Oceanic Anoxic Events

Here is one way of looking at oceanic anoxic events. Assume that the human race/earth releases a huge volume of carbon dioxide during an interval of excessive man-made greenhouse gases/volcanism; global temperatures rise due to the greenhouse effect; global weathering rates and fluvial nutrient flux increase; organic productivity in the oceans increases; organic-carbon burial in the oceans increases (OAE begins); carbon dioxide is drawn down (inverse greenhouse effect); global temperatures fall, and the ocean–atmosphere system returns to equilibrium (OAE ends). In this way, an oceanic anoxic event can be viewed as the Earth’s response to the injection of excess carbon dioxide into the atmosphere and hydrosphere.

Past Occurrence

Oceanic Anoxic Events occurred only during periods of very warm climate characterised by high levels of carbon dioxide (CO2) and mean surface temperatures probably in excess of 25 ° C.

Atmospheric effects

A model put forward by Lee Kump, Alexander Pavlov and Michael Arthur in 2005 suggests that oceanic anoxic events may have been characterised by upwelling of water rich in highly toxic hydrogen sulfide gas which was then injected into the atmosphere. This phenomenon would likely have poisoned plants and animals and caused mass extinctions. Furthermore, it has been proposed that the hydrogen sulfide rose to the upper atmosphere and attacked the ozone layer, which normally blocks the deadly ultraviolet radiation of the Sun. The increased UV radiation caused by this ozone depletion would have amplified the destruction of plant and animal life. Fossil spores from strata recording the Permian extinction show deformities consistent with UV radiation. This evidence, combined with fossil biomarkers of green sulfur bacteria, indicates that this process could have played a role in that mass extinction event, and possibly other extinction events. The trigger for these mass extinctions appears to be a warming of the ocean caused by a rise of carbon dioxide levels to about 1000 parts per million.


Oceanic Anoxic Events have had many important consequences. It is believed that they have been responsible for mass extinctions of marine organisms both in the Paleozoic and Mesozoic. Apart from possible atmospheric effects, many deeper-dwelling marine organisms could not adapt to an ocean where oxygen penetrated only the surface layers.

Another, economically significant consequence of oceanic anoxic events is the fact that the prevailing conditions in so many Mesozoic oceans has helped produce most of the world's petroleum and natural gas reserves. During an oceanic anoxic event, the accumulation and preservation of organic matter was much greater than normal, allowing the generation of potential petroleum source rocks in many environments across the globe.

Some 70 percent of oil source rocks are Mesozoic in age, and another 15 percent date from the Paleogene: only rarely in colder periods were conditions favourable for the production of source rocks on anything other than a local scale. http://en.wikipedia.org/wiki/Anoxic_event

ABC Television screened the documentary ‘Crude – the incredible journey of oil’ (which can be viewed at http://www.abc.net.au/science/crude) on 24/5/07 and features an interview with Dr Lee Kump (re above) about Anoxic Ocean Events.

Crude’ is a ‘must see’ film and Lee Kump’s interview is most enlightening.