A climate event in stone
In the badlands of Wyoming's Bighorn Basin, colorful layers of rock tell a story of ancient, rapid climate change. About 56 million years ago, a massive release of carbon into the atmosphere triggered the Paleocene-Eocene Thermal Maximum (PETM), an intense warming event that lasted around 200,000 years. Global temperatures rose by 5 to 8°C (9 to 14°F). This geological moment is recorded in the Willwood Formation, a sequence of alluvial deposits up to 700 meters thick. The PETM interval itself is visually distinct, often marked by red-banded, oxidized soil horizons, showing a drier, more seasonal climate.
The main scientific evidence for the PETM is a global change in the ratio of carbon isotopes. Scientists find a prominent negative carbon isotope excursion (CIE) in sediment cores from this period worldwide. This means the proportion of the lighter isotope, carbon-12, sharply increased relative to carbon-13, a clear sign that a massive amount of carbon derived from organic matter—between 3,000 and 7,000 billion tons—was released into the atmosphere and ocean. In the Bighorn Basin, this excursion is measured in paleosol (ancient soil) carbonates and organic matter. The onset of this carbon release was geologically instantaneous, occurring over less than 20,000 years. This event is the primary ancient analog for understanding the potential effects of modern, human-caused carbon emissions.