temperatures on continental shelves. The volume of these
solids is unknown, although it is of the same order of magnitude as the Earth’s total quantity of fossil fuels. Methane
leaks into the ocean and atmosphere naturally, for example, as marsh gas. However, if it were released into the
atmosphere in substantial quantities, perhaps as a result
of a bolide impact or a buoyancy change in the methane
ice (for which there are many possible causes), there could
be serious environmental consequences. It would have
a major greenhouse effect (methane has 22 times the
potency of an equal volume of carbon dioxide) and would
also be converted to carbon dioxide by microbes and
chemical oxidization, causing the same adverse effects
characteristic of high levels of carbon dioxide.
Carbon dioxide
Both increasing and decreasing carbon dioxide levels have
been proposed as major contributors to mass extinction
events in the past. Over geological time scales, carbon
dioxide has varied from levels much lower than the
387 ppm of today up to perhaps ten times today’s level
(depending on the reliability of studies of a small number
of fossil soils and whether or not results from single points
in time are representative of longer time intervals). The
effects of high levels of atmospheric carbon dioxide are
pursued below.
Ocean chemistry and pH
This subject can only reliably be assessed via the chemistry of contemporary oceans. Increased atmospheric carbon
dioxide leads to greater amounts of the gas dissolving
in the oceans, which directly reduces pH. This, in turn,
alters the proportions of different forms of inorganic carbon (CO 2 , H 2 CO 3 , HCO 3
–
, and CO 3
2–
) in the ocean
(Buddemeier et al., 2004; Kleypas and Langdon, 2006).
Increasing the concentration of carbon dioxide in the
ocean alters the proportion of the other three forms of carbon. Calcifying organisms use carbonate and/or bicarbonate ions to build skeletons, and thus a decrease in their
availability slows the calcification process. The proportions of each of these carbon components are sensitive to
temperature and pressure, and thus to latitude and depth.
Acidification affects zooxanthellate corals and most
azooxanthellate corals by different paths. The former are
sensitive to acidification of surface waters by atmospheric
carbon dioxide directly; the latter are sensitive to having
their depth range reduced by shallowing of O aragonite .
These surface and deep layers directly influence each
other at high latitudes where temperature gradients are
weak and mixing occurs relatively easily. However, they
are well separated in the warm tropics, especially at the
thermocline, and interact primarily via the sinking of carbonate skeletons of surface-dwelling plankton.
Shallow oceans are currently supersaturated with carbonates, allowing zooxanthellate corals to calcify. However, as O aragonite decreases, calcification requires
increasing amounts of energy. The same applies to
azooxanthellate corals, although here the process depends
on the depth of the aragonite saturation horizon, which, in
turn, varies with temperature as well as chemistry: as
changes in water temperature and chemistry cause the
horizon to rise, the depth range of azooxanthellate corals
shallows.
Acidification
The dependence of calcareous algae on high-magnesium
calcite, the most soluble of all calcium carbonate skeletons, makes them particularly vulnerable to acidification
(Kiessling et al. 2008, Veron, 2008b). Since these algae
are critical to reef consolidation, their removal would, in
the past, reduce the net accretion of reefs before the rate
of coral calcification became limiting. With further
decrease in pH, aragonitic organisms including
Scleractinia would become increasingly affected, further
tipping the balance in favor of reef erosion. Since acidification from elevated atmospheric carbon dioxide affects
oceans on a global scale, the effects on reefs would be
worldwide, although influences from temperature and surface water mixing would create local variations both geographically and with depth. In equatorial regions, adverse
changes in carbonate/bicarbonate ion availability as
a result of decreasing pH would be exacerbated by lack
of mixing between the warm, shallow, carbon dioxideenhanced surface layers, and the buffers of cool, deeper
ocean water. Acidification could thus have a devastating
effect on reef development, leading initially to widespread
inhibition of reef growth and eventually to reef erosion
and dissolution.
The first impact will be on azooxanthellate taxa, which
would be to displace them from deeper oceans as the
O aragonite horizon shallows. This effect is greatest at high
latitudes (Guinotte et al., 2003; Turley et al., 2007). Two
recent studies point toward acidification as the primary
cause of coral extinctions and the reef gaps, despite
deficiencies in the fossil record. (1) Late Cretaceous
azooxanthellate corals of the caryophylliid genus
Coelosmilia have been found to have calcitic skeletons
rather than skeletons of the more soluble aragonite as have
today’s Scleractinia (Stolarski et al., 2007). (2) Two Mediterranean corals, Oculina patagonica and Madracis
pharensis which were placed in acidified aquaria gradually lost their skeletons, then regrew them after being
returned to normal seawater (Fine and Tchernov, 2007).
Unlike the alternative causes of extinctions noted
above, acidification can explain why both azooxanthellate
and zooxanthellate species were affected at the time of
K/T. It can also explain the loss of reefs both locally and
globally, as well as account for the existence of longlasting reef gaps. Although acidification may be difficult
to initiate because of ocean buffers, once achieved it will
persist as long as atmospheric carbon dioxide remains
high. Furthermore, the oceans would remain acidified for
tens of thousand years after carbon dioxide levels had
declined, that being the time required for normal alkalinity
to be restored. This would be an interval long enough for
corals to initiate an evolutionary recovery. Under such
676
MASS EXTINCTIONS, ANOXIC EVENTS AND OCEAN ACIDIFICATION
solids is unknown, although it is of the same order of magnitude as the Earth’s total quantity of fossil fuels. Methane
leaks into the ocean and atmosphere naturally, for example, as marsh gas. However, if it were released into the
atmosphere in substantial quantities, perhaps as a result
of a bolide impact or a buoyancy change in the methane
ice (for which there are many possible causes), there could
be serious environmental consequences. It would have
a major greenhouse effect (methane has 22 times the
potency of an equal volume of carbon dioxide) and would
also be converted to carbon dioxide by microbes and
chemical oxidization, causing the same adverse effects
characteristic of high levels of carbon dioxide.
