Carbon from land contributes to last deglaciation

A new study has confirmed that while the ocean played a significant role in the rise of atmospheric carbon dioxide during the last deglaciation, land-based carbon sources also contributed to abrupt CO2 increases.

The study, published this week in Proceedings of the National Academy of Sciences of the United States, was based on the comparison of carbon isotope ratios in pristine samples of ice mined from the Taylor Glacier in Antarctica, where old ice that flowed from the interior is exposed at the surface.

Although isotopic fingerprinting strategies have been attempted before, the key was detailed work both in the field and in the laboratory that improved the precision to read the record.

"The isotope ratio technique gives us a sort of 'return address' for carbon dioxide," noted Thomas Bauska, the lead author of the study and a former Ph.D. student and post-doctoral researcher at College of Earth, Ocean, and Atmospheric Sciences in Oregon State University (OSU). "The technique is new, extremely precise and gives us one of the best windows into the Earth's past climate."

At the Taylor Glacier, ice that normally would be deep below the surface is available to easily sample in large quantities. And large samples, laboriously cut from the exposed ice layers, allowed the precise measurements

"There wasn't a steady rate of rising carbon dioxide during the last deglaciation," said Edward Brook, an OSU paleoclimatologist and co-author of the study. "It happened in fits and starts ... it is apparent that the early carbon largely came from the ocean, but we think the system got a jolt from an influx of land-based carbon a few times as the climate warmed."

The researchers found that during the initial rise in atmospheric CO2, from 17,600 years ago to 15,500 years ago, the light isotope 12-C increased faster than the heavier isotopes, pointing to a release of carbon from the deep ocean. At about 16,300 years ago and 12,900 years ago, there were abrupt, century-scale perturbations in the carbon ratio that suggested rapid release of carbon from land sources such as plants and soils.

While the region of the CO2 source is not clear, the researchers said at least one of the two events may come from the tropics because methane from tropical swamps rose at the same time.

"One theory," Brook said, "is that an influx of icebergs in the Northern Hemisphere at about 16,300 years ago - from retreating ice sheets - cooled the North Atlantic Ocean and pushed the tropical rain belt southward over Brazil, expanding the wetlands. Swamps in the Southern Hemisphere, in places like Brazil, may have become wetter and produced methane, while plants and soils in the Northern Hemisphere, in places like China, may have been hit by drought and produced CO2."

During the next 4,000 years, the continued rise of CO2, by about 40 parts per million, was marked by small changes in the carbon-13 to carbon-12 ratio, indicating additional sources of carbon from rising ocean temperatures.

The findings may provide hints at what may happen under a new global warming regime, said Alan Mix, an OSU oceanographer and co-author of the study. However, he cautioned, it is not always simple to predict the future based on past events.

"The rise of CO2 is a complicated beast, with different behaviors triggered at different times," Mix said. "Although the natural changes at the end of the ice age are not a direct analogy for the future, the rapid changes do provide a cautionary tale. Manmade warming from CO2 pollution may trigger further release from 'natural sources,' and this could exacerbate greenhouse gases and warming." Endit