tropical marine life was decimated, including a high proportion of calcifying organisms. All remaining ammonites
and belemnites went extinct, as did a high percentage of
bivalves, gastropods, and echinoids, as well as almost all
Foraminifera. Many other unicellular organisms including
radiolarians were also severely affected. However, most
other major taxa – including freshwater fish, amphibians,
turtles, crocodiles, snakes, and lizards – appear to have
been almost unaffected, as were placental mammals.
Criteria used in extant coral taxonomy suggest that onethird of all families and 70% of all genera went completely
extinct. Of the extant families, the Faviidae retained six of
its original 16 genera; the others survived with only one or
two. At least that is what the fossil record appears to say
Veron (1995). However, that record is poor (Rosen and
Turnšek, 1989; Kiessling, 2001) partly because sea levels
were high during most of the Cretaceous and reefs were
eroded away as the seas receded. Importantly, no
branching corals appear to have survived, thus the habitats
maintained by branching corals – those that provide shelter for algae-grazing fish – would have completely
disappeared. On a taxonomic level, there are differences
of opinion based primarily on what are considered to be
valid records. Veron (1995) found that azooxanthellate
corals were affected to the same degree as zooxanthellate
corals, whereas Kiessling and Baron-Szabo (2004) on
other criteria found that they were less affected. There
are also differing views as to whether there was a reef
gap or not. What is beyond dispute is that the fossil record
is poor and few details been gleaned from it (Rosen,
2000).
Globally averaged atmospheric temperatures at the time
of K/T have been estimated to be 6–14
C higher than at
present (Barron, 1983), ranging from a few degrees’ difference at the equator to as much as 20–40
C at the poles
(Barron and Washington, 1985). Estimates of carbon dioxide levels vary greatly; however, the data for K/T are more
reliable than those for more ancient extinctions. By the end
Cretaceous carbon dioxide levels were at least five times,
and perhaps as much as ten times, those of today (Berner,
1994; Tajika, 1999; Gale, 2000; Johnson et al., 2002).
A bolide hitting the Earth near the Yucatán Peninsula of
southeast Mexico was widely thought to have caused the
K/T extinctions (Alvarez et al., 1980). This theory, however, is highly controversial: over 2,000 books and articles
were published about it within a decade of the original
publication (Glen, 1990); the consensus now being that
the bolide was not the primary cause of the extinction.
The impact itself and the outpourings of the shockinduced volcanoes are widely believed to have created
a stratospheric dust cloud that would have plunged the
whole of the Earth into a cold, interminable night. Certainly acid rain (including nitric acid generated from
heat-induced fusion of nitrogen and oxygen), high levels
of carbon dioxide released from the impact site and from
volcanic eruptions everywhere, and high concentrations
of methane released from continental slopes would have
combined to create intense greenhouse warming.
However, the timing of the ultimate extinction of different
animal groups, the time taken for many to go extinct, and
the age of the bolide debris do not correlate well. Some
animal taxa became extinct within an apparently brief
period, whereas others, including the dinosaurs, took hundreds of thousands of years, and their decline started long
before the bolide’s impact.
Causes of mass extinctions
Extrapolations from today’s knowledge of coral biology
to past environments must be cautiously made, especially
as the Scleractinia of today have been in existence for
only two mass extinctions (the end Triassic and K/T).
However, just as a number of physical parameters have
remained effectively constant throughout the Phanerozoic, some physiological processes, while they may vary
in detail over time, would in general have been applicable
to the Scleractinia throughout their existence.
The causes of mass extinctions are divisible into two
classes; those that do not involve the carbon cycle and
those that do (Veron, 2008a, b).
Causes independent of the carbon cycle
Reef development has three prerequisites: (1) corals (and/
or other calcifying organisms) must exist in environments
that are favorable for calcification, (2) the resulting calcium carbonate must be consolidated into reefs, and
(3) the rate of erosion must be less than the rate of accretion. Mass extinctions and reef gaps might occur whenever calcification is decreased, or there is a breakdown in
the mechanisms of consolidation, or there are factors that
increase the erosion or dissolution of calcium carbonate.
In the case of K/T, where there was a simultaneous extinction of both zooxanthellate and azooxanthellate coral species, any explanation of extinction must span wide depth
ranges as well as broad spatial scales.
Direct physical destruction from bolides
A bolide many kilometers across could possibly cause
massive destruction of reefs along the exposed side of an
area the size of a continent; however, it beggars belief that
such an effect, from a single point source, could envelop
the whole Earth (Toon et al., 1997). Nor is there any way
that azooxanthellate species would be equally affected or
that surviving reef corals would take millions of years to
recommence reef building. Be that as it may, the Earth
has several large and well-known craters (13 of which
date to the early Mesozoic or later) (Jablonski, 1986),
and many studies, motivated by the discovery of the K/T
bolide, have sought to establish these as the primary cause
of extinction events. Yet credible links have not emerged
(Rothschild and Lister, 2003). Indirect effects of bolides
are another matter.
“Nuclear Winter” induced by dust clouds
Bolides are presumably capable of creating dust clouds
that may have enveloped the Earth for weeks or months,
creating devastating darkness and (for terrestrial life) cold.
