Scripps Institution of Oceanography / University of California, San Diego
In a research first, an international team of scientists used Carbon-14 (14C) to shed light on why many abrupt climate warming events through the past 100,000 years have been accompanied by increases in atmospheric methane concentration. Their findings suggest that huge meltdowns of frozen methane-which some scientists had proposed as the cause of the methane increases, as well as significant contributors to the warming itself-most likely did not occur during the warming events.
Methane is one of the most important greenhouse gases. Measurements of 14C concentrations in the gas can tell scientists how a sample of methane originated but the difficulty of applying the procedure to ancient air samples had long dissuaded researchers from attempting it. Over the course of five years, Vas Petrenko of Scripps Institution of Oceanography at UC San Diego and his colleagues collected glacial ice from the edge of the Greenland ice sheet that formed around the times of two abrupt climate change events that took place 11,600 years ago and 14,500 years ago. They extracted the ancient air trapped inside bubbles in the ice to analyze the methane they contained.
Every ton of glacial ice the team melted down yielded only about 20 micrograms of ancient methane, a sample size typically too small for 14C analysis. The tiny sample sizes they had to work with required that the team push the boundaries of 14C measurement procedures.
But the arduous task was necessary because of the great potential of 14C to identify methane that came from seafloor sediments. The planet's seafloors bear vast repositories of frozen methane in icelike structures called clathrates. Methane from this source contains no 14C whereas wetland methane contains abundant 14C. The team discovered that the ancient methane contained too much 14C to have a significant part of it come from clathrates.
Instead, said Petrenko, the results suggest that the warmer, wetter conditions associated with the two abrupt climate change events resulted in more methane being produced by wetlands and plants.
In addition, the 14C levels in the ancient methane were so high, ranging from 28 to 35 percent of modern carbon, that they could not be explained by biological methane production alone. The team concluded that some of the 14C in the methane must have originated in the nuclear reactions that take place as cosmic rays bombard glacial ice. This is a new phenomenon that has never previously been observed, Petrenko said.