先史時代の気候のフィードバック・ループ(A Prehistoric Climate Feedback Loop)

古気候学者が発見した、地球最後の温暖化の影響を加速させた古代の気候フィードバックループ Paleoclimatologist uncovers an ancient climate feedback loop that accelerated the effects of Earth’s last warming episode

2022-08-22 カリフォルニア大学サンタバーバラ校(UCSB)

＜関連情報＞

• https://www.news.ucsb.edu/2022/020697/prehistoric-climate-feedback-loop
• https://www.pnas.org/doi/10.1073/pnas.2201871119

第四間氷期温暖化に伴うメタンハイドレート大規模不安定化の証拠 Evidence for massive methane hydrate destabilization during the penultimate interglacial warming

Syee Weldeab, Ralph R. Schneider, Jimin Yu, and Andrew Kylander-Clark
Proceedings of the National Academy of Sciences  Published:August 22, 2022
DOI:https://doi.org/10.1073/pnas.2201871119

Significance

Our results identify an exceptionally large warming of the equatorial Atlantic intermediate waters and strong evidence of methane release and oxidation almost certainly due to massive methane hydrate destabilization during the early part of the penultimate warm episode (126,000 to 125,000 y ago). This major warming was caused by reduced advection of cold water from high latitudes and enhanced downward heat diffusion in response to a brief episode of meltwater-induced weakening of the Atlantic meridional overturning circulation and amplified by a warm mean climate. Our results highlight climatic feedback processes associated with the penultimate climate warming that can serve as a paleoanalog for modern ongoing warming.

Abstract

The stability of widespread methane hydrates in shallow subsurface sediments of the marine continental margins is sensitive to temperature increases experienced by upper intermediate waters. Destabilization of methane hydrates and ensuing release of methane would produce climatic feedbacks amplifying and accelerating global warming. Hence, improved assessment of ongoing intermediate water warming is crucially important, especially that resulting from a weakening of Atlantic meridional overturning circulation (AMOC). Our study provides an independent paleoclimatic perspective by reconstructing the thermal structure and imprint of methane oxidation throughout a water column of 1,300 m. We studied a sediment sequence from the eastern equatorial Atlantic (Gulf of Guinea), a region containing abundant shallow subsurface methane hydrates. We focused on the early part of the penultimate interglacial and present a hitherto undocumented and remarkably large intermediate water warming of 6.8 °C in response to a brief episode of meltwater-induced, modest AMOC weakening centered at 126,000 to 125,000 y ago. The warming of intermediate waters to 14 °C significantly exceeds the stability field of methane hydrates. In conjunction with this warming, our study reveals an anomalously low δ¹³C spike throughout the entire water column, recorded as primary signatures in single and pooled shells of multitaxa foraminifers. This extremely negative δ¹³C excursion was almost certainly the result of massive destabilization of methane hydrates. This study documents and connects a sequence of climatic events and climatic feedback processes associated with and triggered by the penultimate climate warming that can serve as a paleoanalog for modern ongoing warming.