A 60,000-year catastrophe
These unassuming rock outcrops in Wakayama Prefecture hold a globally significant geological record. They preserve the boundary between the Permian and Triassic periods, about 251.9 million years ago. This thin layer of rock records Earth's most severe extinction event, known as "The Great Dying," when up to 96% of all marine species and 70% of terrestrial vertebrate species disappeared. The entire mass extinction occurred over a geologically brief interval of just 60,000 years.
The rocks seen here were originally sediments on the floor of the Panthalassa, the superocean that surrounded the supercontinent of Pangaea. Now part of a geological formation called an accretionary complex, these deep-sea cherts and siliceous claystones were scraped off a subducting oceanic plate and added to the Japanese landmass over millions of years. The layers show a distinct change from grey Permian chert to a black, oxygen-deficient claystone at the boundary, indicating a major environmental collapse. This deep-sea anoxia, or lack of oxygen, lasted for millions of years and was a primary cause of death in marine world. The leading theory for this catastrophe points to massive volcanic eruptions in the Siberian Traps, which released immense volumes of carbon dioxide, leading to rapid global warming and ocean acidification.
Reading the rocks
Scientists use microfossils to precisely date these layers and understand the extinction's progression. Important among these are conodonts, the tooth-like feeding apparatus of an extinct eel-like creature. The first appearance of the conodont species Hindeodus parvus is the official marker for the beginning of the Triassic Period, a definition established using sections like this one. By tracking the disappearance of late-Permian conodont species and the appearance of early-Triassic ones, biostratigraphers can pinpoint the extinction boundary with remarkable accuracy.
Below the boundary, the chert contains abundant fossils of radiolarians, a type of single-celled plankton that builds complex silica skeletons. Right at the boundary, they vanish, and the rocks become barren of these fossils for a long interval, indicating the collapse of the oceanic ecosystem. Analysis of carbon isotopes within the rock layers shows a large, abrupt global decrease in carbon-13, a chemical fingerprint that coincides with the mass extinction and helps correlate this Japanese site with others around the world. These layers in Wakayama, once at the bottom of a lost ocean, now show the planet's most deep biological crisis.