69 , 2016 , Department of Geography, Tokyo Metropolitan University
The geomagnetic polarity reversals, including the Matuyama–Brunhes boundary (MBB), provides an invariant datum plane for sediments and lavas. However, geomagnetic synchronization has complications despite its potential. Its popular age of 780 ka for the MBB is based on astrochronologically-tuned marine sedimentary records, and is supported by ^<40>Ar/^<39>Ar ages of 781–784 ka from Hawaiian lavas using a recent age calibration. Challenging this age, however, younger astrochronological ages using oxygen isotope stratigraphy of highsedimentation-rate marine records, and records of cosmogenic nuclides in marine sediments and an Antarctic ice core have been reported. Moreover, a high-precision U-Pb zircon age of 772.7 ± 7.2 ka is reported from a marine-deposited tephra near the MBB in the Chiba composite section in the Kokumoto Formation, Kazusa Group. U-Pb dating has a distinct advantage over ^<40>Ar/^<39>Ar dating in that it is relatively free from assumptions regarding standardization and decay constants. In addition, a high-resolution oxygen isotope chronology is also obtained in this section through the MBB. Therefore, the Chiba composite section will provide a basis for the first direct comparison between astrochronology, U-Pb dating, and magnetostratigraphy for the MBB, fulfilling a key requirement for calibrating the geological timescale. In this paper, I report these recent achievements according to the MBB.