Changing spectrum of geomagnetic intensity variations in a fragmented 12 My sediment record from the Oligocene

Time series of relative geomagnetic paleointensity variations derived from marine sediments can be calibrated using absolute data derived from igneous materials. The resulting records may be suitable for spectral analysis of geomagnetic dipole variations. This work re-evaluates the 12 My (22.74–34.77 Ma) sediment record from Deep Sea Drilling Project Leg 73, Site 522, that is a key data set for determining the paleomagnetic power spectrum in the frequency range 1–100 My−1. The 12 My record is marred by uneven sampling, with the interval between samples ranging from 1 to 640 ky, and contains several gaps that are considered too long to interpolate. The relative intensity data are calibrated using 129 globally distributed absolute paleointensity data from the same time interval. The power spectrum of the resulting time series is estimated using direct multi-taper spectral estimation with prolate data tapers adapted to deal with missing sections in the time series. The longest record available for analysis is thereby extended from 5.3 to 12 My. The new paleomagnetic power spectrum confirms the presence of a broad spectral peak at around 8 My−1 for the early Oligocene and uncovers a peak around 2.5 My−1 in the late Oligocene. Both peaks may be linked to tiny wiggles in marine magnetic anomalies. The new analysis unambiguously verifies that there is lower overall power in the younger part of the record, where the reversal process appears to dominate the power spectrum of the paleosecular variation. A comparison of the late Oligocene spectrum with that of PADM2M, a model of paleomagnetic axial dipole variations for 0–2 Ma, reveals some broad similarities; both time periods have similar power levels and a reversal rate of 4 My−1. During the early Oligocene the reversal rate is about a factor of two lower, the field strength is higher, and the secular variation is stronger, suggesting that a strong magnetic field inhibits reversals but produces more variability in field strength.

► A 12 Myr relative paleomagnetic intensity record is calibrated using absolute data.
► Gap-adapted multi-tapers are used to estimate the power spectrum of changes in VADM.
► VADM variations differ in style between the early and late parts of the Oligocene.
► During the late Oligocene variations are dominated by reversal processes.
► In the early part, average VADM is stronger and has increased power around 8 Myr-1.