The Carnian Pluvial Event (CPE) was a period of intense global rainfall and climate change during the Late Triassic Period, approximately 234 to 232 million years ago. This episode, lasting between 1 and 2 million years, dramatically reshaped life on Earth.
The Global Climate Shift
The primary proposed cause for the Carnian Pluvial Event was immense volcanic eruptions from the Wrangellia Large Igneous Province (LIP). A LIP refers to vast outpourings of lava that cover extensive areas, such as the Wrangellia LIP across what is now Alaska and British Columbia. These eruptions released massive volumes of greenhouse gases, including carbon dioxide, into the atmosphere.
This led to significant global warming, raising average temperatures by an estimated 3-4 °C. This warming accelerated the global water cycle, drawing vast amounts of moisture from the oceans and enhancing monsoonal circulation across the supercontinent Pangaea. The result was a prolonged and heavy rainy period, shifting Earth’s climate from an arid world to a humid, monsoonal one. Volcanic gases also contributed to acid rain, further impacting terrestrial environments.
A Major Extinction Crisis
Before the Carnian Pluvial Event, Earth’s terrestrial ecosystems were dominated by archosaurs, including crocodile-like reptiles, and synapsids, or mammal-like reptiles. Marine environments supported diverse fauna, including conodonts, ammonoids, and reef-building organisms. Many of these groups were severely impacted by the changing conditions.
In the oceans, the absorption of carbon dioxide led to ocean acidification, dissolving carbonate shells and reducing carbonate deposition. This, coupled with oxygen-poor (anoxic) waters, led to a collapse of marine ecosystems, with high extinction rates among groups like conodonts, many ammonoid species, and various reef-building organisms.
On land, the dramatic shift in vegetation played a significant role in terrestrial extinctions. Prolonged humidity led to a widespread change from drought-tolerant plants, like seed ferns, to water-loving plants such as ferns and conifers. This habitat disruption and changes in food sources led to the decline of major herbivore groups, including large dicynodonts and rhynchosaurs, that could not adapt to the new flora.
The Dawn of a New Era
The environmental upheaval of the Carnian Pluvial Event created “disaster opportunism,” where the extinction of dominant groups cleared ecological niches. This opened opportunities for previously less significant groups to expand and diversify. The most notable beneficiaries were dinosaurs, which had originated earlier in the Triassic but remained rare and ecologically minor.
Following the event, dinosaurs rapidly diversified into the newly available niches, leading to increased diversity and ecological importance. This diversification set them on the path to dominate terrestrial ecosystems for the next 150 million years. Evidence from fossil footprints in places like the Italian Dolomites shows their sudden appearance and proliferation directly following the event.
Beyond dinosaurs, other groups also thrived and radiated during this period of recovery. Modern conifer forests spread across the humid landscapes. New groups of lizards, turtles, crocodiles, and early mammals appeared and diversified, establishing many lineages that form modern terrestrial vertebrate faunas. In the oceans, modern-type scleractinian corals began building reefs for the first time, indicating a reset of marine ecosystems.
Uncovering the Evidence
Scientists have pieced together the story of the Carnian Pluvial Event through various lines of geological and biological evidence. The rock record provides distinct signals of this climatic shift. Sedimentary layers show a clear transition from arid, reddish sandstones to dark, organic-rich shales and coal seams, indicating a widespread shift to wet and humid conditions. These humid conditions promoted the reestablishment of large inland lakes and peat swamps.
The fossil record offers compelling biological evidence of the “faunal turnover” associated with the event. Scientists observe the disappearance of fossils belonging to groups like conodonts and certain archosaurs in rock layers corresponding to the CPE. Simultaneously, there is a sudden appearance and increased abundance of fossils and footprints from newly diversifying groups, such as dinosaurs, immediately following this period.
Geochemical evidence, particularly the analysis of carbon isotopes, provides a chemical fingerprint of the event. Rocks from this period show a specific “negative carbon isotopic excursion,” meaning a distinct shift in the ratio of carbon isotopes. This signature confirms a massive injection of light carbon, likely from volcanic greenhouse gas emissions, into the atmosphere-ocean system, correlating directly with the timing of the Wrangellia eruptions and the climatic changes.