December 5. Paleocene-Eocene Thermal Maximum

To start with today, an update on Archaeopteryx, the first bird. Or maybe not. New research is showing that the transition from dinosaurs to birds was complex, and some lines of thought suggest that Archaeopteryx was a feathered, gliding dinosaur rather than a true bird. This year’s Society of Vertebrate Paleontologists’ meeting in Berlin included new studies of Archaeopteryx. Here's a link to a report from the journal Nature on current thinking about the possible first bird.

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Our Cenozoic topic today is the climate at the end of Paleocene and beginning of Eocene time, about 55 or 56 million years ago. Just about at the boundary between the two epochs there was a short, intense, 170,000-year period of warming called the Paleocene-Eocene Thermal Maximum, or PETM.

Temperatures rose quickly, over about 20,000 years, by about 6°C, and carbon isotope ratios suggest that the oceans underwent acidification because of increased CO2 that was dissolved in ocean water. Sea levels rose because of thermal expansion of the water. Some extinctions are associated with the Thermal Maximum, especially among deep-water marine microorganisms, but some shallow-water calcareous microorganisms actually became more abundant. There’s no evidence for significant extinction on land, and in fact shortly after it began, there is a dramatic radiation of some mammals such as camels, horses, pigs, primates, and other groups. 

We don’t really know what caused the changes in CO2 and temperature during this Thermal Maximum. There are some volcanic eruptions that are at essentially the same time, including some in Canada and Greenland that might have accounted for the spike in carbon dioxide, but the volcanism is more suited to explaining the more gradual warming that was occurring through the Paleocene. Spikes are always challenging to explain.

Speculations about impacts have found no evidence. One good possibility is that warming oceans reached a threshold temperature at which methane hydrates – essentially, methane gas trapped in ice within sea-floor sediment – melted and released their methane, a more potent greenhouse gas than carbon dioxide. Such a release could have occurred over a short period of time – hundreds to thousands of years – making it a reasonable candidate for the spike in temperatures. But changes in carbon isotope ratios suggest that the effects of the event worked their way from shallow waters into deep waters over a span of around 10,000 years, the opposite of what would be expected with a deep-water methane release. Exactly what was going on in the oceans is hard to quantify – things that might support the methane idea include changes in ocean circulation that might have mixed warm tropical waters more thoroughly in the world ocean. The Isthmus of Panama did not yet exist, so there would have been more connection between the Atlantic and Pacific.

The Paleocene-Eocene Thermal Maximum is being studied quite intensely because it may serve as a model for the modern increases in temperature and carbon dioxide that are occurring as a result of human activities. I have a link below to an article in Paleontology Online by Phil Jardine, which is a nice overview of the facts of the Thermal Maximum at the end of the Paleocene.

—Richard I. Gibson

Paleocene-Eocene Thermal Maximum
Carbon releases and PETM