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 describe paleomagnetic records of Matuyama-Brunhes geomagnetic polarity reversal and associated key tephra layers from Early-Middle Pleistocene sediments in the Boso Peninsula. The outcrop is in Terasaki, Chiba, Japan and ~ 25 km northeast of the Chiba Section. The sediment consists of massive silt of the Kokumoto Formation, Kazusa Group, underlaid by thick sand. A tephra layer was identified in the middle of the outcrop with chemical composition comparable to that of the tephra layer Byk-E from the Yoro River section defining the base of the Chibanian Stage. Oriented paleomagnetic samples were taken at intervals of 1–10 cm from the massive silt spanning the vitric tephra layer. To identify the primary remanent magnetization, progressive alternating field demagnetization (PAFD) and progressive thermal demagnetization (PThD) were conducted on pilot samples. Identification of primary magnetization with PAFD was not successful, especially for reversely magnetized samples. However, magnetization during PThD showed sharp drops at 175°C, which decreased gradually between 175°C and ~ 300°C, and became unstable above ~ 350°C. To extract the primary remanent magnetization while avoiding laboratory alteration by heating, a PThD up to 175°C followed by PAFD was conducted. Extraction of primary magnetization was significantly improved by applying a combined analysis of remagnetization circles, which was in agreement with the records reported from the Chiba Section. Rock magnetic experiments were conducted during stepwise heating to understand the magnetic minerals involved and to evaluate the influence of laboratory heating. During heating, FORC-PCA revealed significant changes of magnetic minerals at 200°C, 400°C, 450° and 550°C. Rock magnetic analyses and electron microscopy observations indicate that magnetite and titanomagnetite are magnetic minerals contributing to primary remanent magnetization. Greigite was also identified, which preserve secondary magnetizations during sub-seafloor diagenesis. The presence of feroxyhyte is suggested to contribute to secondary magnetization through the weathering of pyrite by exposure to the air after the Boso Peninsula uplift. The similarity of VGP latitude variations between this study and those from the Chiba section was maximized by providing an age model with sedimentation rates of 30 cm/kyr and 18 cm/kr for the intervals above and below the Byk-E tephra.