The large increase in atmospheric carbon dioxide at the end of the last ice age wasn't released by the oceans, scientists have concluded.
After the last ice age peaked about 18,000 years ago, levels of CO2 rose by about 30 percent, warming the planet and melting the continental ice sheets. It's long been hypothesized that the source of the extra CO2 was the deep ocean.
But a new study using detailed radiocarbon dating of foraminifera found in a sediment core from the Gorda Ridge off the coast of Oregon reveals that the Northeast Pacific was not an important reservoir of carbon during glacial times.
The finding means scientists will have to look elsewhere for potential sources of CO2 during glacial periods.
"Frankly, we're kind of baffled by the whole thing. The North Pacific was such an obvious source for the carbon, but it just doesn't match up," says Oregon State University paleo-oceanographer Alan Mix.
The work involved radiocarbon dating dozens of sediment samples that contained microscopic shells from plankton, and spanning the period from 8,000 to 22,000 years ago.
Over thousands of years, ocean water circulates from the surface to the bottom and back. The radiocarbon results revealed the 'ventilation' rate - the amount of time since the samples were last in contact with the atmosphere.
The team expected to find that the ventilation rate in the basin slowed during glacial times, allowing carbon dioxide to accumulate in the ocean and depleting atmospheric levels of the gas.
But they found that the ventilation rate during glacial times was much the same as it is today, suggesting that the Pacific was not an important carbon reservoir.
“This indicates that the deep Pacific was not an important sink of carbon during glacial times," says University of Michigan paleoclimatologist David Lund.
"Even more intriguing is that we found the ventilation age increased during the deglaciation, at the exact time that atmospheric CO2 levels were rising."
The team says the next step will be to establish whether the missing carbon could have derived from volcanoes.
"At least we've shown where the carbon wasn't," says Mix. "Now we just have to find where it was."