Volcanoes, monsoons and a flooded supercontinent: The climate engine of the Carnian Pluvial Event
Jon Trevelyan (UK)
This is the first of three articles on the Carnian Pluvial Event (CPE), often referred to as an ‘event’, although increasingly recognised as a prolonged episode, as I will discuss) of the Triassic that seems to have grown in importance and interest, and only become more widely known in relatively recent times. In this article, I will introduce the topic, concentrating on what might be called the “climate engine” driving what was happening.
When Pangaea’s climate shifted gear
For much of the Middle Triassic, Earth’s largest ever landmass – the supercontinent Pangaea – experienced a world of climatic stability. Vast continental interiors lay beneath hot, seasonal, semi-arid skies. Red-bed basins stretched for hundreds of kilometres. River systems were sluggish and sandy, lakes were often ephemeral, and vegetation tended to be sparse: conifer scrub, seed ferns and low, drought-tolerant undergrowth. Offshore, along the western margin of the Tethys Ocean, warm, clear waters supported extensive carbonate platforms dominated by sponges and microbial frameworks.
But around 234 million years ago, this seemingly steady world lurched into a completely different mode. Rainfall intensified dramatically, the way sediment moved through the landscape changed equally dramatically, reef growth faltered, successive influxes of clay covered the carbonate platforms, humid forests spread across the continents, and marine as well as terrestrial ecosystems underwent substantial reshuffling.
This episode – the CPE – was not a classic mass extinction. Rather, it was a climatic inflection point: a multi-phase hydrological episode that altered landscapes, ecosystems and even the long-term trajectory of Triassic life. Understanding how and why the CPE happened means understanding how a supercontinent responds to volcanic forcing, and why the geological record captures it so unevenly.
Pangaea: a supercontinent built for monsoons
Pangaea spread across the equator and spanned from pole to pole. This configuration produced enormous seasonal contrasts. Large continental interiors heated strongly in summer and cooled rapidly in winter, creating steep pressure gradients between land and sea. The result was a climate predisposed to powerful monsoonal circulation long before the CPE occurred.
In the region that would one day become the UK, this same climate engine produced landscapes very different from the ones we know. Britain lay close to the subtropics on the western margin of Tethys, positioned between seasonally wet low latitudes to the south and more arid belts to the north. Much of the area consisted of broad alluvial plains, muddy deltas and shallow marginal seas that periodically advanced and retreated. Thick red beds, calcrete horizons (see box below) and ephemeral river deposits show that long dry seasons were punctuated by intense bursts of rainfall – early hints of the strongly seasonal regime that the CPE would later amplify.
| Calcrete horizons in the UK Triassic |
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| Calcrete horizons are hard, calcium-carbonate-rich layers that formed within soils on the semi-arid floodplains of Triassic Britain. They developed when groundwater or soil moisture evaporated during long dry seasons, leaving behind carbonate precipitates that gradually cemented the upper part of the soil profile. In many cases, these appear today as pale, nodular or sheet-like bands cutting through the red mudstones and sandstones of the Sherwood Sandstone Group and the lower Mercia Mudstone Group. Their distribution is broad but especially well documented in the English Midlands, the Cheshire Basin, the Bristol-Gloucester district and the East Devon coast, where quarries, road cuttings and coastal cliffs expose excellent examples. These horizons represent the “dry end” of the climatic spectrum immediately before the CPE: a landscape dominated by strongly seasonal climates, with brief but intense downpours separated by long intervals of evaporation and soil desiccation. Because calcretes only form under conditions of repeated drying, strong evaporation and modest vegetation cover, they serve as sensitive indicators of palaeoclimate. In the British Triassic record, they mark a time when soils were thin and oxidised, rivers intermittent, and the climate prone to rapid swings between drought and sudden floods – conditions that were later transformed as the CPE pushed the system decisively toward wetter, more humid regimes. |
Even before the humid pulses of the Carnian arrived, the British basins were already primed for rapid environmental change: vegetation was sparse and drought-tolerant, soils were thin and oxidised, and river systems were small and easily overwhelmed when wetter conditions set in.
Under normal conditions, this monsoon system was strong but not extreme. Yet even modest perturbations in atmospheric CO₂ could push it into a dramatically more vigorous state. The CPE represents exactly such a shift: a super-monsoon amplified by greenhouse forcing, with rainfall belts widening, shifting latitudinally, and becoming far more intense than before.
