seas of the Super-Tethys and North Africa during the Cretaceous. However, it is inconceivable that any continental
landmass could hold enough fresh water to cause a global
mass extinction.
Disease and toxins
There is strong evidence today that stresses such as mass
bleaching, hyposaline influxes, and a wide range of other
ecological imbalances can greatly increase the incidence
of diseases in corals. Combinations of stresses from high
temperature, high light levels, and disease might lead to
major regional losses of corals for as long as those stresses
persist. These would have to be very widespread and
affect deepwater and nearshore corals equally. The Earth
has no toxins in such quantities, nor any that can exist both
on land and in the oceans.
Extraterrestrial events
Extraterrestrial events apart from bolides have commonly
been suggested as causal factors in extinctions. Episodes
of ultraviolet or cosmic radiation from solar flares or
supernovas (which may be capable of stripping away the
ozone layer, allowing high levels of ultraviolet radiation
to reach terrestrial life) can be discounted as causes of
mass extinctions because the deep ocean would have
shielded bottom-dwelling communities, and these were
not shielded. There is also the often-mentioned finding
that extinctions follow a 26-million-year periodicity, implying an overriding extraterrestrial “supercycle” of unknown
origin. The database of fossils referred to above presents
an avalanche of statistics in support of this conclusion
(Raup and Sepkoski, 1986); nevertheless, although this
long periodicity does not look entirely random, it does not
look distinctly cyclical either, so this concept remains enigmatic and without a plausible mechanism.
Links between mass extinctions and the carbon
cycle
If the above causes of mass extinctions and reef gaps are
discounted as primary factors, there remains an array of
further possibilities that have one aspect in common: they
are either part of the carbon cycle or closely linked to it.
This is hardly surprising, for of all the great matter cycles
of the Earth, the carbon cycle is by far the most important.
The chemical impacts discussed below are all directly
or indirectly linked to the carbon cycle. Severe environmental degradation can occur rapidly in response to bolide
impacts and some geological events such as larva flows
(traps) or supervolcanoes. They may also occur in
response to slower processes created by gases release from
volcanic chains as a result of seafloor spreading and other
tectonic movements. Although slow, these processes may
lead to relatively abrupt extinctions through cumulative
outcomes and synergies.
Importantly, small changes in the concentration of
atmospheric gases can lead to major changes in global
environments, potentially affecting both marine and terrestrial life. These gases include major components of
our atmosphere, notably oxygen and water vapor, as well
as those that are only present in trace amounts. The latter
include the gases that control the atmosphere’s greenhouse warming of which carbon dioxide and methane
being the most important.
Peripheral links to the carbon cycle are considered here
first.
Acid rain
High levels of atmospheric carbon dioxide produce carbonic acid, and high levels of sulphur dioxide produce
sulphuric acid. These substances, together with nitrous
oxide from industry and the draining of tropical marshlands,
are the main sources of acid rain today. Carbon dioxide and
sulphur dioxide are released in massive amounts from some
volcanoes, traps (notably the Deccan Traps of India at the
time of K/T) and other sources, both terrestrial and marine,
both to be ultimately neutralized by ocean buffers. Although
sulphur dioxide and nitrous oxide do not contain carbon, by
the time their products reach the oceans they do, and thus
they are inextricably linked to the carbon cycle. It is not
quantitatively possible for acid rain to be a primary cause
of an extinction event; however, acid rain may well have
acted in synergy with other causes to create extreme environmental degradation.
Hydrogen sulphide
Hydrogen sulphide is another gas released in large quantities into the oceans and atmosphere from volcanoes or
from sulphur-rich geysers and the like. It also has biological origins, which is why it regularly occurs in anoxic
waters of all types, ranging from aquaria to atoll lagoons.
As with sulphur dioxide, although it does not contain carbon, it is inextricably linked to the carbon cycle and could,
in terms of quantity, have been a significant contributor to
an environmental upheaval in synergy with other gases.
Oxygen and anoxia
In a nitrogen-dominated atmosphere, oxygen and carbon
dioxide have an additive relationship, an increase or
decrease of the one creating a parallel effect in the other,
all else being equal. However, all else may sometimes
not be equal, and there may have been conditions, perhaps
the end Permian and K/T, where ocean anoxia may have
been created by primary productivity decrease (through
phytoplankton depletion) as a result of acidification. By
this simple mechanism, high levels of atmospheric carbon
dioxide can work synergistically with low levels of atmospheric oxygen to create ocean anoxia. Whether or not this
has actually happened is not known.
Methane
Methane from geological sources, plant respiration, and
some animal life exists in minute amounts in the atmosphere (currently 1.8 ppm). However, it occurs in much
greater quantities in permafrost, in tropical marshlands,
and in vast quantities as icelike solids (clathrates or
hydrates) that are stable only under pressure and at low
MASS EXTINCTIONS, ANOXIC EVENTS AND OCEAN ACIDIFICATION
675
landmass could hold enough fresh water to cause a global
mass extinction.
