a recovery sequence, reef gaps in the past of millions of
years would be a plausible consequence, depending primarily on the endurance of atmospheric carbon dioxide
and the rate of evolutionary accommodation to it.
Low levels of carbon dioxide
Low levels of carbon dioxide may result in low levels of
photosynthesis, leading to low levels of oxygen and eventually ocean anoxia. This might have triggered the Late
Devonian mass extinction event; the evidence is
inconclusive.
Conclusions
The foregoing discussion argues that ocean acidification
and anoxia as a mechanism explains mass extinctions better than other mechanisms that have been offered. However, studies of ancient environments also suggest that
similar atmospheric carbon dioxide levels may have
occurred at times when reefs actually proliferated rather
than declined. One explanation may lie in the interpretation of information from ancient times for carbon dioxide
data are very uncertain for most of geological time as it is
based on a small number of point samples from widely
different geological intervals. In contrast, as seen in ice
core records and contemporary measurements, carbon
dioxide levels can fluctuate substantially over intervals
as short as millennia. Many possibilities present themselves. (1) Reefs may not have proliferated at all during
carbon dioxide highs; they only appear to have survived
because they were able to resume growth when levels
fell (the boom or bust observation made above). (2) The
high apparent carbon dioxide levels of ancient times may
be an artifact of a lack of data and measuring methods,
which currently produce very conflicting results.
(3) Corals and other taxa may have become partly or
completely askeletal (as noted above) during times of
adverse ocean chemistry. (4) Corals may have had calcitic
skeletons during such times and the calcifying process
may have been more physiologically isolated from ocean
pH than it now is. These options can only be guessed;
there may have been many aspects of coral biology that
allowed ancient corals to tolerate water chemistries that
are lethal to today’s taxa.
Although these deliberations about possible causes of
mass extinction events have highlighted acidification as
the most probable cause of both coral extinctions and reef
gaps, it would be misleading to consider this as the only
cause. Indeed, all mass extinctions appear to be the outcome of several different influences acting synergistically.
Acidification may well have been a primary mechanism
by which marine ecosystems were affected; however,
low light, bleaching, deteriorating water quality owing to
acid rain or anoxia, mechanical damage, and disease
would all have played their parts. Although the time
frames of bolides do not always fit the facts as a singular
cause of extinction, bolides or traps or volcanic chains
may have created severely adverse conditions that became
“the straw that broke the camel’s back.”
Bibliography
Alvarez, L., Alvarez, W., Asaro, F., and Michel, H. V., 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction: experimental results and theoretical interpretation. Science, 208,
1095–1108.
Barron, E. J., 1983. A warm, equable Cretaceous: the nature of the
problem. Earth Science Reviews, 19, 305–338.
Barron, E. J., and Washington, W. M., 1985. Warm cretaceous climates: high atmospheric carbon dioxide as a plausible mechanism. In Sundquist, E. T., and Broecker, W. S. (eds.), The
Carbon Cycle and Atmospheric Carbon Dioxide: Natural Variations, Archaean to Present: American Geophysical Union
Monograph 32, pp. 546–553.
Beauvais, L., 1984. Evolution and diversification of Jurassic
Scleractinia. Palaeontographica Americana, 54, 219–224.
Berner, R. A., 1994. GEOCARB II: a revised Model of Atmospheric carbon dioxide over Phanerozoic time. American Journal of Science, 294, 56–91.
Berner, R. A., 2006. Inclusion of the weathering of volcanic rocks in
the GEOCARBSULF model. American Journal of Science, 306,
295–302.
Buddemeier, R. W., Kleypas, J. A., and Aronson, R. B., 2004. Coral
Reefs and Global Climate Change: Potential Contributions
of Climate Change to Stresses on Coral Reef Ecosystems.
Arlington, VA: Pew Center on Global Climate Change.
Copper, P., 1994. Ancient reef ecosystem expansion and collapse.
Coral Reefs, 13, 3–11.
Copper, P., 2001. Evolution, radiations, and extinctions in Proterozoic to Mid-Paleozoic reefs. In Stanley, G. D. Jr. (ed.), The History and Sedimentology of Ancient Reef Systems. New York:
Academic/Plenum Publishers, pp. 89–119.
Copper, P., 2002. Silurian and Devonian reefs: 80 million years of
global greenhouse between two ice ages. In Kiessling, W.,
Flügel, E., and Galonka, J. (eds.), Phanerozoic Reef Patterns.
