絶対古地磁気強度測定法の進展と新方法による過去５００万年間の平均地球磁場強度―現在の地磁気は異常に強い？―

Abstract

The geomagnetic field generated by the geodynamo is one of the most important properties of the Earth. Since the intrinsic field can be approximated by a dipolar one with time variation, it is essential to know time-averaged value of past geomagnetic dipole moments for evaluating the current status of the geodynamo. From archeological materials and volcanic rocks, we can measure absolute paleointensities and estimate past geomagnetic dipole moments as virtual axial dipole moments (VADMs). The Thellier method with the pTRM check is so far regarded as the most reliable paleointensity determination technique. Many efforts have been made to increase its reliability. Based on the Thellier paleointensities reported so far, the time-averaged VADM for the last five million years is believed to be almost the same as the present geomagnetic dipole moment (8 × 1022 Am2) However, it has recently been revealed that Thelliertype methods have some problems resulting in the overestimation of paleointensities. Instead, the LTD-DHT Shaw method, a lately developed technique in Japan, shows a higher degree of reliability for paleointensity determination of historical lava flows. Applying this method, our study of volcanic rocks from the Society Islands, French Polynesia, gives a new mean VADM of 3.64 ± 2.10 × 1022 Am2, though number of the data is still small. This average is significantly lower than the previous one and is nearly half of the present dipole moment, suggesting that the current status of the geomagnetic field may not be typical of the geodynamo.