11:30 〜 11:45
[SGL22-09] 北西太平洋における後期鮮新世の酸素同位体ー古地磁気複合層序と古気候復元
キーワード：鮮新世、ピアセンジアン期、酸素同位体層序、古地磁気層序、花粉化石
The mid-Piacenzian Warm Period (mPWP), between 3.3 and 3.0 Ma, characterized by the climate warmer than present with the 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. Three paleomagnetic polarity transitions, which are significant stratigraphic markers to correlate between marine and terrestrial sediments, have been recognized during the mPWP, but these marine isotope stage (MIS) assignments in deep-sea sediments are diverse, yet. Additionally, a large portion of paleoclimatic proxies during the mPWP is from the Atlantic and its marginal regions, and thus the paleoclimatic condition during the mid-Piacenzian in the North Pacific is unclear yet. In this study, we construct oxygen isotope–magnetostratigraphy of the mid-Piacenzian marine succession (the uppermost Anno Formation) in the Boso Peninsula and reconstruct paleoclimatic variation in the northwestern Pacific using foraminiferal isotope and fossil pollen records.
Paleomagnetic experiments exhibit that a large part of the studied section is a reversed polarity, and a polarity transition to a normal polarity occurs at the uppermost part within ca 4 m interval. A benthic oxygen isotope (δ18OBen) profile from the studied section shows a pronounced glacial period at the lowermost part and subsequent warm interval. Based on correlating the tephra marker beds, the positive δ18OBen excursion and the polarity transition correspond to marine isotope stage (MIS) M2 and Upper Mammoth transition (UMT), respectively. Although the MIS assignment of the UMT in deep sea sediments is diverse between MIS M2–KM4, a 40Ar/39Ar age of the UMT lavas and our oxygen isotope–magnetostratigraphy indicate that the transition occurred within MIS KM5, which is a time slice of the PlioMIP2. Therefore, the UMT is a significant stratigraphic marker to identify the time slice and to correlate between marine and terrestrial geological archives. The paleoclimatic variation in the studied site will be discussed based on the planktonic δ18O and pollen assemblage records.
Paleomagnetic experiments exhibit that a large part of the studied section is a reversed polarity, and a polarity transition to a normal polarity occurs at the uppermost part within ca 4 m interval. A benthic oxygen isotope (δ18OBen) profile from the studied section shows a pronounced glacial period at the lowermost part and subsequent warm interval. Based on correlating the tephra marker beds, the positive δ18OBen excursion and the polarity transition correspond to marine isotope stage (MIS) M2 and Upper Mammoth transition (UMT), respectively. Although the MIS assignment of the UMT in deep sea sediments is diverse between MIS M2–KM4, a 40Ar/39Ar age of the UMT lavas and our oxygen isotope–magnetostratigraphy indicate that the transition occurred within MIS KM5, which is a time slice of the PlioMIP2. Therefore, the UMT is a significant stratigraphic marker to identify the time slice and to correlate between marine and terrestrial geological archives. The paleoclimatic variation in the studied site will be discussed based on the planktonic δ18O and pollen assemblage records.