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Massive methane release associated with end of severe ice ages


Massive methane release associated with end of severe ice ages

(May 4, 2001)

Enormous releases of methane gas into the oceans followed the extreme ice ages some 600 to 700 million years ago, a period of Earth history when microbes, algae and other simple organisms were the planet's only inhabitants, new research suggests.

Writing in the May issue of the journal Geology, scientists at University of California, Riverside and Columbia University report their findings of an unusual isotopic signature of carbon in rocks formed from sediments that were deposited following at least two major ice ages in the deep past. The characteristic rock layers, called cap carbonates, are found worldwide.

The findings indicate massive amounts of methane gas were released from gas hydrates - crystalline substances composed of ice and gas - as Earth's climate warmed following the ice ages. Gas hydrates are known to exist today along the edges of continents in water about 1,000 feet deep and in the continental permafrost of polar regions like northern Siberia and Alaska. They are formed when methane gas, produced when buried organisms decay, accumulates in sediments where temperatures are sufficiently cold and the overlying pressure is high enough to keep the gas locked into the crystalline hydrate structure.

"Scientists have long wondered why the cap carbonates contain such low carbon isotope values compared to the modern oceans. We think we have found a plausible link between the low carbon isotope values and the release of methane gas into the marine environment," said Martin J. Kennedy, assistant professor of geology at UCR. Co-authors of the study are Nicholas Christie-Blick and Linda E. Sohl, researchers at Columbia University's Lamont-Doherty Earth Observatory.

The scientists analyzed samples from cap carbonate layers in the African nation of Namibia for the characteristic ratio of two carbon isotopes.

Permafrost gas hydrates found on Earth today are known to be especially sensitive to the warming that would have accompanied the end of an ice age.

"We estimate that the amount of gas hydrate needed to produce the carbon signature in the cap carbonates was on the order of 100 times the amount that exists today in permafrost," Kennedy said.

"Although that amount of methane is very large, its hypothesized release comes right on the heels of two of the coldest intervals in Earth's history, when large ice sheets extended all the way to the equator. It makes sense that the extreme cold would allow a great deal of gas hydrate to accumulate over a more extensive region of the planet than in the modern world."

The warming trigger that brought those cold intervals to an apparently sudden end is not yet known. Whatever the cause, the UCR and Columbia scientists said that the rapidly warming climate would have led to glacial melting and flooding of continental shelf permafrost areas. That, in turn, would have caused the gas hydrate to become unstable and melt, freeing methane gas through seeps on the sea floor.

The study has implications for the "snowball Earth" theory, which holds that the oceans were completely covered with a thick layer of ice during the Neoproterozoic, an era of Earth history about 600 million years ago. According to the snowball Earth model of Harvard University scientists Paul F. Hoffman and Daniel P. Schrag, the cap carbonates and their characteristic carbon signature are due to the destruction of marine ecosystems from a thick ice cover during the glacial interval which also interrupted the normal function of the Earth's temperature controller, the global carbon cycle. Build-up in the atmosphere of carbon dioxide derived from volcanoes and the subsequent greenhouse effect led to rapid warming of the Earth and melting of massive glaciers.

In order for the snowball Earth model to function, the oceans would have had to have been completely frozen over to accumulate enough carbon dioxide gas in the atmosphere to warm the planet and break it out of its frozen state. Recent climate modeling experiments conducted by several research groups have suggested that such widespread freezing would have been extremely difficult to achieve, leading Kennedy and his colleagues to seek an alternative explanation.

"We think that the data supporting the gas hydrate hypothesis are pretty compelling," he said. "Given that the cap carbonates and their distinctive isotope values are so closely associated with environmental conditions favoring gas hydrate formation, how can we not consider the role of methane in explaining their existence?"

Additional support for the gas hydrate hypothesis is found in some of the distinctive rock textures and crystal forms of the cap carbonates, according to Kennedy. The features resemble those observed in the localized carbonate deposits associated with modern methane seeps in the Gulf of Mexico and off the coast of Washington and Oregon.

The University of California, Riverside (www.ucr.edu) is a doctoral research university, a living laboratory for groundbreaking exploration of issues critical to Inland Southern California, the state and communities around the world. Reflecting California's diverse culture, UCR's enrollment has exceeded 21,000 students. The campus opened a medical school in 2013 and has reached the heart of the Coachella Valley by way of the UCR Palm Desert Center. The campus has an annual statewide economic impact of more than $1 billion.

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