[S-GL22] Frontier research on chronostratigraphic unit boundaries

[SGL22-08] Precise correlations between geomagnetic reversals and the marine isotopic stratigraphy using a marine Pleistocene succession distributed in the southern Boso Peninsula, central Japan

*Makoto Okada¹, Daiki Kotsuka¹, Takumi Konishi^2,3, Yuta Ebe¹ (1.Department of Earth Sciences, College of Science, Ibaraki University, 2.Graduate School of Science and Engineering, Ibaraki University, 3.Yachiyo Engineering Co.,Ltd.)

Keywords:Gelasian, Geomagnetic reversals, Marine isotope stratigraphy

In this study, we corrected mudstone samples at every 1 m intervals in thickness from a marine sedimentary succession of about 120 m thick, including the lower and upper boundaries of the Feni, and the lower boundary of the Olduvai subchronozones, along the KJ route where Konishi and Okada (2020) has reported a magnetostratigraphy. We extracted foraminiferal fossils from the collected samples and conducted oxygen isotope analyses on the benthic foraminifers (Bolivinita spp., Uvigerina spp.). Analyses were conducted using a MAT 253 mass spectrometer at the National Museum of Nature and Science, Tsukuba, Japan. To construct an age model for the studied succession, we compared the obtained oxygen isotope data with the LR04 standard oxygen isotope curve (Lisiecki and Raymo, 2005). As a result, the age interval of the studied succession is approximately 2.15 Ma to 1.90 Ma, corresponding to the marine isotope stages (MIS) 82 to 73. The average sedimentation rate is about 50 cm/kyr. The ages of the lower and upper boundaries of the Feni and the lower boundary of the Olduvai normal subchronozones in the studied succession are calculated to be 2.141 Ma, 2.116 Ma, and 1.922 Ma, respectively. The corresponding marine isotope stages are MIS 81, 79, and 74, respectively. These results are in excellent agreement with the ages of 2.140 Ma, 2.116 Ma, and 1.925 Ma reported by the Geomagnetic Polarity Time Scale (Ogg, 2020) of the GTS 2020, the current international geologic age scale. In the Gelasian Age, there are few cases where the geomagnetic reversal boundaries and oxygen isotopic curves are obtained from the same sample. In particular, the age and its corresponding MIS of the Olduvai lower boundary have not been agreed upon among several ocean cores from the sedimentary drift body off Iceland, which has been most extensively studied for the Pleistocene magnetostratigraphy. The fact that the ages and MIS correspondences in this study agree with the results of IODP Site U1308 (Channell et al., 2016), which is the basis for GTS 2020, settles the issue of the age of the lower boundary of the Olduvai subchronozone.

References
Channell et al. (2020) https://doi.org/10.1016/j.quascirev.2019.106114
Channell et al. (2016) http://dx.doi.org/10.1016/j.quascirev.2015.10.011
Konishi and Okada (2020) https://doi.org/10.1186/s40645-020-00352-0
Lisiecki and Raymo (2005) https://doi.org/10.1029/2004PA001071
Ogg, J.G. (2020) Chapter 5 - Geomagnetic polarity time scale. In Gradstein, et al. (eds), The Geologic Time Scale 2020. Vol. 1. Elsevier, Boston, MA.