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Measurement of natural remanent magnetization (NRM) of a 15-m interval of nannofossil-chalk cores from DSDP Site 445 revealed the presence of four magnetic-polarity transitions at around 25 Ma. Detailed measurements of NRM variation could be made because of the recovery of unbroken cores. The NRM was found to be stable with respect to alternating-field demagnetization. Magnetism studies on selected specimens indicated the presence of unoxidized titanomagnetite. Two of the four polarity transitions were rapid, and the polarity changed within 6000 years. The other two transitions were slow, and the polarity switched over 25,000 to 30,000 years; these transitions have the following features in common with transition zones observed by several investigators: (1) intermediate directions of magnetization, (2) a drop in the intensity and NRM/susceptibility ratio, and (3) a drop in magnetization intensity coincident with the directional fluctuations. Saturation-magnetization and bulk-susceptibility measurements on selected samples show that the decrease in NRM intensity associated with polarity change is likely not due to low concentration of ferromagnetic minerals. The present results support the model of field reversal in which the main dipole field decays to a low value and then builds up in the opposite direction. The present data show also that the geomagnetic dipole moment of reversed polarity is less stable than that of normal polarity. INTRODUCTION Studies on the geomagnetic-field reversals have been carried out by a number of investigators (see references). There is no need to emphasize that study of field reversals is important to explanation of the origin of the magnetic field of the earth. It is well established that deep-sea-sediment cores present a continuous record of the directional fluctuations and polarity changes of the past geomagnetic field. However, the absolute value of the geomagnetic-field intensity is hardly obtainable from the sedimentary cores. This problem usually is overcome by measuring either the Koenigsberger ratio of the natural remanent magnetization (NRM/[initial susceptibility]) or the ratio of NRM to saturation remanent magnetization. Both ideas are based upon the fact that the intensity of remanent magnetization is proportional to the total amount of ferromagnetic minerals and to the intensity of the geomagnetic field at the moment of sedimentation. During DSDP Leg 58, 892 meters of sediments was penetrated at Site 445, in the southeastern part of the Daito Ridge (25°31.36'N, 133°12.49'E). Below 140 meters sub-bottom, the sedimentary cores consisted mainly of consolidated nannofossil chalks and greenish mudstones. Between 417 and 445 meters sub-bottom, milky-colored, faintly laminated, undisturbed nannofossil chalk cores were recovered with a small number of disordering fractures within a 9-meter-long core. From preliminary NRM measurements, the cores were found to be magnetically stable and applicable to Paleomagnetism studies. MAGNETIC MINERALS A concentrate of ferromagnetic minerals was obtained from each of several portions of the cores by means of a hand magnet. The separated magnetic fractions consisted of dark and a few lighter-colored fine grains. High-field-strength (3000 oe) measurements of Js as a function of temperature were made in a vacuum of 10~ mm Hg; X-ray-diffraction and fluorescence analyses were performed on the magnetic separates to identify the minerals. For Js measurements, the samples were heated and cooled gradually at a rate of 4 to 5 deg/min. Examples of Js curves are shown in Figure 1. It is obvious that no drastic change in the ferromagnetic characteristics of the minerals occurred during heating. Curie temperatures are determinable from these curves. X-ray-diffraction shows lattice parameters of the ferromagnetic minerals of cubic symmetry. In addition, X-ray fluorescence shows that the metallic component

Details of Magnetic-Polarity Transitions Recorded in Sediment Cores from Deep Sea Drilling Project Site 445, Philippine Sea