Three to four million years before the Permian-Triassic extinction, also known as the Great Dying, the seas were already becoming oxygen-starved and sour, said the study in the journal Earth and Planetary Science Letters.
Changqun Cao of the Chinese Academy of Sciences in Nanjing and a team of researchers studied rock samples drilled in central China from the late Permian and early Triassic periods. Rocks from the extinction itself date to 252.2 million years ago, and show several chemical signs of catastrophe.
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The team found that a few of the compounds extend back millions of years before the main extinction event. Known as biomarkers, the chemicals are evidence that Chlorobium — green sulfur bacteria — were living in the oceans.
“These are strict anaerobic organisms,” Roger Summons of the Massachusetts Institute of Technology in Cambridge said. “They use sulfate dissolved in water for respiration instead of oxygen, and produce hydrogen sulfide.”
Hydrogen sulfide is a lethal poison to most oxygen-breathing animals. Today, it is common in lakes or seas when circulation is poor, oxygen levels plummet, and anaerobic bacteria take over.
…The finding gets to the heart of a long debate among extinction researchers: Was hydrogen sulfide from the ocean enough to extinguish up to 90 percent of life on Earth, or was the eruption of the massive Siberian Traps supervolcano responsible for the killing
…The answer may be “both.” If ocean-going bacteria were producing hydrogen sulfide for 3 million years or more, it would have put enormous stress on plants and animals and made them prone to devastation when the volcano finally blew its top.