Reefs in crisis: Marine ecosystem upheaval during the Carnian Pluvial Event
Jon Trevelyan (UK)
This is the second of three articles on the Carnian Pluvial Event (CPE). The first covered the climate engine of the CPE. This article turns to the marine record, tracing how the CPE destabilised reefs and reorganised shallow-marine ecosystems.
When the Triassic seas lost their balance
In the early Late Triassic, the warm epicontinental seas along the western Tethys margin were home to thriving carbonate platforms. These platforms were dominated by microbial-sponge reef systems, with stromatolitic and thrombolitic frameworks forming broad, stable carbonate shelves (see box below). This was a marine world still rebuilding after the devastation of the end-Permian extinction. By the early Carnian, the carbonate factories of the Tethys had regained their vigour, and marine biodiversity was increasing.
| What are stromatolites and thrombolites? |
|---|
| Stromatolites and thrombolites are types of microbial carbonate structures that played a major role in Middle and early Late Triassic reef systems. Stromatolites: These are layered buildups created when microbial mats – mainly cyanobacteria – trap and bind sediment or precipitate calcium carbonate. The result is a laminated structure with fine, distinct layers. They thrive in clear, low-sediment, well-lit water (but some Triassic stromatolites did tolerate slightly elevated stress), and their presence reflects stable, low-nutrient conditions. Thrombolites: Thrombolites are also microbial buildups, but instead of having neat layers, they show a clotted or mottled internal texture. This pattern forms when microbial communities precipitate carbonate in small, irregular patches. They often indicate slightly more energetic or disturbed conditions than stromatolites. Why they matter in the Triassic: During the Middle-early Late Triassic, stromatolites and thrombolites formed the structural foundation of many sponge-microbial reefs. Their sensitivity to water clarity, sediment influx and nutrient levels makes them excellent indicators of environmental stability, and helps explain why these reefs collapsed so abruptly during the CPE. |
Yet within a geologically brief interval, these seemingly stable ecosystems faltered. Clay-rich sediments invaded clear-water platforms, reef communities collapsed in multiple pulses, ammonoid assemblages shifted abruptly, and lagoonal environments swung between stress and recovery. This was the marine expression of the CPE – a volcanically triggered climate episode whose hydrological surges transformed the chemistry, clarity and oxygenation of shallow seas.
The CPE represents one of the most dramatic interruptions to Mesozoic carbonate deposition before the end-Triassic crisis. Understanding what happened on these submerged platforms provides crucial insight into how reefs respond to climatic shocks, and how coral-dominated ecosystems ultimately took their place.
The Middle Triassic carbonate world
Before the onset of the CPE, much of the western Tethys margin supported broad, laterally extensive carbonate platforms with minimal siliciclastic input. These platforms hosted communities dominated not by corals, but by:
- sponges (including demosponges and calcisponges);
- microbial crusts and mats;
- tubiphytes (extinct enigmatic reef-building fossil organisms) and other encrusting organisms;
- small shelly biotas; and
- cryptic reef organisms occupying cavities and sheltered spaces.
This dominance reflected the long recovery from the end-Permian extinction, which had removed most Palaeozoic reef-builders. By the early Carnian, reefs were stabilising, but still relied on clear, low-nutrient, well-lit water. Any disruption to water clarity or chemistry could have immediate and severe consequences.
Sedimentary fingerprints of a crisis
The hallmark of the CPE in marine settings is the presence of marl-limestone alternations – rhythmic successions of fine-grained siliciclastics interrupting once-pure carbonate beds.
These alternations are especially clear in the Dolomites, Slovenia and the Austrian Alps.
During the humid pulses of the CPE, intensified monsoonal rainfall over Pangaea triggered enormous sediment influx from nearby landmasses. Rivers delivered clay, silt and dissolved ions to the western Tethys at far higher rates than usual. The result was:
- temporary suffocation of carbonate producers;
- reduced water clarity;
- enhanced nutrient input;
- intermittent dysoxia (that is, low dissolved oxygen in bottom waters or pore waters, but not totally anoxic) in restricted basins; and
- repeated suppression of reef growth.
These clay-rich intervals are so distinctive that they now serve as the primary marine marker of the event. Between pulses, carbonate sedimentation resumed, although often showing signs of ecological stress.
