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Scientists study ocean to understand global cooling


The depth in the ocean where calcium carbonate dissolves at a faster rate than it is deposited is called the calcite compensation depth (CCD). At present this depth is approximately 4,500 meters (14,700 feet) with some variation between and within ocean basins. Because the CCD is linked to ocean acidity, which is, in turn, linked to atmospheric carbon dioxide concentrations and, hence, to global climate, it is important for scientists to understand the impact of possible changes in its depth.

In the current issue of Nature, URI Graduate School of Oceanography (GSO) visiting scientist Helen Coxall describes how the deepening of the CCD in the Pacific Ocean correlated to global cooling approximately 34 million years ago, when the first significant permanent ice sheets appeared on Antarctica. Other members of the scientific team include Paul Wilson, Southampton Oceanography Center, UK, Heiko Pälike and Jan Backman, University of Stockholm, Sweden, and Caroline H. Lear, Rutgers University, New Jersey.

"This event 34 million years ago marks the transition from a warm ’greenhouse’ climate state, when atmospheric carbon dioxide levels were naturally high and there was no or very little ice at the poles, to the cold glaciated climate state of the modern world that was characterized by lower carbon dioxide," said Coxall. "It is therefore equivalent to global warming in reverse. The results of our study are crucial to the understanding of how climate change works, especially how rapidly major changes in ice-sheet growth and sea level rise and fall occur under altered conditions of atmospheric carbon dioxide."

Coxall and her colleagues analyzed sediment records and found that the deepening of the CCD was more rapid than previously documented and occurred in two jumps of about 40,000 years each, in step with the onset of Antarctic ice-sheet growth. The 40,000-year interval was separated by a plateau of 200,000 years.

The glaciation began after the Earth entered a cooler phase during an interval when the Earth’s orbit of the Sun favored cool summers. The researchers’ observations suggest that it was the prolonged absence of warm summers, inhibiting summer snow melt, not the occurrence of cool winters favoring accumulation, that was important for establishing the first major ice sheets on Antarctica. Although the pattern of Earth’s orbital configuration was the ultimate trigger for creating conditions that led to ice-sheet growth, a natural long-term decrease in atmospheric carbon dioxide levels, which promoted global cooling, was responsible for increasing Earth’s sensitivity to this factor.

In addition, analysis of the data indicates that along with the growth of the Antarctic ice sheet, glaciation in the Northern Hemisphere must also have been taking place.

Lisa Cugini | EurekAlert!
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