Episodes of ocean anoxia to occur and to persist for 105 – 106 years, these OAEs (Ocean Anoxic Events) being particularly associated with warm “greenhouse” climates.

September 2016: Professor Andrew Watson FRS, (University of Exeter, UK) speaks at Royal Society event Ocean ventilation and deoxygenation in a warming world. This video is no longer available.

Abstract
The major biogeochemical cycles which keep the present-day Earth habitable are linked by a network of feedbacks which has led to a broadly stable chemical composition of the oceans and atmosphere over hundreds of millions of years.

This includes the processes which control both the atmospheric and oceanic concentrations of oxygen. However, one notable exception to the generally well-behaved dynamics of this system is the propensity for episodes of ocean anoxia to occur and to persist for 105 – 106 years, these OAEs (Ocean Anoxic Events) being particularly associated with warm “greenhouse” climates.

OAEs are, we believe, amplified by positive feedbacks on the nutrient content of the ocean: low oxygen promotes the release of phosphorus from ocean sediments, which increases ocean productivity and drives more anoxia in the subsurface water, leading to a potentially self-sustaining condition of deoxygenation.

The rapidly increasing degree of ocean deoxygenation occurring today as a result of the warming climate could conceivably result in such a very long-lasting and unpleasant consequence for the Earth’s biogeochemical cycles which underpin the “life support” system of the biosphere. https://royalsociety.org/science-events-and-lectures/2016/09/ocean-ventilation/