Paleocene-Eocene Thermal Maximum
Paleocene-Eocene Thermal Maximum
The Paleocene–Eocene Thermal Maximum (PETM) was a 'blip' in the smooth running of Earth's environment and climate that took place about 56 million years ago (see image to right). It is of great significance because it most probably represents the closest analogue to the changes being forced on the Earth at present by humanity. If we can work out what happened at the PETM, then it will most likely be very useful in helping us understand what is likely to happen in the future.
Paleoclimatologists have collected large amounts of evidence about what happened at the PETM, both on land and also in the oceans. It seems clear that there was rapid and considerable global warming by about 5 °C. There is also evidence of ocean acidification, although this evidence needs to be carefully interpreted. Many species of benthic foraminifera went extinct. There was both rapid extinction and speciation (birth of new species) among mammals. There was a pronounced 'excursion' (temporary shift) to more negative δ13C values in ocean sediments. This is in addition to the temporary shift in δ18O shift shown in the right-hand image.
You can use the carbon model to explore some aspects of this event. The model does not separately represent methane in either the ocean or the atmosphere, but methane has a short lifespan in the oceans and the atmosphere due to rapid conversion to CO2. The model assumes that all methane is instantaneously oxidised to CO2, which may introduce a slight underestimation of its heating effects (methane is a more potent greenhouse gas, molecule-for-molecule, than is carbon dioxide).
A first question is what caused the PETM? A whole raft of hypotheses have been put forward, but out of these there are two main candidates that most people currently favour. The first of these suggests that it was a release of biogenic methane from methane clathrates on the seafloor that caused the global warming (Dickens et al, 1997). The second of these also invokes an input of methane, but this time of thermogenic methane (Svensen et al, 2004). The two different carbon sources have different characteristic δ13C values.
This exercise shows how we can get an idea of the process that initiated the PETM. In reality, however, at the present time we are limited in the data that is available to constrain these three parameters of global change. The change in δ13C of benthic foraminifera is the number that is most well known; the magnitude of global warming is also reasonably well-constrained, but the change in global average lysocline depth is less well determined. We know that it shallowed by at least 2km in the South Atlantic (Zachos et al, 2005), but to accurately constrain this number requires a depth transect of ocean cores - an enormous amount of work. We do not have equivalent data from other ocean basins.
This website is in its early stages of use. If you find it difficult to run a model in the way described, or find any other problems, your feedback will help us improve the site for future users.
- Zachos JC et al. (2005) Rapid Acidification of the Ocean During the Paleocene-Eocene Thermal Maximum. Science, 308: 1611-1615.
- Nunes F & Norris RD (2006) Abrupt reversal in ocean overturning during the Palaeocene/Eocene warm period. Nature, 439: 60 - 63.
- Zeebe RE & Zachos JC (2007) Reversed deep-sea carbonate ion basin gradient during Paleocene-Eocene thermal maximum. Paleoceanography, 22: PA3201, doi:10.1029/2006PA001395.
- Dickens GR, Castillo MM & Walker JCG (1997) A blast of gas in the latest Paleocene; simulating first-order effects of massive dissociation of oceanic methane hydrate. Geology, 25: 259-262.
- Svensen H et al. (2004) Release of methane from a volcanic basin as a mechanism for initial Eocene global warming. Nature, 429: 542 - 545.