University of Wisconsin-Madison, United States

When five Matuyama-Brunhes (M-B) boundary records from the North Atlantic are placed on isotope age models, produced by correlation of the delta (super 18) O record directly or indirectly to an ice volume model, the M-B boundary lies consistently at the young end of marine isotope stage 19 with a mean age for the midpoint of the reversal of 773.1 ka (standard deviation=0.4 kyr), approximately 7 kyr younger than the presently accepted astrochronological age for this polarity reversal (780-781 ka). Two recently proposed revisions of the age of the (super 40) Ar/ (super 39) Ar Fish Canyon sanidine (FCs) standard to 28.201+ or -0.046 Ma and 28.305+ or -0.036 Ma would adjust (super 40) Ar/ (super 39) Ar ages applicable to the M-B boundary (and other reversals and excursions back to 1.2 Ma) to ages older than the new astrochronological ages by 8-24 kyr. The variables used to construct the ice volume models cannot account for the discrepancy. The FCs standard age that best fits the astrochronological ages is 27.93 Ma, which is within the uncertainty associated with the commonly used value of 28.02 (+ or -0.16) Ma but younger than the recently proposed FCs ages. The EDC2 and EDC3 age models in the Dome C (Antarctic) ice core yield ages of 771.7 ka and 766.4 ka, respectively, for the (super 10) Be flux peak that denotes the paleointensity minimum at the reversal boundary, implying that the EDC2 (rather than EDC3) age model is consistent with the observations from marine sediments, at least close to the M-B boundary.