What: Ice ages don't quite conform
Who: Christina Gallup, Assistant Professor UMD Department of Geological Sciences (218) 726-8984
The widely accepted theory that changes in Earth's orbit drive cycles of glaciation can't account for an early thawing of glaciers from the next-to-last ice age, according to research at the University of Minnesota. The failure of the Milankovitch theory, also called orbital forcing, to predict this thawing points to the existence of other factors that can override orbital forcing to influence climate, the researchers said. The work will be published in the Jan. 11 issue of Science.
Relying on extensive examinations of fossil corals, the researchers have tracked sea level changes and volumes of glacial ice, said lead author Christina Gallup, an assistant professor of geology at the University of Minnesota-Duluth. Because sea levels fall when glaciers build up and rise during glacial melting, the elevation at which corals grow depends partly on the glacial cycle. The Minnesota team has dated many ancient corals from Barbados beaches and determined when they grew and at what elevation with respect to current sea level. They have used their findings to compare glacial cycles to cycles in Earth's orbit, which, according to the theory of Serbian scientist Milutin Milankovitch (1879-1958), provide the impetus for ice buildup and melting.
"All previous coral data are in accord with Milankovitch," said Gallup. "Scientists have followed ice ages as far back as three million years and always found frequencies in the marine record of glaciations that are the same as the frequencies of variation in Earth's orbit.
"In this study, we did a more thorough sampling of the coral terrace built during the interglacial [between ice ages] period that occurred between 130,000 and 120,000 years ago. We found corals whose age and position indicated that during the transition to that interglacial, sea level rose too early to be consistent with Milankovitch. This exception tells us something. It implies that other things can override orbital forcing of glacial cycles."
The Milankovitch theory says that summer heating at high northern latitudes drives the melting of glaciers, Gallup explained. At latitude 65 degrees north, changes in Earth's orbit would have caused summer heating to reach a minimum at about 140,000 years ago and increase to a peak at 129,000 years ago. The theory predicts that most of the melting would occur after the halfway point, that is, 134,000 years ago and later. But the corals from Barbados indicate that sea level was quite high--within 20 percent of its all-time high--136,000 years ago. Therefore, much melting must have occurred thousands of years before the halfway point.
The early melting of glaciers could have been due to one or more factors, said Gallup. Scientists have theorized that when ice sheets get heavy enough, they depress the land beneath them and sink. They encounter warmer air at lower elevations, they have less area that's accumulating snow and ice, and sometimes, seawater flows under their bottom layers and helps melt them further. Also, the height of an ice age probably experiences the biggest amounts of floating sea ice. This hinders evaporation of water, shutting off the supply of moisture that fuels snowfalls at high latitudes.
A third factor was suggested by Robert Johnson, a geologist at the University of Minnesota-Twin Cities. Normally, as one moves from tropical to polar latitudes, there is a strong gradient in the amount of solar radiation received. But if this gradient and its accompanying temperature contrasts should weaken, then there will be a lesser energy gradient to drive moisture northward.
"Bob found that there was a minimum in this gradient 140,000 years ago," said Gallup. "This would contribute to glaciers deflating. They also could have been starved by sea ice and destabilized by sinking."
Gallup's colleagues in the study were University of Minnesota-Twin Cities geology professor Lawrence Edwards and postdoctoral fellow Hai Cheng, along with Fred Taylor, a faculty member at the University of Texas at Austin. The work was supported by the National Science Foundation.