Volcanoes to blame for Cretaceous ocean anoxic event

Research published in a letter in Nature Geoscience: around 94.5 million years ago volcanism released large volumes of sulphur into the atmosphere which triggered huge phytoplankton blooms, which in turn deprived the oceans of oxygen and triggered extensive marine extinctions. The abstract tells it like this:

During the Cretaceous period (~145–65 million years ago), there were several periods of global ocean anoxia, each lasting less than one million years. These events, known as ocean anoxic events, were marked by significant increases in organic carbon burial, and are generally attributed to increased primary productivity in surface waters. The details underpinning the initiation, maintenance and termination of these events, however, remain equivocal. Here we present sulphur isotope data spanning the Ocean Anoxic Event 2 (about 94.5 million years ago) from sedimentary rocks in Colorado that were formed in the Western Interior Seaway; this seaway ran north–south, splitting North America during the Cretaceous. Sulphate levels increased rapidly from relatively low background levels at the onset of the event, most likely from the release of sulphur by massive volcanism, and fell during the anoxic event. We infer that the input of sulphate facilitated increased carbon remineralization, which enhanced nutrient recycling and increased global primary productivity, eventually resulting in widespread ocean anoxia. Our scenario indicates that Ocean Anoxic Event 2 may have persisted until sulphate levels were stabilized by the formation and burial of the sulphur mineral pyrite, which returned primary productivity to background levels. We suggest that fluctuations in sulphate levels may have regulated the marine carbon cycle during past periods of low oceanic sulphate concentration.

And that is why geoengineering the climate with artificial volcanoes is a really bad idea: ‘Like the mid-Cretaceous ocean, most modern lakes are poor in sulphate, so it’s possible that geoengineering the climate could trigger blooms and ultimately anoxia in some lakes’ says researcher Matthew Hurtgen of Northwestern University. ‘We hack the climate at our peril’, warns New Scientist. ‘Volcanoes spewed so much sulphate into the atmosphere 94 million years ago that the oceans were starved of oxygen and 27 per cent of marine genera went extinct. Geoengineering our climate could inflict a similar fate on some lakes’. Such is the climate change dilemma: in trying to avoid (for example) climate-induced tectonic-volcanic geo-apocalyptic mega-mayhem we may cause geoengineering-induced volcanic-toxic extinction-anoxic mega-mayhem instead. Agh.

• Derek D. Adams, Matthew T. Hurtgen & Bradley B. Sageman, ‘Volcanic triggering of a biogeochemical cascade during Oceanic Anoxic Event 2’, Nature Geoscience, 31 January 2010 [doi:10.1038/ngeo743]. Abstract.

(There’s more on volcanism triggering the Cretaceous anoxic events in Nature, 17 July 2008, and Ole Nielsen has a great blog post: Late Cretaceous Anoxic Event.)