of the reef is normally the thickest but narrowest. Ramps
often have very low gradients and platforms tend to be
progradational, with minor topography. Greenhouse reefs
can also have a more extensive range as carbonate settings
extend into higher latitudes due to elevated global temperatures. Composition may also be more uniform, often dominated by low-Mg calcite skeletal components.
Carbonate saturation
Living coral reefs are restricted to shallow-water tropical
and subtropical environments characterized by warm temperatures, high light intensities, and also high aragonite
supersaturation. The growth of coral skeletons and other
calcifying organisms precipitates carbonate ions, forcing a
re-equilibration of the bicarbonate-dominated marine inorganic carbon system which also creates a source of carbon
dioxide. A coral reef therefore represents the net accumulation of CaCO 3 and a source of CO 2 , which in turn probably
leads to feedback-controls on the global carbon cycle.
Carbonate saturation has probably varied dramatically
throughout geological time. It appears that Precambrian
oceans were highly oversaturated, but this may have
declined around the beginning of the Phanerozoic with
the appearance of numerous calcareous invertebrates. Since
then, Ca
2þ in seawater has broadly followed sea-level
change such that Ca
2þ levels were reasonably high during
the Cambrian-Mississippian and the Jurassic to Cretaceous.
It has been suggested that the geological distribution of
microbialites might be controlled by physicochemical factors, including the saturation state of seawater and/or
global temperature distribution (Webb, 1996). This may
also explain the decline in abundance of reef microbialite
after the Jurassic due to the increase of pelagic carbonate
that led to a reduced seawater saturation state. This would
have lowered supersaturation levels below a threshold for
abundant automicrite formation, thus restricting its formation to cryptic reef habitats, where abnormal chemistries
could have existed. Such a situation might also explain
the absence of Stromatactis in the late Phanerozoic.
Evolving ocean chemistry
The dominant form of precipitated crystalline CaCO 3 has
oscillated during the geological past, with both inorganic
and organic production of aragonite and high-Mg calcite
dominating carbonate formation during cool periods (icehouse), and low-Mg calcite predominating during warm
periods (greenhouse) (Figure 10). Such mineralogical
shifts are interpreted as markers for major changes in seawater chemistry.
Stanley and Hardie (1998) have proposed that the shifts
in the Mg:Ca ratio have controlled the predominance of
calcite versus aragonite secretors, particularly reef
builders, due to the inhibiting effect of high Mg
2þ concentration on calcite secretion. They suggest that the changing
Mg:Ca ratio of seawater has been controlled by variations
in the rate of production of oceanic crust, because oceanic
hydrothermal alteration is a major sink for Mg and an
important source of Ca. Experimental work has subsequently confirmed the profound influence of Mg:Ca seawater ratios on modern reef builders, including
scleractinian corals (Ries et al., 2006) and the calcareous
green alga Halimeda.
Scleractinian corals were, for example, important reef
builders in the Jurassic, but they did not build extensive
reefs during the greenhouse period (calcite seas) of the Cretaceous. During this period, species diversity remained high
but their abundance on carbonate platforms was low compared to the Jurassic. Distribution shifted to outer platform
settings and higher latitudes ($35–45
N). There are many
hypotheses offered to explain these observations, including
the high temperatures, restricted circulation, and unstable
sediment conditions of Cretaceous platforms, and in particular the favoring of calcite-producing rudist bivalves over
aragonite corals (Wood, 1999). Rudist bivalves with their
outer calcitic skeletons underwent a dramatic radiation in
the late Cretaceous when the Mg:Ca ratio of seawater
reached exceptionally low values so favouring calcite over
aragonite secretors (Steuber, 2002).
Major shifts in the dominant composition of carbonate
skeletal particles through geological time also mirrors in
part these proposed changes in seawater chemistry and climate (Figure 10d), but mass extinctions also play a role by
triggering changes in the predominant form of CaCO 3 produced by marine calcifiers (Kiessling et al., 2008).
Changes in the abundance of aragonitic organisms following mass extinction events appear to have been predominantly driven by selective recovery rather than selective
extinction.
Evidence is also persuasive that the changing seawater
chemistry has influenced the style of early diagenesis in
carbonate regimes, particularly in reefs. The mineralogy
of early marine reef cements also seems to follow the same
secular changes. For example, aragonitic botryoids are
known exclusively from phases of aragonite seas (early
Cambrian, mid-Carboniferous to early Jurassic, and midlate Cenozoic), whereas radiaxial fibrous calcite is common in reefs that grew in calcite seas (particularly the
Ordovician to Devonian); it is virtually unknown from
aragonite seas of the Cenozoic. Enhanced rates of calcite
cementation during calcite seas due to the elevated abundance of calcium ions may have promoted rapid lithification of the reef framework and aided preservation of
cryptic biota that were otherwise vulnerable to disturbance, but this has yet to be documented.
The rise of predation and bioerosion
Many researchers have summarized the importance of herbivores and large marine vertebrates to the healthy functioning of coral reefs, and this importance is
corroborated by analysis of the fossil record (see Wood,
1999, 2002). A dramatic escalation of new organisms with
innovative and destructive feeding methods occurred from
the mid-Jurassic to Miocene (summarized in Vermeij,
1987). In particular, the arrival of piscine herbivores had
464
GENERAL EVOLUTION OF CARBONATE REEFS
often have very low gradients and platforms tend to be
progradational, with minor topography. Greenhouse reefs
can also have a more extensive range as carbonate settings
extend into higher latitudes due to elevated global temperatures. Composition may also be more uniform, often dominated by low-Mg calcite skeletal components.
