The Carnian pluvial episode (CPE), often called the Carnian pluvial event, was an interval of major change in global climate that was synchronous with significant changes in Earth's biota both in the sea and on land. It occurred during the latter part of the Carnian Stage, a subdivision of the late Triassic period, and lasted for perhaps 1-2 million years (around 234-232 million years ago). The CPE represents a significant episode in the evolution and diversification of many taxa that are important today, among them some of the earliest dinosaurs (which include the ancestors of birds), lepidosaurs (ancestors of modern-day snakes and lizards), pterosaurs and true mammals. 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 the ammonoids, bryozoa, 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, there was reduced deposition of carbonate minerals. This 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, which suggest that global warming was prevalent at the time. This climate change was probably linked to the eruption of extensive flood basalts as the Wrangellia Terrane was accreted onto the northwestern end of the North American Plate.
History and nomenclature
Environmental disturbance and high extinction rates were observed for sediments of the Carnian stage long before a global climate perturbation was proposed. Schlager & Schöllnberger (1974) drew attention to a dark siliciclastic layer which abruptly interrupted a long period of carbonate deposition in the Northern Limestone Alps. They termed this stratigraphic "wende" (turning point) the Reingrabener Wende, and it has also been called the Reingraben event or Raibl event. Several Carnian terrestrial formations (namely the Schilfsandstein of Germany and various members of the United Kingdom's Mercia Mudstone Group) are intervals of river sediments enriched with kaolinitic clay and plant debris, despite having been deposited between more arid strata. Humidity-adapted palynomorphs in New Brunswick, karst topography in the U.K., and a carbon isotope excursion in Israel were all reported for the middle of the Carnian prior to 1989. The Julian-Tuvalian boundary experienced high extinction rates among many marine invertebrates, while an extinction among land vertebrates was suggested to occur in the late Carnian.
In 1989, a paper by Michael J. Simms and Alastair H. Ruffell combined these disparate observations into a new hypothesis, pointing to an episode of increased rainfall synchronous with significant ecological turnover in the mid-Carnian. The paper was inspired by a conversation between Simms and Ruffell, on 10 November 1987 at Birmingham University, that connected Ruffell's research on lithological changes in the Mercia Mudstone Group to Simms's research on crinoid extinction. A key aspect of their hypothesis was that the evidence used to demonstrate the climate change was entirely independent of the evidence for biotic change; fossils were not used in any way to infer climate climate change. Their hypothesized climatic disturbance, which they named the Carnian pluvial episode, was tentatively considered to be a result of oceanic and/or volcanic instability related to the early rifting of Pangea, but at that time direct evidence of this was lacking. Simms and Ruffell published several more papers in the coming years, but their hypothesis was not widely accepted. A strong critique by Visscher et al. (1994) argued that aridity-adapted pollen stayed abundant through the entire Carnian of Germany, suggesting that the Schilfsandstein was simply indicative of an invading river system rather than widespread climate change. Their critique also coined the term "Carnian pluvial event", which would eventually become among the most widespread names for the climatic disturbance.