Cretaceous oceans

Nutrient-driven anoxia

Paleoceanography http://doi.org/jsk (2012)

About 93.5 million years ago, the world's oceans became depleted in oxygen. According to numerical simulations, a rise in the availability of nutrients in the surface oceans could have driven much of the oxygen loss at this time.

Fanny Monteiro of the University of Bristol, UK, and colleagues compared geochemical data recording seafloor oxygenation across this interval, known as Ocean Anoxic Event 2, to biogeochemical simulations from an Earth system model. The climate is known to have warmed around the time of the anoxic event, but the modelling showed that oceanic temperature changes alone were not sufficient to reproduce the observed spread of anoxia. Instead, high levels of marine productivity in the surface ocean could have driven the bulk of the oxygen depletion — the decay of these organisms at depth consumes oxygen. This productivity would have been fuelled by the delivery of nutrients by enhanced continental weathering under greenhouse climate conditions and the recycling of phosphorus in increasingly anoxic sediments.

The simulations also imply that at least 50% of the global ocean volume was anoxic at the height of the event, with anoxic conditions even reaching the sunlit surface waters in parts of the east Pacific Ocean.

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Newton, A. Nutrient-driven anoxia. Nature Geosci 5, 844 (2012). https://doi.org/10.1038/ngeo1666

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