Palynological dynamics in the late Permian and the Permian–Triassic transition in southwestern China

Abstract

The Permian-Triassic mass extinction (PTME) is regarded as the largest biotic crisis of the Phanerozoic. However, the influence of the terrestrial ecological disturbance on plants remains controversial. Here we study the late Permian to the Early Triassic palynological successions from three borehole sections drilled through the entire Lopingian (the late Permian) and the Permian–Triassic transitional strata in southwestern China. Analyses of palynomorph composition and relative abundance allow us to identify four distinct palynofloral assemblages, which include, in ascending order, the Tripartites cristatus var. minor - Torispora laevigata (ML) assemblage, the Crassispora orientalis - Anticapipollis tornatilis (OT) assemblage, the Lundbladispora communis - Aratrisporites yunnanensis (CY) assemblage, and the Pteruchipollenites reticorpus - Protopinus fuyuanensis (RF) assemblage. These palynological assemblages, together with recently updated age data, further improve our understanding of vegetation dynamics around the PTME. The ML and OT assemblages from the Xuanwei Formation are dominated by gigantopteris and fern peat-forming rainforest and reflect warm and humid paleoclimate conditions preceding the PTME. At the bottom of the overlying Kayitou Formation, the abrupt replacement of the diverse sporomorph assemblage by the assemblage containing herbaceous communities reveals a dramatic floral disruption. The CY assemblage from the bottom part of the Kayitou Formation marks the destruction of Gigantopteris peat-forming ecosystems, and only a few species of Gigantopteris were retained in this assemblage. Subsequently, in the Kayitou Formation notable quantities of gymnosperm pollen dominate the RF assemblage. Many gymnosperms from the RF assemblage were present in the older assemblages in low abundances but they persisted through the ecological disturbance interval and rebounded in the early Triassic RF assemblage. The drought-tolerant plants growing in uplands were likely the first batch of plants to adapt and recover after the extinction event. In the upper part of the Kayitou Formation and the overlying Dongchuan Formation, spore and pollen fossils are absent, which may suggest the collapse of the local terrestrial ecosystem and may also indicate unsuitable conditions for microfossil preservation. We conclude that the PTME caused a sharp decrease in plant species and changes in vegetation-community compositions, but it did not immediately eradicate the terrestrial ecosystem in southwestern China. More drought-tolerant plants persisted beyond the first stage of the terrestrial crisis, but with gradual environmental degradation, terrestrial vegetation eventually collapsed in the Early Triassic. This observation supports the hypothesis that the extinction process of terrestrial vegetation was a two-staged and longer-duration event in southwestern China.