パナマ地峡の成立と世界的な寒冷化: 第四紀の新しい定義と関連して

摘要

In 2009, the International Union of Geological Sciences (IUGS) recognized the Quaternary Pe­riod as a formal chrono-stratigraphic unit and said that it spans the last 2.58 million years (Ma) of Earth history, instead of the hitherto used 1.81 Ma. In the present paper, I consider the new beginning age of the Quaternary from the viewpoint of oxygen isotopic study. It is well known that the best proxy for the past climatic and relative sea-level changes is an oxygen isotope (δ_(18)O) curve of benthic forami-niferal tests in marine sediment cores. Lisiecki and Raymo (2005) reported a δ_(18)O curve since the beginning of the Pliocene (5.3 Ma) by stack­ing all the δ_(18)O curves of 57 sediment cores recovered from the worldwide ocean floor. Ac­cording to this curve, the climate during the Pliocene (5.3-2.6Ma) was warm and had small amplitudes between the glacial and interglacial periods. On the other hand, the climate of the Quaternary started with a conspicuous cooling during the glacial periods at the beginning, although there was no notable cooling during the interglacial periods. Furthermore, the am­plitudes of the δ_(18)O curve are getting wider between the glacial and interglacial periods. Judging from the amplitudes of the δ_(18)O curve in the early Quaternary, from 2.6 Ma to 2.0 Ma, the interglacial periods were similar to the Holocene. On the other hand, the sizes of con­tinental ice sheets during the glacial periods must be similar to those during marine isotope stages 5a-5d, when the sea level was 50 + 20 m lower than today, if we assume a 120 m sea level drop at the last glacial maximum.%地質年代決定精度の高さで注目されている石灰質ナンノ化石層序に基づいて，パナマ地峡の成立と地球の寒冷化との関連について概説した．高緯度海域一北極海域での石灰質ナンノ化石調査結果は，2.75Maのパナマ地峡の成立とともに北極海域の急激な氷床拡大が発生したことを示唆し，そのタイミングで，寒流系種Coccolithus pelagicusの現在の分布様式が確立する．また，ベーリング海を介した太平洋海水の北極海，東グリーンランド海への流入も，このタイミングで発生し，いわゆるQuaternary-Styleclimateが2.75Maに開始したことを示唆する．