Details of magnetic polarity transitions recorded in a high deposition rate deep-sea core

Abstract

Measurements of the NRM of a 26 m long deep-sea core from the southern Indian Ocean indicated the presence of three transitions of magnetic polarity which have been identified as the upper and lower Jaramillo and the upper Olduvai on the basis of micropaleontological criteria. Detailed studies of the magnetic reversals were made in view of the high deposition rates (~ 9 cm/10J yr) present over sections of the core. The NRM was found stable against alternating fields. Magnetic mineralogy studies indicated the presence of titanomagnetite and magnetite which probably have not undergone any significant low-temperature oxidation. The three polarity changes bad the following features in common : ( 1 ) presence of intermediate directions of magnetization; (2) a pronounced drop in the intensity ol magnetization; (3) the drop in intensity of magnetization was coincident with the large directional fluctuations. Measurements of saturation isothermal and anhysteretic remanence, and bulk susceptibility, show that the decrease in NRM intensity associated with each polarity change is not due to a low concentration of the magnetic minerals. The best estimate for the duration of a polarity transition is approximately 4600 yr. There is evidence for both eastward and westward drift of the non-dipole field, which appears to be dominant during the polarity transition interval. The data presented here support a model of a reversing field in which the main dipole field decays to a low value and then builds up in the opposite direction.