Pinning down the Brunhes/Matuyama and upper Jaramillo boundaries: a reconciliation of orbital and isotopic time scales

Abstract

Until recently, the temporal calibration of the Plio/Pleistocene portion of the geomagnetic reversal time scale (GRTS) was based exclusively on KAr isotopic ages of volcanic rocks. The so-called “chronogram technique” estimates the age that minimizes discrepancies between a given age assignment for a reversal boundary and the population of dates of known polarity. The most widely quoted date, 0.73 Ma, for the Brunhes/Matuyama boundary was derived in such a manner [1].Radioactive decay is not the only source of temporal information in the geologic record, however. Variations in several of the orbital parameters of the Earth (e.g. precession, obliquity, eccentricity) can be calculated for the last several million years and the climatic response approximated. A recent analysis of the so-called “astronomical technique” on oxygen isotopic data from deep-sea sediments suggested an age of 0.78 Ma for the Brunhes/Matuyama boundary [2], some 50,000 years older than the chronogram estimate.We reconcile the two estimates for the age of the last reversal by first presenting new magnetostratigraphic data tied to high-quality40Ar39Ar dates. Our date for the upper Jaramillo boundary is0.992 ± 0.039 Ma, considerably older than the estimate of 0.91 Ma in the standard (chronogram-based) time scale [3]. Furthermore, an age of0.746 ± 0.009 Ma was obtained for sediments immediately overlying the Brunhes/Matuyama boundary. These data are consistent with the astronomical estimates of 0.78 Ma and 0.99 Ma for the Brunhes/Matuyama and upper Jaramillo boundaries, respectively.Finally, we present a new method for estimating the uncertainties of ages calculated using the chronogram technique that employs a simple bootstrap resampling scheme. The 95% confidence region for the chronogram estimate of the Brunhes/Matuyama boundary, using an updated compilation of date/polarity pairs, spans from 0.73 to 0.78 Ma. We find, therefore, that the discrepancies between the isotopic data set and the astronomical calibration of the Brunhes/Matuyama boundary disappear when appropriate uncertainties are applied for the most recent compilation of isotopic data.