Anoxic event
An anoxic event occurs when the Earth's oceans become completely depleted of O2 below the surface levels.
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Occurrence
Anoxic events occur only during periods of very warm climate characterised by high levels of CO2, usually above 1800 ppmv, and mean surface temperatures of around 22 ° C (Quaternary levels are 280 ppmv and 13 ° C). Thus, anoxic events have been concentrated in the Paleogene, Cretaceous and Jurassic, when numoerous ones have been documented, but earlier ones are known from the late Triassic, Devonian and Ordovician.
Major anoxic events occurred between 183 and 180 million years ago, and many times in the period from 120 to 55 million years ago. Typically, each anoxic event lasted for about three or four million years and then oxygenation of the oceans would return - though most likely never to the same level as observed in the "icehouse" world today.
Major anoxic events in the Cretaceous
Of all anoxic events known to have occurred, only those in the Cretaceous (Oceanic Anoxic Event: OAE) have been extensively studied. Some of them are seen as being related not only to changes in climate, but also the opening of the Atlantic Ocean by continental drift.
- OAE1
- Early Aptian (120 million years ago)
- Aptian/Albian boundary (113 to 109 million years ago)
- Late Albian (99 million years ago)
- OAE2
- Turonian (93-91 million years ago)
- OAE3
- Coniacian-Santonian (89 to 84 million years ago; a very long period of permanent anoxia in the oceans but one poorly studied)
- OAE4
- Late Campanian (74 million years ago; poorly studied)
Mechanism
The mechanism by which anoxic events occur is still very poorly understood. It is believed that, with the oceans very warm, and, in the complete absence of polar ice caps they covered large areas that are now dry land. Because of the warmth, much less oxygen could be dissolved in the water, and the small amount was largely used up by marine animals such as corals at very shallow depths, thus preventing oxygen from penetrating to the sea bottom thousands of metres below.
This absence of oxygen from the ocean bottoms meant that the decomposers currently present in deep-sea environments below the Oxygen minimum zone could not survive. Also, because warming during anoxic events was concentrated in high latitudes (which were up to 40 ° C warmer than today) winds were generally much less vigourous than today, so that ocean currents were very weak and upwelling completely absent, especially given the polar water was too warm to sink effectively. Apparently, as far as it did sink, it totally absorbed all the oxygen from the atmosphere so that any organisms from shallower waters that fell into the deep would be broken down in an anoxic environment.
This helped sustain the anoxic event because productivity around continental shelves was probably very low. This was because almost no mountains were being built by continental drift and hence very little new soil was being weathered to fertilise the oceans: thus absorption of CO2 was much reduced and more remained in the atmosphere. It is probable that anoxic events ended because the difference in temperature between low and high latitudes became so low that equatorial air would no longer flow poleward.
Consequences
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. This is natural when one considers that most marine organisms cannot adapt to an ocean where oxygen can - at best - reach only the surface layers.
Another, much more useful, consequence of anoxic events has been the fact that the anoxia of so many Mesozoic oceans has produced most of the world's petroleum and natural gas deposits. During an anoxic event, the conversion of vegetation to oil would be greatly increased in efficiency (the ancient Tethys Sea was always anoxic) so that even with very low productivity fossil fuel formation would become much more efficient relative to the original biomass. This is why some 70 percent of oil deposits are Mesozoic in age, and another 15 percent date from the warm Paleogene: only rarely in colder periods is such conversion effective enough to produce useful deposits.
External links
Hot and stinky: The oceans without oxygen
Seawater strontium isotopes, oceanic anoxic events, and seafloor spreading
Cretaceous climate-ocean dynamics