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    201609201609

    A record of the upper Olduvai geomagnetic polarity transition from a sediment core in southern Yokohama City, Pacific side of central Japan

    Kusu C, Okada M, Nozaki A, Majima R, Wada H

    Geomagnetism, Paleomagnetism, Geomagnetic polarity transition, Olduvai subchron, VGP path, VGP cluster, Reversal, Relative paleointensity, Sediment core

    Paleomagnetic results in the vicinity of the upper Olduvai polarity reversal. (a): VGP path, (b): VGP latitude, (c): relative paleointensity (NRM20-40 mT/ARM20-40 mT) from Core M and Core I. The blue shaded areas in (a) indicate that the vertical component of the NAD field at the Earth’s surface is less than −15 µT (Constable 2007). The stars indicate the core sites.

    A detailed paleomagnetic record of the upper Olduvai polarity transition was obtained from a 106.72 m-long sediment core drilled in southern Yokohama City, located on the northern Miura Peninsula, on the Pacific side of central Japan. The core spans the upper part of the Nojima Formation and the lowermost part of the Ofuna Formation, both of which correspond to the middle Kazusa Group (Lower Pleistocene forearc basin fill). The record was reconstructed using discrete specimens taken throughout mudstone and/or sandy mudstone sequences in the Nojima Formation. In this record, the virtual geomagnetic pole (VGP) fluctuation accompanying the polarity transition was determined to occur between depths of 66.99 and 63.60 m. These depths have been dated at 1784.4 and 1779.9 ka, respectively, and the duration of the polarity transition is estimated to be 4.5 kyr using an age model based on a δ18O record from that core. The VGP paths during the transition do not appear to show any preferred longitudinal bands. However, the VGP positions cluster in five areas: (A) eastern Asia near Japan, (B) the Middle East, (C) eastern North America (North Atlantic), (D) off southern Australasia, and (E) the southern South Atlantic off South Africa. The primary locations of the observed VGP clusters coincide with the areas on the Earth’s surface that possess a strong downward flux of the vertical component of the present geomagnetic non-axial dipole field. The relative paleointensity rapidly decreased approximately 1 kyr before the beginning of the polarity transition and gradually recovered to its initial level in 12 kyr.