“Human beings are now carrying out a large scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future. Within a few centuries, we are returning to the atmosphere and oceans the concentrated organic carbon stored in sedimentary rocks over hundreds of millions of years.”
– Roger Revelle and Hans Suess, 1957
There are five major extinction events known; these events had profound effects on the ecosystems of the Earth and influenced evolutionary processes. Coral reefs, for instance, show ‘reef gaps’ in the geologic record, which is taken to mean that reefs have taken many millions of years to fully recover after such events. Shallow, calcifying corals are particularly useful in exploring these, and other, smaller, background extinctions due to their nature as an ecosystem that actively produces and archives its geologic present and past. In 2008, J.E.N. Veron published a report in Coral Reefs that profiled these events, and their causes, finding that these events are closely tied to the carbon cycle. So when were these five major extinction events (ME) and what do we know about them?
End Ordovician ME When: 434 million years ago (Ma). Why?: This co-occurred with a period of high global temperatures and potentially high levels of atmospheric carbon dioxide. Sea level changes, shifts in ocean chemistry and other causes have also been implicated. Biotic Effects: 60% of all genera of land and sea life were obliterated. Although some corals survived, reefs disappeared for a few millions years, forming the first true ‘reef gap’ known.
Late Devonian ME When: 360 Ma. Why?: Extraterrestrial impact (considered unlikely by most) may have triggered global changes. Atmospheric carbon dioxide dropping (uptake by plants), low temperatures, and shifting sea levels have also been suggested. Biotic Effects: Mostly marine life effected. No real recovery of coral-sponge reefs (the early Devonian hosted extensive reefs globally).
End Permian ME When: 251 Ma. Why? Shifting ocean chemistry, atmospheric carbon dioxide flux, acid rain, deoxygenated surface waters, and massive volcanic venting are all possible (and largely interrelated) causes. Biotic Effects: Big. Really big. 82% of all genera and up to 95% of all marine species went extinct. It is thought that most corals and most other marine calcifiers were among the missing. Reefs did not appear again for another 10 millions years and when they emerged, it was in the form of the Scleractinia, rather than their ancient selves.
End Triassic ME When: 205 Ma. Why? The usual. Sea-level flux, ocean chemistry changes, and high temperatures are implicated, but clear evidence is scant. Biotic Effects: About half of marine invertebrates and up to 80% of land qaudapeds (four-legged things). A third of scleractinian families and 75% of scleractinian genera were included in this (remember your biological classification scheme from junior high: King Philip Came Over For Good Sex—kingdom, phylum, class, order, family, genus, species). Again, a reef gap came after this event (6-8 Ma in duration).
End Cretaceous ME (K/T) When: 65 Ma. Why?: We have wide agreement on a single cause for this one! A bolide (meteoroid) hit near the Gulf of Mexico, causing tsunamis and widespread volcanic activity. Then came the resulting dust clouds that thrust cold darkness onto the world, and the subsequent greenhouse warming from impact-related methane and carbon dioxide release, among other things. Biotic Effects: Everyone knows about this one….the dinosaurs met their end. Very sad. But what about the invertebrates! Some species of corals did survive (70% of all scleractinian genera did not), but reefs did not appear for at least 10 million years. Bivalves, gastropods, forams, and many other taxa were near extinction or went completely extinct (i.e. ammonites). Land animals were also nearly completely decimated.
It is easy to think of extinctions events as just that—single point, acute events. But they are usually an accumulation of processes that have built up over time. For example, the Cretaceous, even before the bolide impact, was extremely volatile in terms of sea level and global temperatures. However, some of these processes that lead to extinctions may be more instrumental in the demise of organisms that others.
Following environmental prerequisites for reef development, Veron rules out causes of mass extinctions not attributable to the carbon cycle and finds common threads in these extinction events in the form of shifting ocean chemistry and reduced pH (acidification). The ocean is a carbon sink, uptaking up to a third of atmospheric carbon. As more carbon dioxide enters the ocean, pH is lowered, and the carbonate chemistry of the oceans shifts. This can reduce calcification in marine organisms, which is one of the defining characteristics of a coral reef. Calcifying algae, which play a major part in reef consolidation (think of them as biotic cement), accrete even more soluble skeletons. Acidification, and other carbon cycle disturbances, is implicated as a cause in these mass extinctions mainly due to the process of elimination.
So in light of its potential role in extinctions, ocean acidification—happening today-– is not something taken lightly. Experimental studies widely show reduced calcification under shifting chemical conditions. It’s true that carbon dioxide levels have been higher in the geologic past, but no evidence exists for the current (human-driven) rate of increase. Organisms may not be able to adapt quickly enough. Even in terms of absolutes, by 2100, atmospheric carbon dioxide is predicted, conservatively, to reach 500 ppm—a value not seen in the at least the past 740,000 years. Oceanic pH is also predicted to drop another 0.3-0.4 units by 2100. We’re running a geophysical experiment without a control and as a result, we’re already looking at a certain amount of committed warming and acidification. Anthropogenic change is beginning to have severe impacts on global ecosystems, and if continued unabated, these impacts will be exacerbated through space and time.
Image: Fossil colonial coral from Anne Burgess on Wikimedia Commons.
This mainly serves as a quick review of Veron’s 2008 article. References (and those therein) drawn upon for this post are below, as well as some suggestions for further reading:
Doney SC, Balch WM, Fabry VJ, Feely R (2009a) Ocean acidification: a critical emerging problem for the ocean sciences. Oceanography 22:16-25
Hoegh-Guldberg O et al. (2007) Coral reefs under rapid climate change and ocean acidification.Science 318:1737-1742
Jackson JBC (2008) Ecological extinction and evolution in the brave new ocean. PNAS 105:11458-11465
Kleypas JA, Buddemeier BW, Archer D, Gattuso JP, Langdon C, Opdyke BN (1999) Geochemical consequences of increased atmospheric carbon dioxide on coral reefs. Science 284:118-120
Royal Society (2005) Ocean acidification due to increasing atmospheric carbon dioxide. The Royal Society: London
Revelle R, Suess H (1957) Carbon dioxide exchange between atmosphere and ocean and the question of an increase in atmospheric CO2 during the past decades. Tellus IX, 1
Sabine CL et al. (2004) The oceanic sink for anthropogenic CO2. Science 305: 367-371
Veron, J. (2008). Mass extinctions and ocean acidification: biological constraints on geological dilemmas Coral Reefs, 27 (3), 459-472 DOI: 10.1007/s00338-008-0381-8