When Earth emerged from its last ice age several thousand years ago, levels of carbon dioxide (CO2) in the atmosphere increased, contributing to the continued warming of the planet.
However, although scientists believe that the primary source of this CO2 was likely the deep ocean, a new study confirms that although the ocean played a large role in this process, there were also land-based carbon sources present in the equation.
"There wasn't a steady rate of rising carbon dioxide during the last deglaciation," said Edward Brook, co-author of the study. "It happened in fits and starts. With the new precise techniques we developed to fingerprint the sources, 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 study examined carbon isotope ratios in samples retrieved from ice gathered, originating from Antarctica's Taylor Glacier. The results revealed that during the initial rise of atmospheric CO2 between 17,600 years ago and 15,500 years ago, the light isotope 12-C increased faster than its heavier counterparts, suggesting that carbon was released from the deep ocean.
However, between 16,300 years ago and 12,900 years ago, the carbon ratio experienced abrupt changes that point to the rapid release of carbon from land sources. Although the team is still not sure what region the CO2 sources came from, at least one of the two events likely came from the tropics due to it taking place at the same time as the methane release from tropical swamps.
"One theory 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," Brook said. "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."
The isotope ratio technique used in the study can help researchers get a glimpse into past global warming trends and help better understand what is to come, although this process is much more difficult than it sounds.
"The rise of CO2 is a complicated beast, with different behaviors triggered at different times," said Alan Mix, co-author of the study. "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."
The findings were published in the March 14 issue of the Proceedings of the National Academy of Sciences.