The Wrangellia Large Igneous Province: a volcanic catalyst
The trigger for the CPE lies thousands of kilometres from the Tethys margin, beneath what is now western Canada and Alaska. During the Carnian, this region formed a buoyant oceanic plateau undergoing massive volcanic outpourings: the Wrangellia Large Igneous Province (LIP).
High-precision geochronology places the main eruptive phases between around 234 to 232 million years ago, matching the timing of the CPE’s major humid pulses. Although these were largely submarine eruptions, the volumes were so vast that significant quantities of CO₂, sulphur compounds, halogens and mercury entered the ocean-atmosphere system.
The geological fingerprints of this activity include:
- Negative carbon-isotope excursions (δ¹³C), reflecting injections of isotopically light volcanic CO₂.
- Mercury anomalies, increasingly used by science today as LIP tracers.
- Enhanced chemical weathering, producing widespread clay influx into marginal marine basins as mentioned earlier.
- Carbonate platform instability, recorded as marl interbeds interrupting clear-water limestone successions.
Together, these indicators build a strong causal link between Wrangellia’s volcanic pulses and the profound climatic shifts that define the CPE.
The four phases of the CPE (A-D)
Although often discussed as a single climatic episode, the CPE was not one continuous humid interval. Instead, high-resolution stratigraphy and geochemical records from the Dolomites, Austria, Slovenia and the eastern Tethys show that it unfolded as a series of discrete humid pulses, separated by partial recoveries. These are conventionally grouped into four phases, labelled A, B, C and D, each reflecting a different balance between rainfall intensity, sediment delivery and environmental stress.
- Phase A marks the onset of humid conditions, with the first significant increase in chemical weathering and clay input into previously carbonate-dominated basins.
- Phase B represents the principal pluvial maximum of the CPE, recording the strongest monsoonal rainfall, the greatest continental runoff and the most widespread suppression of carbonate production.
- Phase C captures a period of partial recovery punctuated by renewed humid pulses, indicating an unstable climate oscillating between wetter and more moderate conditions.
- Phase D records a final, weaker humid interval before a longer-term transition toward drier conditions during the later Carnian and Norian.
Together, these phases show that the CPE was not a single climatic “spike”, but a pulsed hydrological episode, dominated by one major pluvial maximum (Phase B) and framed by earlier and later humid disturbances. Recognition of this structure has become central to correlating CPE successions across Europe and beyond.
Carbon-cycle disturbance and rapid climate feedbacks
A defining feature of the CPE is the sequence of repeated negative δ¹³C excursions. These appear in high-fidelity marine sections across the Tethyan region, especially in the Dolomites, Slovenia and southern China. Each pulse marks a burst of volcanic CO₂ entering the system.
The climate reacted quickly:
- Greenhouse warming increased.
- Land-sea contrasts in temperature between land and ocean surfaces intensified.
- Monsoonal cells strengthened, deepening the seasonal inflow of moisture.
- Rainfall surged, particularly along the western Tethys margin.
- Chemical weathering accelerated, producing more clay and dissolved ions.
- Clay-rich layers accumulated in marine basins, interrupting carbonate production.
This multi-step cascade repeated several times over a period of about two million years, creating the rhythmic structure of the CPE.
A mega-monsoon intensified
Palaeoclimate modelling reinforces what the rocks show: volcanic CO₂ pulses dramatically intensified Pangaea’s monsoon. Key characteristics include:
- An expanded Intertropical Convergence Zone (ITCZ).
- Stronger seasonal wind reversals.
- Broader and more persistent tropical rainbelts.
- Increased convective activity and storms.
- Heavy rainfall relating to the position and form of the mountains along the Tethyan coastal highlands.
The western Tethys margin was ideally placed to record this shift. During humid pulses, enormous quantities of fine sediment were washed from the Pangaean interior into shallow-marine carbonate settings, creating the classic limestone-marl alternations now recognised as the diagnostic sedimentary signal of the CPE.
Why Italy holds the key to understanding the CPE
Among all global sections, the Dolomites preserve the CPE with exceptional clarity. Several factors combine to make this region the global reference point:
- Continuous shallow-marine carbonate deposition sensitive to even small clay influx.
- High sedimentary resolution, with metre-scale alternations reflecting short-lived climatic pulses.
- Abundant ammonoid and conodont biostratigraphy, enabling precise correlation.
- Intercalated volcanic ash layers, providing chronostratigraphic anchors.
- Strong contrast between “normal” limestones and clay-rich pluvial layers, making each pulse visually unmistakable.
