The end-Permian mass extinction, or “great dying”, saw more than 90% of all life wiped out a quarter of a billion years ago – the biggest mass extinction Earth has ever seen – and it took a slow 10 million more to recover.
By looking at changes to the chemistry of pyrite (better known as “fool’s gold”), Guijie Zhang from the University of Science and Technology of China and colleagues suggest sulfur-saturated water from deep in the ocean mixed with shallower fresh water, and essentially poisoned most of what lived in it. The work was published in the Proceedings of the National Academy of Sciences.
To obtain records of Earth conditions all those millions of years ago, the team unearthed pyrite from rocky outcrops in Canada and Japan that were once submerged in the Panthalassic Ocean – the enormous body of water created at the start of the Permian Period when the landmasses dubbed Gondwana and Euramerica collided and formed the supercontinent Pangea.
The pyrite, they found, contained a combination of “light” and “heavy” sulfur isotopes, indicating turbulent oceanic activity “before, during, and after the end-Permian mass extinction”.
The presence of the negatively charged light isotopes, the team concluded, is evidence of shoaling – the process by which waves entering shallower depths change height, affecting both velocity and density. (The behaviour of a tsunami reaching the shore is a perfect, if devastating, example.)
The wave patterns caused periodic changes in the water’s sulfur levels, causing the ocean to fluctuate between euxinic (oxygen-poor) and oxic (oxygen-rich) states. These fluctuations, write Zhang and colleagues, continued beyond the Permian and into “the earliest Triassic, providing evidence of a causal link between incursion of sulfidic waters and the delayed recovery of the marine ecosystem.”
The new data on the elements present in the ocean sediment layer dating from the end-Permian certainly presents another factor to consider when investigating the great dying, according to palaeontologist John Long at Flinders University in Adelaide, Australia, who was not involved in the study.
But he adds that the research doesn’t offer any explanation for why land-based plants and animals were eliminated as well as marine life.
“It’s an interesting paper that presents another piece of evidence into the mix, but to me it’s not a convincing argument that solves what caused the Permian extinction,” he says.
“It’s very important that we’re getting this spike of hydrogen sulfide with water in the ocean but where’s it coming from? Is it coming from volcanic eruptions or from another source?”
Global warming today is contributing to the development of sulfurous zones on continental shelves, threatening marine life, Zhang and colleagues warn.