Earth’s long history includes episodes of dramatic climate change. One such event, a period of prolonged, widespread rainfall, reshaped the planet and life. This event shows how extreme environmental shifts can drive profound transformations across global ecosystems.
The Carnian Pluvial Event
The Carnian Pluvial Episode (CPE) was an extensive period of rainfall. It occurred during the Late Triassic period, approximately 234 to 232 million years ago. Lasting for about one to two million years, the CPE marked a stark departure from the otherwise arid conditions that characterized much of the Triassic.
During this time, all of Earth’s continents were still joined together as the supercontinent Pangea, surrounded by a vast ocean. Evidence for the CPE is found in geological strata worldwide, across both terrestrial and marine environments. This period introduced more humid conditions, with increased rainfall and continental runoff.
Unraveling the Causes
Scientists link the Carnian Pluvial Event primarily to massive volcanic eruptions. These eruptions originated from the Wrangellia Large Igneous Province, located in what is now western North America. Such volcanic activity released large quantities of greenhouse gases, including carbon dioxide, into the atmosphere.
This influx of greenhouse gases led to an increase in global temperatures, further warming the oceans. Warmer oceans would have intensified the hydrological cycle, leading to greater evaporation and more water vapor in the atmosphere. This process created the conditions for the widespread and persistent rainfall observed during the CPE.
Transforming Earth
The Carnian Pluvial Event altered Earth’s environment. Increased rainfall led to continental weathering and runoff, resulting in the deposition of siliciclastic sediments in basins. On land, the prevailing arid climate shifted to a hotter and more humid environment, with arid lands transforming into lush, green landscapes as moisture-loving plants, such as ferns and early conifers, proliferated. This change in vegetation also contributed to the development of new soil types.
The event also had an impact on life, influencing the evolution and diversification of various animal groups. While dinosaurs existed before the CPE, they were relatively rare. The environmental changes created new opportunities, leading to a proliferation of plant life that supported herbivorous dinosaurs and, in turn, carnivorous ones. This period is associated with the rise of early dinosaurs, along with other groups such as turtles, crocodiles, lizards, and early mammals, as they filled newly available ecological niches. Conversely, many marine species, including ammonoids and conodonts, experienced extinctions due to rising temperatures, ocean acidification, and reduced oxygen levels in deep waters.
Reading the Rocks
Researchers have pieced together the story of the Carnian Pluvial Event by studying geological records from around the world. Sedimentary rock layers provide key evidence, with dark grey siliciclastic layers indicating very wet conditions within otherwise dry carbonate rock sequences. These distinct layers have been identified globally, from the Eastern Alps to the UK, Germany, the US, and the Himalayas.
Fossil evidence also offers insights into the environmental shifts. Changes in plant fossils show a transition from drought-resistant species to those that thrive in wetter conditions. The appearance of abundant dinosaur footprints in rock layers immediately following the CPE further supports the timing of their diversification. Scientists use radiometric dating of volcanic ash beds found within these sedimentary sequences to precisely pinpoint the age and duration of the event.