The Carnian Pluvial Episode (ca. 234 to 232 Ma; CPE) was a significant global environmental and climatic event during the Carnian Stage of the Late Triassic (Simms and Ruffell, 1989; Sun et al., 2016; Dal Corso et al., 2020; Lu et al., 2021). It is considered an important turning point in the long-term recovery of the ecosystem and organism diversity after the global-scale devastation of the Permian–Triassic mass extinction (PTME) and its extended aftermath (Dal Corso et al., 2020, 2022a) and has been hailed as a dawn of modern ecosystems (Dal Corso et al., 2020). The CPE is characterized by dramatic climatic warming, increased global rainfall, and relatively humid climatic conditions (e.g., Dal Corso et al., 2020) and is accompanied by significant fluctuations in the carbon cycle (Sun et al., 2016; Miller et al., 2017; Dal Corso et al., 2018, 2020; Shi et al., 2019; Lu et al., 2021; Li Q. et al., 2022), the shutdown of carbonate platform productivity (Shi et al., 2017), eustatic sea-level fall and modifications in carbonate platforms (Jin et al., 2020, 2022a), widespread anoxia in oceans and lakes (Sun et al., 2016; Dal Corso et al., 2020; Lu et al., 2021), elevated extinction rates (especially in ammonoids, conodonts and crinoids) (Simms and Ruffell, 1989; Bernardi et al., 2018; Dal Corso et al., 2020), and increased biodiversity including the origination of major groups of modern conifers, dinosaurs, and calcium nanobiotes (Simms and Ruffell, 1989; Bernardi et al., 2018; Dal Corso et al., 2020). The temporal coincidence of the CPE and the Wrangellia Large Igneous Province (LIP) suggests a causal relationship through release of large amounts of CO₂ into the atmosphere-marine system, resulting in an enhancement of the global atmospheric circulation and hydrological cycle (Dal Corso et al., 2020; Lu et al., 2021; Mazaheri-Johari et al., 2021; Zhao et al., 2022; Jin et al., 2023).

Enhanced atmospheric circulation and hydrological cycles have been considered potential drivers for major changes in global floras during the CPE (Dal Corso et al., 2020; Lu et al., 2021). In the Gondwana supercontinent, previous studies have shown that the terrestrial macroflora is dominated by corystosperm seed ferns (pteridosperms), and spore and pollen fossils (Kustatscher et al., 2018). In Laurasia, a transition from arid to humid floras occurred during the Carnian stage and was accompanied by thick coal seams (Pott et al., 2008). These coals mark the re-commencement of peat accumulation after the global coal gap (Retallack et al., 1996) that followed the collapse of Permian–Triassic terrestrial ecosystems (Retallack et al., 1996; Dal Corso et al., 2020, 2022a,b; Zhang et al., 2023a; Figure 1). Palynological studies have independently identified increases in hygrophytic plant abundance through the CPE in the boreal realm (Mueller et al., 2016a), the northeastern Tethys region (Mazaheri-Johari et al., 2022), the western Tethys region (Roghi, 2004; Roghi et al., 2010; Mueller et al., 2016b; Baranyi et al., 2019b; Fijałkowska-Mader et al., 2021), and the eastern Tethys region (Lu et al., 2021; Li L. et al., 2022; Peng et al., 2022), collectively indicating that a relatively humid climate prevailed (Figure 1). However, palynological studies from the Denmark Basin (Lindström et al., 2017) and the United Kingdom (Baranyi et al., 2019a) did not show significant floral changes or increased humid climatic conditions during the CPE, showing that the climatic effects varied spatially. Thus, the global extent of the effects of CPE and the relatively humid climatic conditions that prevailed during this period remains somewhat controversial.

In this study, we review the current evidence for terrestrial floral changes during the CPE and evaluate the information on climatic and environmental change this provides. We also investigate the relationship between floral changes and massive volcanism during the CPE.

