Rapid global warming and oceanic oxygen deficiency during the Early Jurassic Toarcian Oceanic Anoxic Event at around 183 Ma is associated with a major turnover of marine biota linked to volcanic activity. The impact of the event on land-based ecosystems and the processes that led to oceanic anoxia remain poorly understood. Here we present analyses of spore–pollen assemblages from Pliensbachian–Toarcian rock samples that record marked changes on land during the Toarcian Oceanic Anoxic Event. Vegetation shifted from a high-diversity mixture of conifers, seed ferns, wet-adapted ferns and lycophytes to a low-diversity assemblage dominated by cheirolepid conifers, cycads and Cerebropollenites-producers, which were able to survive in warm, drought-like conditions. Despite the rapid recovery of floras after Toarcian global warming, the overall community composition remained notably different after the event. In shelf seas, eutrophication continued throughout the Toarcian event. This is reflected in the overwhelming dominance of algae, which contributed to reduced oxygen conditions and to a marked decline in dinoflagellates. The substantial initial vegetation response across the Pliensbachian/Toarcian boundary compared with the relatively minor marine response highlights that the impacts of the early stages of volcanogenic global warming were more severe for continental ecosystems than marine ecosystems.
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The authors declare that the data supporting the findings of this study are available within the article and Supplementary Information. All materials (rock samples and slides) are housed in the collections of the Natural History Museum, London.
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We are grateful to W. Foster, D. Murphy and M.-E. Clemence for their help with fieldwork. We thank P. von Knorring for artwork in Fig. 4. This research was funded by the Wenner-Gren Foundation (grant no. UPD2017-0155), the Swedish Research Council (grant no. VR 2015-04264), Lund University Carbon Cycle Centre and a Natural Environment Research Council (NERC) grant to R.J.T. (grant no. NE/I005641/1).
Supplementary figures, tables and references
Raw palynological counts. ‘p’ refers to taxa that are present in slides but not in counts
Raw palynofacies counts
nMDS axis scores of samples for Fig. 3 and Supplementary Fig. 3.