Carnian pluvial episode

The Carnian pluvial episode (CPE), often called the Carnian pluvial event, was a period of major change in global climate that coincided with significant changes in Earth's biota both in the sea and on land. It occurred during the latter part of the Carnian Stage, the first subdivision of the Late Triassic Epoch, and lasted for perhaps 1–2 million years (around 234–232 million years ago). Volcanic activity off the coast of North America led to global warming and increased rainfall on land, alongside a reduction of carbonate platforms in the oceans. Pluvial means "of or relating to rain; characterized by much rain, rainy."

The CPE corresponds to a significant episode in the evolution and diversification of many taxa that are important today. The earliest dinosaurs (which include the ancestors of birds), lepidosaurs (the ancestors of modern-day lizards, snakes, and the tuatara) and potentially mammaliaforms (ancestors of mammals) all diversified during the event. In the marine realm it saw the first appearance among the microplankton of coccoliths and dinoflagellates, with the latter linked to the rapid diversification of scleractinian corals through the establishment of symbiotic zooxanthellae within them. The CPE also saw the extinction of many aquatic invertebrate species, especially among ammonoids, bryozoans, and crinoids.

Evidence for the CPE is observed in Carnian strata worldwide and in sediments of both terrestrial and marine environments. On land, the prevailing arid climate across much of the supercontinent Pangea shifted briefly to a hotter and more humid climate, with a significant increase in rainfall and runoff. In the oceans, carbonate minerals such as limestone saw reduced deposition, leaving mud-rich layers as prominent geological markers. Carbonate disruption may reflect the extinction of many carbonate-forming organisms, but may also be due to a rise in the carbonate compensation depth, below which most carbonate shells dissolve and leave few carbonate particles on the ocean floor to form sediments.

Climate change during the Carnian pluvial event is reflected in chemical changes in Carnian strata across the CPE. Major disruption to the carbon cycle and other natural systems show that global warming was prevalent at the time. This climate change was most likely linked to the eruption of extensive flood basalts and volcanic CO2 offgassing as the Wrangellia Terrane was accreted onto the northwestern end of the North American Plate.

History and nomenclature

Scientists first noticed big changes in rocks from the Carnian stage long before they thought about global climate changes. They saw dark layers in the Northern Limestone Alps that broke a long time of calm, sunny weather. These layers were called the Reingrabener Wende, or the Reingraben event. Other places, like Germany and the United Kingdom, also showed signs of more rain and rivers, even though the weather usually stayed dry.

In 1989, two scientists, Michael J. Simms and Alastair H. Ruffell, put all these clues together. They suggested that a time of heavy rain changed many plants and animals in the middle of the Carnian. They called this time the Carnian pluvial episode. At first, many scientists did not agree with them. But in the late 2000s, more evidence from places like Italy started to support their idea. Even today, scientists are still learning more about what caused this big change in the weather.

Environmental disruption

The Carnian pluvial episode brought much wetter weather around the world, changing the dry climate of the Late Triassic period. This extra wetness caused more rainfall, which we can see from things like siliciclastic sediments and special kinds of fossil soils that grow in humid places.

The world also got warmer during this time, likely because of big volcanic eruptions. This warmth changed the oceans and the air, making everything a bit hotter. The extra rain and warmth caused rocks and soil to wear away faster, sending more dirt and sand into the seas. This change affected special ocean areas made mostly of calcium carbonate, turning them into flatter shapes and changing the tiny sea creatures that lived there.

Effect on life

During the Carnian pluvial episode, many changes happened in the world’s plants and animals. In the oceans, creatures like conodonts, ammonoids, crinoids, bryozoa, and green algae died out in large numbers. But other groups, such as dinosaurs, calcareous nannofossils, corals, and conifers, grew and spread during this time.

On land, the climate changes helped some animals thrive while others struggled. For example, some reptiles called rhynchosaurs that were common earlier mostly disappeared, but a few specialized types survived for a short time. Other animals, like aetosaurs and early sauropodomorph dinosaurs, became more diverse after the climate event. Plants such as conifers, ferns, and bennettitaleans also spread widely, and large lakes and peat swamps began to form again after a long pause.

Possible causes and influences

The Carnian Pluvial Episode was likely caused by the activity of the Wrangellia Large Igneous Province, a large area of volcanic activity in the Panthalassan Ocean. This volcanic activity released large amounts of CO₂ into the atmosphere, leading to global warming and increased rainfall on land. This warming also affected the oceans, reducing carbonate platforms.

Other factors may have played a role, such as the uplift of a new mountain range, the Cimmerian orogen, which could have created strong rains similar to modern monsoons. Additionally, Earth's orbital cycles might have influenced the timing and severity of these changes.