[S-EM13] Geomagnetism, paleomagnetism and rock magnetism

[SEM13-P01] Brunhes-Matuyama geomagnetic reversal record from Terasaki, Boso Peninsula

*Hirokuni Oda¹, Hiroomi Nakazato², Futoshi Nanayama¹, Yumiko Harigane¹ (1.Institute of Geology and Geoinformation, Geological Survey of Japan, AIST, 2.National Agriculture and Food Research Organization)

Keywords:Paleomagnetism, Boso Peninsula, Geomagnetic Reversal, greigite, combined analysis of remagnetization circles

We report Brunhes-Matuyama geomagnetic reversal records from Terasaki, which is several km northeast of Chiba Section, Boso Peninsula. Oriented paleomagnetic drill core samples were taken from ~3.5m thickness silt layers at intervals of a few to ten centimeters. Three volcanic ash layers were identified and the middle ash layer could be recognized as Byk-E, which is assigned as a key bed for Chibanian. Paleomagnetic analyses were conducted using (1) stepwise alternating field demagnetization (AFD), (2) stepwise thermal demagnetization (ThD), (3) stepwise ThD up to 175-300°C followed by stepwise AFD, etc. Although the third method was most successful in identifying primary magnetization, it is not always successful. In order to extract primary magnetization from most of the samples, we conducted combined analysis of remagnetization circles (McFadden&McElhinny, 1988) using a software paleomagnetism.org (Koymans et al., 2016). The calculated paleomagnetic directions (VGP latitudes) are consistent with those reported by Haneda et al. (2020). Correlation of VGP latitude in combination with rock magnetic results suggest the sedimentation rate at the studies site is about 3 times lower than that at Chiba Section. Further, we plan to discuss on the origin of magnetic minerals including magnetite and greigite, and its relation to decay/alterlation during thermal demagnetization.