EVIDENCE FOR A SYSTEMATIC ERROR IN GRIP ICE-FLOW CHRONOLOGY

Dansgaard-Oeschger (D-O) cycles recorded in the GRIP ice core oxygen isotope series are spectrally bifurcated over 0-100 Ka, while their counterparts in the neighboring GISP2 ice core are not. Time-frequency analysis reveals that this bifurcation is caused by a frequency modulation affecting the cycles that is phased with the Earth's obliquity variation. The frequency-independent nature of the modulation suggests that it is the result of a systematic error in the GRIP ice-flow chronology. For times less than 50 Ka, this error can be estimated by correlation of the GRIP isotope series with the varve-calibrated GISP2 isotope series. For times previous to 50 Ka, where GISP2 chronology is not determined by varves, the error can be tracked by frequency modulation analysis of the GRIP D-O cycles over 0-100 Ka. This estimated error is then used to adjust the GRIP chronology over the entire 0-100 Ka interval. The adjustment brings the GISP2 and GRIP isotope series into close alignment over 0-50 Ka; timing deviations between the two series that remain prior to 50 Ka are assumed to be due to errors in GISP2 chronology. This adjusted GRIP chronology realigns the Earth’s orbital modes in the isotope series to their proper frequency positions, whereas without the adjustment, spectral analysis detects obliquity and precession components with frequencies that are slightly higher than predicted. This discovery of an apparent systematic error in the GRIP ice-flow chronology raises the possibility that similar errors may exist in other ice core chronologies that rely on ice-flow dating techniques, for example, Antarctic ice stratigraphy, where annual accumulations are generally too low for varve resolution.