A rare coincidence of orbital cycles may have caused sudden global cooling
23 million years (Ma) ago, according to scientists, who used high resolution
records and new techniques that allow astronomical calibration to be extended
much further back in time.

The late Oligocene to early Miocene Earth (20-26 Ma ago) experienced a complex
climate history, including a stepwise transition towards cooler climate, with
partial glaciations of the Antarctica. At the boundary between the two periods,
scientists have also discovered a blip in isotope records that could only have
been caused by a short but expansive glaciation of Antarctica, coupled with
several degrees of sea cooling. The question is — what caused these climatic

Using information from Ocean Drilling Project (ODP) sites 926 and 929 (western
equatorial Atlantic), the team of US and UK scientists has put together an
uninterrupted and high-fidelity chronology of Oligocene and early Miocene
isotope geochemistry that enables them to reconstruct the climatic conditions
of that time. Results of their work will be shared at the Earth Systems
Processes conference on Wednesday, June 27, in Edinburgh, Scotland. The
Geological Society of America and the Geological Society of London will
co-convene the June 24-28 meeting.

The Earth’s climate varies in a cyclic manner first defined by the scientist
after whom the cycles are named, Milutin Milankovitch. The cores studied show
persistent periodic climatic variations as the Milankovitch theory would
suggest. However there were unusually strong signals where the variability in
isotope signals increased considerably — between 21.4 and 22.9 Ma ago, and
weaker signals where the isotope signal decreased — between 22.9 and 23.3 Ma

Astronomical calculations suggest that over this 0.4 million-year period there
were four consecutive cycles involving low amplitude variance in orbital
obliquity (the inclination of the Earth’s orbit to the plane of the ecliptic)
during a period of low orbital eccentricity (relatively less elliptical
orbits). The net result of this was a 200,000-year period of unusually low
seasonality. This, the researchers believe, could have been responsible for
the step-like growth of the Antarctic ice-sheet at the time, and the prolonged
period of cooling.

Lead Author James Zachos (University of California, Santa Cruz) said “This
unique isotope record provides a rare window into how the climate system
responded to orbital forcing during the Earth’s more distant past.”


During the Earth System Processes meeting, June 25-28, contact the GSA/GSL
Newsroom at the Edinburgh International Conference Centre for assistance and
to arrange for interviews: +44 (0) 131 519 4134

Ted Nield, GSL Science and Communications Officer
Ann Cairns, GSA Director of Communications

The abstract for this presentation is available at:

Post-meeting contact information:

James Zachos

Earth Sciences

University of California, Santa Cruz

Santa Cruz, CA 95054 USA

+01 (0) 831 459 4644


Ted Nield
Geological Society of London

+44 (0) 20 7434 9944


Ann Cairns

Geological Society of America

+01 303 447 2020 ext. 1156