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.

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. Climate change during the Carnian pluvial event is reflected in chemical changes in Carnian strata across the CPE. This climate change was most likely linked to the eruption of extensive flood basalts and volcanic CO2 off the coast of what is now North America.

History and nomenclature

Scientists first noticed big changes in Earth’s rocks from the Carnian time long before they thought about climate changes. They saw a dark layer in the Northern Limestone Alps that broke a long time of calm, sunny rock layers. They called this change the Reingrabener Wende.

In 1989, two scientists, Michael J. Simms and Alastair H. Ruffell, put many clues together and suggested that a time of lots of rain happened in the middle of the Carnian. They called this the Carnian pluvial episode. Their idea slowly gained support, especially after studies in Italy in the late 2000s. Even today, scientists discuss what caused this rainy time and how it changed life on Earth.

Environmental disruption

The Carnian pluvial episode brought much wetter weather around the world, changing the dry climate of the Late Triassic period. This increase in rain is shown by many clues, such as special types of soil and rocks that form in wet places, along with fossil pollen that suggests plants lived in humid areas.

During this time, the Earth also got warmer, likely because of big volcanic eruptions far off the coast of what is now North America. This warmth changed the oceans and made rocks form in new ways. The wet weather and warm temperatures caused more rain and rivers to carry dirt and sand into the seas, which changed how limestone formed and affected many sea creatures.

Effect on life

Conodonts, ammonoids, crinoids, bryozoa and green algae died out at high rates during this time. However, other creatures such as dinosaurs, calcareous nannofossils, corals and conifers grew and spread.

The climate changes affected land animals greatly. Some older reptiles, like rhynchosaurs, disappeared, while others, like hyperodapedontines, became common for a short time. New groups, including early aetosaurs and sauropodomorph dinosaurs, began to spread. Scientists debate whether these changes were directly caused by the climate event or happened over longer periods.

Plants also changed significantly. Conifers, ferns and other plant groups grew more common. Wet areas like swamps returned, allowing more plants to thrive. This plant growth may have helped balance the atmosphere. The first large deposits of tree resin, known as amber, appeared during this time and preserved tiny animals and plants in remarkable detail.

Possible causes and influences

The main reason for the Carnian Pluvial Episode was the activity of a large area of volcanic rocks called the Wrangellia Large Igneous Province. This volcanic activity released lots of gases into the air, which caused the Earth to warm up and rain more. This change in climate affected both the oceans and the land.

Scientists have found clues in rocks that show this time was different. For example, they found changes in the types of carbon in the rocks, which tells us that lots of new carbon was added to the air and oceans. They also found small amounts of a metal called mercury, which often comes from volcanic activity. These clues help us understand that the big volcanoes were a big part of what caused these changes in the Earth's climate.