[HQR06-04] 下部更新統上総層群大田代層に対するアルケノン古水温計の適用
キーワード：大田代層、上総層群、アルケノン、古水温
Long Chain Alkenones (LCAs), a class of C35-C42 unsaturated ketones produced by Isochrysidales haptophytes, have been widely used as proxies for sea surface temperatures based on alkenone unsaturation indices (UK’37). Application of the alkenone paleotemperature proxy for lithified and uplifted marine sequences exposed on land is usually constrained by the poor preservation due to denaturation and subaerial weathering. Therefore, it is rare that LCAs are discovered in terrestrial outcrops, and only one previous study in Vrica outcrops, Italy has clearly shown that UK’37 reflected temperature variations between glacial and interglacial periods.[1]
In this study, we discovered that LCAs are well preserved in the lower Pleistocene Otadai Formation of the Kazusa Group, Boso Peninsula, Japan. The Kazusa group is well-exposed marine sequences consisting of hemipelagic siltstones interbedded by sandstone beds, which has been studied for stratigraphy and paleoenvironment using microfossils and pollen analysis. Detailed oxygen isotope stratigraphy to determine the marine isotope stage (MIS) from the drilled core is available for the upper Otadai Formation, which can be correlated to the outcrops along the Yoro River using the intercalated tephra layers [2]. We collected siltstones from the outcrops along the Yoro River covering from MIS 29 to 33.
LCAs can be detected from all collected samples, probably because they have never been exposed to high temperatures or consolidation stress in the process of burial and uplift [3]. Possible LCAs producers should be Pseudoemiliania lacunosa and Reticulofenestra asanoi, because modern alkenone producers Gephyrocapsa spp. and Emiliania huxleyi are almost absent in this section [2]. In spite of the difference in production species, LCA distributions in the Otadai Formation are quite similar to those in modern marine settings, i.e. di- and tri- unsaturated C37 and C38 dominate with a ratio of almost 1:1. On the assumptions that UK’37-temperature calibrations of ancient alkenone producers were the same as those of G. oceanica and E. huxleyi, the temperatures between MIS 29 and 33 fluctuated between 23.15 °C and 26.60 °C, which were perfectly synchronized with the glacial-interglacial cycles. Although we need to carefully consider whether the modern UK’37-temperature calibration can be applied because these temperatures seem to be slightly high compared to the present (MIS 1) alkenone temperatures around the Boso Peninsula [4, 5], the UK’37 in the Otadai formation certainly record the paleotemperature information.
[1] Cleaveland and Herbert, (2009) Geology 37(2), 179-182. [2] Tsuji et al. (2005) Journal of Geological Society of Japan 111(1), 1-20. [3] Kamiya et al. (2017) Tectonophysics 710-711, 69-80. [4] Yamamoto et al. (2004) Geophysical Research Letters 31, L16311. [5] Kajita et al. (2020) Quaternary Science Reviews 230, 106160.
In this study, we discovered that LCAs are well preserved in the lower Pleistocene Otadai Formation of the Kazusa Group, Boso Peninsula, Japan. The Kazusa group is well-exposed marine sequences consisting of hemipelagic siltstones interbedded by sandstone beds, which has been studied for stratigraphy and paleoenvironment using microfossils and pollen analysis. Detailed oxygen isotope stratigraphy to determine the marine isotope stage (MIS) from the drilled core is available for the upper Otadai Formation, which can be correlated to the outcrops along the Yoro River using the intercalated tephra layers [2]. We collected siltstones from the outcrops along the Yoro River covering from MIS 29 to 33.
LCAs can be detected from all collected samples, probably because they have never been exposed to high temperatures or consolidation stress in the process of burial and uplift [3]. Possible LCAs producers should be Pseudoemiliania lacunosa and Reticulofenestra asanoi, because modern alkenone producers Gephyrocapsa spp. and Emiliania huxleyi are almost absent in this section [2]. In spite of the difference in production species, LCA distributions in the Otadai Formation are quite similar to those in modern marine settings, i.e. di- and tri- unsaturated C37 and C38 dominate with a ratio of almost 1:1. On the assumptions that UK’37-temperature calibrations of ancient alkenone producers were the same as those of G. oceanica and E. huxleyi, the temperatures between MIS 29 and 33 fluctuated between 23.15 °C and 26.60 °C, which were perfectly synchronized with the glacial-interglacial cycles. Although we need to carefully consider whether the modern UK’37-temperature calibration can be applied because these temperatures seem to be slightly high compared to the present (MIS 1) alkenone temperatures around the Boso Peninsula [4, 5], the UK’37 in the Otadai formation certainly record the paleotemperature information.
[1] Cleaveland and Herbert, (2009) Geology 37(2), 179-182. [2] Tsuji et al. (2005) Journal of Geological Society of Japan 111(1), 1-20. [3] Kamiya et al. (2017) Tectonophysics 710-711, 69-80. [4] Yamamoto et al. (2004) Geophysical Research Letters 31, L16311. [5] Kajita et al. (2020) Quaternary Science Reviews 230, 106160.