Approximately 55 million years ago it was very bad to be a deep-sea animal. First the ocean temperature was rising. At the surface, temperatures rose anywhere from 5-10 degrees and in the deep around 5 degrees. The chemistry of the water also changed significantly. Oxygen became depleted. The ocean became more acidic. Global currents were altered such that deep-water upwelled in the Northern Hemisphere instead of the Southern for ~100,000 years.
This event is used to mark the end of the Paleocene and the start of the Eocene and thus the event is called the Paleocene-Eocene Thermal Maximum. A old instructor of mine use to refer to it as the P.E. S*&^ Storm. I digress but you get the point.
The warmer, hypoxic, acidic ocean was essentially an inhospitable place leading to a mass extinction event. 30-40% of calcareous benthic forams went extinct. Many other groups like gastropods are also hypothesized to went extinct. In general, the PETM was considered a resetting of the deep-sea fauna especially for those organisms possessing calcareous shells which don’t fair well under increased acidity. However, some groups such as isopods are predicted to have made it through the PETM. Oh yeah, mammals died off too.
Another way to grasp the PETM event is by looking at C-13:C-12. It appears that a few metabolic enzymes, but not all, in organisms prefer C-12 to C-13. The reason appears to be that C-12 is slightly more energetic and thus more likely to react. Thus organisms are differentially made up of Carbon-12 as opposed to Carbon-13. If more phytoplankton growth then theC-12 decreases. Less C-12 indicates less photosynthesis and production. Thus shells below made up of of calcium carbonate formed in high C-12 waters would also high C-12 and low C-13:C-12. A sample of marine sediments from the PETM event with the tons of foram shells would give an indication of the the phytoplankton production at this point. Samples from the PETM have very high levels of C-12. But are far greater than that would be expected just from a stop of all photosynthetic activity. Even if you burned every living thing on the planet, returning on the 12C, it would only be 1/3 that of the PETM13C:12C ratio.
What caused the PETM and the sudden rise in C-12? Greenhouse gases other than carbon dioxide. It has been argued that methane, a far more effective greenhouse gas, was released into the atmosphere causing the PETM. It is proposed that the methane source was ice in deep seafloor sediments one of the only massive pools of light carbon (C-12), the methane burp hypothesis. In once scenario, erosion or seismic activity along the continental margin may have allowed methane to escape from gas reservoirs. In an other, a gradual surface change in temperature lead to a snowball effct by leading to a thermal dissociation of methane hydrate. However, a very recent study in Science indicates that the PETM may be linked to a massive volcanic eruptions that pushed Greenland and northwest Europe apart to create the North Atlantic Ocean.
“That prehistoric volcanic activity released more than 2000 gigatonnes (billion metric tons) of carbon into the oceans and atmosphere in the form of methane and carbon dioxide – two potent greenhouse gases,” said Michael Storey of Roskilde University in Denmark, the study’s lead author. “The carbon probably came from the heating of earlier deposits of decayed organic matter – similar to deposits in the Atlantic and North Sea we tap today for oil and gas.”
You can have some confidence in the dating of the ash layer as…
That dating method, performed in labs at the three universities, renders precise ages in geological time frames, based on known decay rates of potassium to argon trapped in the volcanic material.
Foram image from http://www.palmod.uni-bremen.de/FB5/geochron/Robert/RPSpeb.html