Mesozoic C-cycle perturbations and climate: evidence for increased resilience of the Cretaceous biosphere to greenhouse pulses1

The Mesozoic C-isotope record traces the history of the global carbon cycle. Major perturbations of the carbon cycle triggered by extraordinary volcanic activity are recorded in negative spikes coupled with positive C-isotope excursions. Prominent examples are the C-isotope anomaly events at the Permian–Triassic boundary and at the Triassic–Jurassic boundary, in the Toarcian or in the Aptian. While the major volcanic pulses at the Permian–Triassic and Triassic–Jurassic boundaries are considered as the main trigger of mass extinctions, carbon cycle perturbations in the Jurassic and Cretaceous were not accompanied by comparable extreme changes in marine or terrestrial biota. The data suggest either that changes in degassing of large igneous provinces explain the difference in the biota’s response to perturbation or (and) that the resilience of the Mesozoic biosphere to volcanic pulses changed through time. It is hypothesized that two factors contributed to the increased resilience of the biosphere in the Late Mesozoic: (1) starting in the Middle Jurassic, pelagic carbonate developed into an important sink of CO₂ in the long-term carbon cycle, contributing to increased resilience of the carbon cycle to perturbations, and (2) increasing fragmentation of Pangea resulted in the establishment of a transequatorial current system coupled with equatorial upwelling. This circulation pattern was intensified during greenhouse pulses. Increased marine productivity and widespread basinal anoxia favoured burial of organic carbon. Increased resilience of the Cretaceous biosphere against volcanic activity may explain why Deccan Trap volcanism was no longer sufficient as a trigger of a mass extinction at the Cretaceous–Paleogene boundary.