Carnian pluvial episode

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. 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 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 possibility was lacking. but their hypothesis was not widely accepted. Their critique also coined the term "Carnian pluvial event", which would eventually become among the most widespread names for the climatic disturbance. The obscurity of Simms and Ruffell's hypothesis began to dissipate in the late 2000s, as further support accumulated from studies on Carnian sites in Italy. Carnian humid episode, Carnian pluvial phase, and Carnian crisis. == Environmental disruption ==

Climate during the Carnian pluvial episode The Carnian pluvial episode introduced markedly more humid conditions across the globe, interrupting the otherwise arid climate of the Late Triassic period. This humidity was related to increased rainfall during the CPE, evidenced by: • Siliciclastic (high silica-content) sediment in sedimentary basins, reflecting a high level of continental weathering and runoff; • Significant karst conduits (caves) in Palaeozoic limestone inliers beneath the Late Triassic terrestrial unconformity. The topographic context of these caves is consistent with a Carnian age, although some research groups instead support a Rhaetian age based on localised occurrence of microfossils. • The development of histic and spodic palaeosols, fossil soils which are typical of a tropical humid climate with more water entering through precipitation than leaving through evapotranspiration; • Hygrophytic palynological (fossil pollen) assemblages that reflect vegetation more adapted to a humid climate; • The widespread presence of amber. One climate simulation argues that the interior of Pangaea actually became drier during the CPE, even as its eastern margin and high-latitude regions became rainier. The onset of the episode may have been very rapid (~15,800 years), amplified by carbon cycle feedback effects. Global warming Global warming was also prevalent during the Carnian pluvial event. This is evidenced by oxygen isotope analyses performed on conodont apatite from the CPE, which show an approximately 1.5‰ negative shift in the stable isotope δ18O. This result suggests global warming of at least 3–4 °C during the CPE and/or a change in seawater salinity. Wider sampling supports warming on the order of 4–8°C. This volcanic activity was in turn probably related to the formation of the Wrangellia Large igneous province around the same time, which created vast quantities of igneous (volcanic) rocks that were accreted onto the northwest end of the North American Plate (now the Wrangell Mountains, Alaska, and an estimated 6km thick layer underlying most of Vancouver Island). Weathering was also seen in prehistoric coastal Europe, enhancing runoff of terrestrial sediments responsible for local mud-rich marine layers. High rates of weathering continued even after the earth returned to a drier climate, suggesting that the CPE eroded enough sediment in the region to expose basement rock. Effects on carbonate platforms of the Italian Dolomites. The sloped layer represents the Carnian Pluvial Episode, a time when carbonate platforms (vertical limestone cliffs) were interrupted by an influx of more easily eroded shale and mudstone. The onset of the CPE marks a sharp change in the shape and composition of carbonate platforms across the entire Tethys Ocean, which extended from Central Europe to East Asia. Some of the fossil-rich layers overlap with the Carnian Pluvial Episode, such as the transition from the Zhuganpo to Xiaowa formations. Nevertheless, black shales are not as abundant in the CPE compared to other extinctions (which typically coincide with oceanic anoxic events). There is some evidence for seabed euxinia (no oxygen and high toxic sulfide concentrations) during the CPE. Limestones are enriched in manganese ions near the top of the Zhuganpo Formation. Manganese ions are concentrated and soluble in deep euxinic waters, but precipitate in carbonates at the base of the oxygenated zone. Increasing manganese concentrations indicate a narrowing of the oxygenated zone and a corresponding expansion of euxinic water. == Effect on life ==

File:Marine extinctions during the CPE.jpg|Marine extinctions Dinosaurs (top) and Eoraptor'' (bottom) were among the earliest well-preserved dinosaurs. They are from the Ischigualasto Formation of Argentina, deposited only a few million years after the CPE. Some studies interpret the CPE as a key geobiological event allowing dinosaurs to diversify. The oldest well-constrained geological units with dinosaur fossils are the Santa Maria Formation of Brazil and the Ischigualasto Formation of Argentina. The latter's earliest dinosaur-bearing layers are radiometrically dated back to 230.3 to 231.4 million years ago. This is similar to early minimum age estimates for the CPE (≈230.9 million years ago). More recent studies place the CPE a few million years earlier, near the start of the underlying Los Rastros Formation. However, while avemetatarsalian diversity, diversification rate, and size disparity does increase through the Carnian, it increases faster in the Ladinian and Norian, suggesting that the CPE was not a major influence on the rise of dinosaurs. Precipitation has no apparent correlation with dinosaur diversity across the Late Triassic, with latitude as a better proxy instead. Other tetrapods , leaving only the specialized hyperodapedontine rhynchosaurs (such as Beesiiwo, pictured), which were vulnerable to later extinctions. The CPE had a profound effect on the diversity and morphological disparity (shape variety) of herbivorous tetrapods. This is exemplified in rhynchosaurs, a group of reptiles with strong shearing and grinding jaws. Rhynchosaur lineages which were common in the Middle Triassic went extinct, leaving only the specialized hyperodapedontines as representatives of the group. Immediately after the CPE, hyperodapedontines were widespread and abundant in the late Carnian world, suggesting that they benefited from the climate fluctuations or floral turnover. Hyperodapedontine abundance was not sustained for long, and they too would die out in the early Norian. By cutting rhynchosaurs off from more generalized niches, the CPE would have reduced their versatility and increased their long-term vulnerability to extinction. Similar trends are observed in dicynodonts, though they would survive until much later in the Triassic. Conversely, more versatile and generalist herbivores such as aetosaurs and sauropodomorph dinosaurs would diversify after the CPE. while Adelobasileus is "no older than 225 Ma". Mammaliaforms and their closest relatives, the buck-toothed tritylodonts, together make up the group Mammaliamorpha. Mammaliamorphs were the first fully endothermic cynodonts, and their ancestry can be traced back to the CPE. In the subsequent Norian stage, unambiguous mammaliaforms appeared on the scene, with morganucodonts, haramiyids, and other forms throughout Europe and Greenland. Plants Conifers, ferns, and the now-extinct bennettitaleans all diversified greatly during and after the CPE, establishing themselves as mainstays of Mesozoic flora. Most regions show a higher proportion of hygrophytic (moisture-loving) plants during the episode compared to earlier parts of the Triassic. Spores of ferns and freshwater algae are frequently abundant in palynological samples. The Carnian saw the reestablishment of large inland lakes and peat swamps, ending the Early-Middle Triassic "coal gap" caused by the Permian-Triassic mass extinction. Increased plant growth and coal burial probably helped to draw down CO2, returning the atmosphere to a more normal state after the CPE. Though tiny amber traces can be found in rocks as old as the Carboniferous, the earliest widespread amber deposits date back to the CPE. Carnian amber droplets from Italian paleosols are the oldest amber deposits known to preserve arthropods and microorganisms. Amber would not reappear in the fossil record until the Late Jurassic, though it would take until the Early Cretaceous for amber to occur in concentrations equivalent to or exceeding Carnian amber. The first planktonic calcifiers occurred just after the CPE and might have been calcareous dinocysts, i.e., calcareous cysts of dinoflagellates. Coastal ostracod communities in Hungary experienced major changes across the CPE. Through Julian 2, land-based sediments isolated and filled in marine basins, replacing carbonate-specialists such as bairdiids and healdiids with Bektasia, a platycopid tolerant of shallow siliciclastic seas. Further shallowing across the Julian-Tuvalian boundary left only a few aberrant limnocytherids (Renngartenella, Simeonella) and cytherurids (Kerocythere) which could manage severe salinity fluctuations in the restricted coastal basins. Bairdiids returned in force at the end of the crisis when the basins deepened, reacquiring carbonate and better ventilation. == Possible causes and influences ==