By contrast, continental settings such as the UK were less responsive to short-term rainfall changes. Red-bed basins, oxidising soils and coarse fluvial systems tend to mute climatic signals (for example, those of the Mercia Mudstone Group basins of England and Wales, including Somerset, Devon and the Bristol Channel referred to above). In these environments, the CPE is recorded only subtly, in gleyed palaeosols (see box below), modest facies changes and shifts in palynological assemblages.
| Gleyed palaeosols |
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| In soil science, the word “Gleyed” describes soil that is waterlogged and oxygen-deprived, causing a chemical change (reduction) in iron minerals, which results in a characteristic grey, blue or greenish colour, often with rusty mottling when exposed to air. |
Continental and marine signatures: two sides of the same event
Although the CPE is best known from marine records, its impact on land was equally transformative.
Marine responses included:
- Temporary drowning of carbonate platforms.
- Repeated marl layers during humid pulses.
- Local oxygen-depleted bottom waters.
- Turnover among ammonoids, conodonts and benthic faunas.
- Post-CPE rise of scleractinian coral reefs in the Norian.
Continental responses included:
- Widespread soil wetting and gleying.
- Increase in chemical weathering and smectite production. (Smectite is a group of swelling clay minerals known for their layered structure, high surface area, and ability to absorb large amounts of water, causing significant swelling and shrinkage).
- Expansion of humid conifer forests, fern spikes in pollen records.
- Floodplain widening, more stable lakes, finer-grained fluvial deposits.
- Proliferation of amphibian- and reptile-dominated wetland ecosystems.
Both realms reflect the same underlying driver: an intensified monsoon delivering more rainfall and sediment across vast areas of Pangaea.
End of the episode: Norian drying and stabilisation
As Wrangellia’s volcanic activity waned, CO₂ levels gradually stabilised. Weathering feedbacks drew down atmospheric carbon, and the monsoon weakened. The climate shifted back toward drier conditions during the Norian Stage, allowing carbonate platforms to rebound. This recovery set the stage for the expansion of extensive coral-dominated reef systems, notably the Dolomia Principale. (The Dolomia Principale is a major, widespread rock formation in the Alps, representing a vast, shallow carbonate platform from the Late Triassic period that developed in warm, evaporative seas and tidal flats.)
Although the climate returned to a drier baseline, the world that emerged was fundamentally restructured. Vegetation, faunal communities, soil systems and sediment pathways had all been reshaped. In many ways, the Late Triassic ecological landscape was born out of this episode.
Conclusion: a climatic turning point with lasting consequences
The Carnian Pluvial Event is a vivid example of how volcanic greenhouse forcing can reorganise the Earth system. It was not a catastrophic extinction but a profound climatic and ecological pivot. The combination of volcanic CO₂, a supercontinent-scale monsoon and sensitive marginal-marine basins produced one of the most distinctive climatic signatures of the Mesozoic.
Its legacy – from expanded forests and altered river systems, to reorganised marine faunas and the rise of coral-dominated reefs – shaped the trajectory of life for millions of years afterwards. These are discussed in the following two articles.
Further reading
Benton, M.J. (2018). The impact of the Carnian Pluvial Event on Late Triassic ecosystems. Proceedings of the Geologists’ Association.
Dal Corso, J. et al. (2012). A volcanic trigger for the Carnian Pluvial Event? Earth and Planetary Science Letters.
Dal Corso, J. and Gianolla, P. (2020). Landscape and biotic dynamics during the Carnian Pluvial Episode. Palaeogeography, Palaeoclimatology, Palaeoecology.
Kustatscher, E., et al. (2021). Floral evolution and humidity change in the Carnian. Earth-Science Reviews.
Preto, N., Gianolla, P., Kustatscher, E., Wignall, P.B. (2010). Triassic climates, the Carnian Pluvial Episode, and the origin of the dinosaurs. Journal of the Geological Society.
Roghi, G., Gianolla, P., Van der Eem, J. (2007). Palynological evidence for the Carnian Pluvial Event. Rivista Italiana di Paleontologia e Stratigrafia.
| Other articles in this series |
|---|
| Volcanoes, monsoons and a flooded supercontinent: The climate engine of the Carnian Pluvial Event |
| Reefs in crisis: Marine ecosystem upheaval during the Carnian Pluvial Event |
| Forests, floodplains and the first amber: Terrestrial transformations during the Carnian Pluvial Event |