Refining the date of the K/T boundary and the dinosaur extinction

At Zumaia in the Basque country of northern Spain, sediments laid down around the end of the Cretaceous period show layers of light limestone and dark marl reflecting warm and cool periods, respectively, in Earth's climate. These alternating climatic periods are caused by 100,000-year and 405,000-year cycles in Earth's orbital eccentricity. Because Earth's orbit, and thus the relative ages of the sediment layers, can be precisely calculated, dating of the sediments by the argon-argon method provided a much-needed calibration of the method and made it possible to pinpoint the Cretaceous/Tertiary boundary at 65.95 million years ago. (Image courtesy of PNAS)