Based on the available evidence from biostratigraphy (including fossil ammonoids, conodonts, and palynomorphs) and carbon isotope chemostratigraphy, markers for the commencement of the CPE are recognized in marine, marine-continental transitional, and continental strata (Dal Corso et al., 2020; Lu et al., 2021; Li Q. et al., 2022; Figures 1, 2). The onset of the CPE coincided with the first appearance of the ammonoid genus Austrotrachyceras in the Julian substage of the Carnian (Simms and Ruffell, 1989; Dal Corso et al., 2012, 2015; Sun et al., 2016) and was accompanied by a negative carbon isotope excursion (CIE-I) (Dal Corso et al., 2018, 2020) and an increase in the proportion of terrestrial hygrophyte plants (Mueller et al., 2016b; Lu et al., 2021; Figures 2, 3). Correlation between biostratigraphic data and the high-resolution C-isotope patterns suggests that ∼3–4 separate CIEs punctuated the CPE interval (Miller et al., 2017; Dal Corso et al., 2018; Lu et al., 2021; Tomimatsu et al., 2021; Li Q. et al., 2022; Figure 2). In contrast, the timing of the end of the CPE is less well constrained, with current evidence placing this at the base or within the Tuvalian 2 substage of the Carnian, based on sedimentological (end of terrigenous sediment supply) and chemostratigraphic (CIE-IV) evidence (Dal Corso et al., 2020; Lu et al., 2021; Figure 2).

Based on cyclostratigraphy, biostratigraphy, magnetostratigraphy, chemostratigraphy, and zircon U–Pb dating, previous studies have determined that the CPE occurred between ca. 234 and 232 Ma and lasted ∼1.09 to 1.7 Ma from the late Julian 1 to the Tuvalian 2 substages of the Carnian (Zhang et al., 2015; Miller et al., 2017; Bernardi et al., 2018; Lu et al., 2021). In South China, the CPE has been constrained by sedimentology (transition from carbonate-rich to clastic-rich facies), magnetostratigraphy (an interval of reversed polarity), and cyclostratigraphy (405 kyr long-eccentricity cycle from spectral gamma-ray intensity curve) from which the CPE is estimated to have approximately lasted 1.3 Ma (Zhang et al., 2015). In the United Kingdom, the CPE has been constrained by chemostratigraphy (four CIEs) and cyclostratigraphy (405 kyr long-eccentricity of Ca/Ti elements) and is interpreted to have lasted ∼1.09 Ma (Miller et al., 2017). In the Dolomites (Italy), the duration of the CPE has been constrained by sedimentology (significant pulse of humid climatic conditions inferred from four sudden increases in terrigenous material entering the basin interior), chemostratigraphy (four CIEs), and biostratigraphy (restriction by Aonoides/Austriacum and Subbullatus), showing that the CPE may have lasted 1.6 to 1.7 Ma (Bernardi et al., 2018; Dal Corso et al., 2018). In North China, CPE has been constrained by biostratigraphy (increased hygrophyte plants), chemostratigraphy (four CIEs), two zircon U–Pb dates, and calculations made from deposition rates calibrated by zircon U–Pb dates, that show the CPE may have lasted ∼1.6 Ma (Lu et al., 2021).

Previous studies have shown that the enhancement of atmospheric circulation and hydrological cycles during the CPE resulted in significant changes in the terrestrial flora from xerophyte to hygrophyte-dominated (Dal Corso et al., 2020; Lu et al., 2021; Figure 3). During the CPE, climatic changes also facilitated the radiation and diversification of plant groups including gymnosperms (Bennettitales and conifers) and ferns (Hymenophyllaceae, Matoniaceae and Dipteridaceae) (Figure 3). Each of these groups subsequently became major components in Mesozoic and, in some cases, modern floras (Dal Corso et al., 2020 and references therein) (Figure 3).

