Paleointensity record from Pleistocene sediments (1.4–0 Ma) off the California Margin

Abstract

High resolution magnetic measurements of 45 m of sediment from the Delgada Fan area at Ocean Drilling Program Site 1021 (39°5′N/127°46′W) were performed in order to extract a reliable signal of relative paleointensity. A detailed chronology was established by correlation of the magnetic susceptibility variations to the insolation curve for the past 1.4 Myr covered by the record. The remanence carrier is pseudo‐single‐domain‐sized magnetite that varies in concentration by a factor <3. Stepwise alternating field demagnetization isolated a very stable natural remanent magnetization (NRM) which showed the succession of the Cobb and Jaramillo Subchrons and directional changes associated with the Punaruu event (1.105 Ma). Two techniques have been used to extract the signal of relative paleointensity. A significant climatic component remained present in the curve obtained from the slopes of the NRM versus the anhysteretic remanent magnetization(ARM). In contrast the NRM and the susceptibility (klf) both being driven by orbital forcing, normalization by susceptibility is not contaminated by climatically driven changes and is thus taken as our best estimate of relative paleointensity. Comparison with other high‐resolution records and compilations reinforces the actual geomagnetic origin of the variations displayed by the NRM/k signal. Overall, this record confirms the succession of distinct highs and lows which are concomitant with full field reversals and short‐term geomagnetic events present during the Matuyama Chron. The presence of large oscillations of the field with no periodicity confirms that the geomagnetic field (at least his dipolar part) is highly unstable and may have remained in this mode for a long period of time.