CONFIRMATION OF THE ASTRONOMICAL CALIBRATION OF THE MAGNETIC POLARITY TIMESCALE FROM SEA-FLOOR SPREADING RATES

THE dating of the history of reversals of the Earth's magnetic field since the Jurassic period has been based primarily on radiometric dating of a small number of reversals, with most ages of individual reversals interpolated or extrapolated on the basis of the widths of marine magnetic anomalies. Recently, however, Pliocene and Pleistocene reversals have been dated by an entirely independent method, which relies on correlating climate proxies (such as oxygen isotope ratios) in sediments with calculated variations of the Earth's orbit and inclination1-4. The dates obtained from this astronomical calibration are approximately 3-8% older than radiometrically calibrated ages published between 1979 and 1989 (see, for example, refs 5 and 6), although recent dating by the Ar-40/Ar-39 method tends to confirm the astronomical calibration7-10, at least for the past 3.3 Myr. Here I use sea-floor spreading rates determined from high-precision plate-rotation solutions for five plate pairs to test the astronomically calibrated timescale over the past 5.32 Myr. The results suggest that the errors in the astronomical calibration are no greater than 0.02 Myr, and also show that spreading rate can remain constant for several million years, even for fast-moving plates.