A long‐term geomagnetic excursion from Plio‐Pleistocene sediments in Java

Abstract

A paleomagnetic record from the Plio‐Pleistocene sediments in Sangiran, central Java represents a long‐period change of the geomagnetic field and reveals an existence of large‐scale swing of declination between the Olduvai and Jaramillo events. The field directions in reverse polarity show a gradual clockwise swing to almost west‐pointing directions, followed by a rapid return to south‐pointing directions. The geomagnetic behavior has been well duplicated from the Plio‐Pleistocene sediments in Mojokerto, 200 km west of Sangiran. At both sites, the declinations have maximum deflection more than 70° from the axial dipole field direction and the inclinations remain low during the swing. The full duration of the swing, recorded over sections about 40 m thick at both sites, was estimated about 130,000 years and a zone of most deviated fields 50,000 years, assuming a constant sedimentation rate. The phenomenon was regarded as an excursion. Neither geomagnetic excursion nor geomagnetic event corresponding to the entire duration of the excursion has been reported at present, although only the final stage, a duration of most deviated fields, may be associated with the geomagnetic events reported from North America and Europe. An upward‐pointing eccentric radial dipole model rather than a downward‐pointing one seems plausible to explain the geomagnetic behavior during the excursion. The former becomes one possible source for the inclination anomalies in the time‐averaged fields of the Plio‐Pleistocene times in the equatorial areas. A short sequence of the Upper Olduvai transitional fields obtained from Mojokerto exhibits a VGP path confined to a band of longitude 150°W ± 20°, inconsistent with those from other sites on the globe. This suggests that a dipolar component was not dominant globally in the Upper Olduvai transitional fields.