Matuyama–Brunhes geomagnetic reversal record and associated key tephra layers in Boso Peninsula: extraction of primary magnetization of geomagnetic fields from mixed magnetic minerals of depositional, diagenesis, and weathering processes

Abstract We report paleomagnetic records of Matuyama–Brunhes geomagnetic polarity reversal and associated key tephra layers from the Early–Middle Pleistocene marine sedimentary succession in Boso Peninsula. The outcrop is Terasaki, Chiba, Japan ~ 25 km northeast Chiba section. sediment consists a massive siltstone layer Kokumoto Formation, Kazusa Group. A was identified middle with chemical composition comparable to that Byk-E section defining base Chibanian Stage. Oriented samples were collected at intervals 1–10 cm siltstone. To identify primary remanent magnetization, progressive alternating field demagnetization (PAFD) thermal (PThD) conducted on pilot samples. Identification magnetization PAFD not successful, especially for reversely magnetized In addition, during PThD showed sharp drops around 175 °C, which decreased gradually between °C 300 became unstable above 350 °C. extract while avoiding laboratory alteration by heating, up followed conducted. Combined analysis remagnetization circles enables extraction improved reliability. Rock magnetic experiments stepwise heating understand minerals involved evaluate influence heating. During FORC-PCA revealed significant changes 200 400 450 550 analyses electron microscopy indicate titanomagnetite/magnetite are contributing magnetization. Greigite also preserving secondary magnetizations sub-seafloor diagenesis. presence feroxyhyte suggested as through weathering pyrite exposure air after Peninsula uplift. correlation relative paleointensity provides an age model sedimentation rates 30 cm/kyr 18 cm/kr below tephra. VGP latitudes highly consistent those based model, assigns main directional swing reversed normal polarities 772.8 ± 0.5 ka. Graphical