Life on Earth has never been so close to an end as during the environmental catastrophe that marked the Permian-Triassic boundary – 252 million years ago. Scientists have long speculated what could have triggered the sudden disappearance of so many organism groups – more than 95% of marine and 70% of terrestrial species went extinct. Among the favoured hypotheses have been large-scale volcanism, methane release from hydrate mounds on the seafloor, and an asteroid impact similar to that which ended the reign of the dinosaurs 66 million years ago. The latter has been, however, largely rejected in the recent years as no reliable evidence, direct or indirect, of the impact has been found.
To illuminate the causes and consequences of the extinction, we used an innovative approach to reconstruct the seawater pH (acidity) from boron isotope measurements in well-preserved fossil brachiopod shells. Seawater pH is a critical parameter; first, because it has direct implications for marine life. Second, because the ocean and the atmosphere are closely coupled and CO2 is readily exchanged between them, we can use data on ocean pH to directly reconstruct the atmospheric CO2 levels. We paired our pH data with the global carbon isotope records, and assimilated it into a model that quantified both the source and volume of CO2 over the extinction period.
Our results showed that the extinction was initiated by an unprecedented pulse of CO2 to the atmosphere, the result of organic-rich sediments combusted by volcanic intrusions. It was hence not the CO2 released directly by the volcanic activity from magma degassing, for this would have been likely insufficient to produce such a rapid and deadly mass extinction. It was when the magma reached and started burning extensive ancient coal deposits that a sudden and immense amount of CO2 was released. Our estimated total emissions amount is more than 40 times the amount of carbon available in modern fossil fuel reserves, including that already burned since the industrial revolution. This is likely the biggest event of carbon release since the evolution of animal life.
To further understand how the CO2 release led to the extinction, we studied its feedbacks in the model and reconstructed a scenario that integrated our data with available geological, geochemical and palaeontological evidence. Our results showed that the CO2 release led to a strong greenhouse effect on the marine environment. This severe heating and acidification of the oceans wiped-out shallow-water organisms in the initial stage of the extinction. Hot climate and high atmospheric CO2 levels also had a dramatic effect on chemical weathering on land – the process of decomposition of minerals and rocks. As a result, over thousands of years more and more nutrients reached the oceans. This allowed plankton communities to thrive and accumulate, which also aided to remove a large part of the CO2 excess from the atmosphere. However, dying and sinking plankton decomposed at depth consuming oxygen from the oceans. This subsequently killed-off deeper dwelling organisms.
Jurikova, H. et al. Permian-Triassic mass extinction pulses driven by major marine carbon cycle perturbations. Nat. Geosci. 13, 745-750 (2020).
Hana Jurikova, The Science Breaker, 21 June 2021. Full article.