Comparison of plant fossil assemblages across the Permian–Triassic Boundary with simulated biomes in alternative climatic states

Terrestrial floras underwent major changes across the Permian–Triassic Boundary (PTB), as observed in the distribution of plant fossil assemblages before (Wuchiapingian, Changhsingian) and after (Induan, Olenekian, Anisian) the PTB [1]. While marine animals suffered the most severe mass extinction event at the Permian–Triassic transition, terrestrial plants were marked by extreme shifts in their distribution and composition which can arise when abiotic drivers reach a critical threshold or tipping point [2].  Interestingly, using a coupled atmosphere-ocean-sea ice-land configuration of the MIT general circulation model, three alternative climatic states have been found for the Permian–Triassic paleogeography [3], namely a cold state with perennial ice in the northern hemisphere reaching 40°N and global mean surface air temperature (SAT) of 16–18 °C, a hot state without ice and SAT larger than 30 °C, and an intermediate warm state. These states turn out to be stable over a common range of atmospheric CO2 content, thus allowing for hysteresis paths and tipping points in abiotic drivers, such as the global surface air temperature. Through asynchronous coupling with the vegetation model BIOME4, the distribution of the biomes corresponding to each climatic state has been obtained. Here, we perform a detailed comparison between the biomes corresponding to the alternative climatic states and the distribution of plant fossil assemblages from the Wuchiapingian to the Anisian. For each observed assemblage, the geodetic distance to the closest simulation grid point with the same biome is calculated. This allows us to quantify, through statistical significance tests, the resemblance between the simulated biomes and the observed distribution of plant fossil assemblages, and thus to determine the climatic state which minimizes the mean distance at each geological period. We find a clear signature of climatic shifts from a cold state in the Changhsingian to a hot state in the Olenekian, whereas during the earliest Triassic (Induan) the attribution to a particular climatic state is not univocal, strong climatic oscillations being still present in the aftermath of the PTB.    References [1] H. Nowak, C. Vérard, E. Kustatscher, Frontiers in Earth Science 8, 613350 (2020) [2] E. Schneebeli-Hermann, Frontiers in Earth Science 8, 588696 (2020) [3] C. Ragon, C. Vérard, J. Kasparian, M. Brunetti, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2023-1808, 2023