A record of the lower Mammoth geomagnetic polarity reversal from a marine succession in the Boso Peninsula, central Japan

Abstract

SUMMARY Palaeomagnetic records from geological archives provide significant information about the nature of geomagnetic polarity reversals; however, there are few detailed palaeomagnetic records of pre-Pleistocene reversals. The lower Mammoth Subchron boundary (late Pliocene) is recorded in a 10-m interval of a marine succession deposited at high accumulation rates (9–66 cm kyr–1) in the Boso Peninsula, central Japan. Here, we report a continuous palaeomagnetic record of the lower, normal to reverse boundary interval of the Mammoth Subchron, including the geomagnetic field direction and relative palaeointensity, with an average temporal resolution of ca. 800 yr. A hybrid method of thermal demagnetization at 200 °C and progressive alternating field demagnetization were used to effectively extract the primary palaeomagnetic component, which is carried by magnetite. The lower Mammoth transition is characterized by palaeomagnetic direction of instability and decay of the relative palaeointensity, and occurred from late Marine Isotope Stage MG3 (3351 ka) to MG2 (3336 ka) or MG1 (3331 ka), spanning 15–20 kyr. Virtual geomagnetic poles (VGPs), calculated from primary palaeomagnetic directions, rapidly rebounded twice from southern latitudes to northern latitudes within the transition. In contrast to the complex lower Mammoth reversal behaviour recorded in the Boso Peninsula succession, records from a lava sequence in O'ahu (Hawai'i) reveal a rebound following a 180° directional change, and those from a marl succession in Sicily (Italy) indicate a single rapid directional change. Diverse geomagnetic field evolution among these three sections is reflected resolution difference among the records likely in combination with an influence of non-axial dipole field.