Carbonate saturation
Living coral reefs are restricted to shallow-water tropical
and subtropical environments characterized by warm temperatures, high light intensities, and also high aragonite
supersaturation. The growth of coral skeletons and other
calcifying organisms precipitates carbonate ions, forcing a
re-equilibration of the bicarbonate-dominated marine inorganic carbon system which also creates a source of carbon
dioxide. A coral reef therefore represents the net accumulation of CaCO 3 and a source of CO 2 , which in turn probably
leads to feedback-controls on the global carbon cycle.
Carbonate saturation has probably varied dramatically
throughout geological time. It appears that Precambrian
oceans were highly oversaturated, but this may have
declined around the beginning of the Phanerozoic with
the appearance of numerous calcareous invertebrates. Since
then, Ca
2þ in seawater has broadly followed sea-level
change such that Ca
2þ levels were reasonably high during
the Cambrian-Mississippian and the Jurassic to Cretaceous.
It has been suggested that the geological distribution of
microbialites might be controlled by physicochemical factors, including the saturation state of seawater and/or
global temperature distribution (Webb, 1996). This may
also explain the decline in abundance of reef microbialite
after the Jurassic due to the increase of pelagic carbonate
that led to a reduced seawater saturation state. This would
have lowered supersaturation levels below a threshold for
abundant automicrite formation, thus restricting its formation to cryptic reef habitats, where abnormal chemistries
could have existed. Such a situation might also explain
the absence of Stromatactis in the late Phanerozoic.
Evolving ocean chemistry
The dominant form of precipitated crystalline CaCO 3 has
oscillated during the geological past, with both inorganic
and organic production of aragonite and high-Mg calcite
dominating carbonate formation during cool periods (icehouse), and low-Mg calcite predominating during warm
periods (greenhouse) (Figure 10). Such mineralogical
shifts are interpreted as markers for major changes in seawater chemistry.
Stanley and Hardie (1998) have proposed that the shifts
in the Mg:Ca ratio have controlled the predominance of
calcite versus aragonite secretors, particularly reef
builders, due to the inhibiting effect of high Mg
2þ concentration on calcite secretion. They suggest that the changing
Mg:Ca ratio of seawater has been controlled by variations
in the rate of production of oceanic crust, because oceanic
hydrothermal alteration is a major sink for Mg and an
important source of Ca. Experimental work has subsequently confirmed the profound influence of Mg:Ca seawater ratios on modern reef builders, including
scleractinian corals (Ries et al., 2006) and the calcareous
green alga Halimeda.
Scleractinian corals were, for example, important reef
builders in the Jurassic, but they did not build extensive
reefs during the greenhouse period (calcite seas) of the Cretaceous. During this period, species diversity remained high
but their abundance on carbonate platforms was low compared to the Jurassic. Distribution shifted to outer platform
settings and higher latitudes ($35–45
N). There are many
hypotheses offered to explain these observations, including
the high temperatures, restricted circulation, and unstable
sediment conditions of Cretaceous platforms, and in particular the favoring of calcite-producing rudist bivalves over
aragonite corals (Wood, 1999). Rudist bivalves with their
outer calcitic skeletons underwent a dramatic radiation in
the late Cretaceous when the Mg:Ca ratio of seawater
reached exceptionally low values so favouring calcite over
aragonite secretors (Steuber, 2002).
Major shifts in the dominant composition of carbonate
skeletal particles through geological time also mirrors in
part these proposed changes in seawater chemistry and climate (Figure 10d), but mass extinctions also play a role by
triggering changes in the predominant form of CaCO 3 produced by marine calcifiers (Kiessling et al., 2008).
Changes in the abundance of aragonitic organisms following mass extinction events appear to have been predominantly driven by selective recovery rather than selective
extinction.
Evidence is also persuasive that the changing seawater
chemistry has influenced the style of early diagenesis in
carbonate regimes, particularly in reefs. The mineralogy
of early marine reef cements also seems to follow the same
secular changes. For example, aragonitic botryoids are
known exclusively from phases of aragonite seas (early
Cambrian, mid-Carboniferous to early Jurassic, and midlate Cenozoic), whereas radiaxial fibrous calcite is common in reefs that grew in calcite seas (particularly the
Ordovician to Devonian); it is virtually unknown from
aragonite seas of the Cenozoic. Enhanced rates of calcite
cementation during calcite seas due to the elevated abundance of calcium ions may have promoted rapid lithification of the reef framework and aided preservation of
cryptic biota that were otherwise vulnerable to disturbance, but this has yet to be documented.
The rise of predation and bioerosion
Many researchers have summarized the importance of herbivores and large marine vertebrates to the healthy functioning of coral reefs, and this importance is
corroborated by analysis of the fossil record (see Wood,
1999, 2002). A dramatic escalation of new organisms with
innovative and destructive feeding methods occurred from
the mid-Jurassic to Miocene (summarized in Vermeij,
1987). In particular, the arrival of piscine herbivores had
464
GENERAL EVOLUTION OF CARBONATE REEFS