Carbon dioxide
Both increasing and decreasing carbon dioxide levels have
been proposed as major contributors to mass extinction
events in the past. Over geological time scales, carbon
dioxide has varied from levels much lower than the
387 ppm of today up to perhaps ten times today’s level
(depending on the reliability of studies of a small number
of fossil soils and whether or not results from single points
in time are representative of longer time intervals). The
effects of high levels of atmospheric carbon dioxide are
pursued below.
Ocean chemistry and pH
This subject can only reliably be assessed via the chemistry of contemporary oceans. Increased atmospheric carbon
dioxide leads to greater amounts of the gas dissolving
in the oceans, which directly reduces pH. This, in turn,
alters the proportions of different forms of inorganic carbon (CO 2 , H 2 CO 3 , HCO 3
–
, and CO 3
2–
) in the ocean
(Buddemeier et al., 2004; Kleypas and Langdon, 2006).
Increasing the concentration of carbon dioxide in the
ocean alters the proportion of the other three forms of carbon. Calcifying organisms use carbonate and/or bicarbonate ions to build skeletons, and thus a decrease in their
availability slows the calcification process. The proportions of each of these carbon components are sensitive to
temperature and pressure, and thus to latitude and depth.
Acidification affects zooxanthellate corals and most
azooxanthellate corals by different paths. The former are
sensitive to acidification of surface waters by atmospheric
carbon dioxide directly; the latter are sensitive to having
their depth range reduced by shallowing of O aragonite .
These surface and deep layers directly influence each
other at high latitudes where temperature gradients are
weak and mixing occurs relatively easily. However, they
are well separated in the warm tropics, especially at the
thermocline, and interact primarily via the sinking of carbonate skeletons of surface-dwelling plankton.
Shallow oceans are currently supersaturated with carbonates, allowing zooxanthellate corals to calcify. However, as O aragonite decreases, calcification requires
increasing amounts of energy. The same applies to
azooxanthellate corals, although here the process depends
on the depth of the aragonite saturation horizon, which, in
turn, varies with temperature as well as chemistry: as
changes in water temperature and chemistry cause the
horizon to rise, the depth range of azooxanthellate corals
shallows.
Acidification
The dependence of calcareous algae on high-magnesium
calcite, the most soluble of all calcium carbonate skeletons, makes them particularly vulnerable to acidification
(Kiessling et al. 2008, Veron, 2008b). Since these algae
are critical to reef consolidation, their removal would, in
the past, reduce the net accretion of reefs before the rate
of coral calcification became limiting. With further
decrease in pH, aragonitic organisms including
Scleractinia would become increasingly affected, further
tipping the balance in favor of reef erosion. Since acidification from elevated atmospheric carbon dioxide affects
oceans on a global scale, the effects on reefs would be
worldwide, although influences from temperature and surface water mixing would create local variations both geographically and with depth. In equatorial regions, adverse
changes in carbonate/bicarbonate ion availability as
a result of decreasing pH would be exacerbated by lack
of mixing between the warm, shallow, carbon dioxideenhanced surface layers, and the buffers of cool, deeper
ocean water. Acidification could thus have a devastating
effect on reef development, leading initially to widespread
inhibition of reef growth and eventually to reef erosion
and dissolution.
The first impact will be on azooxanthellate taxa, which
would be to displace them from deeper oceans as the
O aragonite horizon shallows. This effect is greatest at high
latitudes (Guinotte et al., 2003; Turley et al., 2007). Two
recent studies point toward acidification as the primary
cause of coral extinctions and the reef gaps, despite
deficiencies in the fossil record. (1) Late Cretaceous
azooxanthellate corals of the caryophylliid genus
Coelosmilia have been found to have calcitic skeletons
rather than skeletons of the more soluble aragonite as have
today’s Scleractinia (Stolarski et al., 2007). (2) Two Mediterranean corals, Oculina patagonica and Madracis
pharensis which were placed in acidified aquaria gradually lost their skeletons, then regrew them after being
returned to normal seawater (Fine and Tchernov, 2007).
Unlike the alternative causes of extinctions noted
above, acidification can explain why both azooxanthellate
and zooxanthellate species were affected at the time of
K/T. It can also explain the loss of reefs both locally and
globally, as well as account for the existence of longlasting reef gaps. Although acidification may be difficult
to initiate because of ocean buffers, once achieved it will
persist as long as atmospheric carbon dioxide remains
high. Furthermore, the oceans would remain acidified for
tens of thousand years after carbon dioxide levels had
declined, that being the time required for normal alkalinity
to be restored. This would be an interval long enough for
corals to initiate an evolutionary recovery. Under such
676
MASS EXTINCTIONS, ANOXIC EVENTS AND OCEAN ACIDIFICATION