5:15 PM - 7:15 PM
[MIS17-P03] Precise correlations between geomagnetic reversals and the marine isotopic stratigraphy using a marine Plio-Pleistocene succession
★Invited Papers
Keywords:Chronostratigraphy, Paleomagnetism , Marine Isotope Stages
Geomagnetic reversal events are essential calibration points for geologic ages because they provide a globally correctable chronostratigraphic surface and are detectable in igneous rocks that can be radiometrically dated. Especially for the Pliocene and later ages, the age of the geomagnetic reversal boundary can be precisely determined by comparing it with a marine isotopic variation curve. However, the contrast between paleomagnetic signals and marine isotopic records is not always possible because few sediments can simultaneously provide both signals and marine isotopic records. Even when such a comparison is possible, the “lock-in depth” of sedimentary remanent magnetization causes the age of the geomagnetic reversal boundary to be systematically calculated several thousand years older in the case of typical deep-sea sediments with a sedimentation rate of a few centimeters per thousand years. To solve this problem, the Chikura Group of the marine Pliocene to Pleistocene at the southernmost part of the Boso Peninsula is used to precisely determine the age of geomagnetic reversal boundaries that occurred between 1.5 and 3.5 Ma in the past by comparing with marine isotope records obtained from the same stratum. The Chikura Group has a strong and stable magnetic signal, is rich in various microfossils, and has an average sedimentation rate of more than 50 cm per thousand years, which limits the delay of paleomagnetic ages due to lock-in depth to less than a few hundred years.
In this study, we have constructed a composite magneto-oxygen isotope stratigraphy in the Hata, Minamikasai, and Mera Formations of the Chikura Group. The upper and lower boundaries of the Olduvai normal subchronozone in the Hata Formation are correlated with Marine Isotope Stage (MIS) 63 and 73, respectively, the upper and lower boundaries of the Feni normal subchronozone with MIS 79/80 and 81, respectively, and the Gauss-Matsuyama boundary with MIS 103 in the Minamiasai Formation. The upper and lower boundaries of the Kaena reversed subchronozone and the lower boundary of the Mammoth reversed subchronozone have been identified in the Mera Formation and are being compared with marine isotope curves. Since there are few cases in which geomagnetic reversal boundaries and marine isotope curves are obtained from the same sample during the Gelasian to early Calabrian periods, the geomagnetic reversal boundary ages and MIS correspondences in this study can significantly contribute to the global chronostratigraphy of this age interval.
In this study, we have constructed a composite magneto-oxygen isotope stratigraphy in the Hata, Minamikasai, and Mera Formations of the Chikura Group. The upper and lower boundaries of the Olduvai normal subchronozone in the Hata Formation are correlated with Marine Isotope Stage (MIS) 63 and 73, respectively, the upper and lower boundaries of the Feni normal subchronozone with MIS 79/80 and 81, respectively, and the Gauss-Matsuyama boundary with MIS 103 in the Minamiasai Formation. The upper and lower boundaries of the Kaena reversed subchronozone and the lower boundary of the Mammoth reversed subchronozone have been identified in the Mera Formation and are being compared with marine isotope curves. Since there are few cases in which geomagnetic reversal boundaries and marine isotope curves are obtained from the same sample during the Gelasian to early Calabrian periods, the geomagnetic reversal boundary ages and MIS correspondences in this study can significantly contribute to the global chronostratigraphy of this age interval.