バッケン頁岩層は、地球の地質史に新たな洞察をもたらし、デボン紀後期のいくつかの生物的危機の主要な引き金を明らかにしました: ユークシニア、または大きな水域での酸素枯渇と硫化水素の膨張. この研究は、地球の過去を理解するのに役立つだけでなく、多様性の減少や絶滅率の増加など、地球温暖化の潜在的な影響について警告しています。
北米の主要な石油資源は、地球上で最も壊滅的な大量絶滅の 1 つに関する洞察を明らかにします。
カナダとノースダコタ州の一部の地下 200,000 平方マイルに広がるバッケン頁岩層は、過去 70 年間、北米の石油と天然ガスの豊富な供給源でした。 最近の調査結果は、これらの岩石が私たちの惑星の複雑な地質学的歴史へのユニークで洞察に満ちた一瞥を提供することを明らかにしました.
メリーランド大学、ジョージ メイソン大学、ノルウェーの石油ガス会社 Equinor の地質学者で構成される研究者チームは、地層の岩石から収集された古生物学的および生物地球化学的情報を研究するための新しいアプローチを考案しました。
この技術を使用して、研究チームは、ほぼ 3 億 5000 万年前のデボン紀後期にいくつかの近接した生物危機の主要な引き金を突き止めました: ユークシニア、または大きな水域での酸素の枯渇と硫化水素の膨張です。 最近雑誌に掲載された 自然、チームの調査結果は、海面、気候、海洋化学、および生物的混乱の間のリンクを示しています。
バッケン頁岩層から採取した岩石サンプルを観察し、議論する研究者たち。 クレジット: Alan Jay Kaufman
「初めて、デボン紀後期の一連の重要な生物的混乱の原因となった特定の殺害メカニズムを指摘することができます」と、論文の上級著者であるUMD地質学教授のアラン・ジェイ・カウフマンは述べています。 「おそらく硫化水素の膨張によって引き起こされたと思われる大量絶滅は以前にもありましたが、地球の歴史のこのような重要な時期に、この死滅メカニズムの影響をこれほど徹底的に研究した人はいません。」
カウフマンによれば、デボン紀後期は、今日の地球のあり方に大きな役割を果たした要因の「完璧な嵐」でした。 維管束植物と樹木は、このプロセスにとって特に重要でした。 植物が陸上で拡大するにつれて、植物は土壌構造を安定させ、栄養素を海洋に拡散するのを助け、二酸化炭素を大気から排出しながら酸素と水蒸気を大気に加えました。
「可能な陸生植物の導入[{” attribute=””>photosynthesis and transpiration stimulated the hydrological cycle, which kick-started the Earth’s capacity for more complex life as we know it today,” Kaufman said.
The Devonian Period ended around the same time the Bakken sediments accumulated, allowing the layers of organic-rich shale to ‘record’ the environmental conditions that occurred there. Because the Earth’s continents were flooded during that time, various sediments including black shale gradually accumulated in inland seas that formed within geological depressions like the Williston Basin, the preserved the Bakken formation.
Undergraduate laboratory assistant Tytrice Faison (B.S. ’22, geology)—who joined Kaufman’s lab after taking a course with him through the Carillon Communities living-learning program—prepared and analyzed more than 100 shale and carbonate samples taken from the formation. After analyzing the samples, Kaufman, Faison, and the rest of the Bakken team deciphered clear layers of sediment representing three key biotic crises known as the Annulata, Dasberg, and Hangenberg events, with the last crisis associated with one of the greatest mass extinctions in Earth history.
“We could see anoxic events distinctly marked by black shale and other geochemical deposits, which are likely linked to a series of rapid rises in sea level,” Kaufman explained. “We suspect that sea levels may have risen during the pulsed events due to the melting ice sheets around the South Pole at this time.”
Higher sea levels would have resulted in the flooding of interior continental margins, or the transitional region between oceanic and continental crusts. In these settings, high levels of nutrients, such as phosphorous and nitrogen, could have triggered algal blooms which create low oxygen zones in large bodies of water. These zones in turn would have increased toxic hydrogen sulfide right where most marine animals would have lived. Under those conditions, animals in the oceans and on land around the shoreline would have died during these late Devonian events.
The team’s research is not exclusive to global biotic disruptions from hundreds of millions of years ago. Kaufman suggests that their findings are not just applicable to the shallow inland seas of the Devonian Period, but perhaps also to the oceans of today affected by global warming. He compared the ocean’s circulatory system to a “conveyor belt” carrying nutrients, oxygen, and microorganisms from place to place.
“Cold, salty water develops in the North Atlantic region before it sinks and eventually makes its way to the Indian and Pacific Oceans, cycling around the globe. This oceanic jet stream helps to spread life-sustaining oxygen through the oceans,” Kaufman explained. “If that conveyor belt were to be slowed down due to global warming, parts of the ocean might be deprived of oxygen and potentially become euxinic.”
The collateral damage caused by global warming might then promote animal migration out of dead zones or put Earth on a path to decreased diversity and increased rates of extinction, he added.
“Our study helps us to understand several things about Earth’s growing pains across a critical transition from a world we would not recognize today to one we would find more familiar,” Kaufman said. “It provides evidence for a kill mechanism that may be general to many of the many mass extinctions that occurred in the past, but also explains the origin of a major source of oil and gas to the United States.”
Reference: “Basin-scale reconstruction of euxinia and Late Devonian mass extinctions” by Swapan K. Sahoo, Geoffrey J. Gilleaudeau, Kathleen Wilson, Bruce Hart, Ben D. Barnes, Tytrice Faison, Andrew R. Bowman, Toti E. Larson and Alan J. Kaufman, 8 March 2023, Nature.
DOI: 10.1038/s41586-023-05716-2
The study was funded by Equinor.