calcifiers. They can form reefs as well as binding and stabilizing reef frameworks being prolific producers of sediment. Such algae probably occupied these niches in the
past. Calcareous algae have been part of the carbonate factory since the Ordovician but were not significant in reef
communities until the early Carboniferous (Mississippian). Phylloid (leaf-like) algae were conspicuous reef
builders in the late Palaeozoic.
Microbes
Microbes are highly diverse single-celled prokaryotes and
eukaryotes. Carbonate precipitation associated with them
can be inferred by the presence of often encrusting microcrystalline micrite (lime mud) with peloidal, clotted or
laminated fabrics, or by calcified microbial sheaths
(calcimicrobes).
Microbes are non-obligate calcifiers in that they induce
carbonate precipitation by their metabolic processes or
postmortem chemistry: they are incapable of calcification
under unfavorable conditions such as low carbonate saturation. The attributes of microbial textures are therefore
thought to depend upon seawater carbonate saturation
and postmortem preservation.
Calcification often takes place associated with a biofilm
consisting of layers of organic matter (EPS – extracellular
polysaccharide) that contains a variety of living and dead
microbial communities within a matrix of degrading
organic matter and mucilage. The precipitated microcrystalline micrite is sometimes called automicrite because it is
produced in-place, generally on a substrate, and it can be
an important binding or cementing agent.
It is also possible that some micrite formation in reefs
relies mainly on early diagenetic organomineralization
processes to aid calcite nucleation on dead organic matter,
particularly via molecules derived from decayed sponges.
The presence of abundant, reactive Ca
2þ -binding fulvic
acids have been confirmed in well-preserved Cretaceous
mud mounds and modern coral reef caves.
Automicrite is the main constructor of layered stromatolites and clotted thrombolites. Stromatolites, composed
of automicrite and synsedimentary cement, were the
only reef builders throughout the Precambrian and were
augmented by thrombolites and calcimicrobes in the
Neoproterozoic. They have continued to be a variable
contributor to reefs throughout the Phanerozoic.
Calcimicrobes, once established in the Neoproterozoic,
have continued to be a significant and sometimes dominant element of reefs throughout the Phanerozoic, particularly the Paleozoic. Whereas automicrite is essentially an
encrusting or binding element in most reefs, often within
cavities, it may nevertheless contribute substantially to
the reef rock volume.
The most common calcimicrobes are tubular
(Girvanella, Rothpletzella), subspherical (Renalcis), or
branching (Epiphyton). They are generally interpreted
as sheaths of bacteria that have undergone variably
taphonomic alteration. Many other similar structures of
complex affinity such as the Mesozoic Tubiphytes (probably a foraminfer–cyanobacteria association) have also
been placed in this category. Calcimicrobes were
also important sediment producers.
Internal cavity systems
Much of most modern and ancient reefs are void space as
metazoan-algal-microbe growth geometries often produce
interconnected cavities, even in those reefs almost wholly
composed of mud. These cavities are often shaded from
light and suffer less predation or grazing and reduced
wave energy compared to the open reef surface. Cavities
are often populated by cryptic organisms that encrust
walls and hang down from ceilings. They range from photic organisms near the openings to relatively fragile heterotrophs in the dark, lightless interiors. Automicrite
often preferentially coats inner surfaces of cavities and
encrusts the cryptic community. Cavities are also sites of
fine-grained sediment accumulation, material composed
of skeletal debris and mud that forms a geopetal internal sediment fill. Reef cavities first appeared in the
Neoproterozoic coincident with calcimicrobes and
thrombolites. Thereafter, they were a significant element
of most reefs.
Synsedimentary lithification
Most reefs are lithified immediately below the living surface by a variety of calcareous precipitates. Much of such
cement is microcrystalline but other cements are spectacular, growing as millimeter of centimeter-sized crystal
arrays from the walls and ceilings of cavities or between
sediment grains. Such cements not only confer rigidity
to reefs but also occlude much original pore space.
The location of the reef determines the amount of
cementation. Platform-margin and down-slope reefs usually contain marine cements, particularly in high-energy
areas, whereas lagoonal or inner-ramp reefs rarely do
and so may be unlithified.
Synsedimentary cementation is an attribute of reefs of all
ages but the type and mineralogy is controlled in part by
changing seawater chemistry. The most intensive ancient
cementation documented is found in Precambrian reefs,
where some are volumetrically dominated by cements.
The amount of crystalline cement in reefs decreases with
the appearance of calcareous organisms in the Cambrian
and then again following the evolution of calcareous plankton in the Jurassic: Many Palaeozoic reefs contain conspicuously more synsedimentary cement than Jurassic and
younger examples.
Bioerosion and predation
Living calcareous reef builders and reef rock are grazed
upon by a variety of bioeroders, particularly limpets, echinoids, and fish that not only consume soft parts but also
remove reef carbonate, which is reduced to fine sediment.
