Vegetation response to an Early Jurassic climate change and environmental perturbation

The Toarcian oceanic anoxic event (T-OAE, ~183 Ma) was marked by a rapid global warming, pronounced negative carbon-isotope excursion, ocean deoxygenation, volcanic activity and turnover of marine and terrestrial biota. This warming world affected global weather patterns leading to increased seasonality and storminess, with erratic precipitation patterns, climatic extremities, enhanced hydrological cycle and runoff. The biotic feedbacks during these Mesozoic hyperthermals are important archives of how the Earth’s ecosystems respond to severe environmental crisis. We have investigated the link between geochemical climate proxies Sr/Cu, Rb/Sr and the vegetation composition from the palynological dataset (fossil spore and pollen) in the lacustrine Anya section, from the Ordos Basin, North China. The Ordos Basin was situated at low-middle latitudes during the Mesozoic and spread across the warm-temperate to subtropical climate belts. The rise in the thermophilic drought-adapted Cheirolepidiaceae conifer and cycad pollen during the T-OAE negative carbon excursion is paralleled by the decrease in more water-dependent ferns and seed ferns simultaneously with a shift to a more seasonal semihumid–semiarid climate indicated by the Sr/Cu, Rb/Sr ratios. The vegetation experienced only temporal biodiversity losses but the decrease of mainly mid-canopy and understory elements indicates loss on functionality in the terrestrial ecosystem. This shift to a more open landscape reduced the slope stabilization function of the vegetation that increased the run-off rate from the hinterland during high rain periods. In addition, the proliferation of Cheirolepidiaceae conifers with low stomatal density and lower transpiration rate may have enhanced the weathering and transport of sediments and nutrients into the lake. The T-OAE scenario resonates well with climate model projections that suggests an increase in the number of extreme precipitation events, and longer and more frequent droughts for the future of forest ecosystems. The results even suggest that changes in vegetation composition may increase the severity and consequences of the ongoing climate change.