Tracking Ancient Ocean 'Burps'

Oceanographers and climatologists speculate that large amounts of carbon dioxide entered the atmosphere from reservoirs in the deep ocean at the end of the last ice age, which occurred about 19,000 years ago. Long-awaited evidence that traces the redistribution of the gas comes from a new study published in Science (DOI: 10.1126/science1142326).

What scientists have been trying to explain is the rise in atmospheric CO₂ from 180 to 265 ppm accompanied by a puzzling 35% drop in the gas's radiocarbon (¹⁴C) content, according to Ralph F. Keeling of Scripps Institution of Oceanography at the University of California, San Diego. In a commentary accompanying the Science paper, he writes that a simple but unproven explanation to link both changes is a well-known process called ventilation, by which the ocean's surface waters are exchanged with subsurface waters.

"A ventilation increase could increase atmospheric CO₂ concentration by releasing excess CO₂ that had accumulated in subsurface waters by the decomposition of detritus," Keeling writes. In addition, CO₂ from subsurface waters would have lower ¹⁴C content than that of surface waters because ¹⁴C is produced naturally in the atmosphere, and the ¹⁴C in CO₂ isolated from the atmosphere can only decay rather than accumulate.

Researchers hypothesize that during the last ice age, an isolated carbon reservoir existed at a depth below 2,500 meters. But obtaining good-quality sediment cores—which serve as a chronicle of the radiocarbon evidence—from such depths is difficult.

Now, paleoclimatologist Thomas M. Marchitto Jr. at the University of Colorado, Boulder, and colleagues believe they have tracked CO₂ "burped" from that deep reservoir as it made its way into the atmosphere. Their findings are based on analysis of a high-quality sediment core collected just 700 meters below the ocean surface off the southern tip of Baja California. They measured the amounts of ¹⁴C in bottom-dwelling foraminifera, a ubiquitous marine microorganism, found in the different layers of the core. In combination with other previously collected data, they constructed a record of ¹⁴C concentrations in that part of the ocean. Two very low levels of ¹⁴C in the oceanic record correlated to the time periods when CO₂ concentrations abruptly increased in the atmosphere.

"Marchitto and colleagues have the first ocean data to show that the carbon exhalation happened twice, after each of the most severe cold episodes of the past 20,000 years," says??Lloyd Keigwin, a marine geologist at Woods Hole Oceanographic Institution, in Massachusetts.

But Keeling wants to see more data. "The study nevertheless leaves the skeptics with arrows in their quiver," he writes in his commentary. For example, he notes that the results indicate pockets of water with so little ¹⁴C "that they start to stretch credibility." But if follow-up work supports the findings, this study could be a turning point, he writes.

Marchitto says additional ¹⁴C evidence should be available at other sites around the world. Next, he and his colleagues plan to obtain and analyze a core from a site near the southern tip of South America.