Five times in Earth's history, the majority of species died during a relatively short period of time in geological terms. Scientists have carefully studied each of these five great extinction events by examining different types of fossils found in rocks.
For example, 201 million years ago, at the end of the Triassic period, many types of marine and land organisms disappeared from the fossil record. During the slow recovery of life in the subsequent Jurassic period, dinosaurs gradually emerged as dominant organisms. They ruled the Earth until another mass extinction at the end of the Cretaceous period, 66 million years ago.
Fossils found in rocks show what happened during each great extinction event. These rocks have been accurately dated using the rate of decay of radioactive uranium isotopes to stable isotopes of lead.
However, scientists hotly debate the causes of mass extinctions. Most theories involve climate disruption -- abrupt cooling or heating -- but it is unclear if this was triggered by volcanic eruptions, meteorite impacts, and/or massive releases of greenhouse gases such as carbon dioxide or methane.
The greatest of all extinctions -- the "Great Dying" -- took place 252 million years ago at the end of the Permian period. The end-Permian extinction eliminated 95 per cent of all species -- including the complete disappearance of once-dominant marine organisms such as trilobites.
Sedimentary rocks indicate that very large changes occurred in the Earth's carbon cycle quickly (in geologic terms) at the end of the Permian. Of the two non-radioactive carbon (C) isotopes in the atmosphere, heavier C-13 comprises just over 1 per cent, and C-12 the remainder. Photosynthetic organisms (plants, algae, some bacteria) selectively incorporate carbon dioxide containing the lighter C-12 isotope into their tissues -- their "organic" carbon is depleted in C-13. Other organisms (animals, fungi, some bacteria) consume plant-derived carbon and release it as inorganic carbon. If C-13-depleted inorganic carbon is released in high amounts over hundreds to thousands of years, rocks formed during that time will be depleted in C-13.
During a 20,000-year period at the end of the Permian, rocks became increasingly depleted in C-13. This suggests that some rapid and mysterious change occurred in the Earth's carbon cycle that involved a major release of inorganic carbon. And this was not a one-time event, as might be triggered by a volcanic eruption. The rate of growth of inorganic carbon was exponential, increasing through time.
A new study in the journal Proceedings of the National Academy of Sciences, entitled "Methanogenic burst in the end-Permian carbon cycle," has unravelled this mystery. The authors provide evidence that an organism of the genus Methanosarcina developed a new way to tap into a relatively abundant source of organic carbon -- acetate. Acetate is familiar to us in the form of acetic acid, or vinegar, as an end-product of fermentation. When energy-rich compounds such as cellulose and sugars are available, bacteria excrete acetate as a waste product. When times get tough, some bacteria can switch their metabolism and start scavenging acetate from the environment.
Methanosarcina -- found today in sewage plants and the rumen of cattle -- is related to bacteria but is classified in a group called the archaea. This group includes species that can tolerate extremes of high temperature, acidity and anaerobic conditions (lack of oxygen). At the end of the Permian, Methanosarcina apparently acquired new acetate-conversion genes from a cellulose-decomposing bacterium, giving it a new and efficient way to metabolize acetate without oxygen.
Gene transfer occurs only rarely among archaea and bacteria, but can have major consequences. The new, genetically altered Methanosarcina did something no other organism could: efficiently consume acetate in the oxygen-depleted depths of the ocean. In the process, it released large amounts of methane, which rose to the surface and was eventually oxidized to carbon dioxide.
Greenhouse gases increased, the planet heated up, and biological activity increased as well -- but only among a much reduced number of species. Oxygen levels likely declined, creating an even better habitat for Methanosarcina and other strange organisms that thrive in anaerobic conditions, including those that convert sulfate to sulfide. Few terrestrial organisms could have survived the release of large amounts of toxic hydrogen sulfide gas to the atmosphere by sulfate-reducing bacteria.
Fast forward 250 million years. One particular species, Homo sapiens, has acquired a new ability to utilize previously untapped organic carbon resources -- namely, fossil fuel deposits accumulated over hundreds of millions of years. As occurred with Methanosarcina, release of inorganic carbon by Homo sapiens is occurring at an exponential rate. This has already depleted atmospheric carbon-13. The oceans are becoming increasingly acidic and oxygen-depleted. The Earth's sixth great extinction is well underway.
Skeptics who claim that humans can't trigger mass climate disruption and mass biological extinction may find their position increasingly uncomfortable. If the humble Methanosarcina virtually wiped out life at the end of the Permian, who's to say Homo sapiens can't do it as well -- or even better?
Ole Hendrickson is a retired forest ecologist and a founding member of the Ottawa River Institute, a non-profit charitable organization based in the Ottawa Valley.
Photo: Taro Taylor/flickr
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