What was responsible for the end of the last ice age?
The last ice age was lengthy, taking place over a period of 25,000 years. It covered over a third of the earth and ended over 10,000 years ago. How the glacial period ended has been a matter of dispute, but one accepted theory was that a significant release of carbon dioxide from the ocean was the cause. It was hypothesised that this carbon dioxide entered the atmosphere and, via the greenhouse effect, warmed the world. The levels of the gas were particularly high after the peak of the last ice age. While the theory that carbon dioxide was responsible has not been disproven, the source of the gas has been brought into doubt.
The source of this carbon dioxide was believed to be the sediment formed from the calcite shells of plankton. The production of the microscopic shells is a process that takes carbon from the organism's environment where it is kept until released by reaction with naturally occurring acidic compounds. Carbon dating was used on numerous samples of deep-ocean sediment extracted near the coast of Oregon. Extracts of various ages were taken, covering a 14,000 year period starting 22,000 years ago.
Scientists sampling a deep-sea sediment core. Credit: Alan Mix
The researchers were looking for signs that revealed the last time that the sediment had been in contact with the atmosphere. This information would demonstrate that while the glacial period was active the sediment would have less contact with the atmosphere, in effect building a store of captured carbon dioxide. It was thought that water circulating back to the surface would slow during these periods. The researchers were surprised to discover that their analysis did not support this theory and the circulation of sediment was no less than that found today.
David Lund, the lead author, was cautious about completely dismissing the original hypothesis. He expressed a need to look again and take into account the volcanic activity in the zone which could affect radiocarbon ratios, and he also said that it was, "... conceivable that we are misunderstanding the radiocarbon signal by assuming it is controlled by ocean mixing."
The researchers have remained positive about their findings. Alan Mix, another of the study's authors commented, "At least we've shown where the carbon wasn't. Now we just have to find where it was."
Top Image Credit: Moreno glacier in southern Patagonia icefield © ChaseinDC
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