Earth and Moon. Photo: NASA/GSFC

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 excursions?

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 were shared at the Earth Systems Processes conference last week in Edinburgh, Scotland. The Geological Society of America and the Geological Society of London co-convened 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 ago.

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."