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Published radioisotopic (K/Ar, 40Ar/39Ar, and Rb/Sr) and astronomical ages for the Eocene-Oligocene boundary are essentially consistent at ca. 33.8 ± 0.1 Ma, but the 40Ar/39Ar ages have been calculated relative to an outdated age of 27.83–27.84 Ma for the Fish Canyon Tuff sanidine dating standard. Application of a revised age of 28.02 Ma, or the new astronomically calibrated age of 28.201 Ma, leads to significant discrepancies, while others are eliminated. In particular, the astronomically tuned ages of ca. 33.79 Ma at Ocean Drilling Program (ODP) Site 1218 and of 33.90–33.95 Ma at Massignano–Monte Cagnero are now in good agreement with recalculated (alternative) 40Ar/39Ar sanidine ages for the boundary as derived from the volcanic ignimbrite complex in New Mexico and for the Persistent White Layer (PWL) ash bed in North America, which is supposed to closely correspond to the boundary. This mutual consistency suggests that the tuning is correct at the scale of the 400 k.y. eccentricity cycle.

Evidently, additional single-crystal 40Ar/39Ar sanidine dates from the tuffs in North America and independent checks on the astronomical tuning and the intercalibration between the astronomical and 40Ar/39Ar dating methods are needed to definitively solve the problem of the numerical age of the Eocene-Oligocene boundary. It is anticipated that such analyses and tests will be carried in the coming years as part of the international Earthtime initiative and associated projects to significantly improve the geological time scale. Clearly, an accurate and precise dating of the Eocene-Oligocene boundary is crucial if we are to unravel the underlying cause of the major climate transition associated with it.

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