Approximately 252 million years ago, when the continents were still fused together as Pangea, our planet experienced an unparalleled loss of plant and animal life. While the mechanism for this "Great Permian Extinction" (where 70% of land species died off) has largely been unknown, a new study has uncovered that the disappearance of 96% of sea life may have been due to warm, oxygen-depleted oceans. To arrive at this conclusion, the study innovatively combined the fossil record, experiments involving animal physiology, and cutting-edge ocean models.
"The Permian extinction was the perfect storm of catastrophic events; carbon dioxide emissions and subsequent global warming wiped out 95% of all species on Earth," says Veronica Padilla Vriesman, a PhD student in geology at the University of California, Davis studying past changes in the ocean environment using archaeological shells. "The Earth turned into one massive dead zone in a geologically short period of time. Recovery following the extinction took millions of years, forever altering Earth’s biota."
While ocean and atmospheric conditions during the Permian Period were similar to today, a series of intense volcanic eruptions in Siberia spewed high volumes of greenhouse gases into the atmosphere. Carbon dioxide levels were nearly 12 times greater than they are today, thereby enveloping the Earth in a thick heat-trapping blanket. The authors determined that the Permian oceans warmed by 20 °F (11 °C) and lost up to 80% of the oxygen they contained (when ocean temperatures rise, they lose their ability to hold oxygen). Additionally, waters closest to the seafloor became completely devoid of oxygen.
To understand the impact of these stressful conditions on the animals that lived in the oceans towards the end of the Permian Period, the authors referenced laboratory studies that tested the responses of 61 modern marine animals to changes in temperature and oxygen levels. They specifically selected animals that evolved under similar conditions as the extinct species, such as corals, shellfish, and sharks.
The authors found that the warm, oxygen-depleted waters were the primary source of extinctions in the Permian oceans. As ocean temperatures rose, many sea creatures' metabolisms will have accelerated. However, the oxygen-deprived oceans were probably incapable of meeting the physiological needs of Permian plants and animals, ultimately suffocating them. While carbon dioxide emissions from the Siberian volcanoes may have dissolved in seawater causing the oceans to become more acidic and contributed to the loss of some species, warming and oxygen loss are considered the strongest drivers of this massive die-off.
Further, species in the tropics did not suffer as much as species found in colder waters near the poles. This is likely because sea life closest to the equator evolved to survive in warm, tropical waters.
"Since tropical organisms' metabolisms were already adapted to fairly warm, lower-oxygen conditions, they could move away from the tropics and find the same conditions somewhere else," said co-author Dr. Curtis Deutsch, "But if an organism was adapted for a cold, oxygen-rich environment, then those conditions ceased to exist in the shallow oceans."
This ocean-wide extinction event that occurred 252 million years ago may provide some insight into the consequences of modern human-driven changes to the Earth's climate and oceans. Currently in the Anthropocene (the Human Epoch), carbon dioxide levels are higher than they have been in 15 million years and the planet is the warmest it has been in 120,000 years. Further, if greenhouse gas emissions continue at their current rate, then ocean waters close to the surface could be 20% as warm as they were at the end of Permian Period by the year 2100 (and up to 50% as warm by the year 2300).
According the Padilla Vriesman, "We can look at the lead-up to the end-Permian extinction as an analog for what we are seeing on Earth today. Humans—rather than volcanoes—are triggering a global event with features alarmingly similar to those leading up to the end-Permian."
