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Sulphate the suspect in ancient ocean die-off

Just how volcanoes managed to wipe out much of the life in the oceans some 94.5 million years ago has been teased out by a new paper in Nature Geoscience.

The Cretaceous period saw a number of bouts of very low oxygen levels in the world’s oceans, with obvious negative consequences for animals dwelling in those oceans. Previous work has pegged magma as responsible for one of these ‘anoxic events’ and now Matthew Hurtgen and colleagues at Northwestern University in Illinois suggest exactly why what the volcanoes were doing created the problem.

They looked at sulphur in sediments from around the time of the Oceanic Anoxic Event 2 and found a huge increase in sulphate at the start of this event, levels then fell off as the event continued. Their conclusion is that volcanoes released a huge amount of sulphate into the Cretaceous seas, where levels of this element may have been very low compared with today.

More sulphate enhanced the recycling of nutrients in the oceans and enabled more life to grow in surface waters. Then the bodies of these animals drifted down to the depths, feeding further life in the depths. This deep living beastie bloom couldn’t last though: the animals used up all the oxygen and the anoxic event was upon them.

“Sulphates help the ocean hang on to its phosphorous,” Hurtgen told the Times. “Along with nitrogen and iron, phosphorous is a key limiting nutrient in the ocean. Without it phytoplankton cannot grow. But when massive volcanism delivered more, it changed the amount of phosphorus available, and drove these anoxic events.”

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