Forests, floodplains and the first amber: Terrestrial transformations during the Carnian Pluvial Event
Jon Trevelyan (UK)
This is the last of three articles on the Carnian Pluvial Event (CPE). The first covered the climate engine of the CPE and the second described the marine ecosystem upheaval during those times. This final article turns to the continents, examining how the CPE transformed terrestrial landscapes and ecosystems.
When the Triassic continents turned green
At the start of the Late Triassic, much of Pangaea’s interior was a world of seasonal dryness, red-bed floodplains, ephemeral lakes and sparsely vegetated uplands. Thin, oxidised soils covered broad plains, and much of the vegetation comprised drought-tolerant conifers, seed ferns and low, scrubby undergrowth. River systems tended to be affected by flash floods and be braided, with coarse sediment loads reflecting highly seasonal rainfall.
But around 234 million years ago, this landscape changed abruptly. Persistent, repeated pulses of rainfall swept across Pangaea as a result of volcanic greenhouse forcing from the Wrangellia Large Igneous Province. These humid episodes – the CPE – transformed continental ecosystems. Floodplains expanded into wide, muddy plains. Lakes became longer-lived. Vegetation flourished. Rivers shifted their styles. Soil formation intensified. Amphibians and reptiles adapted to wetlands proliferated. And in some conifer forests, resin flowed so abundantly that the first major amber deposits in Earth history began to form.
The terrestrial impact of the CPE is not as visually abrupt as the marine marl-limestone cycles of the Dolomites (as discussed in the second article of this series), but it was no less profound. It represents a climatic reset that re-engineered continental environments and underpinned key ecological transitions of the Late Triassic.
The dry-world of Pangaea before the CPE
Before the onset of the pluvial pulses, Pangaea’s continental interiors were dominated by:
- thin, oxidised palaeosols (the classic red beds);
- limited vegetation cover;
- braided rivers and ephemeral channels;
- closed or intermittent lake basins; and
- highly seasonal rainfall.
Soils were generally immature: calcareous nodules and mottling reflecting alternating short wet seasons and long dry periods. Vegetation cover was sparse, limiting organic accumulation and stabilisation of floodplains. Wildfires were common, especially in seasonally arid belts.
This fragile landscape was primed for rapid transformation should the hydrological regime change – and, during the CPE, it certainly did.
Soil transformations under intensified rainfall
The first evidence of the CPE in many continental basins occurs in palaeosols. With sustained increases in rainfall, soils:
- became thicker and more deeply weathered;
- developed well-defined soil layers reflecting stronger chemical weathering;
- displayed widespread gleying (grey-green colours from reduced iron; see Part 1);
- contained higher proportions of clay, especially smectite; and
- showed stronger root traces and bioturbation.
These changes reflect wetter conditions, more persistent soil moisture and greater vegetative cover. In some settings, the appearance of organic-rich horizons marks the development of proto-swampy conditions, which were rare in pre-CPE red-bed terrains.
The shift is particularly striking in basins of Germany, the eastern USA and parts of the UK, where, in the latter, gleyed palaeosols, mottled red–green weathering profiles and sharp increases in smectite (see Part 1) content appear in several Carnian successions. These are particularly well developed within the Mercia Mudstone Grou of the English Midlands and the Penarth/Bristol Channel outcrops, which preserve some of the clearest continental signatures of heightened humidity during the CPE.
Rivers, lakes and expanding floodplains
Increased rainfall during the CPE profoundly altered drainage systems. Runoff became stronger and more consistent, affecting river style, channel geometry and floodplain dynamics.
Key hydrological changes included:
- A shift from braided to more sinuous or meandering channels in some basins.
- Finer-grained overbank deposition, forming widespread muddy floodplains.
- Expansion and deepening of lakes and wetlands, some becoming perennial.
- Greater floodplain stability, linked to increased vegetation cover.
- Development of crevasse splays, levees and swampy depressions.
Together, these changes reflect a fundamental reshaping of how sediment moved across the continent. While not all basins responded in the same manner, many show a clear shift towards finer sedimentation, greater aggradation and longer residence times of surface water.
Vegetation change: fern spikes and conifer forests
Changes in pollen and spore records offer some of the sharpest clues to how the CPE reshaped plant life across the continents: including fern spikes, indicating rapid expansion of these plants in response to increased moisture. These spikes occur globally and coincide closely with the humid pulses.
Other changes include:
- Spread of humid-adapted conifers, particularly voltzialeans. (Voltzialeans were early conifer-like gymnosperms that emerged after the Carboniferous and form part of the stem lineage leading to modern conifers.)
- Greater abundance of seed ferns, cycads and ginkgophytes. (Ginkgophytes were a long-lived ancient group of seed plants – gymnosperms – characterised by distinctive fan-shaped leaves, today represented by a single living species, Ginkgo biloba.)
- Development of more structured forest ecosystems with layered canopies.
