Carnian Pluvial Event

The Carnian Pluvial Event (CPE) is a major global climate change and biotic turnover that occurred during the Carnian,^[1] early Late Triassic, ~ 230 million years ago.^[2]

The base of the CPE is marked by a ~ 0.4% negative shift in carbon stable isotopes (δ¹³C) of fossil molecules (n-alkanes) from higher plants and total organic carbon.^[3] A ~ 0.15% negative shift in oxygen stable isotopes (δ¹⁸O) of conodont apatite suggests a global warming.^[4][5] Major changes in organisms responsible for calcium carbonate production occurred during the CPE.^[6][7][8] A halt of carbonate sedimentation is observed in deep water settings of Southern Italy that was probably caused by the rise of the Carbonate Compensation Depth (CCD).^[9] High extinction rates occurred among ammonoids, conodonts, bryozoa, and crinoids.^[1] Major evolutionary innovations followed the CPE, as the first occurrence of dinosaurs, calcareous nannofossils and scleractinian corals.^[2][3]

Naming

The CPE is also known as "Reingrabener Wende" (meaning Reingrabener turnover),^[10] or "Raibl event".^[11]

Climate during the Carnian Pluvial Event

The arid climate of the Late Triassic was interrupted by the markedly more humid conditions of the Carnian Pluvial Event (CPE). Evidences of increased rainfall during the CPE are 1) the development of palaeosols (histic and spodic soils) typical of tropical humid climate with a positive water budget throughout the year; 2) hygrophytic palynological assemblages that reflect a vegetation more adapted to humid climate; 3) siliciclastic sediment input into the basins due to increased continental weathering and runoff; 4) the widespread presence of amber.

Oxygen isotope analyses performed on conodont apatite show a ~ 0.15% negative shift. This negative δ¹⁸O excursion suggests a global warming of 3-4°C during the CPE and/or a change in seawater salinity.

Biological Turnover

Extinctions: conodonts, ammonoids, bryozoa, and green algae were severely hit by the CPE and experienced high extinction rates. But most noticeable were the radiations of, among other groups, dinosaurs, calcareous nannofossils, corals and crinoids. Dinosaurs: the radiometric age of the most ancient known Dinosaurs (Eoraptor) found in the Ischigualasto Formation of Argentina dates back to 230.3-231.4 million years ago. This age is very similar to the minimum age calculated for the CPE (~ 230.9). Calcareous nanofossils: the first planktonic calcifiers occurred just after the CPE and might have been calcareous dinocysts, i.e., calcareous cysts of dinoflagellates.

Effects on carbonate platforms

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) which 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). These anoxic levels are often fossil Lagerstätten, very rich in crinoids and reptiles (e.g. Ichthyosaurs).

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 Wrangellia Large 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 as CO₂, 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 enough rapid, 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).

Uplift of the Cimmerian orogen

According to an alternative hypothesis, the Carnian Pluvial Event is a regional climatic perturbation mostly visible in the western Tethys and related to the uplift of a new mountain range, the Cimmerian orogen, 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, 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 sediments of western Tethys.

References