Although present in the Palaeozoic, this component of inplace reef removal, bioerosion, and sediment production
454
GENERAL EVOLUTION OF CARBONATE REEFS
past. Calcareous algae have been part of the carbonate factory since the Ordovician but were not significant in reef
communities until the early Carboniferous (Mississippian). Phylloid (leaf-like) algae were conspicuous reef
builders in the late Palaeozoic.
Microbes
Microbes are highly diverse single-celled prokaryotes and
eukaryotes. Carbonate precipitation associated with them
can be inferred by the presence of often encrusting microcrystalline micrite (lime mud) with peloidal, clotted or
laminated fabrics, or by calcified microbial sheaths
(calcimicrobes).
Microbes are non-obligate calcifiers in that they induce
carbonate precipitation by their metabolic processes or
postmortem chemistry: they are incapable of calcification
under unfavorable conditions such as low carbonate saturation. The attributes of microbial textures are therefore
thought to depend upon seawater carbonate saturation
and postmortem preservation.
Calcification often takes place associated with a biofilm
consisting of layers of organic matter (EPS – extracellular
polysaccharide) that contains a variety of living and dead
microbial communities within a matrix of degrading
organic matter and mucilage. The precipitated microcrystalline micrite is sometimes called automicrite because it is
produced in-place, generally on a substrate, and it can be
an important binding or cementing agent.
It is also possible that some micrite formation in reefs
relies mainly on early diagenetic organomineralization
processes to aid calcite nucleation on dead organic matter,
particularly via molecules derived from decayed sponges.
The presence of abundant, reactive Ca
2þ -binding fulvic
acids have been confirmed in well-preserved Cretaceous
mud mounds and modern coral reef caves.
Automicrite is the main constructor of layered stromatolites and clotted thrombolites. Stromatolites, composed
of automicrite and synsedimentary cement, were the
only reef builders throughout the Precambrian and were
augmented by thrombolites and calcimicrobes in the
Neoproterozoic. They have continued to be a variable
contributor to reefs throughout the Phanerozoic.
Calcimicrobes, once established in the Neoproterozoic,
have continued to be a significant and sometimes dominant element of reefs throughout the Phanerozoic, particularly the Paleozoic. Whereas automicrite is essentially an
encrusting or binding element in most reefs, often within
cavities, it may nevertheless contribute substantially to
the reef rock volume.
The most common calcimicrobes are tubular
(Girvanella, Rothpletzella), subspherical (Renalcis), or
branching (Epiphyton). They are generally interpreted
as sheaths of bacteria that have undergone variably
taphonomic alteration. Many other similar structures of
complex affinity such as the Mesozoic Tubiphytes (probably a foraminfer–cyanobacteria association) have also
been placed in this category. Calcimicrobes were
also important sediment producers.
Internal cavity systems
Much of most modern and ancient reefs are void space as
metazoan-algal-microbe growth geometries often produce
interconnected cavities, even in those reefs almost wholly
composed of mud. These cavities are often shaded from
light and suffer less predation or grazing and reduced
wave energy compared to the open reef surface. Cavities
are often populated by cryptic organisms that encrust
walls and hang down from ceilings. They range from photic organisms near the openings to relatively fragile heterotrophs in the dark, lightless interiors. Automicrite
often preferentially coats inner surfaces of cavities and
encrusts the cryptic community. Cavities are also sites of
fine-grained sediment accumulation, material composed
of skeletal debris and mud that forms a geopetal internal sediment fill. Reef cavities first appeared in the
Neoproterozoic coincident with calcimicrobes and
thrombolites. Thereafter, they were a significant element
of most reefs.
Synsedimentary lithification
Most reefs are lithified immediately below the living surface by a variety of calcareous precipitates. Much of such
cement is microcrystalline but other cements are spectacular, growing as millimeter of centimeter-sized crystal
arrays from the walls and ceilings of cavities or between
sediment grains. Such cements not only confer rigidity
to reefs but also occlude much original pore space.
The location of the reef determines the amount of
cementation. Platform-margin and down-slope reefs usually contain marine cements, particularly in high-energy
areas, whereas lagoonal or inner-ramp reefs rarely do
and so may be unlithified.
Synsedimentary cementation is an attribute of reefs of all
ages but the type and mineralogy is controlled in part by
changing seawater chemistry. The most intensive ancient
cementation documented is found in Precambrian reefs,
where some are volumetrically dominated by cements.
The amount of crystalline cement in reefs decreases with
the appearance of calcareous organisms in the Cambrian
and then again following the evolution of calcareous plankton in the Jurassic: Many Palaeozoic reefs contain conspicuously more synsedimentary cement than Jurassic and
younger examples.
Bioerosion and predation
Living calcareous reef builders and reef rock are grazed
upon by a variety of bioeroders, particularly limpets, echinoids, and fish that not only consume soft parts but also
remove reef carbonate, which is reduced to fine sediment.
Although present in the Palaeozoic, this component of inplace reef removal, bioerosion, and sediment production
454
GENERAL EVOLUTION OF CARBONATE REEFS