The Late Triassic plant macrofossil record is sporadic and generally only recognizable at the stage level, making it difficult to accurately track floral changes through the CPE interval (from Julian to late Tuvalian substages) (Dal Corso et al., 2020). However, there is an overall transition from xerophytes to hygrophytes in the flora of the Carnian stage (Roghi, 2004; Roghi et al., 2010; Baranyi et al., 2019b; Figure 3). In South China, sphenophytes, ferns, seed ferns, and cycads/bennettitaleans are common in the Carnian Daqiaodi flora in Sichuan-Yunnan provinces and the Jiuligang flora in Hubei province (Zhou and Zhou, 1983). In Europe, the Lunz flora is representative of the Carnian macroflora and is dominated by an abundance of ferns and cycads/bennettitaleans, with sphenophytes common and ginkgophytes and conifers less abundant (Pott et al., 2008; Mueller et al., 2016b; Kustatscher et al., 2018). In North China, xerophytic floras dominated by the conifer Voltzia were widespread in the Middle Triassic Ermaying Formation (Liu et al., 2018). However, the Late Triassic Yanchang Formation flora (Sun et al., 2020) lacked these xerophytic elements and reflected more humid conditions. Using U–Pb dating and the change to more humid floras allows the CPE to be placed in the Yangchang Formation (Sun et al., 2020).

Plant macrofossils are strongly susceptible to taphonomic bias affecting their distribution and preservation potential in different sedimentary environments (e.g., DiMichele et al., 2020), so caution must be applied to literal readings of the plant macrofossil record. In contrast, habitat heterogeneity affects palynological spores and pollen fossils less, as they can be readily transported by wind and water and tend to have better taphonomic preservation potential than plant macrofossils (e.g., Marchetti et al., 2022). Spore and pollen fossils more accurately reflect broad patterns of floral change than plant macrofossils during the CPE. Palynological studies of the Western Tethys region have shown significant changes in plant types and climatic conditions during the CPE. In northeastern Italy, an increase in abundance of lycopod and fern spores from the late Julian to early Tuvalian substages indicates that relatively humid climatic conditions prevailed (Roghi, 2004). In northern Norway, elevated spore abundances (including Concavisporites and Deltoidispora) and coal seams occur in the De Geerdalen Formation, suggesting that relatively warm and humid climate conditions prevailed during the late Julian substage (Hounslow et al., 2007; Mueller et al., 2016a). In Austria, increases in hygrophyte sporomorphs (including all spores and Cycadopites, Alisporites, and Aulisporites pollen) and the hygrophyte/xerophyte (H/X) ratio were recorded in the Julian 2 substage, indicating relatively humid climatic conditions and widespread intensification of the hydrological cycle (Roghi et al., 2010; Mueller et al., 2016b; Figure 4). In Western Hungary, an increase in hygrophyte sporomorphs (including all spores identified and Cycadopites and Aulisporites pollen) and elevated H/X ratio accompanies high kaolinite concentrations in the Julian 2 substage (Dal Corso et al., 2018; Baranyi et al., 2019b). Kaolinite forms in terrestrial settings as a weathering product of felsic minerals under humid conditions (e.g., Zhang et al., 2022a), thus indicating relatively humid climatic conditions and intensive terrestrial runoff (Dal Corso et al., 2018; Baranyi et al., 2019b; Figure 4). In Poland, a marked shift occurs from xerophyte (including all spores and Aulisporites pollen) to hygrophyte (including all other pollen except Cycadopites and Monosulcites) dominated flora during the CPE, accompanied by paleosol changes from aridisols to poorly drained hydric soils, suggest relatively humid climatic conditions prevailed (Fijałkowska-Mader et al., 2021). In the Aghdarband Basin of northeast Iran, the dominance of hygrophytic plant spores and coal seams in the lowest part of the Miankuhi Formation suggest that relatively humid climatic conditions prevailed during the Julian 2 substage (Mazaheri-Johari et al., 2022).

