Instead of a sudden crisis, the largest mass extinction in Earth's history might have been a prolonged event that stretched over hundreds of thousands of years. That's what a new study of ancient sediments suggests. If confirmed, the findings could send researchers scrambling to find a new explanation for the greatest of mass die-offs on land.
For some reason, most life on Earth died out about 252 million years ago at the end of the Permian period. About 90% of the species in the ocean and about 70% of land-dwelling critters disappeared in relatively short order. Ever since scientists discovered the Great Dying, they have been searching for the trigger. Physical evidence such as fossil remains collected from layers of sedimentary rocks, particularly from sites in South Africa's Karoo Basin, suggested that plants and animals began to vanish, and then some sort of catastrophic event such as prolonged volcanic eruption polished off most of Earth's inhabitants. But some researchers remained skeptical that a catastrophe had occurred at all and kept searching for evidence.
One such team, led by paleontologist Robert Gastaldo of Colby College in Waterville, Maine, decided to examine the sedimentary record in greater detail. During six expeditions to the South African locations, he and his undergraduate students visited every site where previous researchers had identified the distinctive chemical deposits signifying the boundary between the Permian period and the subsequent Triassic period. "We spent days walking kilometers throughout the [sites] trying to trace it from every angle and couldn't," Gastaldo says, adding that they repeated their surveys over several years "to ensure that our data were good." He says the so-called unique layer associated with the event "couldn't be traced more than about 100 meters laterally," meaning it could not represent a global phenomenon.
As a result of their field surveys, the team reports in the March issue of Geology that some of the sediments thought to be associated with the Great Dying are actually located about 8 meters below the Permian boundary, meaning they were deposited long before the catastrophe is thought to have occurred. "Because the boundary event bed doesn't occur at the same position in the rock record," Gastaldo says, "there can be no one, unique event."
Sedimentary geochemist Neil Tabor of Southern Methodist University in Dallas, Texas, agrees. It's a fundamental study that should have an "important influence" on the perception of the end-Permian extinction, he says, because Gastaldo and his team "actually walked out and documented what they observed" in the Karoo Basin. The findings, Tabor says, have exposed a major misconception about the Permian extinction.