The Carnian Pluvial Episode (CPE) represents a geologically brief yet dramatic upheaval in Earth’s climate history, occurring during the Late Triassic Period. This event fundamentally interrupted the otherwise arid conditions that dominated the supercontinent Pangea. The episode is defined by a sustained period of global hydrological intensification, meaning the planet experienced prolonged, heavy rainfall and high humidity across diverse landmasses. This profound climatic shift triggered a significant turnover in both marine and terrestrial ecosystems. The search for the ultimate driving force behind this global soaking points directly to a massive, singular geological event that injected vast amounts of greenhouse gases into the atmosphere.
Defining the Carnian Pluvial Episode
The Carnian Pluvial Episode took place within the Carnian Stage, the earliest subdivision of the Late Triassic Epoch. Geochronological data pin the event to a window approximately 234 to 232 million years ago. High-resolution studies suggest the CPE lasted for around 1 to 2 million years, with one detailed analysis constraining the maximum length to about 1.09 million years.
This episode represents a profound shift from the generally arid climate that characterized the interior of Pangea. The term “pluvial” refers to the high levels of precipitation, which transformed dry plains into landscapes dominated by swamps, lakes, and increased river runoff. The evidence for this shift is found globally, indicating a systemic enhancement of the water cycle driven by elevated temperatures.
The CPE was not a single, continuous downpour but rather an interval characterized by at least four distinct pulses of intensified rainfall. These wet phases were interspersed with drier periods. Researchers identify this period as a time of global climate warming, which directly enhanced evaporation from the oceans, leading to the observed, prolonged humid and monsoonal conditions across the globe.
The Volcanic Trigger
The consensus among geoscientists is that the Carnian Pluvial Episode was caused by an immense outpouring of lava known as the Wrangellia Large Igneous Province (LIP). This massive volcanic activity occurred off the coast of what is now North America, with the flood basalts currently preserved in parts of Alaska and British Columbia. The Wrangellia LIP is estimated to have erupted a minimum volume exceeding one million cubic kilometers of basaltic rock into the Panthalassan Ocean.
The immediate consequence of this large-scale volcanism was the injection of vast quantities of greenhouse gases, primarily carbon dioxide (\(text{CO}_2\)) and possibly methane, into the atmosphere. This sudden gas release perturbed the global carbon cycle, a change chemically recorded in the rock record as a pronounced negative shift in carbon isotopes (\(delta^{13}text{C}\)). This isotopic signature indicates a rapid, external input of volcanically-derived carbon into the atmosphere-ocean system.
The influx of these gases led to rapid global warming, with oxygen isotope data suggesting a temperature rise of about 4 to 8 degrees Celsius. This warming accelerated the global hydrological cycle, increasing evaporation and subsequently intensifying the monsoonal system over Pangea. The resulting hot, humid climate created the sustained, heavy rainfall that defines the CPE, directly linking the volcanic eruptions to the global pluvial conditions.
Global Environmental Upheaval
The environmental consequences of the volcanic trigger and subsequent rainfall are visible in the geological record across the planet. The intensified continental weathering, driven by the heavy rainfall, resulted in a massive influx of siliciclastic sediments into marine basins. This material was deposited over previously carbonate-dominated seafloors, demonstrating increased erosion and runoff from the landmasses.
In the oceans, the CPE is associated with the widespread deposition of black shales, particularly in restricted marginal marine basins of the Tethys realm. These dark, fine-grained sedimentary rocks are indicative of anoxic conditions, meaning the deep waters became severely depleted in oxygen. The warming climate and increased nutrient runoff from the continents likely contributed to the expansion of oxygen-depleted zones, disrupting marine ecosystems.
The volcanic carbon dioxide injection also had a detrimental effect on ocean chemistry, leading to a shutdown of marine carbonate platforms. This change is marked by a shift where the production of carbonate minerals by organisms was severely curtailed. On land, the humid conditions fostered the growth of vast, dense plant communities, evidenced by the formation of extensive coal beds in various locations.
The climate during the CPE shifted from a prevailing arid state, characterized by red beds and evaporites, to a hot, humid, monsoonal pattern. This shift is recorded in the sedimentary stacks, where layers indicative of wet conditions are sandwiched between strata representing dry periods. This geological evidence confirms the event was a profound interruption of the long-term Triassic climate regime.
Evolutionary Consequences
The environmental upheaval of the CPE triggered a significant biological turnover, which cleared ecological space for new groups to diversify. Marine ecosystems suffered a notable reduction in diversity, with approximately one-third of all marine genera disappearing. Among the hardest hit were invertebrates like ammonoids, bryozoans, and crinoids, which struggled to cope with the changing ocean chemistry and widespread anoxia.
On land, the climatic shift caused a restructuring of terrestrial vertebrate communities. Large herbivorous reptiles, such as the rhynchosaurs and dicynodonts, saw their dominance decline as the flora they depended upon was replaced. This ecological vacuum provided an opportunity for the diversification and ascendancy of several clades that would become prominent in the later Mesozoic Era.
The first true dinosaurs benefited greatly, with their diversification event coinciding with the CPE. This allowed dinosaurs to expand rapidly in diversity and ecological impact across Pangea, moving from small, sparse populations to becoming common components of terrestrial faunas. Other groups that diversified and rose to prominence include the early crocodylomorphs, lizards, turtles, and the ancestors of modern mammals.
The CPE also marked the origin and expansion of several modern flora and marine groups. Modern conifers began their diversification. In the oceans, the event is linked to the rise of the first scleractinian coral reefs and the appearance of new plankton groups like coccoliths and dinoflagellates. This biological reorganization set the stage for the ecosystems that would define the rest of the Mesozoic Era.