About 23 million years (Ma) ago, a huge ice sheet spread
over Antarctica, temporarily reversing a general trend of
global warming and decreasing ice volume. Now a team of
researchers has discovered that this climatic blip at the
boundary between the Oligocene and Miocene epochs corresponded
with a rare combination of events in the pattern of Earth's
In a paper published in today’s issue of the journal
Science, the researchers show that the transient
glaciation and other climatic variations during a period from
about 20Ma to 25.5Ma ago correspond with variations in Earth's
orbit known as Milankovitch cycles. Although the concept of
such relationships is not new, some of the results were
surprising, says James Zachos, a professor of Earth sciences
at the University of California, Santa Cruz, and lead author
of the paper.
"When we began examining the temporal relationship of the
orbital oscillations relative to the oscillations in the
climate record, we never suspected that the transient
glaciation at 23Ma ago had anything to do with orbital
anomalies" Zachos says.
The astrophysicist Milutin Milankovitch first proposed that
cyclical variations in certain elements of Earth-Sun geometry
could cause major changes in Earth's climate. The main
variables are eccentricity, obliquity, and precession.
Eccentricity refers to the changing shape of Earth's orbit
around the Sun, which varies from nearly circular to
elliptical over a cycle of about 100,000 years. Obliquity
refers to the angle at which Earth's axis is tilted with
respect to the plane of its orbit, varying between 22.1
degrees and 24.5 degrees over a 41,000-year cycle. And
precession is the gradual change in the direction Earth's axis
is pointing, which completes a cycle every 21,000 years.
"Because there are several components of orbital
variability, each with lower frequency components of amplitude
modulation, there is the potential for unusual interactions
between them on long timescales of tens of millions of years"
Zachos says. "What we found at 23Ma ago is a rare congruence
of a low point in Earth's eccentricity and a period of minimal
variation in obliquity."
The result of this rare coincidence was a period of about
200,000 years when there was unusually low variability in the
planet's climate, with reduced extremes of seasonal warmth and
coldness. Earth's orbit was nearly circular, so its distance
from the Sun stayed about the same throughout the year. In
addition, the tilt of Earth's axis, which gives rise to the
seasons, varied less than usual. In other words, the tilt does
not always vary between the same extremes in its 41,000-year
cycles; the obliquity cycle itself varies in amplitude over a
longer period of about 1.25Ma. Similarly, the eccentricity
cycle peaks every 400,000 years.
The combination of a low-amplitude "node" in the obliquity
cycle and a minimum in eccentricity would have caused only
several degrees difference in summer temperatures at the
poles, but it was probably enough to allow the Antarctic ice
sheet to expand, Zachos says.
Zachos's collaborators on the paper were Nicholas
Shackleton and Heiko Pälike of Cambridge University, Justin
Revenaugh of UC Santa Cruz, and Benjamin Flower of the
University of South Florida.
The researchers obtained detailed climate records for the
late Oligocene and early Miocene by analysing sediment cores
drilled out of the ocean floor. Cutting through layers of
sediments laid down over millions of years, such cores contain
a chronological record of past climates written in the
chemistry of fossil shells left behind by tiny marine
organisms. Oxygen isotopes in the shells, for example, reflect
ocean water temperatures and the amount of ice trapped in
In the 1970s, scientists using these techniques obtained
the first good evidence in support of Milankovitch's theory,
almost 50 years after he had proposed it. According to Zachos,
researchers are still trying to get a handle on the
relationships between climate cycles and orbital variations.
Since most of the research has focused on the past five
million years, the new paper is valuable because it looks
through a more distant window in time when conditions on the
planet were different.
In the period they examined, (late Oligocene and early
Miocene) Zachos and his collaborators found evidence of
several climate cycles with frequencies corresponding to the
Milankovitch cycles. But the correspondence of the orbital
anomaly with the transient glaciation event at the boundary
between the two epochs is especially interesting, Zachos says.
The climate system seems to have undergone a fundamental shift
at this boundary, which also marks a major break in the
"I'm not sure everyone will be convinced that the orbital
anomaly alone is responsible" Zachos says. "But the congruence
of those orbital cycles is a very rare event, and the fact
that it exactly corresponds with this rare climatic event is