Low absolute paleointensity during Late Miocene Noma excursion of the Earth’s magnetic field

Abstract

Paleointensity variation is investigated for a sequence of andesitic lavas which records the Noma geomagnetic excursion at 6.66 ± 0.45 Ma. Sixty-six samples are collected from 11 lava flows at the Kamegaoka Mountain (31° 21′N, 130° 13′E) on the Noma Peninsula, Kyushu Island, Japan. Rock magnetic experiments confirm the presence of Ti-poor titanomagnetite grains with pseudo-single-domain or a mixture of single and multi-domains as the carrier of the remanent magnetization. The application of the Tsunakawa-Shaw method yields reliable geomagnetic paleointensity data set of 29 samples from 9 lavas that cover the Noma excursion. The mean paleointensities of lava flows vary from 1.9 to 10.3 µT. Paleointensity variation reveals a ‘V -shape’ of high–low–high paleointensities during the excursion. The highest value of 10.3 ± 3.5 µT appears prior to the excursion, the paleointensity decreases down to 1.9 ± 0.3 µT as the excursion progresses, and the paleointensity finally regenerates to 5.1 ± 3.0 µT at the end of the excursion. The virtual dipole moment (VDM) during the Noma excursion (0.3–1.1 × 1022 Am2) is less than 16% of the mean VDM during the period between 4 and 8 Ma in Miocene, suggesting that this weak geomagnetic field intensity is ascribed to the nature of excursions rather than the Miocene geomagnetic field properties. The weakest VDM (0.3 × 1022 Am2) in the Noma excursion is detected at the pole position that is located at fairly high latitude (58.9°N) in the opposite hemisphere. The Noma excursion is characterized by a large shift in virtual geomagnetic poles of more than 140° from the geographic pole and by a weak VDM of 1.0 × 1022 Am2 or less.