09:45 〜 10:00
[SCG51-10] Piacenzian oxygen isotope-magnetostratigraphy constrained by U-Pb dating of tephra beds from a marine sucession in central Japan
キーワード：酸素同位体層序、古地磁気層序、ウランー鉛年代、テフラ、鮮新世、古地磁気境界
The mid-Piacenzian Warm Period (mPWP), between 3.3 and 3.0 Ma, is characterized by a climate warmer than present with comparable atmospheric CO2 to the modern level. Therefore, the mPWP is one of the best analogs for a globally warmer future climate and is vigorously investigated using geological archives and climatic modeling. However, the inherent age uncertainty of the time slab nature could generate inconsistency between models and proxies. The mPWP is preceded by one polarity transition and contains three polarity transitions: the lower Mammoth transition, and the upper Mammoth and the lower and upper Kaena transitions, respectively. Although these polarity transitions are significant stratigraphic markers to correlate between marine and terrestrial sediments, these marine isotope stage (MIS) assignments in deep-sea sediments are diverse, yet. This is likely because of the lock-in depth and smoothing effects due to low sedimentation rates, stability of paleomagnetic signals, sampling resolution, the definition of the “polarity boundary”, and long-term dominance of the transition field across polarity switches. Therefore, it is necessary to refine the stratigraphic relationship between MIS and the paleomagnetic polarity transitions using marine successions characterized by high sedimentation rate, stable paleomagnetic signal, and abundance of calcareous microfossils. In this study, we constructed highly precise oxygen isotope-magnetostratigraphy of the mid-Piacenzian with U-Pb zircon age of tephra beds from the Pliocene Anno Formation in the Boso Peninsula.
Paleomagnetic and benthic foraminiferal oxygen isotope (δ18OBen) analyses and U-Pb dating of tephra beds were conducted on the TN section, which is the uppermost Anno Formation. Additionally, in the Shikoma River (SR) section, we performed U-Pb dating for a tephra bed just below the lower Mammoth transition (LMT), which has been reconstructed by Haneda and Okada (2022).
Stratigraphic profiles of the paleomagnetic direction and δ18OBen with tephra correlation to the SR section exhibit that the upper Mammoth transition (UMT) in the TN section occurred within MIS KM5. The U-Pb age of the tephra in the SR section strongly indicates the LMT has not occurred during MIS MG3–MG2, which was suggested by Haneda and Okada (2022), but during late MIS MG1, consistent with the Geomagnetic Polarity Time Scale 2020 (Ogg, 2020). Additionally, the revised δ18OBen profile in the SR section shows a negative excursion intercepting the inception of MIS M2. However, our oxygen isotope stratigraphy does not detect MIS KM4, which is a glacial period after MIS KM5. Thus, the upper limit of our oxygen isotope stratigraphy has large temporal uncertainty. It needs to complement the oxygen isotope stratigraphy above MIS KM5 using another upper Pliocene marine succession.
Paleomagnetic and benthic foraminiferal oxygen isotope (δ18OBen) analyses and U-Pb dating of tephra beds were conducted on the TN section, which is the uppermost Anno Formation. Additionally, in the Shikoma River (SR) section, we performed U-Pb dating for a tephra bed just below the lower Mammoth transition (LMT), which has been reconstructed by Haneda and Okada (2022).
Stratigraphic profiles of the paleomagnetic direction and δ18OBen with tephra correlation to the SR section exhibit that the upper Mammoth transition (UMT) in the TN section occurred within MIS KM5. The U-Pb age of the tephra in the SR section strongly indicates the LMT has not occurred during MIS MG3–MG2, which was suggested by Haneda and Okada (2022), but during late MIS MG1, consistent with the Geomagnetic Polarity Time Scale 2020 (Ogg, 2020). Additionally, the revised δ18OBen profile in the SR section shows a negative excursion intercepting the inception of MIS M2. However, our oxygen isotope stratigraphy does not detect MIS KM4, which is a glacial period after MIS KM5. Thus, the upper limit of our oxygen isotope stratigraphy has large temporal uncertainty. It needs to complement the oxygen isotope stratigraphy above MIS KM5 using another upper Pliocene marine succession.