TOWARD A REVISED CALIBRATION OF THE PALEOGENE TIMESCALE

Despite improvements in ⁴⁰Ar/³⁹Ar dating that have resulted in ages routinely accompanied with relative analytical uncertainties of a few tenths of a percent, considerable uncertainty persists as to “absolute” age calibration as derived from first principle and intergeochronometer calibrations. Long term reproducibility of ⁴⁰Ar/³⁹Ar ages and intercalibration of various ⁴⁰Ar/³⁹Ar standards likewise have been shown to vary within a few percent, although interlaboratory variations may be significantly greater. Current variation in the “absolute” age of commonly used ⁴⁰Ar/³⁹Ar dating standards, unfortunately, remains between one and three percent. To minimize confusion in the literature, Earthtime Ar geochronologists in 2005, proposed an interim solution, by reporting ⁴⁰Ar/³⁹Ar ages relative to Fish Canyon Sanidine at 28.02 Ma. Unfortunately, currently used geologic timescales are not in agreement with this calibration adding confusion amongst “users” of geochronologic data. Current Cenozoic timescales for example, mix ages of various derivatives including astronomically derived ages for the Neogene and isotopically derived ages for the Paleogene. As a result, ages for the Paleogene appear not to be in sync with those used for the Neogene. To address this issue, we report new ⁴⁰Ar/³⁹Ar ages on key Paleogene calibration points, intercalibrating these results directly with aliquots of Fish Canyon Sanidine and the astronomically tuned A1 Ash (Faneromeni). Results indicate about a one percent underestimate in the age of the Paleogene compared with the Neogene, much of this accounted for by the age of Fish Canyon Sanidine at 28.02 Ma rather than 27.84 Ma (BKSA 95), less discrepancy is noted in the recalibrated GOS 2004 timescale. Intercalibration with the astronomically tuned Faneromeni A1 Ash indicates an age of 28.13 Ma for Fish Canyon Sanidine.