrite has been
reported from stranded organic-matter-rich Holocene
estuarine mud in eastern Australia (Bush et al., 2004) –
here diagenetic pyrite occurs throughout the marine to
freshwater-brackish sediments, but marcasite only occurs
in the upper part of the estuarine sequence (which comprises sediments accumulated under freshwater-brackish
conditions), forming overgrowths on earlier-formed pyrite
crystals.
Synsedimentary diagenetic crystallographic overgrowths of pyrite were recorded by Wilkin and Barnes
(1997a), as mentioned earlier, in the estuary of the
Pettaquamscutt River. Framboids of pyrite that have
settled on the sediment surface and have been shallowly
buried are infilled and crystallographically overgrown,
locally to have euhedral overgrowths as a result of nucleation and continued pyrite growth on framboids originally
formed above the sediment-water interface.
As an example of synsedimentary diagenesis, Clark
and Lutz (1980) reported that living molluscs in a tidal
marsh are being pyritized (not by framboids but by
euhedral pyrite crystals), indicating that pyritization can
commence in a shell even before the death of the organism. In one shell Clark and Lutz (1980) found pyrite
replacing aragonite in parts of its outer shell layer, especially around fractures, borings, and concentrations of
organic matrix.
Summary
In estuarine environments there are five types of mineralization (Figure 1). The most prevalent is the transformation by biochemical breakdown, including by
microorganisms, of organic molecules of plants and animals to inorganic molecules such as PO 4
3À , SO 4
2À , S
2À ,
NH 4
+ , NH 3 , NO 3
À
, and NO 2
À and the oxidation of
organic matter. With these processes, free molecules and
ions are available for plant uptake or for further chemical
processing. Mineralization specifically by sulfate reduction, resulting in the widespread oxidation of organic
matter in estuarine sediments and in precipitation of
fine-grained pyrite as a mineral product, quantitatively is
also an important process. Estuarine environments accumulate organic matter in their sediments, and sulfatereducing bacteria in anaerobic environments utilizing
sulfate ions to aid in the oxidation and degradation of this
organic material produce sulfide which, in combining with
any available Fe, produces iron sulfides, including FeS 2 .
As such, pyrite as fine-grained framboids, formed through
biomediation of sulfate-reducing bacteria, is a common
and widespread product in estuarine sedimentary deposits.
There is a range of other minerals that form in estuarine
environments. Many are biominerals formed by organisms as skeletal structures for support, defense, tissue
strengthening, feeding apparatus, or navigation; these
include calcite, aragonite, bone, and magnetite. Upon
death of the organism, these biominerals can be incorporated into the sediments of estuaries. In this regard, this
aspect of biomineralization contributes to the sediment
accumulation in estuaries, from gravel-sized particles to
mud-sized particles.
Some of the minerals in the estuarine environment are
direct inorganic precipitates from estuarine waters and
accumulate on the sediment surface or within sediment
interstices, the latter potentially cementing estuarine sediments. Some of the more common minerals include calcite, aragonite, gypsum, apatite, and pyrite. A range of
other minerals are diagenetic, formed as alteration products of earlier-formed minerals; these include goethite
and hematite as products of the oxidation of pyrite, crystallographic overgrowths of framboidal pyrite, marcasite
formed by alteration of pyrite, dolomite formed by alteration of calcite/aragonite, phosphatization of carbonate
minerals or clay minerals, and the pyritization of molluscan carbonate.
The various minerals, their diagenetic alterations, and
biominerals often are specific to sub-environments in the
estuary. For instance, mineral precipitates can be environment specific in their occurrence: freshwater seepage is
the specialized location for shoreline primary carbonate
precipitation and for diagenetic formation of dolomite;
hypersaline tidal flats and tidal zones are the specialized
location for carbonate precipitation as calcite, aragonite,
and gypsum; dark gray sandy and muddy sediments,
abundant with fine-grained pyrite framboids, are ubiquitous in estuaries wherever there is accumulating organic
matter and anaerobic conditions; and as such, this form
of pyrite mineralization is widespread in estuaries. In
terms of biominerals, leaving aside the factor that biogenic
skeletal materials can be fragmented and transported,
often environment-specific biotic assemblages result in
skeletal suites with either diagnostic shell types and their
fragments (Semeniuk et al., 2000) and possibly mineralogy, or a specific proportion of foraminiferal tests and diatom frustules (Revets, 2000), or vertebrate skeletal parts,
all with their geochemical/mineralogical signatures
related to sub-environments in the estuary.
Interesting minerals that form in anoxic estuarine
environments are pyrite framboids and, while they have
been much studied, the details of their formation are still
in debate (Wilkin and Barnes, 1997b). At one extreme
they are viewed to be wholly biogenic, i.e., pyritized bacteria (Schneiderhohn, 1923; Love, 1957), or pyritized
organic spherical globules or gaseous vacuoles
MINERALIZATION
443