Survival estimates for corals under conditions of very low
light vary from death in just a few weeks to survival of at
least parts of colonies for six months or more. Significantly, this strongly suggests that, if today’s reef-building
species were suddenly subjected to conditions of very low
light for periods of years rather than weeks or months,
99% of species would go extinct. No such extinction
has ever happened to the Scleractinia. The proportion
of zooxanthellate corals that went extinct at K/T is not
on its own sufficient to explain the end of reef building
or the reef gap that followed: (1) Many taxa that survived,
including members of the Poritidae and Faviidae, are among
the best reef builders (Veron, 1995). (2) Azooxanthellate
taxa fared no better than zooxanthellate taxa. (3) The K/T
mass extinction caused a much higher rate of extinction in
corals than in many other photosynthetic organisms, notably terrestrial vascular plants, which would have been more
vulnerable to prolonged darkness.
From a biological point of view, it can be concluded
that very low light, even if maintained for months, is
exceedingly unlikely to have been the principal cause of
the K/T mass extinction event. The dust cloud created by
the K/T bolide would also have caused atmospheric temperatures to plunge, but only for as long as the cloud
persisted. The thermal inertia of the oceans would have
protected marine environments from significant temperature change.
Sea-level changes
The major sea-level changes throughout Earth’s history
have been due to plate tectonics. These are very different
from the sea-level changes that occurred during the Pleistocene, primarily in being much slower and of immensely
longer duration. During the last glacial cycle, the sea level
dropped 130 m (Lambeck and Chappell, 2001; Siddall
et al., 2003). This fall is more than the depth range of all
but a few deepwater zooxanthellate corals, which means
that almost all corals had to relocate. Throughout the
Pleistocene, such falls were repeated many times, yet only
about 10% of all corals went extinct over this entire interval (Veron and Kelly, 1988). This is no more than
a background extinction rate. Thus sea-level changes of
any kind, let alone those resulting from tectonic movements, can be discounted as a primary cause of coral species extinctions.
Loss of area during sea-level regression
Loss of reef area during sea-level changes of the past has
been suggested as a cause of coral extinctions, but the
Pleistocene history of modern corals offers a number of
reasons why this would not be so. There were major losses
of reef area during the last glacial cycles; however, these
had no significant effect on today’s coral diversity. At the
opposite extreme, small areas may have most of the species
complement of an entire province. The great dispersal capabilities of corals would ensure that recolonization would
take place swiftly from surviving niches. Area loss is thus
highly unlikely to have been a primary cause of any
extinction. Furthermore, azooxanthellate species would
have been unaffected by a loss of reef area.
Loss of biodiversity as a cause of reef gaps
Reef building does not depend on high species diversity.
Many of the corals that survived K/T belonged to genera
well able to build reefs in the absence of any other coral
species. We can therefore discount loss of biodiversity as
a primary cause of reef gaps.
It has also been suggested that reefs do not reestablish
until peak evolution rates of new species are reached
(Jablonski, 1986). Although this may have been true after
some mass extinctions (there is no evidence either way), it
was certainly not the case after K/T as coral diversity
remained low long after widespread reef building had
reestablished.
Low temperatures
Conceptually, low temperatures could explain reef gaps
and coral extinctions, but the data available for global temperatures give no credence to this. During the K/T extinction, ocean temperatures were at or above, rather than
below, present levels. Furthermore, the suite of reefbuilding genera that survived indicates no selection for
cold tolerance. Nor does low temperature explain why
azooxanthellate species were lost. Even during a full glacial cycle, the world’s oceans cool only around 6
C, and
less than this at the equator. Although temperature reductions of this extent would be enough to contract the latitudinal range of species today, such a decrease could not
have initiated extinctions in equatorial regions on the scale
of any mass extinction.
High temperatures
Elevated temperatures can cause mass bleaching of corals,
creating widespread devastation when reef ecosystems
become so degraded that they are taken over by
macroalgae and bacterial slime. However, mass bleaching
only occurs in zooxanthellate organisms as it is due to an
overproduction of photosynthetic oxygen. Over geological time, corals growing under such conditions have the
evolutionary option of avoiding the issue by becoming
azooxanthellate. In such circumstances, any reduced
capacity for calcification would be in part offset by the
effects of temperature. Furthermore, a mass extinction
from temperature would require that such temperatures
extend to highest latitudes. This would never have
been possible, even under the most extreme conditions.
High temperatures cannot explain the extent or global
nature of mass extinction events, nor can they account
for the species that survived K/T, nor the loss of
azooxanthellate taxa.
