11:00 〜 11:15
[MIS18-18] 北西太平洋における中期ピアセンジアン温暖期の古海洋－古気候変動復元
キーワード：中期ピアセンジアン温暖期、酸素同位体層序、古地磁気層序、花粉化石、房総半島
The mid-Piacenzian Warm Period (mPWP), between 3.3 and 3.0 Ma, characterized by global average temperature ~2–3℃ warmer than preindustrial with atmospheric CO2 concentration between 350–450 ppm. Therefore, the mPWP is one of the best analogs for the climate of a globally warmer future, and is vigorously investigated using geological archives and climatic modeling.
The Kuroshio Current, which is a western boundary current in the Pacific, transports large amount of heat and water from low to middle latitudes, and strongly affects East Asian and global climates. During Pleistocene, the variations in the position of the Kuroshio Extension Front (KEF), which is the northern limit of the Kuroshio Current, occured at the glacial–interglacial and millennial time scales, and affected both marine and terrestrial environments. However, the variation in the KEF and its influence on terrestrial environments in the East Asin margin during the mPWP are unclear. In this study, we reconstruct marine and terrestrial environments in the KEF region during the mPWP at the glacial–interglacial cycle time scale.
We conducted paleomagnetic, oxygen isotope, and pollen analyses for a 50-m marine sedimentary succession in the upper Anno Formation, Awa Group, distributing in the Boso Peninsula. Resulting magnetostratigraphy and oxygen isotope stratigraphy were correlated to the chronostratigraphy of the Anno Formation by Haneda & Okada (2019), based on tephra marker beds.
Progressive alternating field demagnetization and progressive thermal demagnetization were performed on paleomagnetic specimens, which have been obtained from 55 stratigraphic levels. A polarity transition from a reversed to a normal polarity occurs at the 41.9–46.4 m interval, corresponding to the upper boundary of the Mammoth reversed subchron on 3.207 Ma. Additionally, we observed a short-term normal polarity at the 14.6–15.9 m interval.
Oxygen isotope analyses were performed on benthic foraminifera Melonis pompiloides and Bolivinita quadrilatera, and planktonic foraminifera Globigerina bulloides from 46 stratigraphic levels. Resultant benthic oxygen isotope (δ18OBen) profile shows a pronounced glacial period and subsequent warm interval, and thus is correlated to the age interval between Marine Isotope Stage (MIS) M2 and KM5, with assistance from the magnetostratigraphy. On the other hand, resultant planktonic oxygen isotope (δ18Obul) profile gradually decreases from the base to the upper limit of the studied sequence with no variation corresponding to MIS M2.
Examined samples for the pollen analysis predominantly yield Taxodiaceae with Cathaya and Quercus (subgen. Cyclobalanopsis) as minor components. This flora indicates a climate condition warmer than present. Samples from the MIS M2 interval yield genus Picea, Tsuga, and Alnus, likely indicating cooler climate condition relative to other samples, consistent with the δ18OBen profile. Therefore, the terrestrial vegetation around Boso Peninsula during the early mPWP may related to the glacial–interglacial cycle rather than to the change in the sea surface environment.
Gradual warming of the sea surface temperature in the northwestern Pacific was also reported from planktonic oxygen isotope records in the Shatsky Rise (Lam et al., 2021), and is attributed to poleward shifts of the westerlies and Kuroshio Extension (Abell et al., 2021). It is thought that the decrease in the δ18Obul from the Anno Formation indicates an increase in sea surface temperature due to the northward shift of the KEF.
References
Abell et al., 2021, Nature, 589, 70–75.
Haneda & Okada, 2019, PEPS, 6,6.
Lam et al., 2021, Paleoceanography and Paleoclimatology, 36, e2021PA004318.
The Kuroshio Current, which is a western boundary current in the Pacific, transports large amount of heat and water from low to middle latitudes, and strongly affects East Asian and global climates. During Pleistocene, the variations in the position of the Kuroshio Extension Front (KEF), which is the northern limit of the Kuroshio Current, occured at the glacial–interglacial and millennial time scales, and affected both marine and terrestrial environments. However, the variation in the KEF and its influence on terrestrial environments in the East Asin margin during the mPWP are unclear. In this study, we reconstruct marine and terrestrial environments in the KEF region during the mPWP at the glacial–interglacial cycle time scale.
We conducted paleomagnetic, oxygen isotope, and pollen analyses for a 50-m marine sedimentary succession in the upper Anno Formation, Awa Group, distributing in the Boso Peninsula. Resulting magnetostratigraphy and oxygen isotope stratigraphy were correlated to the chronostratigraphy of the Anno Formation by Haneda & Okada (2019), based on tephra marker beds.
Progressive alternating field demagnetization and progressive thermal demagnetization were performed on paleomagnetic specimens, which have been obtained from 55 stratigraphic levels. A polarity transition from a reversed to a normal polarity occurs at the 41.9–46.4 m interval, corresponding to the upper boundary of the Mammoth reversed subchron on 3.207 Ma. Additionally, we observed a short-term normal polarity at the 14.6–15.9 m interval.
Oxygen isotope analyses were performed on benthic foraminifera Melonis pompiloides and Bolivinita quadrilatera, and planktonic foraminifera Globigerina bulloides from 46 stratigraphic levels. Resultant benthic oxygen isotope (δ18OBen) profile shows a pronounced glacial period and subsequent warm interval, and thus is correlated to the age interval between Marine Isotope Stage (MIS) M2 and KM5, with assistance from the magnetostratigraphy. On the other hand, resultant planktonic oxygen isotope (δ18Obul) profile gradually decreases from the base to the upper limit of the studied sequence with no variation corresponding to MIS M2.
Examined samples for the pollen analysis predominantly yield Taxodiaceae with Cathaya and Quercus (subgen. Cyclobalanopsis) as minor components. This flora indicates a climate condition warmer than present. Samples from the MIS M2 interval yield genus Picea, Tsuga, and Alnus, likely indicating cooler climate condition relative to other samples, consistent with the δ18OBen profile. Therefore, the terrestrial vegetation around Boso Peninsula during the early mPWP may related to the glacial–interglacial cycle rather than to the change in the sea surface environment.
Gradual warming of the sea surface temperature in the northwestern Pacific was also reported from planktonic oxygen isotope records in the Shatsky Rise (Lam et al., 2021), and is attributed to poleward shifts of the westerlies and Kuroshio Extension (Abell et al., 2021). It is thought that the decrease in the δ18Obul from the Anno Formation indicates an increase in sea surface temperature due to the northward shift of the KEF.
References
Abell et al., 2021, Nature, 589, 70–75.
Haneda & Okada, 2019, PEPS, 6,6.
Lam et al., 2021, Paleoceanography and Paleoclimatology, 36, e2021PA004318.