COLUMBUS, Ohio - An international team of scientists reported this week that a rock core drilled from the seafloor off the coast of Antarctica is the first to show cyclic climate changes in polar regions that are linked to cores taken from the ocean bottom in both temperate and tropical zones.
These records show ice sheet advances and retreats that match Milankovitch cycles - variations in the Earth's orbit around the sun, in the tilt of the Earth's axis and in the direction the planet's axis is pointing. The finding, reported in the British journal Nature, suggests a link between these orbital oscillations and the timing of Antarctic ice ages.
The core was drilled in 1998-99 as part of the Cape Roberts Project, an effort by scientists from seven nations to retrieve climate histories trapped in millions of years of sediment beneath the floor of the Ross Sea. Drill sites located just offshore from the Transantarctic Mountains and near McMurdo Station, the main U.S. base in the Antarctic, have retrieved cores from three drill holes. The report in Nature discusses sediments found in the second of these cores. While the Antarctic ice sheets formed approximately 34 million years ago, the parts of the core described in this paper were deposited during a period lasting about 400,000 years, approximately 24.1 to 23.7 million years ago.
Global temperatures at that time were perhaps 3 to 4 degrees C higher than they are today, similar to those predicted for the next century by current climate models that incorporate global warming effects. The amount of carbon dioxide in the air at that time is believed to have been approximately twice current levels.
For years, researchers examining deep-ocean cores from tropical and temperate parts of the oceans have used indirect evidence to propose that variation in the volume of the ice sheets in the polar regions was driven by so-called Milankovitch cycles.
But none of the cores drilled on the Antarctic continental shelf had provided the high-quality data needed to rigorously test that theory. And interpreting changes in polar climate based on evidence recovered so far-removed from the region in question makes many scientists uneasy.
These new findings, however, show that Antarctic ice sheets advanced and retreated at regular intervals during a 400,000-year period between 24.1 and 23.7 million years ago. The records in the core showed the cycles lasted approximately 100,000 years and 40,000 years -- the same time spans characteristic of some Milankovitch cycles.
"It appears that the Antarctic ice sheet has responded in a very major and rhythmic way during this period," explained Peter Webb, professor of geological sciences at Ohio State University and co-chief scientist on the project. "The growth and reduction of the Antarctic ice sheet at its margins is similar to that of the Quaternary Ice Sheets in the Northern Hemisphere."
That is important since most scientists believe that the more recent formation of the large Quaternary ice sheets, some 2.5 million years ago in the Northern Hemisphere, stockpiled water on the continents and caused sea levels to drop by as much as several hundred feet. Webb says the sea level drop indicated by these new Antarctic core is of similar magnitude.
Overall, the Cape Roberts Project cores record approximately 15 million years of Antarctic history. Within that history, Webb said that the team had identified approximately 46 sediment cycles, each of which contained a similar pattern of sediment layers. Each records a major glacial advance, followed by ice sheet retreat, and concludes when ice advanced again from the land into the into the marine continental shelf area.
"This is exactly what we would expect from a growing and receding ice sheet over time," explained Larry Krissek, an associate professor of geological sciences at Ohio State and a member of the Cape Roberts team.
What sets the new finding apart from other work is that the three sediment sequences described in the Nature paper contained known time markers that allowed researchers to date them precisely. The time markers included deposits of volcanic ash from eruptions of known dates; microfossils known to live during a specific period; and episodes when the Earth's polarity was reversed - all elements that helped to date the cores. Previous drill cores lacked the precise dating needed to test any paleoclimatic signal for a potential Milankovitch effect.
Within the Cape Roberts Project drillcores, researchers recognized climate changes that lasted a few tens of thousands of years. That observation let them identify climatic variations dating 17 to 34 million years ago. Previously, that kind of change had only been known in Antarctic ice cores for only the past half-million years.
Both Krissek and Webb were surprised with how rapidly global climate changed, based on the sequences in the core. Like evidence from cores below the seafloor in the North Atlantic, these segments suggest a transition from intense glaciations to a wide-scale glacial retreat may have taken less than 100 years.
"It should catch people's attention now since the change appears to occur in about a human lifespan," Krissek said. Both agree that the discovery places polar seafloor core research on a level with similar work from sites in the mid-latitudes, a significant accomplishment given the short time such work has been underway. Significant seafloor drilling for climate records only began in 1972.
"We've shown now that the Antarctic continent has a valuable archival record," Webb said. "Now we need to go to other parts of the continent and see if the entire ice sheet is behaving in this manner, or if our new record reflects only a small part of it." He added that researchers also must fill in the gap between 17 million years ago and the present, a time when the Earth has been considerably colder than the 15 million years before that.
The Cape Roberts project involves scientists from Australia, Germany, Italy, Netherlands, New Zealand, the United Kingdom and the United States and is supported by the scientific programs of each of those nations. Cores retrieved during the project are divided and stored at two sites - the Alfred Wegener Institut in Bremerhaven, Germany and Florida State University.
Materials provided by Ohio State University. Note: Content may be edited for style and length.
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