SEPM Special Publication, 72, pp. 181–238.
Erwin, D. H., 2006. Extinction: How Life on Earth Nearly Ended 250
Million Years Ago. Princeton, NJ: Princeton University Press.
Fine, M., and Tchernov, D., 2007. Scleractinian coral species survive and recover from decalcification. Science, 315, 1811.
Flügel, E., and Senowbari-Daryan, B., 2001. Triassic reefs of the
Tethys. In Stanley, G. D. Jr. (ed.), The History and Sedimentology of Ancient Reef Systems. New York: Academic/Plenum Publishers, pp. 217–249.
Gale, A., 2000. The Cretaceous World. In Culver, S. J., and Rawson,
P. F. (eds.), Biotic Response to Global Change: The Last 145 Million Years. Cambridge: Cambridge University Press, pp. 4–19.
Glen, W., 1990. What killed the dinosaurs? American Scientist, 78,
354–370.
Grice, K., et al., 2005. Photoc zone euxinia during the PermianTriassic superanoxic event. Science, 307, 706–709.
Guinotte, J. M., Buddemeier, R. W., and Kleypas, J. A.,
2003. Future coral reef habitat marginality: temporal and spatial
effects of climate change in the Pacific basin. Coral Reefs 22,
551–558.
Hallum, A., and Wignall, P. B., 1997. Mass Extinctions and Their
Aftermath. New York: Oxford University Press.
Hautmann, M., 2004. Effect of End-Triassic carbon dioxide maximum on carbonate sedimentation and marine mass extinction.
Facies, 50, 257–261.
Hautmann, M., Benton, M. J., and Tomašových, A., 2008. Catastrophic ocean acidification at the Triassic-Jurassic boundary.
Neues Jahrbuch für Geologie und Paläontologie.
Abhandlungen, 249, 119–127.
Jablonski, D., 1986. Mass extinctions: new answers, new questions.
In Kaufman, L., Mallory, K. (eds.), The Last Extinction.
Cambridge: MIT Press, pp. 43–61.
MASS EXTINCTIONS, ANOXIC EVENTS AND OCEAN ACIDIFICATION
677
years would be a plausible consequence, depending primarily on the endurance of atmospheric carbon dioxide
and the rate of evolutionary accommodation to it.
Low levels of carbon dioxide
Low levels of carbon dioxide may result in low levels of
photosynthesis, leading to low levels of oxygen and eventually ocean anoxia. This might have triggered the Late
Devonian mass extinction event; the evidence is
inconclusive.
Conclusions
The foregoing discussion argues that ocean acidification
and anoxia as a mechanism explains mass extinctions better than other mechanisms that have been offered. However, studies of ancient environments also suggest that
similar atmospheric carbon dioxide levels may have
occurred at times when reefs actually proliferated rather
than declined. One explanation may lie in the interpretation of information from ancient times for carbon dioxide
data are very uncertain for most of geological time as it is
based on a small number of point samples from widely
different geological intervals. In contrast, as seen in ice
core records and contemporary measurements, carbon
dioxide levels can fluctuate substantially over intervals
as short as millennia. Many possibilities present themselves. (1) Reefs may not have proliferated at all during
carbon dioxide highs; they only appear to have survived
because they were able to resume growth when levels
fell (the boom or bust observation made above). (2) The
high apparent carbon dioxide levels of ancient times may
be an artifact of a lack of data and measuring methods,
which currently produce very conflicting results.
(3) Corals and other taxa may have become partly or
completely askeletal (as noted above) during times of
adverse ocean chemistry. (4) Corals may have had calcitic
skeletons during such times and the calcifying process
may have been more physiologically isolated from ocean
pH than it now is. These options can only be guessed;
there may have been many aspects of coral biology that
allowed ancient corals to tolerate water chemistries that
are lethal to today’s taxa.
Although these deliberations about possible causes of
mass extinction events have highlighted acidification as
the most probable cause of both coral extinctions and reef
gaps, it would be misleading to consider this as the only
cause. Indeed, all mass extinctions appear to be the outcome of several different influences acting synergistically.
Acidification may well have been a primary mechanism
by which marine ecosystems were affected; however,
low light, bleaching, deteriorating water quality owing to
acid rain or anoxia, mechanical damage, and disease
would all have played their parts. Although the time
frames of bolides do not always fit the facts as a singular
cause of extinction, bolides or traps or volcanic chains
may have created severely adverse conditions that became
“the straw that broke the camel’s back.”
