Subdivision of the Triassic according to the
ICS, as of 2021.[5] Vertical axis scale: millions of years ago.
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).[6][7]
The CPE corresponds to 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 (the ancestors of modern-day snakes and lizards) and
mammaliaforms (ancestors of mammals). In the marine realm it saw the first appearance among the microplankton of
coccoliths and
dinoflagellates,[8][7][9] 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.[6]
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
supercontinentPangea shifted briefly to a hotter and more humid climate, with a significant increase in rainfall and runoff.[6][10][8][11][12] 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.[13][14][15][16]
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.[10]
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.[17] They termed this stratigraphic "wende" (turning point) the Reingrabener Wende, and it has also been called the Reingraben event or Raibl event.[14][18] 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
kaoliniticclay 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.[6]
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.[6] 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.[19] 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 was lacking.[6] Simms and Ruffell published several more papers in the coming years,[20][21] but their hypothesis was not widely accepted.[19] 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.[22] Their critique also coined the term "Carnian pluvial event", which would eventually become among the most widespread names for the climatic disturbance.[16][23]
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.[16][24][19] Interest in the hypothesis was greatly enhanced by a 2008 meeting and workshop on Triassic climate at the
Museum of Nature South Tyrol in
Bolzano, Italy.[23][19] However, even as the global nature of the CPE became increasingly accepted, its ultimate cause was still hotly debated going into the 2010s. Even its nomenclature was not agreed upon, with various authors applying names such as the middle Carnian wet intermezzo,[25][26]Carnian humid episode,[20][27][28]Carnian pluvial phase,[29][30] and Carnian crisis.[31] Carbon and
Osmium isotope records published over the coming years supported a strong link between the Carnian climate disturbances and the Wrangellia large igneous province, but many questions remain unanswered.[32][10] A geological workshop focusing on the CPE met in 2018 at the
Hanse-Wissenschaftskolleg (HWK) Institute for Advanced Study in
Delmenhorst, Germany. The workshop was intended to spur further research on the mechanisms, impact, and stratigraphy of the CPE, as well as its relevance for understanding modern climate change. It also attempted to standardize the nomenclature of the CPE, rejecting descriptors such as "event" (typically applied to geological processes under a million years in duration) or "middle Carnian" (a nebulous term with no equivalent geological substage).[33]
Geological evidence
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, evidence of which includes:
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 [34] although some claim a Rhaetian age based on localised occurrence of microfossils[35])
This usually wet climate of the CPE was periodically interrupted by drier climates typical of the rest of the Late Triassic period.[29]
Global warming was also prevalent during the Carnian pluvial event. This is evidenced by oxygen
isotope analyses performed on
conodontapatite from the CPE, which show an approximately 1.5
‰ negative shift in the stable isotope
δ18O, suggesting global warming of 3–4
°C during the CPE and/or a change in
seawatersalinity.[31][36] This warming was probably related to extensive volcanic activity at the time, evidenced by carbon isotope trends across the CPE.[10] This volcanic activity was in turn probably related to the formation of the
WrangelliaLarge 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).[10]
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 of south China. 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.[28]
At the onset of the CPE a sharp change in
carbonate platform geometries is recorded in western
Tethys. High relief, mainly isolated, small carbonate platforms surrounded by steep slopes, typical of the early Carnian, were replaced by low-relief carbonate platforms featuring low-angle slopes (i.e., ramps). This turnover is related to a major change in the biological community responsible for calcium carbonate precipitation (i.e. carbonate factory). The highly-productive, mainly bacterial-dominated biological community (M-factory) whose action led to the carbonate production on high-relief platforms was substituted by a less productive mollusc-metazoan-dominated community (C-T factories).
In the South
China block, the demise of carbonate platforms is coupled with the deposition of sediments typical of
anoxic environments (black
shales). Thanks to low oxygen levels, animal remains were often well-preserved in sedimentary deposits called
Lagerstätten. These Lagerstätten are rich in crinoids and reptiles, such as
ichthyosaurs.
Geochemical traces
Carbon
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.[37] This excursion was first mentioned regarding carbonates in Israel,[6] and was later reported in more detail from fragments of carbonized wood in the Dolomites.[10] It has been confirmed in various carbon-based sediments throughout Europe and Asia.[37][28][38][39] 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.[40]
Osmium
Norwegian shale and
Japanesechert 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 LIP 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.[32][41][42]
Mercury
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.[43] Mercury spikes are also found alongside carbon cycle disruptions in both marine[44] and lake[45] sediments in China. These mercury spikes have no trace of
mass-independent fractionation, meaning that their isotope distribution is most consistent with a volcanic origin and atmospheric deposition.[44]
The oldest dinosaur-bearing fossil assemblage, the
Ischigualasto Formation of
Argentina, has been radiometrically dated back to 230.3 to 231.4 million years ago. This age is very similar to the minimum age calculated for the CPE (≈230.9 million years ago).
Ichnofossil comparisons of various tetrapods from before, during and after the CPE suggest an explosive radiation of dinosaurs due to the Carnian humid phase.[46] 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.[47]
Other tetrapods
The CPE had a profound effect on the diversity and
morphologicaldisparity of herbivorous tetrapods.[48] 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.[49] Hyperodapedontine abundance was not sustained for long, and they too would die out in the early Norian. By cutting rhynchosaurs off from a greater variety of niches, the CPE would have reduced their versatility and increased their 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.[48]
Plants
The oldest widespread
amber deposition occurred during the CPE.[50] Carnian amber droplets from Italian
paleosols are the oldest amber deposits known to preserve
arthropods and
microorganisms.[51] 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.[52][50]
Marine life
The first
planktonic calcifiers occurred just after the CPE and might have been calcareous dinocysts, i.e., calcareous
cysts of
dinoflagellates.
Possible causes
Eruption of Wrangellia flood basalts
The recent discovery of a prominent δ13C negative shift in higher plants' n-alkanes suggests a massive
CO2 injection in the
atmosphere-
ocean system at the base of the CPE. The minimum radiometric age of the CPE (≈230.9 Ma) is similar in age to the
basalts of the
Wrangellialarge igneous province (LIP). In the geological record, LIP
volcanism is often correlated to episodes of major climate changes and extinctions, which may be caused by pollution of
ecosystems with massive release of volcanic gases such as CO2 and
SO2.
Large release of CO2 in the atmosphere-ocean system by Wrangellia can explain the increased supply of siliciclastic material into basins, as observed during the CPE. The increase of CO2 in the atmosphere could have resulted in
global warming and consequent acceleration of the hydrological cycle, thus strongly enhancing the continental
weathering. Moreover, if rapid enough, a sudden rise of pCO2 levels could have resulted in
acidification of seawater with the consequent rise of the
carbonate compensation depth (CCD) and a crisis of carbonate precipitation (e.g. demise of carbonate platforms in the western
Tethys). On top of all that, the global warming brought on by the flood basalt event was likely exacerbated by the release of methane clathrates.[53]
Uplift during the Cimmerian orogeny
According to an alternative hypothesis, the Carnian pluvial episode was a regional climatic perturbation mostly visible in the western Tethys and related to the
uplift of a new
mountain range, the
Cimmerian orogen, which resulted from the closing of a Tethyan northern branch, east of the present European continent.
The new mountain range was forming on the southern side of
Laurasia, and acted then as the
Himalayas and
Asia do today for the
Indian Ocean, maintaining a strong
pressure gradient between the ocean and continent, and thus generating a
monsoon. Summer monsoonal winds were thus intercepted by the Cimmerian mountain range and generated strong rain, thus explaining the switch to humid climate recognized in western Tethys sediments.[31][14]
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