PETM Part 2??

The Cenozoic began with the Paleocene period which started with a relatively cool climate but temperatures would abruptly rise towards its end. This is known as the Paleocene-Eocene Thermal Maximum (PETM). The PETM occurred around 55 million years ago and is one of the most intensely studied climatic events in geological history, characterised by a remarkable temperature rise of 5°-8°C.

The above image depicts major climatic events throughout the Cenozoic, including the PETM. Taken from Zachos et al. 2001.

The exact cause of the PETM remains controversial, although it is generally agreed that there was a mass injection of GHG’s into the, yet the source of the gases remains elusive. One suggestion is the emplacement of a series of kimberlite pipes around northern Canada which may have triggered an increase of ocean temperatures, leading to the release of methane from deep sea clathrates (see the Permian extinction). Other causes of mass GHG include intensive volcanism, bolide impacts or changes in oceanic circulation.

Whilst the cause of this temperature rise may be disputed, its effects are evident. The global climate became warmer and more humid and some areas of the Atlantic ocean suffered from anoxia (possibly due to disruptions in thermohaline circulation resulting from increasing temperatures). Interestingly, organisms responded differently to the PETM; angiosperms diversified with tropical rainforests flourishing in the new climate whereas calcareous marine organisms (such as foraminifera) suffered high levels of extinction in warmer, more acidic oceans.

The PETM is an interesting analogue to modern anthropogenic warming and environmental change. Whilst there are clear similarities between the two (e.g. increasing temperatures, atmospheric GHG concentrations and ocean acidification), there are also important differences (e.g. the rate of warming during the PETM was far slower than modern rates). The use of the PETM as an analogue of global warming should be done so with caution as the world was very different during the Paleocene; North and South America were not yet connected and Antartica and South America were not yet apart (see previous post on the Drake Passage). This meant that the Atlantic and Pacific oceans were free to mix and Antarctica was not isolated and as a result, there were no large expanses of ice sheets during the period.

The PETM is surprising in that extinction levels were relatively low (with the exception of certain marine benthic taxa). Despite a dramatic temperature rise, this raises the question of whether our predictions for future species extinction may be exaggerated, or that global warming is not the biggest cause of modern extinctions.

For further reading McInerny and Wing 2011 provide a great summary of the PETM and its potential in predicting future climates.