Magnetostratigraphy and relative palaeointensity of late Neogene sediments at ODP Leg 167 Site 1010 off Baja California

Abstract

Sediments recovered from the most seaward site of the Baja Transect of the California Margin (ODP Leg 167) yielded a high-fidelity magnetostratigraphic record spanning the last 6 Myr. Using a long-core cryogenic magnetometer, we made NRM measurements of 59 u-channel samples, which cover the interval from 0 to 78.5 mcd (metres composite depth) of the spliced section of Holes 1010C and 1010E. Stepwise AF demagnetization between 20 and 60 mT showed that the remanence is essentially composed of a single stable component, which was not affected by a coring-induced magnetization. The investigated section can be divided into at least 25 polarity intervals, which are assigned to the chrons from C3A.n2 to the Brunhes (C1n). The sedimentation rate was nearly constant at about 13 m Myr-1, except for a reduction to 9 m Myr-1 between 4 and 5 Ma. ARM and IRM measurements of the u-channels revealed that the sediments below 23 mcd exhibit sharp drops in magnetic concentration at several horizons, but that the upper part has a relatively uniform magnetic concentration, containing magnetite of pseudo-single-domain size. The upper interval yielded consistent relative intensity records when normalized with ARM, IRM and shipboard susceptibility data. Several peaks in the relative intensity variations correspond to drops in the S ratio, in particular at 0.1 T, or to anomalous peaks in the IRM and magnetic susceptibility profiles, suggesting that the normalized intensity record is partly influenced by episodic changes in the flux of magnetic grains. The normalized intensity for the intervals younger than 1.2 Ma shows similarities with published palaeointensity records from the equatorial Pacific, including results from ODP Leg 138. In particular, the intensity minima at 0.05, 0.11, 0.18 and 0.29 Ma and those associated with the onset and termination of the Jaramillo subchron are well correlated.