Limestone recording Earth's worst mass extinction. Chemicalsignatures in the rock show oceans suffocated as 96% of marine species died.
A 252-million-year-old crime scene
In the hills of Savannakhet Province, an unassuming sequence of limestone contains the chemical fingerprints of Earth's most severe extinction event. These rocks at Phou Xang He are a geological ledger from the end of the Permian Period, approximately 251.9 million years ago. At that time, this region was a shallow, tropical seabed in the Paleo-Tethys Ocean, an ancient body of water that once separated the supercontinents of Gondwana and Laurasia. The limestone here formed from the accumulation of marine organisms' skeletons and shells.
The boundary between the Permian and the overlying Triassic layers is a division of rock; it shows a deep biological catastrophe. Known as the Permian-Triassic (P-Tr) extinction or "The Great Dying," this event eliminated an estimated 81% of all marine species and 70% of terrestrial vertebrate species. The evidence at Phou Xang He allows scientists to perform a chemical autopsy on this ancient crisis by analyzing the elemental and isotopic composition of the rock layers, a discipline called chemostratigraphy.
Reading the chemical clues
The story of the extinction is written in carbon. Specifically, it is told through the ratio of stable carbon isotopes, carbon-13 (¹³C) and carbon-12 (¹²C). Scientists who study the Phou Xang He section, and others like it worldwide, observe a sharp, negative "carbon isotope excursion" right at the P-Tr boundary. This means the rocks contain a much lower ratio of ¹³C to ¹²C than the layers below them. This isotopic signature points to a massive injection of ¹²C-rich carbon into the atmosphere and oceans.
The primary culprit for this carbon surge was the Siberian Traps, a colossal large igneous province in modern-day Russia. Over a period of two million years, volcanic eruptions poured out an estimated 4 million cubic kilometers of basaltic lava. The magma intruded into vast Siberian coal basins, burning the organic material and releasing immense quantities of carbon dioxide and methane into the atmosphere.
This sudden greenhouse gas release triggered lethal global warming, with equatorial ocean temperatures potentially exceeding 40°C (104°F). The warmer water held less dissolved oxygen, leading to widespread ocean anoxia, or suffocation. The rocks also show evidence of ocean acidification from the absorption of atmospheric CO2, which would have made it difficult for organisms with calcium carbonate shells and skeletons to survive. The chemical data in the Phou Xang He limestone provides a direct record of these deadly marine conditions that persisted for hundreds of thousands of years.
💡Fun Facts
The Siberian Traps eruptions released a volume of lava that could cover an area the size of Russia.
The term "traps" used for the Siberian Traps comes from the Swedish word "trappa," meaning stairs, which describes the step-like hills of the volcanic landscape.
It took life on Earth more than 5 million years to recover its former diversity after the Permian-Triassic extinction.
The ancient Tethys Ocean, where these rocks formed, existed for over 400 million years before closing as the African, Arabian, and Indian plates collided with Eurasia.
This is a natural, remote geological outcrop with no official hours. Access is unrestricted but requires local guidance.
Admission
Free.
Accessibility
The site is in a rural, hilly area. Reaching the specific rock exposures requires overland hiking on potentially difficult terrain. It is not wheelchair accessible.
Phou Xang He Mass Extinction RecordSavannakhet Province, Laos