INTERCALIBRATION OF ASTRONOMICAL AND RADIOISOTOPIC TIME

The Ar-40/Ar-39 radioisotopic dating technique is one of the most prec ise and versatile methods available for dating events in Earth's histo ry, but the accuracy of this method is limited by the accuracy with wh ich the ages of neutron-fluence monitors (dating standards) are known. Calibrating the ages of standards by conventional means has proved di fficult and contentious. The emerging astronomically calibrated geomag netic polarity time scale (APTS) offers a means to calibrate the ages of Ar-40/Ar-39 dating standards that is independent of absolute isotop ic abundance measurements. Seven published Ar-40/Ar-39 dates for polar ity transitions, nominally ranging from 0.78 to 3.40 Ma, are based on the Fish Canyon sanidine standard and can be compared with APTS predic tions. Solving the Ar-40/Ar-39 age equation for the age of the Fish Ca nyon sanidine that produces coincidence with the APTS age for each of these seven reversals yields mutually indistinguishable estimates rang ing from 27.78 to 28.09 Ma, with an inverse variance-weighted mean of 27.95 +/- 0.18 Ma. Normalized residuals are minimized at an age of 27. 92 Ma, indicating the robustness of the solution.