Determination of Geomagnetic Palaeointensities in Vacuum

Summary. Specimens with low and intermediate Curie points from six Hawaiian historic basalt flows were used for the determination of the intensity of the historic geomagnetic field by the Thelliers' method. The specimens were heated either in air or in a vacuum of 10⁻⁵ torr. The regional intensity of the Earth's magnetic field at the time of extrusion of these rocks is known from direct observations. The palaeointensities determined in vacuum from four low and intermediate Curie point flows are correct within the uncertainty caused by the local field anomalies, whereas those determined in air from the same four flows are of lesser quality and accuracy. Unaltered submarine basalts have similarly low Curie points and thus may also be amenable to palaeointensity determination in vacuum. The behaviour of the remaining two flows, which had higher Curie points, was more erratic, and they yielded less accurate palaeointensities regardless of whether they were determined in air or vacuum.     

Geomagnetic secular acceleration

Summary. A spherical harmonic model of the second time-derivative of the geomagnetic field is determined, for the first time, directly from measures of the secular acceleration based on observatory annual mean data. The data span the interval 1964.5–1975.5, and 165 observatories are included. The model comprises the 32 coefficients of degree and order up to 6 that are significant at the 5 per cent level. Its primary purpose is to aid in the reduction of data to epoch for the 1980 series of navigational charts. The model is compared with earlier estimates of secular acceleration, derived by less direct methods.     

Geomagnetic excursion in the late Cretaceous

Summary. Two sedimentary cores from the western Pacific display a palaeo-magnetic record of the late Cretaceous long normal interval and the boundary reversed interval corresponding to seafloor spreading anomalies 33–34. Near the young end of this reversed interval, a systematic excursion of inclinations is observed in both cores. Samples are very stable to both alternating field and thermal demagnetization. Blocking temperatures and Curie points suggest that the remanence is carried primarily by magnetite, but with an additional contribution from hematite. Approximate sedimentation rates derived from biostratigraphy suggest that the excursion had a duration of between 46 000 and 54 000yr and occurred about 236000–303000 yr before the succeeding polarity reversal. The excursion, thus, may represent an aborted geomagnetic field reversal.