[H-QR06] 地球惑星科学へのルミネッセンス・ＥＳＲ年代測定の応用

[HQR06-P04] Post-IR IRSL dating of an 80-m-long sediment core collected in the northeastern Kanto Plain, eastern Japan

*張 逸之¹、中里 裕臣²、田村 亨^1,2、岡崎 浩子³ (1.東京大学、2.産業技術総合研究所、3.深田地質研究所)

キーワード：光ルミネッセンス、更新世、ポスト赤外線刺激ルミネッセンス、海水準変動、堆積相、テクトニクス

The post-infrared infrared stimulated luminescence (pIRIR) dating has recently become popular in the study of Quaternary sediments. The dating range can reach hundreds of thousands of years. When applied to coastal sedimentary stratigraphic sequences, it may aid in understanding long-term changes in sea level and sedimentary environment. The Niihari Upland is located northwest of Kasumigaura Lake and considered to be a part of the extensive marine terraces that developed during the last interglacial period over the Kanto Plain. In this study, we tested and obtained pIRIR dating data of sediment samples in an 80-meter-long sediment core GS-KUR-1 collected from the Niihari Upland. We integrated the data with the sediment sequences identified in the core to analyze the sedimentary environment and processes. Eleven depositional units A-K were identified. The sedimentary facies include shallow marine and shelf, shoreface, foreshore and backshore, lagoon, and fluvial channel to flood plain. Tephra Yb1.1 was newly identified in the core and used as an independent age control. Equivalent dose (De) plateau test and dose recovery test were applied to two representative samples. According to the pretests and a comparison with tephra Yb1.1, pIRIR measured at 260ºC after prior infrared stimulated luminescence (IRSL) at 50ºC was chosen for age determination. The modified single aliquot regenerative-dose protocol was used for determining the equivalent dose of 26 samples. The fading test was also carried out for eight representative samples out of 26 to characterize the average fading rate for correcting the pIRIR ages. Twenty-six fading-corrected ages showed a consistent trend with the stratigraphy. The sedimentary facies and pIRIR₂₆₀ ages show temporal changes in the sedimentary environment over the last 400,000 years. These sequences are correlated to the conventional stratigraphy of the Shimosa Group and attributed to glacial sea-level fluctuations. These results indicate 1) two sea-level peaks may have occurred during MIS 11 in contrast to an existing view of a single peak, 2) MIS 5e strata is absent at the core site, 3) the Niihari Upland was identified as a fluvial terrace formed during MIS 5c, and 4) ages and levels of fluvial and coastal facies constrain the range of uplift rate of the core site as 0.07–0.27 m/kyr.