Palynological studies of the Eastern Tethys region also have shown significant changes in plant types and climatic conditions during the CPE. In North China, high abundance of fern spores (including Cyclogranisporites, Osmundacidites, and Punctatisporites) and freshwater algae, as well as increases in hygrophyte sporomorphs (including all spores identified and Cycadopites, Alisporites, and Aulisporites pollen) and the H/X ratio, show significant increases from the Julian 2 to early Tuvalian substages and indicate humid climatic conditions (Lu et al., 2021; Figure 4). In South China, an abundance of ferns (especially Dipteridaceae/Matoniaceae) and an increase in the hygrophyte group (including ferns, horsetails, lycopsids, mosses, and seed ferns) and H/X ratio in the Ma’antang Formation during the Julian 2 substage also indicates relatively humid climatic conditions (Li L. et al., 2022; Figure 4). In the Junggar Basin of northwest China, the Huangshanjie Formation recorded a shift from a conifer-dominated forest community (with common ginkgophytes and bennettites) to a fern-dominated community, showing a change from arid to relatively humid climatic conditions during the CPE (Peng et al., 2022).

In summary, most palynological studies from Western and Eastern Tethys regions show an increase in the abundance of hygrophyte plants and an increase in the H/X ratio during the CPE, suggesting relatively humid climatic conditions were globally widespread at that time. This agrees with the sedimentary evidence from the Central European Basin (Simms and Ruffell, 1989), the Newark Basin (North America; Olsen, 1997), Morocco (Mader et al., 2017), East Greenland (Andrews and Decou, 2019), the Wessex Basin (United Kingdom; Baranyi et al., 2019a), the Ischigualasto Basin (Argentina; Mancuso et al., 2020), and the Jiyuan Basin (North China; Lu et al., 2021) that show increases in terrestrial runoff, lake deepening and/or area expansion, and prevailing relatively humid climatic conditions during the CPE. Therefore, we consider that the significant changes in terrestrial floras during the CPE represent a global humid event.

In recent years LIP eruptions have increasingly been identified as major drivers for changes in terrestrial environments and floras globally at different stratigraphic times (Dal Corso et al., 2020; Lu et al., 2020, 2021; Shen et al., 2022; Jin et al., 2022b, 2023; Zhang et al., 2022b, 2023a,b). As outlined above (see Section 3), studies of widely distributed plant macro- and microfossils have revealed significant synchronous changes in terrestrial flora during the CPE. Time-dependent coupling relationship among these changes and Hg concentration anomalies and Carbon Isotope Excursions (CIEs) in sedimentary successions and global warming (Figures 2, 3) identifies the Wrangellia LIP as a likely driver for climate changes during the CPE (Dal Corso et al., 2018, 2020; Lu et al., 2021; Mazaheri-Johari et al., 2021; Zhao et al., 2022; Jin et al., 2023).

Sedimentary Hg records serve as proxies for volcanism in deep time and reveal that there were 3–4 pulses of volcanism during the CPE interval (Dal Corso et al., 2020; Lu et al., 2021; Mazaheri-Johari et al., 2021; Zhao et al., 2022; Jin et al., 2023). Each eruption released large amounts of isotopically light CO₂ into the reservoir of the exogenic carbon cycle, which led to global warming and a significant carbon cycle fluctuation (Dal Corso et al., 2020; Lu et al., 2021; Figure 5). As observed in other regions including Western Tethys, South China, North China, and Japan, the close correspondence between Hg concentrations (or Hg/TOC peaks) and CIEs during the CPE interval indicates the entry of large amounts of isotopically light carbon into the atmosphere–ocean system directly from volcanism (Lu et al., 2021; Mazaheri-Johari et al., 2021; Zhao et al., 2022; Jin et al., 2023; Figure 2). Similarly, multiple CIEs and oxygen isotope variations from the western and eastern Tethys (Hornung et al., 2007a; Trotter et al., 2015; Sun et al., 2016; Dal Corso et al., 2020), as well as results from biogeochemical box models based on the CIEs and Hg/TOC ratios (Dal Corso et al., 2022b), provide evidence for global carbon cycle fluctuations and warming climatic conditions during the CPE. Higher temperatures promoted the evaporation of water vapor, thus increasing the water saturation in the atmosphere and the temperature differences between the oceans and the land, leading to the intensification of atmospheric circulation and hydrological cycle (Figure 5). Increased hydrological cycle and atmospheric circulation led to increased terrestrial precipitation, that in turn may have culminated in a significant shift from arid to humid climatic conditions (see Section 3).

