Earth’s largest mass extinction: How did it happen?
About 250 million years ago, at the end of the Permian and start of the Triassic period, Earth experienced the most severe environmental crisis to date. Over 95 % of its marine species and 70 % of its terrestrial species disappeared, resulting in the greatest mass extinction seen in geologic time. According to scientists, the movement of magma under Earth’s crust in the volcanic rock region known as the Siberian Traps played an important role in the Permian-Triassic extinction. However, a clear picture of the precise cause and subsequent chain of events that led to the mass extinction had yet to be formed. Using advanced analytical methods and geochemical modelling, researchers supported by the EU-funded BASE-LiNE Earth project have now been able to biochemically reconstruct the events that led to Earth’s biggest mass extinction. Their findings have been published in the journal ‘Nature Geoscience’.
The story told by seawater pH
To create a new seawater pH record, the BASE-LiNE Earth researchers used fossil brachiopod shells. Brachiopods “are clam-like organisms that have existed on Earth for more than 500 million years,” explained first author Dr Hana Jurikova in an article posted on the ‘SciTechDaily’ website. “We were able to use well-preserved brachiopod fossils from the Southern Alps for our analyses. These shells were deposited at the bottom of the shallow shelf seas of the Tethys Ocean 252 million years ago and recorded the environmental conditions shortly before and at the beginning of extinction,” she observed. The seawater pH record, which was derived by measuring boron isotopes in the fossil shells, shows a marked decline in pH levels at the time the mass extinction occurred. The above results were achieved using high-precision isotope analyses together with high-resolution microanalyses using a large-geometry secondary ion mass spectrometer. “With this technique, we can not only reconstruct the evolution of the atmospheric CO2 concentrations, but also clearly trace it back to volcanic activity,” stated co-author Dr Marcus Gutjahr in the same article. “The dissolution of methane hydrates, which had been suggested as a potential further cause, is highly unlikely based on our data,” he went on to say.
A clearer picture through geochemical modelling
The team then fed this data and additional carbon isotope data into a model that simulated the chemical reactions affecting the geological systems at the time. They found that the initial ocean acidification was closely linked to substantial carbon degassing from the Siberian sill intrusions – bodies of igneous rock that formed between surrounding rock layers. Already fatal to a lot of marine life, the release of CO2 into the atmosphere also resulted in higher temperatures and increased rates of chemical weathering on land. Over time, this caused large-scale deoxygenation and scattered sulfide poisoning of the oceans. “This domino-like collapse of the inter-connected life-sustaining cycles and processes ultimately led to the observed catastrophic extent of mass extinction at the Permian-Triassic boundary,” noted Dr Jurikova. Thanks to the new techniques developed as part of the BASE-LiNE Earth (Brachiopods As SEnsitive tracers of gLobal marINe Environment: Insights from alkaline, alkaline Earth metal, and metalloid trace element ratios and isotope systems) project, it’s now possible to create detailed reconstructions of environmental processes that happened millions of years ago. The 4-year project ended in 2018. For more information, please see: BASE-LiNE Earth project website
Keywords
BASE-LiNE Earth, mass extinction, Permian, Triassic, fossil