June 15, 2011

Start Of Ancient Global Warming Episode Discovered

Researchers have pinpointed the timing of the start of an ancient global warming episode known as the Paleocene-Eocene thermal maximum (PETM).

The early part of the Cenozoic era witnessed a series of transit global warming events called hyperthermals.  The most severe of these was the PETM at the Paleocene-Eocene boundary, which took place around 56 million years ago.

Over a 20,000-year period, ocean temperatures rose globally by about 41 degrees Fahrenheit. 

The team said one possibility of this temperature rise is that these hyperthermals were driven by cyclic variations in the eccentricity of the Earth's orbit around the sun.  Increased temperatures at the cycle peaks could have caused methane hydrate deposits in the deep sea to release large amounts of methane. 

The researchers also said it may have been geological processes that could have been the culprit for the warming associated with the PETM.

Magmatism in this scenario would have caused the baking of marine organic sediments, leading to the massive release of methane and/or carbon dioxide.

"Determining exactly what triggered the PETM requires very accurate dating of the event itself, to determine whether it occurred during a known maximum in the Earth's orbital eccentricity" Adam Charles, a University of Southampton PhD student supervised by Dr Ian Harding, and first author of the newly published report in the journal Geochemistry Geophysics Geosystems, said in a statement.

The researchers, based at the National Oceanography Centre, Southampton, measured radioisotopes of uranium and lead in the mineral zircon to get a better grip on the numerical age of the Paleocene-Eocene boundary.

The team collected these rocks from two locations in Spitsbergen, the largest island of the Svalbard Archipelago in the Arctic.

The researchers dated the Paleocene-Eocene boundary at between 55.728 and 55.964 million years ago, which they believe to be the most accurate estimate to date.

Their analyses indicated that the onset of the PETM did not occur at the peak of a 400 thousand year cycle in the Earth's orbital eccentricity.

"Compared to other early Eocene hyperthermals, it appears that the PETM was triggered by a different mechanism, and thus may have involved volcanism. However, a thorough test of this hypothesis will require further detailed dating studies," Adam said in a statement.

The researchers are Adam Charles, Ian Harding, Heiko Pälike and John Marshall (Ocean and Earth Science, University of Southampton), Daniel Condon (British Geological Survey), and Ying Cui and Lee Kump (Pennsylvania State University). The research was supported by the Natural Environment Research Council (NERC) and Shell UK, as well as a Philip Leverhulme Prize to Heiko Pälike.

Reference: Charles, A. J., Condon, D. J., Harding, I. C., Pälike, H., Marshall, J. E. A., Cui, Y. & Kump, L. Constraints on the numerical age of the Paleocene"“Eocene boundary. Geochemistry Geophysics Geosystems 12, Q0AA17, doi:10.1029/2010GC003426


Image Caption: The image shows a location in Longyearbyen, Spisbergen, where the researchers carried out field work. Credit: Ian Harding


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