Disease and toxins
There is strong evidence today that stresses such as mass
bleaching, hyposaline influxes, and a wide range of other
ecological imbalances can greatly increase the incidence
of diseases in corals. Combinations of stresses from high
temperature, high light levels, and disease might lead to
major regional losses of corals for as long as those stresses
persist. These would have to be very widespread and
affect deepwater and nearshore corals equally. The Earth
has no toxins in such quantities, nor any that can exist both
on land and in the oceans.
Extraterrestrial events
Extraterrestrial events apart from bolides have commonly
been suggested as causal factors in extinctions. Episodes
of ultraviolet or cosmic radiation from solar flares or
supernovas (which may be capable of stripping away the
ozone layer, allowing high levels of ultraviolet radiation
to reach terrestrial life) can be discounted as causes of
mass extinctions because the deep ocean would have
shielded bottom-dwelling communities, and these were
not shielded. There is also the often-mentioned finding
that extinctions follow a 26-million-year periodicity, implying an overriding extraterrestrial “supercycle” of unknown
origin. The database of fossils referred to above presents
an avalanche of statistics in support of this conclusion
(Raup and Sepkoski, 1986); nevertheless, although this
long periodicity does not look entirely random, it does not
look distinctly cyclical either, so this concept remains enigmatic and without a plausible mechanism.
Links between mass extinctions and the carbon
cycle
If the above causes of mass extinctions and reef gaps are
discounted as primary factors, there remains an array of
further possibilities that have one aspect in common: they
are either part of the carbon cycle or closely linked to it.
This is hardly surprising, for of all the great matter cycles
of the Earth, the carbon cycle is by far the most important.
The chemical impacts discussed below are all directly
or indirectly linked to the carbon cycle. Severe environmental degradation can occur rapidly in response to bolide
impacts and some geological events such as larva flows
(traps) or supervolcanoes. They may also occur in
response to slower processes created by gases release from
volcanic chains as a result of seafloor spreading and other
tectonic movements. Although slow, these processes may
lead to relatively abrupt extinctions through cumulative
outcomes and synergies.
Importantly, small changes in the concentration of
atmospheric gases can lead to major changes in global
environments, potentially affecting both marine and terrestrial life. These gases include major components of
our atmosphere, notably oxygen and water vapor, as well
as those that are only present in trace amounts. The latter
include the gases that control the atmosphere’s greenhouse warming of which carbon dioxide and methane
being the most important.
Peripheral links to the carbon cycle are considered here
first.
Acid rain
High levels of atmospheric carbon dioxide produce carbonic acid, and high levels of sulphur dioxide produce
sulphuric acid. These substances, together with nitrous
oxide from industry and the draining of tropical marshlands,
are the main sources of acid rain today. Carbon dioxide and
sulphur dioxide are released in massive amounts from some
volcanoes, traps (notably the Deccan Traps of India at the
time of K/T) and other sources, both terrestrial and marine,
both to be ultimately neutralized by ocean buffers. Although
sulphur dioxide and nitrous oxide do not contain carbon, by
the time their products reach the oceans they do, and thus
they are inextricably linked to the carbon cycle. It is not
quantitatively possible for acid rain to be a primary cause
of an extinction event; however, acid rain may well have
acted in synergy with other causes to create extreme environmental degradation.
Hydrogen sulphide
Hydrogen sulphide is another gas released in large quantities into the oceans and atmosphere from volcanoes or
from sulphur-rich geysers and the like. It also has biological origins, which is why it regularly occurs in anoxic
waters of all types, ranging from aquaria to atoll lagoons.
As with sulphur dioxide, although it does not contain carbon, it is inextricably linked to the carbon cycle and could,
in terms of quantity, have been a significant contributor to
an environmental upheaval in synergy with other gases.
Oxygen and anoxia
In a nitrogen-dominated atmosphere, oxygen and carbon
dioxide have an additive relationship, an increase or
decrease of the one creating a parallel effect in the other,
all else being equal. However, all else may sometimes
not be equal, and there may have been conditions, perhaps
the end Permian and K/T, where ocean anoxia may have
been created by primary productivity decrease (through
phytoplankton depletion) as a result of acidification. By
this simple mechanism, high levels of atmospheric carbon
dioxide can work synergistically with low levels of atmospheric oxygen to create ocean anoxia. Whether or not this
has actually happened is not known.
Methane
Methane from geological sources, plant respiration, and
some animal life exists in minute amounts in the atmosphere (currently 1.8 ppm). However, it occurs in much
greater quantities in permafrost, in tropical marshlands,
and in vast quantities as icelike solids (clathrates or
hydrates) that are stable only under pressure and at low
MASS EXTINCTIONS, ANOXIC EVENTS AND OCEAN ACIDIFICATION
675