Palynology and vegetation dynamics across the Permian–Triassic boundary in southern Tibet

Abstract

The end-Permian mass extinction (EPME) is regarded as the most severe biotic crisis in Earth history, which is particularly well-documented in the marine fossil record. Despite compelling evidence that dramatic global environmental changes probably drove this extinction event, its effects on terrestrial plants are still highly controversial. Here we present Changhsingian (latest Permian) to late Smithian (latest Early Triassic) composite palynological successions from the lagoonal to open marine Qubu and Tulong sections in southern Tibet. Quantitative analyses of palynomorph distributions (range charts and relative abundance data) allow us to differentiate five distinct, informal palynofloral assemblages in ascending order: the Scheuringipollenites ovatus–Vitreisporites pallidus (OP) assemblages, the Reduviasporonites catenulatus (С) assemblage, the Endosporites papillatus–Pinuspollenites thoracatus (PT) assemblages, the Lundbladispora brevicula–Densoisporites nejburgii (BN) assemblages and the Falcisporites nuthallensis–Alisporites landianus (NL) assemblages. These palynological assemblage intervals and the associated ammonoid and conodont biostratigraphy resolve vegetation changes across the Permian–Triassic boundary (PTB) on the northern Tethyan margin of Gondwanaland in great detail and high resolution. Principal Component Analysis (PCA) of palynological data indicates that the OP assemblages, which are dominated by non-taeniate bisaccate pollen, represent a vegetation in a cool and dry climate during the Changhsingian. In the Qubu Section, the palynological turnover from the OP assemblages to the C assemblage 0.81 m below the PTB, coincides with abrupt sea-level fluctuations in the latest Permian. As this turnover further coincides with an extreme negative δ13Corg shift, a great abundance of amorphous organic matter (AOM) and marine acritarchs as well as the frequent occurrence of Reduviasporonites catenulatus, the C assemblage is tentatively interpreted to represent the “fungal spike” reported worldwide at the onset of the EPME. In southern Tibet, the EPME appears to result only in a short-term disturbance and range contraction of land-plant communities. Surprisingly, the early Dienerian PT assemblages, postdating the EPME by about 500Kyrs, closely resemble the late Changhsingian OP assemblages, suggesting a resurgence of those land plant groups that were dominant during the late Changhsingian. This recovery was disrupted with the onset of the late Dienerian BN assemblages in the Qubu Section, which is represented by a peak abundance of cavate trilete spores, reflecting large-scale deforestation. For nearly one million years, the regional vegetation appears to have been dominated by pioneer communities of quillworts and spike mosses. Only towards the end of the Smithian, bisaccate gymnosperm pollen regained dominance with the onset of the NL assemblages in the Tulong Section. Integrating the results of the Principal Component Analysis on our new palynological data and conodont-apatite oxygen-isotope data from neighbouring sections in Pakistan, the contraction of gymnosperm forests during the early Dienerian–Smithian in southern Tibet is interpreted to be accompanied by an abrupt change to an episode with a warmer and more humid climate.This observation supports the hypothesis that the secondary eruptions of the Siberian Large Igneous Province during the Induan led to long-term global warming and, as a side effect, to more humid conditions in the southern temperate zone as a result of altered precipitation and evaporation patterns.