Abstract
The Pliocene climate is one of the best analogs for the climate of a globally warmer future. Here, we present a new Pliocene integrated chronostratigraphy from the Anno Formation in the uppermost Awa Group, which is distributed throughout the Boso Peninsula, central Japan, based on paleomagnetic and benthic foraminiferal oxygen isotope records. This new chronostratigraphy provides valuable constraints for paleoceanographic and paleoclimatic studies in the northwestern Pacific Ocean, where the number of paleoceanographic records is limited due to the lack of calcareous microfossils from deep-sea sediment cores, with the exception of some plateaus at water depths above the calcite compensation depth (CCD). Paleomagnetic results indicate that the Anno Formation corresponds to the period extending from the Nunivak normal polarity subchronozone (4.493–4.631 Ma) to Chron C2An.2n (3.116–3.207 Ma), which is just above the Mammoth reversed polarity subchronozone. Although foraminifera are not found in the middle Anno Formation, our oxygen isotope records from the upper and lower Anno Formation demonstrate the recording of glacial–interglacial cycles. However, the amplitude of our δ18O profile is much larger than that of the LR04 stack, with similar to slightly lower glacial values and much lower interglacial values. This observation implies that the bottom water had lower δ18O values and/or a warmer water mass during interglacials compared with global average deep-water regions.
Highlights
During the Pliocene, Earth’s climate was substantially warmer; it was characterized by limited variations in ice volume, global temperatures that were ~ 2–3 °C higher relative to the present day, CO2 concentrations that were comparable to or higher than those today, suppressed Northern Hemisphere glaciation, and fully established East and West Antarctic Ice Sheets (Lisiecki and Raymo 2005; Naish et al 2009; Lunt et al 2010; Seki et al 2010; Salzmann et al 2011; De Schepper et al 2014)
The low-temperature remanence curves (Fig. 4c, d) indicate the presence of a Verwey transition, where the magnetite transforms from a monoclinic to a cubic spinel structure between 110 and 120 K (Verwey 1939; Özdemir et al 1993); the curves are characterized by a rapid remanence decline of 30–40% from 10 to 50 K, with a broad Verwey transition from 100 to 120 K
Based on paleomagnetic and oxygen isotopic data, the new, integrated chronostratigraphy of the Pliocene Anno Formation, which is located in the Boso Peninsula, central Japan, reveals the relationship between the tephra marker beds, Pliocene marine isotope stage (MIS), and geomagnetic reversal boundaries within the Anno Formation
Summary
During the Pliocene, Earth’s climate was substantially warmer; it was characterized by limited variations in ice volume, global temperatures that were ~ 2–3 °C higher relative to the present day, CO2 concentrations that were comparable to or higher than those today, suppressed Northern Hemisphere glaciation, and fully established East and West Antarctic Ice Sheets (Lisiecki and Raymo 2005; Naish et al 2009; Lunt et al 2010; Seki et al 2010; Salzmann et al 2011; De Schepper et al 2014). We present a new Pliocene integrated chronostratigraphy based on new magnetostratigraphy and oxygen isotope stratigraphy results, which we constrain using existing calcareous nannofossil data and the radiometric dating results obtained from tephra beds in the Anno Formation of the uppermost
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