Reconciling astrochronological and 40Ar/39Ar ages for the Matuyama-Brunhes boundary and late Matuyama Chron

When Matuyama-Brunhes (M-B) boundary records from the North Atlantic are placed on isotope age models, produced by correlation of the δ18O record directly or indirectly to an ice volume model, the M-B boundary lies consistently at the young end of marine isotope stage (MIS) 19 with a mean age for the mid-point of the reversal of 773.2 ka (std deviation = 0.3 kyr). This M-B boundary age is ~7 kyr younger than the traditional astrochronological age for this polarity reversal (780-781 ka) that was based on lower resolution (lower sedimentation rate) records. Two recently proposed revisions of the age of the 40Ar/39Ar Fish Canyon sanidine (FCs) standard to 28.201 ± 0.046 Ma and 28.305 ± 0.036 Ma would adjust 40Ar/39Ar 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 traditional (Renne et al., 1998) age for this standard (28.02 ± 0.16 Ma), but younger than the recently proposed ages of 28.201 and 28.305 Ma. 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 10Be 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. Sub-orbital oscillations in benthic δ18O in the upper part of MIS 19 at ODP Site 983 appear out-of-phase with similar features in the planktic δ18O record from the same core. If Site 983 benthic δ18O is synchronized with δDice oscillations at Dome C, by shifting the δDice record, consistent with the “bi-polar seesaw” implied by some δ18O records from the North Atlantic (e.g. Portuguese Margin), the ice-core 10Be flux peak coincides with the marine M-B boundary, thereby supporting the revised age for the M-B boundary. The revised astrochronological ages for the M-B boundary and late Matuyama reversals/excursions appear, however, to be inconsistent with recently proposed FCs ages of 28.201 Ma and 28.305 Ma.