Ocean anoxia more likely consequence than cause of Permian-Triassic mass extinction

When it comes to explaining Earth’s mass extinctions some scientists point to space. They could be close – but perhaps should consider pointing to the sky instead.

On Sunday we had an article on the millions-of-years aftermath of the Permian-Triassic mass extinction. Possible causes we discussed in August. Most scientists think a combination of pyroclastic volcanic eruptions, flood basalts, coal fires and eventually large-scale biomass degradation (through the disappearance of forests) led to a cascading rise in atmospheric and oceanic carbon concentrations – and that is what caused The Great Dying.

Now a new publication in yesterday’s PNAS seems to offer some additional support for that hypothesis, if we interpret the findings of the team of Earth scientists of the University of Arizona correctly. Through isotope measurements in the end-Permian fossil record they have reconstructed ocean oxygen conditions over 250 million years ago – and conclude anoxia seemed to follow events, not direct them:

“The intensification of oceanic anoxia coincided with, or slightly preceded, the EH [Extinction Horizon – geological layer boundary] and persisted for an interval of at least 40,000 to 50,000 y following the EH. These findings challenge previous hypotheses of an extended period of whole-ocean anoxia prior to the end-Permian extinction.”

Anoxia is both a cause and a consequence of biomass decline. It can be brought about by warming of water [carbon], but of course also by a phytoplankton decrease – one that could in turn be caused by ocean acidification [again, carbon]. Positive feedbacks could play an important role – and for all we now know in the end-Permian likely carbon started events, and ocean anoxia was one of the killer-mechanisms, gradually worsening with time – and hurting the chances for metazoan life till well into the Triassic.

What’s in it to learn for modern times? Add too much CO2 to the climate system – and anoxia can again become an ocean killer.

© Rolf Schuttenhelm | www.bitsofscience.org

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