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

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 degrees 21'N, 130 degrees 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 mu T. Paleointensity variation reveals a 'V -shape' of high-low-high paleointensities during the excursion. The highest value of 10.3 +/- 3.5 mu T appears prior to the excursion, the paleointensity decreases down to 1.9 +/- 0.3 mu T as the excursion progresses, and the paleointensity finally regenerates to 5.1 +/- 3.0 mu T at the end of the excursion. The virtual dipole moment (VDM) during the Noma excursion (0.3-1.1 x 10(22) Am-2) 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 x 10(22) Am-2) in the Noma excursion is detected at the pole position that is located at fairly high latitude (58.9 degrees N) in the opposite hemisphere. The Noma excursion is characterized by a large shift in virtual geomagnetic poles of more than 140 degrees from the geographic pole and by a weak VDM of 1.0 x 10(22) Am-2 or less.