New study describes workings of deep oceanduring the Last Glacial Maximum
Scientists know quite a bit about surface conditions during the Last Glacial Maximum (LGM), a period that peaked about 18,000 years ago, when ice covered significant portions of Canada and northern Europe.
But to really understand the mechanisms involved in climate change, scientists need to have detailed knowledge of the interaction between the ocean and the atmosphere. And until now, a key component of that knowledge has been lacking for the LGM because of limited understanding of the glacial deep ocean.
In a paper published in the November 29 issue of the journal Science, researchers from the California Institute of Technology and Harvard University report the first measurements for the temperature-salinity distribution of the glacial deep ocean. The results show unexpectedly that the basic mechanism of the distribution was different during icy times.
"You can think of the global ocean as a big bathtub, with the densest water at bottom and the lightest at top," explains Jess Adkins, an assistant professor of geochemistry and global environmental science at Caltech and lead author of the paper. Because water that is cold or salty--or both--is dense, it tends to flow downward in a vertical circulation pattern, much like water falling down the sides of the bathtub, until it finds its correct density level. In the ocean today, this circulation mechanism tends to be dominated by the temperature of the water.
In studying chlorine data from four ocean drilling program sites, the researchers found that the glacial deep ocean's circulation was set by the salinity of the water. In addition, a person walking on the ocean bottom from north to south, 18,000 years ago, would have found that the water tended to get saltier as he proceeded (within an acceptable margin of error, both north and south waters were the same temperature). Taking that into account, the water in the north would have been less dense. The exact reverse is true today, with the waters at low southern latitudes being very cold and relatively fresh, while those in the high northern latitudes being warmer and saltier.
Adkins says there is a good explanation for the change. The seawater "equation of state" dictates that the density of water near the freezing point is about two-to-three times more sensitive to changes in salinity relative to changes in temperature, as compared to today's warmer deep waters.
So, the equation demands that the density-layering of the ocean "bathtub" be set by the water's salt content at the last glacial maximum. Temperature is still crucial, in that colder waters are more sensitive to salinity changes than warmer water, but Adkin's results show that the deep water circulation mechanism must have operated in a fundamentally different manner in the past.
"This observation of the deep ocean seems like a strange place to go to study Earth's climate, but this is where you find most of the mass and thermal inertia of the climate system," Adkins says.
The ocean's water temperature enters into the complex mechanism affecting the climate, with water moving about in order for the ocean to equalize its temperature. Too, the water and air interact to further complicate the weather equation.
Thus, the results from the glacial deep ocean shows that the climate in those days was operating in a very different way, Adkins says. "Basically, the purpose of this study is to understand the mechanisms of climate change."
In addition to Adkins, the other authors are Katherine McIntyre, a postdoctoral scholar in geochemistry at Caltech; and Daniel P. Schrag of the Department of Earth and Planetary Sciences at Harvard University.
Contact: Robert Tindol (626) 395-3631