Platform drowning and light stress
In many Tethyan basins, carbonate platforms were temporarily drowned during the strongest pluvial pulses. Drowning occurred not from rising sea level, but from:
- increased turbidity reducing light penetration;
- sediment blanketing killing benthic communities (that is, those in the ecological region at the lowest level of a body of water, including the sediment surface and subsurface layers); and
- nutrient enrichment driving ecological instability.
Reef frameworks are especially vulnerable to light starvation. Sponges and microbial mats tolerate some turbidity, but sustained high sediment levels overwhelm them. Decline can be rapid and recovery slow, producing the sharp alternations seen in the stratigraphic record. And restricted lagoons frequently developed oxygen-poor bottom waters as clay and organic matter accumulated. Such settings often preserve microbial-dominated or stressed faunal assemblages.
Faunal turnover: winners and losers
The CPE caused substantial turnover among marine faunas. Although not a mass extinction, certain groups declined while others diversified.
- Ammonoids. Assemblages shifted rapidly during each surge of intensified rainfall; and some groups dwindled, while others took advantage of the changing conditions and diversified into the gaps they left behind.
- Conodonts. Sensitive to chemistry and salinity, they show pronounced species turnover and provide fine-scale correlation of humid pulses.
- Bivalves and gastropods. Shallow-marine molluscs experienced stresses from lowered salinity, high sedimentation and nutrient fluctuations; and opportunistic taxa sometimes flourished.
- Microbial mats. These expanded during stressed intervals, mirroring patterns seen during other major global environmental upheavals.
[FIG 7]
The rise of the scleractinian corals
Scleractinian corals are stony, skeleton-forming corals that build modern reefs using hard calcium-carbonate frameworks; and one of the most profound outcomes of the CPE was the expansion of such coral reefs. Corals were present before the event but played a minor ecological role. In the stable conditions of the Norian, they rose to dominance.
Contributing factors likely included:
- reduced sediment input after the humid interval;
- clearer water and improved light penetration;
- stabilised nutrient cycling; and
- the re-establishment of the partnerships corals rely on to build their skeletons.
The Dolomia Principale (Figs. 5 and 10; and see Part 1), an extensive Norian carbonate unit in the Dolomites, records this resurgence, forming towering platform complexes contrasting with the marl-rich Raibl Beds below. (The Raibl Beds are significant geological rock formations, primarily limestone and marl, from the Carnian period, famous for rich fossil plants, microfossils, vertebrates in the Eastern Alps.)
Sensitivity and resilience: lessons from the CPE
The CPE demonstrates that carbonate platforms are highly sensitive to changes in sediment input, light penetration, nutrients and oxygenation. Yet it also shows that reef systems can recover, often in dramatically altered ecological configurations. The transition from sponge-microbial frameworks to coral-dominated reefs is among the most striking examples of ecological reassembly in the Mesozoic.
Modern reefs exhibit similar vulnerabilities to turbidity, eutrophication (excessive richness of nutrients) and climate-driven disturbance. Although not a direct analogue for today’s crises, the CPE highlights how rapidly environmental changes can destabilise shallow-marine ecosystems, and how profoundly they can reorganise during recovery.
A marine turning point in Triassic history
The CPE was a time of profound marine disruption. Intensified monsoon rainfall delivered unprecedented sediment loads to shallow seas, drowning and destabilising carbonate platforms. Reefs collapsed in multiple pulses, faunas reorganised, and lagoonal environments swung between stressed and recovering states.
However, from this disturbance came renewal. In the aftermath, scleractinian corals rose to dominance, reshaping the architecture of reefs for the rest of the Mesozoic. The CPE was not just a climatic episode: it was a pivot in the history of marine ecosystems, captured most vividly in the stratigraphy of the Dolomites.
Further reading
Dal Corso, J. et al. (2018). Volcanism, climate and reef response during the Carnian. Earth-Science Reviews.
Gianolla, P., Roghi, G., Preto, N. (2012). The Carnian Pluvial Event: a life-changing episode. Rendiconti della Società Geologica Italiana.
Hornung, T. and Brandner, R. (2005). Lithostratigraphy and facies of Carnian deposits in the Dolomites. Facies.
Kustatscher, E. et al. (2019). Floral and marine responses to the CPE. Palaeogeography, Palaeoclimatology, Palaeoecology.
Preto, N., Kustatscher, E., Wignall, P.B. (2010). Triassic climates and the Carnian Pluvial Episode. Journal of the Geological Society.
Stanley, G.D. (2003). Origins of modern corals. Science.