The "Great Dying" at the end of the Permian period, some 250 million years ago, was the most catastrophic of the five mass extinctions in Earth’s fossil record. More than half of the families of living things died out, and as many as 95 percent of the planet’s marine species were lost. At the same time, perhaps 70 percent of the land’s reptile, amphibian, insect, and plant families became extinct.
What caused the calamity has long been a source of scientific controversy, due both to the extreme remoteness of the event, and to an absence of geological evidence – the world’s seas retreated at the same period, reducing the amount of sedimentary rock entering the geological record.
The recent discovery of sections from the Permo-Triassic boundary in South China has, however, allowed more detailed analysis to take place, leading to a conclusion that the Great Dying was probably caused by poisonous gas, released by a massive volcanic event in Siberia, known as the Siberian Traps, which spewed about 3 million cubic kilometers of basalt lava over vast swathes of the Siberian landmass (to put this in perspective, the largest eruption in historical time, of Mt Pinatubo in Iceland in 1783, released 12 cubic km of lava onto the island).
Payne, a paleobiologist who joined the Stanford faculty in 2005, studies the Permian-Triassic extinction and the following 4 million years of instability in the global carbon cycle.
"People point to the fossil record as a place where we can learn about how our actions today may affect the future course of evolution," Payne said. "That's certainly true: The deep geologic record provides context for modern events. We may miss very important processes or underestimate the magnitude of changes in the future by using only the past couple thousand years as a baseline."
Payne has spent the past five years unearthing the deep geologic record in south China's kilometer-thick, limestone fossil beds at the Great Bank of Guizhou, which were formed in shallow ocean waters during the late Permian and early Triassic periods. As the ocean floor sank, new, younger layers of limestone formed on top of deeper, older ones.
Continental plate tectonics have since flipped these marine fossil beds on their side, allowing Payne to walk back in time across the formerly horizontal layers. Because ocean waters cover large areas for long periods of time and somewhat protect the underlying rocks from erosion, marine fossil beds tend to be physically larger and cover a longer period of time with finer temporal resolution.
More than 90 percent of all marine species disappeared from the Great Bank of Guizhou and other end-Permian fossil formations.
Douglas Erwin, curator of the Paleozoic invertebrates collection at the Smithsonian National Museum of Natural History, has dubbed this extinction event "the greatest biodiversity crisis in the history of life."
An unusually long period of time passed before biological diversity began to reappear. Scientists disagree on the causes of this extinction. However, nearly all explanations cite the high levels of greenhouse gases, including carbon dioxide, low levels of oxygen in the oceans and high levels of toxic gases.
In 1991, scientists reported that the largest known volcanic event in the past 600 million year -known as the Siberian Traps (image below)- occurred at the same time as the end-Permian extinction. Magma extruded through coal-rich regions of the Earth's crust and blanketed a region the size of the continental United States with basalt to a depth of up to 6 kilometers.
The eruptions that formed the Siberian Traps not only threw ash, debris and toxic gases into the atmosphere but also may have heated the coal and released vast quantities of carbon dioxide and methane into the atmosphere.
Rapid release of these greenhouse gases would have caused the oceans first to become acidic and then to become supersaturated with calcium carbonate.
The researchers believe that the volcanic gases from the eruption, which would have have heated the coal and released vast quantities of carbon dioxide and methane into the atmosphere depleting the Earth’s protective ozone layer and acidified the land and sea, killed rooted vegetation. This meant that soil was no longer retained, and was eventually washed into the surrounding oceans. Soils in the oceans would have blocked out light and soaked up oxygen. Analysis of rock chemistry suggests that after the soil crisis on land, the marine ecosystem succumbed to the stresses of environmental change and oceanic life faltered, completing a global catastrophe.
Payne presents evidence that underwater limestone beds around the world eroded at the time of the end-Permian extinction. This finding, coupled with geochemical evidence for changes in the relative abundances of carbon isotopes, strongly suggests an acidic marine environment at the time of the extinction.
"This end-Permian extinction is beginning to look a whole lot like the world we live in right now," Payne said. "The good news, if there is good news, is that we have not yet released as much carbon into the atmosphere as would be hypothesized for the end-Permian extinction. Whether or not we get there depends largely on future policy decisions and what happens over the next couple of centuries."
Today, land degradation is a worsening global problem thanks to human activity, and soil erosion has caused the loss of a third of arable land over the last forty years. Thirty five percent of the Earth’s land is now soil-free. Identifying the nature of the end Permian crisis may help us understand what is in store for us in the years ahead.
Casey Kazan via Stanford University News