MASS EXTINCTIONS, ANOXIC EVENTS AND OCEAN ACIDIFICATION
673
and belemnites went extinct, as did a high percentage of
bivalves, gastropods, and echinoids, as well as almost all
Foraminifera. Many other unicellular organisms including
radiolarians were also severely affected. However, most
other major taxa – including freshwater fish, amphibians,
turtles, crocodiles, snakes, and lizards – appear to have
been almost unaffected, as were placental mammals.
Criteria used in extant coral taxonomy suggest that onethird of all families and 70% of all genera went completely
extinct. Of the extant families, the Faviidae retained six of
its original 16 genera; the others survived with only one or
two. At least that is what the fossil record appears to say
Veron (1995). However, that record is poor (Rosen and
Turnšek, 1989; Kiessling, 2001) partly because sea levels
were high during most of the Cretaceous and reefs were
eroded away as the seas receded. Importantly, no
branching corals appear to have survived, thus the habitats
maintained by branching corals – those that provide shelter for algae-grazing fish – would have completely
disappeared. On a taxonomic level, there are differences
of opinion based primarily on what are considered to be
valid records. Veron (1995) found that azooxanthellate
corals were affected to the same degree as zooxanthellate
corals, whereas Kiessling and Baron-Szabo (2004) on
other criteria found that they were less affected. There
are also differing views as to whether there was a reef
gap or not. What is beyond dispute is that the fossil record
is poor and few details been gleaned from it (Rosen,
2000).
Globally averaged atmospheric temperatures at the time
of K/T have been estimated to be 6–14
C higher than at
present (Barron, 1983), ranging from a few degrees’ difference at the equator to as much as 20–40
C at the poles
(Barron and Washington, 1985). Estimates of carbon dioxide levels vary greatly; however, the data for K/T are more
reliable than those for more ancient extinctions. By the end
Cretaceous carbon dioxide levels were at least five times,
and perhaps as much as ten times, those of today (Berner,
1994; Tajika, 1999; Gale, 2000; Johnson et al., 2002).
A bolide hitting the Earth near the Yucatán Peninsula of
southeast Mexico was widely thought to have caused the
K/T extinctions (Alvarez et al., 1980). This theory, however, is highly controversial: over 2,000 books and articles
were published about it within a decade of the original
publication (Glen, 1990); the consensus now being that
the bolide was not the primary cause of the extinction.
The impact itself and the outpourings of the shockinduced volcanoes are widely believed to have created
a stratospheric dust cloud that would have plunged the
whole of the Earth into a cold, interminable night. Certainly acid rain (including nitric acid generated from
heat-induced fusion of nitrogen and oxygen), high levels
of carbon dioxide released from the impact site and from
volcanic eruptions everywhere, and high concentrations
of methane released from continental slopes would have
combined to create intense greenhouse warming.
However, the timing of the ultimate extinction of different
animal groups, the time taken for many to go extinct, and
the age of the bolide debris do not correlate well. Some
animal taxa became extinct within an apparently brief
period, whereas others, including the dinosaurs, took hundreds of thousands of years, and their decline started long
before the bolide’s impact.
Causes of mass extinctions
Extrapolations from today’s knowledge of coral biology
to past environments must be cautiously made, especially
as the Scleractinia of today have been in existence for
only two mass extinctions (the end Triassic and K/T).
However, just as a number of physical parameters have
remained effectively constant throughout the Phanerozoic, some physiological processes, while they may vary
in detail over time, would in general have been applicable
to the Scleractinia throughout their existence.
The causes of mass extinctions are divisible into two
classes; those that do not involve the carbon cycle and
those that do (Veron, 2008a, b).
Causes independent of the carbon cycle
Reef development has three prerequisites: (1) corals (and/
or other calcifying organisms) must exist in environments
that are favorable for calcification, (2) the resulting calcium carbonate must be consolidated into reefs, and
(3) the rate of erosion must be less than the rate of accretion. Mass extinctions and reef gaps might occur whenever calcification is decreased, or there is a breakdown in
the mechanisms of consolidation, or there are factors that
increase the erosion or dissolution of calcium carbonate.
In the case of K/T, where there was a simultaneous extinction of both zooxanthellate and azooxanthellate coral species, any explanation of extinction must span wide depth
ranges as well as broad spatial scales.
Direct physical destruction from bolides
A bolide many kilometers across could possibly cause
massive destruction of reefs along the exposed side of an
area the size of a continent; however, it beggars belief that
such an effect, from a single point source, could envelop
the whole Earth (Toon et al., 1997). Nor is there any way
that azooxanthellate species would be equally affected or
that surviving reef corals would take millions of years to
recommence reef building. Be that as it may, the Earth
has several large and well-known craters (13 of which
date to the early Mesozoic or later) (Jablonski, 1986),
and many studies, motivated by the discovery of the K/T
bolide, have sought to establish these as the primary cause
of extinction events. Yet credible links have not emerged
(Rothschild and Lister, 2003). Indirect effects of bolides
are another matter.
“Nuclear Winter” induced by dust clouds
Bolides are presumably capable of creating dust clouds
that may have enveloped the Earth for weeks or months,
creating devastating darkness and (for terrestrial life) cold.
MASS EXTINCTIONS, ANOXIC EVENTS AND OCEAN ACIDIFICATION
673