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新しい証拠は、地球史上最も厳しい氷河期の一つであるマリノアン氷河期が、これまで考えられていたほど地球を完全に凍結させなかった可能性を示唆している。 むしろ、中緯度の浅い海に生命を支える外水域が点在する「スラッシュボールアース」のようなものでした。 この発見は、 ネイチャーコミュニケーションズ、6億年以上前の地質サンプルと古代の藻類に基づいています。


地球は少なくとも 5 回の氷河期を経験しており、そのうちの 1 つは 6 億 3,500 万年前に起こり、その結果、極から極まで氷河が形成されました。 マリノアン氷河期として知られるこの特定の氷河期は、1970 年代に最初の地質学的証拠が発見されたオーストラリアの地域にその名前が由来しています。


しかし、中国湖北省東部の神農嘉林業区で収集された新たな証拠は、地球が完全に凍っていなかった、少なくとも氷河期の終わり頃ではなかったことを示唆している。 代わりに、その時代に遡る地質サンプルに基づいて、中緯度の浅い海の一部に開いた水域がいくつかありました。

「私たちはこの氷河期を『スノーボール・アース』と呼んでいました」とシンシナティ大学文理学部の地球科学教授トーマス・アルジオ氏は語る。 「私たちは、この長い氷河期の間に地球は完全に凍ってしまったと信じていました。 でも、もしかしたらそれはもっと『スラッシュボール・アース』だったかもしれない。」


研究者らは、6億3500万年前のマリノアン氷河期に地球が完全に凍って固体ではなかったという証拠を発見した。 中緯度の浅い海には氷がなかったので、おそらく生命の存続に貢献したのでしょう。 クレジット: マイケル・ミラー

研究は雑誌に掲載されました photosynthesis.

A team of geoscientists from China, the United Kingdom, and the United States conducted an isotopic analysis and found that habitable open-ocean conditions were more extensive than previously thought, extending into oceans that fall between the tropics and the polar regions and providing refuge for single-celled and multi-celled organisms during the waning stages of the Marinoan ice age.

Thomas Algeo

University of Cincinnati Geology Professor Thomas Algeo and his co-authors discovered isotopic evidence that some mid-latitude seas remained ice-free during the Marinoan Ice Age known as Snowball Earth. Credit: Andrew Higley

Lead author Huyue Song from the China University of Geosciences said while deep water likely did not contain oxygen to support life during this period, the shallow seas did.

“We present a new Snowball Earth model in which open waters existed in both low- and mid-latitude oceans,” Song said.

Song said the ice age likely saw many intervals of freezing and melting over the span of 15 million years. And under these conditions, life could have persisted, Song said.

“We found that the Marinoan glaciation was dynamic. There may have existed potential open-water conditions in the low and middle latitudes several times,” Song said. “In addition, these conditions in surface waters may have been more widespread and more sustainable than previously thought and may have allowed a rapid rebound of the biosphere after the Marinoan Snowball Earth.”

Thomas Algeo Examines Rock Cores

University of Cincinnati Geology Professor Thomas Algeo examines rock cores in his lab. Credit: Andrew Higley

Paradoxically, UC’s Algeo said, these refuges of life likely helped to warm the planet, ending the Marinoan ice age. The algae in the water released carbon dioxide into the atmosphere over time, gradually thawing the glaciers.

“One of the general take-home messages is how much the biosphere can influence the carbon cycle and climate,” he said. “We know that carbon dioxide is one of the most important greenhouse gases. So we see how changes in the carbon cycle have an impact on the global climate.”

Algeo said the study raises tantalizing questions about other ice ages, particularly the second one during the Cryogenian Period that scientists also believe created near-total glaciation of the planet.

“We don’t know for sure what triggered these ice ages, but my suspicion is it was related to multicellular organisms that removed carbon from the atmosphere, leading to carbon burial and the cooling of the Earth,” Algeo said. “Today, we’re releasing carbon quickly in huge amounts and it is having a big impact on global climate.”

Reference: “Mid-latitudinal habitable environment for marine eukaryotes during the waning stage of the Marinoan snowball glaciation” by Huyue Song, Zhihui An, Qin Ye, Eva E. Stüeken, Jing Li, Jun Hu, Thomas J. Algeo, Li Tian, Daoliang Chu, Haijun Song, Shuhai Xiao and Jinnan Tong, 4 April 2023, Nature Communications.
DOI: 10.1038/s41467-023-37172-x

The study was funded by the National Natural Science Foundation of China and the China Geological Survey.

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