Bibliography
Alvarez, L., Alvarez, W., Asaro, F., and Michel, H. V., 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction: experimental results and theoretical interpretation. Science, 208,
1095–1108.
Barron, E. J., 1983. A warm, equable Cretaceous: the nature of the
problem. Earth Science Reviews, 19, 305–338.
Barron, E. J., and Washington, W. M., 1985. Warm cretaceous climates: high atmospheric carbon dioxide as a plausible mechanism. In Sundquist, E. T., and Broecker, W. S. (eds.), The
Carbon Cycle and Atmospheric Carbon Dioxide: Natural Variations, Archaean to Present: American Geophysical Union
Monograph 32, pp. 546–553.
Beauvais, L., 1984. Evolution and diversification of Jurassic
Scleractinia. Palaeontographica Americana, 54, 219–224.
Berner, R. A., 1994. GEOCARB II: a revised Model of Atmospheric carbon dioxide over Phanerozoic time. American Journal of Science, 294, 56–91.
Berner, R. A., 2006. Inclusion of the weathering of volcanic rocks in
the GEOCARBSULF model. American Journal of Science, 306,
295–302.
Buddemeier, R. W., Kleypas, J. A., and Aronson, R. B., 2004. Coral
Reefs and Global Climate Change: Potential Contributions
of Climate Change to Stresses on Coral Reef Ecosystems.
Arlington, VA: Pew Center on Global Climate Change.
Copper, P., 1994. Ancient reef ecosystem expansion and collapse.
Coral Reefs, 13, 3–11.
Copper, P., 2001. Evolution, radiations, and extinctions in Proterozoic to Mid-Paleozoic reefs. In Stanley, G. D. Jr. (ed.), The History and Sedimentology of Ancient Reef Systems. New York:
Academic/Plenum Publishers, pp. 89–119.
Copper, P., 2002. Silurian and Devonian reefs: 80 million years of
global greenhouse between two ice ages. In Kiessling, W.,
Flügel, E., and Galonka, J. (eds.), Phanerozoic Reef Patterns.
SEPM Special Publication, 72, pp. 181–238.
Erwin, D. H., 2006. Extinction: How Life on Earth Nearly Ended 250
Million Years Ago. Princeton, NJ: Princeton University Press.
Fine, M., and Tchernov, D., 2007. Scleractinian coral species survive and recover from decalcification. Science, 315, 1811.
Flügel, E., and Senowbari-Daryan, B., 2001. Triassic reefs of the
Tethys. In Stanley, G. D. Jr. (ed.), The History and Sedimentology of Ancient Reef Systems. New York: Academic/Plenum Publishers, pp. 217–249.
Gale, A., 2000. The Cretaceous World. In Culver, S. J., and Rawson,
P. F. (eds.), Biotic Response to Global Change: The Last 145 Million Years. Cambridge: Cambridge University Press, pp. 4–19.
Glen, W., 1990. What killed the dinosaurs? American Scientist, 78,
354–370.
Grice, K., et al., 2005. Photoc zone euxinia during the PermianTriassic superanoxic event. Science, 307, 706–709.
Guinotte, J. M., Buddemeier, R. W., and Kleypas, J. A.,
2003. Future coral reef habitat marginality: temporal and spatial
effects of climate change in the Pacific basin. Coral Reefs 22,
551–558.
Hallum, A., and Wignall, P. B., 1997. Mass Extinctions and Their
Aftermath. New York: Oxford University Press.
Hautmann, M., 2004. Effect of End-Triassic carbon dioxide maximum on carbonate sedimentation and marine mass extinction.
Facies, 50, 257–261.
Hautmann, M., Benton, M. J., and Tomašových, A., 2008. Catastrophic ocean acidification at the Triassic-Jurassic boundary.
Neues Jahrbuch für Geologie und Paläontologie.
Abhandlungen, 249, 119–127.
Jablonski, D., 1986. Mass extinctions: new answers, new questions.
In Kaufman, L., Mallory, K. (eds.), The Last Extinction.
Cambridge: MIT Press, pp. 43–61.
MASS EXTINCTIONS, ANOXIC EVENTS AND OCEAN ACIDIFICATION
677