17:15 〜 18:30
[MIS16-P21] Preliminary results of oxygen isotopic, paleomagnetic, and pollen analyses from a mid-Piacenzian marine succession in the Boso Peninsula, central Japan.
キーワード：酸素同位体層序、古地磁気層序、花粉化石、中期ピアセンジアン温暖期、房総半島、安野層
The Pliocene climate is one of the best analogs for the climate of a globally warmer future. In this presentation I show preliminary results of oxygen isotopic, paleomagnetic, and pollen analyses from the uppermost part of the Anno Formation, which is a forearc basin-fill deposit distributing in the Boso Peninsula. Lithological observations were conducted along the Minato River and its branch stream in Futtsu city, Chiba. The examined interval consists of silty stone, and pumice, scoria, and fine volcanic ash beds. Fifty-five samples for oxygen isotopic, paleomagnetic, and pollen analyses were collected from silty beds.
Progressive alternating field and thermal demagnetizations were conducted for each specimen to remove secondary remanence. Paleomagnetic results show a directional change from a reversed polarity to a normal polarity with transitional directions in the uppermost part of the examined interval. Oxygen isotope analyses were conducted for benthic foraminifera Melonis pompiloides, and Bolivinita quadrilatera. Resultant oxygen isotope record shows a glacial period in the lowermost part of the examined interval (δ18O = ca. 3.70 ‰), and subsequent warm periods with a minor fluctuation of δ18O between 3.07 ‰ and 3.24 ‰ in the upper interval. When comparing the oxygen isotope and paleomagnetic records of this study to the chronostratigraphy of the Anno Formation (Haneda & Okada, 2019), the examined interval ranges from Marine Isotope Stage (MIS) M2 to KM5 in the Mammoth Reversed Polarity Subchron.
A preliminary analysis of pollen fossil was conducted for 6 samples. All samples well preserve pollen fossils, and predominantly yield Taxodiaceae with Cathaya and Quercus (subgen. Cyclobalanopsis) as minor components. This flora indicates a warmer climate than present. Two samples from the MIS M2 interval yield genus Picea, Tsuga, and Alnus, likely indicating cooler climate condition relative to other 4 samples, consistent with the benthic oxygen isotope record. The oxygen isotope, paleomagnetic, and pollen records in this study indicate that the uppermost part of the Anno Formation is a potentially remarkable archive, recording paleoclimatic–paleoceanographic variations in the East Asian margin during the mid-Piacenzian warm period.
Progressive alternating field and thermal demagnetizations were conducted for each specimen to remove secondary remanence. Paleomagnetic results show a directional change from a reversed polarity to a normal polarity with transitional directions in the uppermost part of the examined interval. Oxygen isotope analyses were conducted for benthic foraminifera Melonis pompiloides, and Bolivinita quadrilatera. Resultant oxygen isotope record shows a glacial period in the lowermost part of the examined interval (δ18O = ca. 3.70 ‰), and subsequent warm periods with a minor fluctuation of δ18O between 3.07 ‰ and 3.24 ‰ in the upper interval. When comparing the oxygen isotope and paleomagnetic records of this study to the chronostratigraphy of the Anno Formation (Haneda & Okada, 2019), the examined interval ranges from Marine Isotope Stage (MIS) M2 to KM5 in the Mammoth Reversed Polarity Subchron.
A preliminary analysis of pollen fossil was conducted for 6 samples. All samples well preserve pollen fossils, and predominantly yield Taxodiaceae with Cathaya and Quercus (subgen. Cyclobalanopsis) as minor components. This flora indicates a warmer climate than present. Two samples from the MIS M2 interval yield genus Picea, Tsuga, and Alnus, likely indicating cooler climate condition relative to other 4 samples, consistent with the benthic oxygen isotope record. The oxygen isotope, paleomagnetic, and pollen records in this study indicate that the uppermost part of the Anno Formation is a potentially remarkable archive, recording paleoclimatic–paleoceanographic variations in the East Asian margin during the mid-Piacenzian warm period.