Eruption of Wrangellia flood basalts , and temperature (△T). The Wrangellia Large Igneous Province is the red patch at (26) Geochemical traces for volcanic activity The CPE is marked by disruptions to geochemical cycles, most notably the carbon cycle. Sediments corresponding to the base of the episode show a prominent –2 to –4‰ δ13C excursion, indicating the release of a lightweight carbon isotope, carbon-12, into the atmosphere. This excursion was first mentioned for carbonates in Israel, More precise stratigraphic evaluation of European outcrops has resolved this excursion into three or possibly four major pulses, spanning the late Julian and early Tuvalian. Each pulse can be equated with an interval of abnormal sedimentation on land and sea. The third excursion, at the Julian-Tuvalian boundary, is correlated with major ammonoid and conodont extinctions. A four-pulse episode is also strongly supported in terrestrial systems, particularly lake and river sediments in North China. Norwegian shale and Japanese chert from the Ladinian-Carnian boundary show a marked change in the ratio of seawater osmium isotopes. The relative abundance of osmium-187 over osmium-188 declines strongly through most of the Julian before rebounding and stabilizing in the Tuvalian. The decline is attributed to early phases of the Wrangellia large igneous province enriching the ocean with osmium-188. Osmium-188 is preferentially sourced directly from the mantle, while osmium-187 is a radiogenic isotope supplied from eroded land. In the Alps, moderate to high concentrations of mercury occur alongside carbon cycle disruptions, just prior to the sediment disruption which marks the CPE. These mercury spikes occur in well-oxygenated mudstones, meaning that they are not a consequence of redox fluctuations. The ratio of mercury to organic carbon is stronger and occurs earlier in areas corresponding to open marine environments. Although the mercury spikes do not correlate with any indicators of terrestrial runoff, runoff could help maintain high mercury concentrations in the ocean through the CPE. The most parsimonious explanation is that the mercury was initially derived from a pulse of volcanic activity, particularly the Wrangellia LIP. This further supports a volcanic cause of the Carnian pluvial episode. Mercury spikes are also found simultaneous to carbon cycle disruptions in both marine and lake These mercury spikes have no trace of mass-independent fractionation, meaning that their isotope distribution is most consistent with fallout from volcanic eruptions. Influence from orbital cycles In marine sediments of Tibet, a periodic pattern emerges from carbon isotope fluctuations, sea level highstand layers, and marine crises during the CPE. Astrochronology finds a strong link to a 1.2-million year obliquity modulation cycle (the wobble in the tilt of Earth's axis). This Milankovitch cycle may have enhanced biotic turnovers in the marine realm. In the Junggar Basin, 405-kyr eccentricity cycles (Earth's distance from the Sun) mark environmental fluctuations on land, similar to warm-cold climate cycles during the Oligocene and Miocene. Eccentricity cycles are also apparent in the Jiyuan Basin of North China, with four evenly-spaced spikes of carbon and mercury isotope disruption over the course of 1.65 million years. The primary driver of the CPE would have been four pulses of volcanic activity in the Wrangellia LIP, with orbital forcing helping to amplify each pulse. Coal swamp recovery Coal swamps recovered in the early Carnian, about 15 million years after their demise in the Permian-Triassic mass extinction. Prior to the recovery, the Early and Middle Triassic was a time of low biological productivity. Global temperature, atmospheric CO2 levels, erosion, and soil oxidation were all very high. The return of the forests reversed the paradigm, establishing a productive pathway for sequestering CO2 into stable soils. According to one model, an early Carnian drop in global CO2 would increase the relative severity of the CPE. This is because CO2 has a logarithmic influence on earth's climate, so any new spike in CO2 levels (such as during the Wrangellia eruptions) would have a greater effect if it is preceded by lower background levels in the atmosphere. == References ==