ミオセンの中期 南洋の冷却と南極の冷凍圏の膨張
Amelia E Shevenell1, James P Kennett, David W Lea
1Department of Geological Sciences and Marine Science Institute, University of California, Santa Barbara, CA 93106-9630, USA. ashevenell@umail.ucsb.edu
まとめ
中期のミオセンの気候移行は,南西太平洋で CO2 の引き下げではなく,海洋循環の変化によって引き起こされた顕著な冷却を目撃しました. この冷却は南極の氷の膨張に先行し,複雑な気候フィードバックを明らかにした.
科学分野:
- パレオセアノグラフィー
- 気候科学 気候科学
- 海洋地質学 海洋地質学
背景:
- 中期のミオセンの気候移行 (14.2〜13.8百万年前) は,地球温暖化の重要な時期でした.
- 以前の仮説では,大気中の二酸化炭素 (CO2) の排出が,この移行の主要な原動力であると示唆されていた.
- 軌道強制,海洋循環,冷凍圏の膨張の相互作用を理解することは,過去の気候の動態を再構築するための鍵です.
研究 の 目的:
- ミオセンの中期気候移行期における高緯度海面温度 (SST) を再構築する.
- 南洋の冷却と南極の凍土圏の膨張の要因を調査する.
- 二酸化炭素排出量削減がケノゾイク時代の気候変動に果たした役割に関する既存の仮説に異議を唱えるか,支持する.
主な方法:
- 南洋のプランクトン型フォラミニフェラにおけるマグネシウム/カルシウム比の分析により,古気温を決定した.
- 再構築されたSSTを世界の炭素サイクルプロキシと氷の量動向と比較.
- 軌道 (離心) フォッシングが気候の変動に及ぼす影響の評価.
主要な成果:
- 高緯度南西太平洋のSSTは,ミオセンの気候移行期中に6~7°C冷却された.
- 漸進的な冷却は,偏心力の強制によって加速され,南極の冷凍圏の膨張の約6万年前に発生しました.
- 海洋循環の変化が,エピソード的なCO2排出量減少ではなく,冷却と氷の増加を誘発したことを示唆している.
結論:
- 海洋循環の軌道のペースの変化は,メリディアン熱と蒸気輸送を大幅に変化させた.
- これらの循環のシフトは,南極の氷の増加と,その後の地球的な冷却を引き起こし,CO2の減少仮説に異議を唱えました.
- この発見は,大気中のCO2濃度が低い期間の気候の調節において,海洋・大気・氷のフィードバックの重要性を強調している.
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