Keywords:geomagnetic reversal, Boso Peninsula, FORC PCA, magnetite, greigite, thermal decomposition
We report paleomagnetic records of Matuyama-Brunhes geomagnetic polarity reversal associated key tephra layers from sediments of an outcrop in Boso Peninsula. The outcrop is ~4m height along a roadside in Terasaki, Chiba Prefecture, Japan. The sediment mainly consists of massive silt of Early-Middle Pleistocene Kokumoto Formation, Kazusa Group underlaid by thick sand. A 2mm-thick vitric tephra layer could be identified in the middle of the outcrop about 2 m above the boundary between silt and sand. Chemical compositions of the vitric tephra could be correlated to TNTT (Byk-E) tephra taken along Yoro River section, which is associated with Matuyama-Brunhes polarity transition in the previous studies. Drill cores were taken at intervals of 1~10 cm from the massive silt spanning the vitric tephra layer and 2-3 paleomagnetic samples were cut from each drill core. Neither progressive alternating field demagnetization nor thermal demagnetization was not successful in revealing primary remanent magnetization. In general, progressive thermal demagnetization up to 175°C in vacuum followed by alternating field demagnetization was successful. In order to understand magnetic mineral change during thermal demagnetization, progressive heating experiments while monitoring FORC distributions were conducted. Principle component analysis of FORC revealed considerable change of FORC components by heating starting from 200°C to 400°C. The change is indicative of decomposition of higher coercivity mineral, which is considered as greigite. In the presentation, we will discuss magnetic mineral changes more in detail.