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Chilly Coincidence in Earth History
13 April 2001 7:00 pm
A rare combination of the shape of Earth's orbit and the tilt of its rotation axis some 23 million years ago led to a brief climatic cooling, researchers report in the 13 April issue of Science. This confluence can explain a longstanding mystery: the ultimate origin of an episode of sediment erosion that geologists use to mark the division between two time periods, the Oligocene and Miocene. The research also identifies a new way to make an ice age.
The coming and going of ice ages is linked to regular variations in the shape or eccentricity of Earth's orbit around the sun. Another factor is the changing tilt of the planet, which influences how much the summer sun warms the high latitudes--and how much snow accumulates into ice sheets. These steady changes in eccentricity and tilt are indirectly recorded in ocean sediment, which can varyies accordingly in color and other properties. The sediment also contains climate records in the microscopic skeletons of animals called foraminifera. The changing ratio of oxygen-18 to oxygen-16 along a core reflects varying bottom-water temperature as well as changes in the volume of glacial ice in the world.
These types of records have now yielded new insights into climatic events of the Oligocene-Miocene transition, between 20 million and 25.5 million years ago. From two cores in the western equatorial Atlantic, paleoceanographer James Zachos of the University of California, Santa Cruz, and colleagues extracted a complete, finely detailed 5-million-year record. After inspecting it, Zachos and colleagues found one geologic moment when the orbital forces for climatic cooling and ice sheet building came together. At 23.0 million years ago, eccentricity dropped to low levels and variations in tilt nearly disappeared. Simultaneously with these ideal orbital conditions for ice sheet formation, bottom waters cooled and ice volume increased. An ice age had arrived. That also drew down sea level, which exposed shallow-water sediments to erosion. When the orbital confluence disappeared, 200,000 years later, so did the extra ice.
The convergence of orbital variations would be "a very reasonable explanation" for the glacial boundary event, says paleoclimate modeler Thomas Crowley of Texas A&M University in College Station, as well as further support for the power of orbital variations to influence climate.