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   February 2000 

Evidence for a 'methane burp'
A period of 10–20 thousand years marked the Late Paleocene Thermal Maximum (LPTM), a time 55.5 million years ago when Earth experienced major global warming. It marked the appearance of modern mammals, including primates and horses. At the same time, many deep-sea microfossils, such as foraminifera and ostracods, temporarily disappeared from the fossil record or became extinct. Evolution hurtled forward at an astonishing pace during a time when other archaic species were obliterated by a powerful and unknown force now believed to be … bubbles.

How can the disappearance and emergence of so many species really be attributed to the release of gas into the ocean? A team of researchers led by Miriam Katz of Rutgers University, in conjunction with the Ocean Drilling Program (ODP), made a significant discovery published in the Nov. 19 Science supporting the “methane burp” theory. 

Junior, an orangutan at the National Zoo in Washington, D.C., is a descendant of the first primates that appeared after the Late Paleocene Thermal Maximum. Photo from Jessie Cohen, National Zoo.

Born in the mid-1990s, the theory tells of a gradual warming of Earth to the point where ocean circulation could have changed as increasingly salty and dense equatorial water sank toward the sea floor of the continental shelves, thereby replacing the cold bottom water. The benthic forams that lived in the cold, deep water became extinct and the sediment temperature slowly increased under warmer water at the sea floor. As the upper few hundred meters of continental slope sediment warmed, the pressure of the methane trapped in gas hydrates would have increased. Eventually the crystalline structure in which the gas was trapped ruptured and released large amounts of methane into the ocean. Because methane and its oxidation product, carbon dioxide, are both greenhouse gases, a massive release would have spurred further warming of the ocean and atmosphere and propagated the entire global warming cycle.

It is estimated that presently over 15 trillion tons of methane are buried in sea-floor sediments. Assuming that the Paleocene sediments contained less methane than they do today, if only 8 percent of the total Paleocene methane was released into the ocean the carbon isotope ratio would have been lighter by 2.3 parts per thousand. The results obtained from analysis of the foram samples taken off the coasts of Florida and Antarctica revealed that ratios of carbon-13 to carbon-12 decreased by 2.5 parts per thousand, according to a study led by Santo Bains of Oxford University and published in the July 30, 1999, issue of Science. The isotopic carbon ratio of the sediments is a major factor in promoting this theory because the methane byproduct of many benthic bacteria contains the most isotopically light form of carbon. Other sources of gas include volcanic degassing, which would not produce the volume or light isotopic ratio that has been observed.

While this theory has been around for several years, no evidence for a release site had been found until recently. The team of researchers led by Katz discovered a potential release site in the subtropical North Atlantic located 400 kilometers east of Tallahassee, Fla., on the continental slope. This site is known as Blake Nose, ODP site 1051 (the same site used by Bains’ team). Corings revealed that 55 percent of foram taxa disappeared in the late Paleocene and 60 percent of the last appearances occurred immediately below the appearance of the carbon isotope excursion. Below the carbon isotope excursion (the change in ratio of carbon-13 to carbon-12) lies a mud clast interval indicative of sediment disturbance upslope, as well as a drop in faunal diversity from 25–30 taxa to 5–10 taxa per sample. The heavier carbon-13 isotope occurrence is reduced in favor of the lighter, biogenic carbon-12.

Because methane can only escape from buried gas hydrate reservoirs through sediment failure, there should be an accompanying slumping and disturbance of the sediments on the middle and lower continental slope at the beginning of the LPTM. As predicted in the “methane burp” theory, the Blake Nose core samples exhibit evidence of sediment disturbance and transport.

As a result of the drastic global warming at the LPTM, higher latitude migration routes opened up to animals that couldn’t previously have survived in the inhospitable cold. Evolution on land took a huge leap as many of the ancestors of modern mammals appeared in the fossil record while, deep in the ocean, species were dying off.

—Laura Wright