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, both researchers with Ohio State’s Byrd
Polar Research Center, 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.

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Contact: Peter Webb, (614) 292-7285; Webb.3@osu.edu.
Additional telephone numbers for Webb include his home at (740) 927-2275,
and the OSU Department of Geological Sciences at (614) 292-2721.
Written by Earle Holland, (614) 292-8384; Holland.8@osu.edu.