Salinity
Corals, and by extension reefs, are adversely affected by
low salinity and low salinity may well have been a major
cause of regional extinctions at various times, especially
in partly landlocked regions such as the epicontinental
674
MASS EXTINCTIONS, ANOXIC EVENTS AND OCEAN ACIDIFICATION
light vary from death in just a few weeks to survival of at
least parts of colonies for six months or more. Significantly, this strongly suggests that, if today’s reef-building
species were suddenly subjected to conditions of very low
light for periods of years rather than weeks or months,
99% of species would go extinct. No such extinction
has ever happened to the Scleractinia. The proportion
of zooxanthellate corals that went extinct at K/T is not
on its own sufficient to explain the end of reef building
or the reef gap that followed: (1) Many taxa that survived,
including members of the Poritidae and Faviidae, are among
the best reef builders (Veron, 1995). (2) Azooxanthellate
taxa fared no better than zooxanthellate taxa. (3) The K/T
mass extinction caused a much higher rate of extinction in
corals than in many other photosynthetic organisms, notably terrestrial vascular plants, which would have been more
vulnerable to prolonged darkness.
From a biological point of view, it can be concluded
that very low light, even if maintained for months, is
exceedingly unlikely to have been the principal cause of
the K/T mass extinction event. The dust cloud created by
the K/T bolide would also have caused atmospheric temperatures to plunge, but only for as long as the cloud
persisted. The thermal inertia of the oceans would have
protected marine environments from significant temperature change.
Sea-level changes
The major sea-level changes throughout Earth’s history
have been due to plate tectonics. These are very different
from the sea-level changes that occurred during the Pleistocene, primarily in being much slower and of immensely
longer duration. During the last glacial cycle, the sea level
dropped 130 m (Lambeck and Chappell, 2001; Siddall
et al., 2003). This fall is more than the depth range of all
but a few deepwater zooxanthellate corals, which means
that almost all corals had to relocate. Throughout the
Pleistocene, such falls were repeated many times, yet only
about 10% of all corals went extinct over this entire interval (Veron and Kelly, 1988). This is no more than
a background extinction rate. Thus sea-level changes of
any kind, let alone those resulting from tectonic movements, can be discounted as a primary cause of coral species extinctions.
Loss of area during sea-level regression
Loss of reef area during sea-level changes of the past has
been suggested as a cause of coral extinctions, but the
Pleistocene history of modern corals offers a number of
reasons why this would not be so. There were major losses
of reef area during the last glacial cycles; however, these
had no significant effect on today’s coral diversity. At the
opposite extreme, small areas may have most of the species
complement of an entire province. The great dispersal capabilities of corals would ensure that recolonization would
take place swiftly from surviving niches. Area loss is thus
highly unlikely to have been a primary cause of any
extinction. Furthermore, azooxanthellate species would
have been unaffected by a loss of reef area.
Loss of biodiversity as a cause of reef gaps
Reef building does not depend on high species diversity.
Many of the corals that survived K/T belonged to genera
well able to build reefs in the absence of any other coral
species. We can therefore discount loss of biodiversity as
a primary cause of reef gaps.
It has also been suggested that reefs do not reestablish
until peak evolution rates of new species are reached
(Jablonski, 1986). Although this may have been true after
some mass extinctions (there is no evidence either way), it
was certainly not the case after K/T as coral diversity
remained low long after widespread reef building had
reestablished.
Low temperatures
Conceptually, low temperatures could explain reef gaps
and coral extinctions, but the data available for global temperatures give no credence to this. During the K/T extinction, ocean temperatures were at or above, rather than
below, present levels. Furthermore, the suite of reefbuilding genera that survived indicates no selection for
cold tolerance. Nor does low temperature explain why
azooxanthellate species were lost. Even during a full glacial cycle, the world’s oceans cool only around 6
C, and
less than this at the equator. Although temperature reductions of this extent would be enough to contract the latitudinal range of species today, such a decrease could not
have initiated extinctions in equatorial regions on the scale
of any mass extinction.
High temperatures
Elevated temperatures can cause mass bleaching of corals,
creating widespread devastation when reef ecosystems
become so degraded that they are taken over by
macroalgae and bacterial slime. However, mass bleaching
only occurs in zooxanthellate organisms as it is due to an
overproduction of photosynthetic oxygen. Over geological time, corals growing under such conditions have the
evolutionary option of avoiding the issue by becoming
azooxanthellate. In such circumstances, any reduced
capacity for calcification would be in part offset by the
effects of temperature. Furthermore, a mass extinction
from temperature would require that such temperatures
extend to highest latitudes. This would never have
been possible, even under the most extreme conditions.
High temperatures cannot explain the extent or global
nature of mass extinction events, nor can they account
for the species that survived K/T, nor the loss of
azooxanthellate taxa.
Salinity
Corals, and by extension reefs, are adversely affected by
low salinity and low salinity may well have been a major
cause of regional extinctions at various times, especially
in partly landlocked regions such as the epicontinental
674
MASS EXTINCTIONS, ANOXIC EVENTS AND OCEAN ACIDIFICATION