The Carnian Pluvial Event (CPE) was an episode of global climate change that interrupted the Late Triassic Period, occurring between 234 and 232 million years ago. During this time, planet-wide arid conditions gave way to one to two million years of intense and sustained rainfall. This prolonged humid phase reshaped environments across the supercontinent of Pangea.
The Volcanic Trigger
The catalyst for this climatic shift was a series of volcanic eruptions in the Wrangellia Large Igneous Province, located in what is now western North America. A Large Igneous Province (LIP) is a region where a large volume of lava erupts over a geologically brief interval. These eruptions pumped vast quantities of greenhouse gases, particularly carbon dioxide (CO2), into the atmosphere, triggering a significant global warming event.
This atmospheric warming directly impacted the planet’s water cycle. Higher global temperatures increased the atmosphere’s capacity to hold moisture, leading to a supercharged hydrological system. As the oceans warmed, more water evaporated, which then moved over the supercontinent of Pangea, cooled, and fell as relentless, heavy rain.
The volume of basaltic lava released by the Wrangellia LIP forms a substantial part of the western coast of North America today. The sustained release of CO2 from this volcanic activity was the primary driver of the environmental changes that characterized the Carnian Pluvial Event.
A World Under Rain and Heat
The defining feature of this period was nearly continuous, heavy rainfall that saturated the supercontinent of Pangea. This “pluvial” phase altered terrestrial landscapes. The constant downpour led to extensive soil erosion, stripping away ancient soils and changing river systems across the globe.
The planet’s oceans also experienced major changes. The influx of atmospheric CO2 led to significant ocean acidification, making it difficult for many marine organisms to build their calcium carbonate shells and skeletons. Increased rainfall also led to greater nutrient runoff from land into the oceans, which in some areas may have contributed to anoxic events, creating oxygen-depleted dead zones hostile to most marine life.
Geological evidence for this humid period is found worldwide in rock layers from this time. Scientists observe a shift from arid-environment deposits to layers of sediments rich in silica and gray rock. These layers indicate high levels of continental weathering and runoff from persistent rainfall, confirming the event was a global phenomenon.
Life’s Transformation
The environmental stress of the Carnian Pluvial Event triggered a biodiversity crisis and extinction event on land and in the sea. In the oceans, many groups of invertebrates like ammonoids, crinoids, and reef-building organisms suffered losses, with some estimates suggesting the disappearance of up to 33% of all marine genera. On land, dominant herbivorous reptile groups struggled as the vegetation they depended on was wiped out and replaced.
This wave of extinction, however, created an opportunity for diversification. The collapse of established ecosystems opened ecological niches, allowing new groups of organisms to expand. Among the primary beneficiaries were the dinosaurs, which were previously a minor group but began to diversify rapidly after the event, expanding from their origins in South America to achieve a global distribution.
The sudden shift to arid conditions following the humid episode appears to have been advantageous for the dinosaurs. This time of transformation also saw other groups thrive and spread, including:
- The first turtles
- Crocodiles
- Lizards
- Early mammals
Additionally, the event spurred the evolution of modern conifer forests, which replaced the flora wiped out by the climate shift.
Echoes of the Past
The Carnian Pluvial Event serves as a natural experiment in rapid, CO2-driven climate change. The volcanic eruptions released greenhouse gases, triggering environmental effects that mirror concerns of the modern era. Studying this ancient event provides insights into the potential long-term consequences of our current climate trajectory.
The parallels between the past and present are notable. The global warming, ocean acidification, and mass extinction that characterized the CPE are the same phenomena scientists track today from anthropogenic carbon emissions. By examining the geological and fossil records from this period, researchers can better understand how Earth’s systems respond to a large influx of CO2. This knowledge helps refine climate models and predict impacts on biodiversity.