Relative geomagnetic field intensity across the Jaramillo subchron in sediments from the California margin: Ocean Drilling Program Leg 167

Two relative paleointensity records of the Jaramillo subchron were obtained during Ocean Drilling Program Leg 1.67 at Sites 1010 and 1021 on the Calif ornia margin. The two sites are similar to 1300 km apart and differ in sedi ment composition and deposition rate. The demagnetization of the natural re manent magnetizations (NRM) of discrete samples revealed a vertical drillin g overprint at both sites which could be removed by alternating field demag netization at 20 mT for the sediments from Site 1010. This was confirmed by a positive reversal test on the characteristic remanent magnetization of t he discrete samples across the Jaramillo subchron. Rock magnetic criteria f or determining relative paleointensities are fulfilled for the samples from Site 1010. :Because of variations in the rockmagnetic properties, two shor t intervals had to be excluded from relative paleointensity determinations at Site 1021. Normalization of the NRM at several demagnetization steps wit h various magnetic parameters yielded similar results only for Site 1010, a nd contributions from viscous remanent magnetization could not be detected in the sediments from this site using the pseudo-Thellier method of Tauxe e t al. [1995]. The relative paleointensity data of Site 1021 show variations of up to 50% between normalizations with different magnetic parameters and therefore had to be excluded from further analysis. Several fine-scale int ensity features are detectable in the record of Hole 1010C, displaying a ma ximum in relative paleointensity shortly above the lower Jaramillo transiti on, a prominent local minimum in the middle of the subchron, and a gradual decrease toward the upper Jaramillo reversal. A comparison of tl lis record from the California margin with several published records of relative pale ointensity across the Jaramillo subchron with different sedimentation rates and sediment composition shows reasonable agreement between the paleointen sity patterns but differences in amplitudes. This suggests an influence of postdepositional magnetization processes which were not removed during norm alization.