During the CPE, changes in terrestrial climatic conditions from arid and semi-arid to humid were favorable to plant growth and would have increased terrestrial primary productivity. Increased primary productivity promoted global peat accumulation and significantly increased organic carbon amount and burial rate in the geosphere (e.g., Retallack et al., 1996; Dal Corso et al., 2020, 2022b; Lu et al., 2021). Such a change would have been critical to recommencing thick peat accumulation worldwide after the coal-gap (Figures 1, 5). Furthermore, each of the separate pulses of CPE volcanism resulted in the same general pattern, including increases in temperature and humidity, changes in floral composition with an increase in the proportion of hygrophytic plants, and also increases in primary organic productivity (Dal Corso et al., 2020; Lu et al., 2021). Increases in humidity resulted in elevating lake levels and changing large parts of sedimentary basin interiors into swamps (Dal Corso et al., 2020; Lu et al., 2021). The humid climate conditions in the CPE also promoted enhanced continental chemical weathering rates (e.g., Baranyi et al., 2019b; Pecorari et al., 2023), which led to increased nutrients yields in fluvial, swamp, and lacustrine environments. Increased nutrient levels in terrestrial watercourses, in turn, led to increased primary organic productivity and blooms of freshwater algae and eutrophication, as evidenced by the increased abundance of freshwater algae in North China and the UK (Baranyi et al., 2019a; Lu et al., 2021), and a decrease in the Carbon/Nitrogen (C/N) ratio in North China reflecting a shift from terrestrial (C-rich) to fluvial/lacustrine (N-rich) sources of organic matter in lakes (Lu et al., 2021). The increase in plant and freshwater algae primary productivity would have increased the draw-down of additional atmospheric CO₂ through plant photosynthesis and most likely played an essential role in controlling the atmospheric composition by counteracting the warming effects of greenhouse gas emissions. This may have helped drive climatic conditions toward a steady state after pulses of volcanism subsided, which may have ended the CPE once Wrangellian volcanism ceased (Figure 5).

Studies of plant macro- and microfossils widely distributed around the Tethys suggest a global nature of the environmental and floral effects of the CPE. During the CPE, significant increases in the abundance of terrestrial hygrophytes (including ferns) and freshwater algae and increased H/X ratios indicate that relatively warm and humid climatic conditions prevailed. These are best explained by an intensification of global atmospheric circulation and the hydrological cycle caused by the release of large amounts of greenhouse gases from the Wrangellia LIP. Changes in climatic conditions from arid to humid enabled increased terrestrial primary productivity and an increased proportion of hygrophytic plants and also led to changes in sedimentary environments, including rising lake water levels, lake area expansion, and forming extensive swamps in continental basin interiors. Collectively these changes promoted the reoccurrence of global peat accumulation after the “coal gap” and led to increased organic carbon abundance and burial in terrestrial sediments. Increased terrestrial primary productivity and burial would have helped offset global warming caused by greenhouse gas emissions from volcanism through a more significant draw-down of CO₂ by photosynthesis. Increased draw-down of CO₂ through the CPE likely played an important role in returning post-volcanism climates to steady states ultimately leading the end of the CPE post-volcanism.

PZ, MY, JL, LS, and JH designed the research. JL, PZ, MY, LS, and JH wrote the paper. All authors contributed to the article and approved the submitted version.

Financial support was provided by the National Key Research and Development Program of China (2021YFC2902000), the Open Fund of National Energy Shale Gas Research and Development Centre (2022-KFKT-14), the Fund of Henan University of Urban Construction (K-Q2023019), the National Natural Science Foundation of China (Grant nos. 42172196, 41772161, and 41472131), and the National Science and Technology Major Project (Award no. 2017ZX05009-002).

MY was employed by PetroChina.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.