- Increased biotic activity in soils, stabilising landscapes.
The appearance of extensive humid forests had cascading ecological consequences. With denser vegetation came more shade, more niches, more organic matter and more opportunities for insects and other arthropods. This is the ecological backdrop to the earliest major amber formation.
The first amber: resin flow in humid conifer forests
One of the most fascinating consequences of the CPE is the formation of some of the earliest abundant amber deposits. Resin production in conifers is often triggered by:
- humidity;
- fungal attack;
- storm damage; and
- increased insect activity.
The humid forests of the CPE provided ideal conditions for exuding resin. In the Dolomites, thin but widespread amber layers occur within Carnian deposits, preserving microscopic inclusions of fungi, spores and arthropods. These represent some of the oldest windows into forest-floor ecosystems and resin-based ecological interactions.
The CPE marks the transition from minor, sporadic resin occurrences to the first significant episodes of amber formation in Earth history – a development with far-reaching implications for palaeoecology and taphonomy.
Terrestrial fauna: wetland expansion and ecological shifts
The CPE reshaped terrestrial animal communities. Wetter conditions favoured amphibious and semi-aquatic vertebrates, while altering habitats for reptiles and early dinosaurs.
Major faunal responses (see box below) included:
- Metoposaurs, large amphibians, flourishing in expanded lakes and wetlands.
- Phytosaurs thriving in river systems as apex semi-aquatic predators.
- Rhynchosaurs and dicynodonts declining in many areas, possibly linked to habitat turnover.
- Small insects, mites and spiders increasing in diversity within humid forest ecosystems.
- Early dinosaurs remaining minor components of faunas – the CPE did not cause dinosaur dominance, but helped restructure the ecosystems they later thrived in.
| Some key vertebrates of the CPE |
|---|
| Metoposaurs: Large, flat-headed amphibians related to temnospondyls. Metoposaurs lived in lakes, rivers and floodplains, where they hunted fish and small tetrapods. They were well adapted to the expanded wetlands that developed during the CPE. Phytosaurs: Semi-aquatic archosaur predators that strongly resemble crocodiles, although they are not closely related. They had long snouts, powerful jaws and a largely aquatic lifestyle, thriving in the humid river systems of the Carnian. Rhynchosaurs: Robust, beaked herbivores with deep skulls and powerful jaw muscles. They fed on tough vegetation such as seed ferns and cycad-like plants, making them some of the most abundant plant-eaters in many Late Triassic ecosystems. Dicynodonts: Large, tusked herbivorous synapsids, which were distant relatives of mammals. Although past their peak by the Carnian, some lineages survived in wetter floodplain environments, feeding on soft vegetation and occupying niches similar to modern grazing mammals. |
Overall, fauna that was adapted to wetter and vegetated environments expanded, while species tied to dry-world habitats contracted.
[FIG 7]
A transformed post-CPE landscape
After the pluvial pulses ended, climates gradually returned to a drier baseline in the Norian Stage. But the landscape did not return to its pre-CPE form. Instead, continents had been profoundly re-engineered:
- New drainage networks persisted.
- Forest ecosystems remained more developed.
- Soil profiles were thicker and more weathered.
- Some basins retained longer-lived lakes and wetlands.
- Amber-producing forests continued into the Norian.
The CPE acted as a reset button for continental environments. Its influence can be traced in the sedimentary, palaeobotanical and palaeoecological record long after humidity declined.
A continental climate pivot with lasting consequences
The Carnian Pluvial Event was one of the most important climatic episodes of the Mesozoic. On land, it transformed soils, rivers, floodplains and forests, enabling ecological innovations that would shape the Late Triassic world. It nurtured the earliest major amber-producing forests, expanded wetland ecosystems and laid the groundwork for evolutionary trajectories that continued into the Jurassic, including the environments in which early dinosaurs later diversified.
Far from being a brief wet spell, the CPE was a continent-scale environmental transformation, whose terrestrial legacy is written in soils, floras, faunas and the oldest amber on Earth.
Further reading
Benton, M.J. (2018). The impact of the Carnian Pluvial Event on terrestrial ecosystems. Proceedings of the Geologists’ Association.
Dal Corso, J. et al. (2021). Palaeoclimate and environmental change during the Carnian. Palaeogeography, Palaeoclimatology, Palaeoecology.
Kubar, M. et al. (2021). Triassic soils and weathering under changing climates. Sedimentary Geology.
Kustatscher, E., Bernardi, M. & Dal Corso, J. (2020). The terrestrial Carnian Pluvial Episode. Earth-Science Reviews.
Roghi, G., Gianolla, P., Kustatscher, E. (2010). Carnian amber and forest ecosystems in the Dolomites. Rivista Italiana di Paleontologia e Stratigrafia.
Vajda, V. & McLoughlin, S. (2007). Palynological signatures of humidity events. Palaeogeography, Palaeoclimatology, Palaeoecology.