An improved new age for the C33n-C32r paleomagnetic reversal, San Juan Basin, NW New Mexico and SW Colorado
— J.E. Fassett and M.T. Heizler

Three new ⁴⁰Ar/³⁹Ar sanidine dates of tephra deposits, along with existing stratigraphic and paleomagnetic data, are used to constrain the magnetochron C33n-C32r reversal to 73.7 Ma. These 3 ages agree well with previously published dates when the later are corrected to equal ⁴⁰Ar/³⁹Ar flux monitor age and ⁴⁰K decay constant. This age contrasts significantly with the Geologic Time Scale of Gradstein et al. (2012) that places the boundary at 74.6 Ma; 600 ka older. A wide range of error estimates (0.05-0.4 Ma) for the reversal age depend on the statistical approach chosen and demonstrate the difficulty in unambiguously determining the uncertainty of magnetochron boundaries that are based on ages of ash layers that stratigraphically bracket the reversal. Both discrepancies in reported ages of reversal boundaries and their uncertainties plague global time scales and impact the ability to unambiguously determine rates of important evolutionary processes and to make stratigraphic correlations. Here we recommend that our new age of 73.7±0.4 Ma (a conservative error) be adopted for the C33n/C32r paleomagnetic reversal and to be the established age for this reversal in future global time scales.