RELATIVE PALEOINTENSITY OF THE GEOMAGNETIC FIELD OVER THE LAST 200,000 YEARS FROM ODP SITES 883 AND 884, NORTH PACIFIC OCEAN

Abstract

Ocean Drilling Program Sites 883 and 884 were cored as part of a three-site depth transect down the slopes of Detroit Seamount in the North Pacific Ocean. Continuous mineral magnetic and paleomagnetic measurements were made using u-channel samples for sediments that span the last 200 kyr. Thermomagnetic and high-field hysteresis data indicate that the magnetic mineralogy of the clay-rich sediments at the two sites is dominated by pseudo-single domain magnetite in a narrow range of grain sizes and concentrations, thereby meeting established criteria for relative paleointensity studies. The natural remanent magnetization (NRM) has been normalized using the anhysteretic remanent magnetization (ARM) and the low field magnetic susceptibility (χ). The NRM/ARM and NRM/χ records are nearly identical for both cores. Furthermore, coeval horizons in the two cores can be correlated by matching >100 magnetic susceptibility features over the 15 m length of both cores. Coherence function analysis indicates that the records are not significantly affected by local environmental conditions. These factors suggest that the large-scale variations in normalized remanence are most likely due to geomagnetic paleointensity fluctuations. Dating constraints are provided by a δ18O stratigraphy from Site 883. Our North Pacific paleointensity versus age curve is similar to a published record from the western Caroline basin and a recently proposed global paleointensity curve. Offsets (up to 10 kyr) in the timing of paleointensity features between our composite North Pacific record and the global curve may result from imprecisions in the dating of our record. Nevertheless, the correspondence between the paleointensity records suggests that, in suitable sediments, paleointensity of the geomagnetic field can give a globally coherent, dominantly dipolar, signal.