造成恐龍難以在地球生存的原因是小行星撞擊，而非火山爆發

原文網址：https://www.imperial.ac.uk/news/198595/asteroid-impact-volcanoes-made-earth-uninhabitable/

造成恐龍難以在地球生存的原因是小行星撞擊，而非火山爆發

by Hayley Dunning

模擬6600萬年前的希克蘇魯伯小行星撞擊事件之後，結果顯示地球大部分地方因此變得不適合恐龍生存。

這顆小行星在6600萬年前的白堊紀末期撞擊墨西哥外海。科學家長久以來都認為它是恐龍滅絕的元凶，只有那些演化成鳥類的恐龍得以倖存下來。

然而，也有一些研究人員提出這場消滅將近75%地球物種的滅絕事件，真兇其實是持續數萬年的大型火山爆發。

最近，由倫敦帝國學院、布里斯托大學、倫敦大學學院組成的研究團隊，證實了唯有小行星撞擊才能造成世界各地的環境條件都變得不利於恐龍生存。

他們還證明大型火山活動長期來說有助於生命從小行星撞擊事件中恢復過來。這項結果發表在《美國國家科學院院刊》(Proceedings of the National Academy of Sciences)。

唯一合理的解釋

主要作者Alessandro Chiarenza博士在倫敦帝國學院地球科學與工程系攻讀博士時進行了這項研究。他說：「我們證實小行星導致了長達數十年的撞擊冬天，它對環境造成的影響使得適合恐龍居住的環境不復存在。相較而言，劇烈火山爆發造成的影響並不足以強到能大幅干擾全世界的生態系。」

「我們的研究首度以定量化的方式證明滅絕事件唯一合理的解釋，是撞擊冬天摧毀了全世界的恐龍棲地。」

小行星撞擊可以把大量微粒和氣體送到高層大氣，遮住未來數年的陽光而讓冬季一直持續下去。不過火山爆發也可以製造出遮蔽陽光的微粒和氣體，恰好在大滅絕的時間點附近，現今印度的德干暗色岩區發生了為時數萬年的火山爆發。

為了判斷小行星和火山活動何者對氣候的影響力更強，研究人員的傳統研究方法是運用地質記錄和強大的數學模型。在這篇新的論文當中，團隊除了上述方法還結合了每種恐龍生存所需的環境條件，像是雨量和氣溫等資訊。

他們接著畫出小行星撞擊以及大型火山活動之後，世界上還有哪些地方仍然具備能讓恐龍生活的環境條件。他們發現只有小行星撞擊能把恐龍可以生存的棲地給全數消滅，而火山活動則會在赤道附近留下一些還能居住的地方。

協助新生命演化

布里斯托大學的Alex Farnsworth博士是共同主要作者，他說：「我們不只利用地質紀錄來模擬小行星和火山活動對全世界的氣候可能造成的影響，還更進一步地把生態這個維度給加進來，藉此展示氣候的波動如何對生態系造成嚴重影響。」

共同作者，倫敦大學學院的Philip Mannion博士補充：「在這項研究中，我們模擬時加入了關鍵的地質和氣候資料，結果得出小行星撞擊對全世界的棲地造成了重大浩劫。對於恐龍來說，這基本上就像全世界忽然徹底停止運作一樣。」

雖然火山會釋放出遮擋陽光的氣體和微粒，但也能釋放出二氧化碳這種溫室氣體。火山爆發過後，短期來說這些阻擋陽光的物質有較大的影響——它們會造成「火山冬天」。但長期來看，這些微粒和氣體會從大氣中逐漸消失，然而二氧化碳仍然可以待在大氣當中並持續累積，造成地球溫度升高。

團隊的模型顯示在小行星最初讓全球陷入酷寒之後，火山造成的暖化作用長期下來可以幫助環境棲地回復過來，使得從災難中倖存下來的生命開始演化。

Chiarenza博士表示：「我們提出的新證據認為與小行星撞擊差不多同一時間發生的火山爆發，或許可以降低撞擊造成的環境影響，特別是因為火山可以加快氣溫從撞擊冬天回暖的速度。這種由火山引起的暖化可以提升動植物從滅絕事件中倖存並復原的機會，許多生物族群，像是鳥類和哺乳類便是事件不久之後種類因此而大幅增加的生物。」

 

Asteroid impact, not volcanoes, made the Earth uninhabitable for dinosaurs

Modelling of the Chicxulub asteroid impact 66 million years ago shows it created a world largely unsuitable for dinosaurs to live in.

The asteroid, which struck the Earth off the coast of Mexico at the end of the Cretaceous era 66 million years ago, has long been believed to be the cause of the demise of all dinosaur species except those that became birds.

However, some researchers have suggested that tens of thousands of years of large volcanic eruptions may have been the actual cause of the extinction event, which also killed off almost 75% of life on Earth.

Now, a research team from Imperial College London, the University of Bristol and University College London has shown that only the asteroid impact could have created conditions that were unfavourable for dinosaurs across the globe.

They also show that the massive volcanism could also have helped life recover from the asteroid strike in the long term. Their results are published today in Proceedings of the National Academy of Sciences.

The only plausible explanation

Lead researcher Dr Alessandro Chiarenza, who conducted this work whilst studying for his PhD in the Department of Earth Science and Engineering at Imperial, said: “We show that the asteroid caused an impact winter for decades, and that these environmental effects decimated suitable environments for dinosaurs. In contrast, the effects of the intense volcanic eruptions were not strong enough to substantially disrupt global ecosystems.

“Our study confirms, for the first time quantitatively, that the only plausible explanation for the extinction is the impact winter that eradicated dinosaur habitats worldwide.”

The asteroid strike would have released particles and gases high into the atmosphere, blocking out the Sun for years and causing permanent winters. Volcanic eruptions also produce particles and gases with Sun-blocking effects, and around the time of the mass extinction there were tens of thousands of years of eruptions at the Deccan Traps, in present-day India.

To determine which factor, the asteroid or the volcanism, had more climate-changing power, researchers have traditionally used geological markers of climate and powerful mathematical models. In the new paper, the team combined these methods with information about what kinds of environmental factors, such as rainfall and temperature, each species of dinosaur needed to thrive.

They were then able to map where these conditions would still exist in a world after either an asteroid strike or massive volcanism. They found that only the asteroid strike wiped out all potential dinosaur habitats, while volcanism left some viable regions around the equator.

Helping new life evolve

Co-lead author of the study Dr Alex Farnsworth, from the University of Bristol, said: “Instead of only using the geologic record to model the effect on climate that the asteroid or volcanism might have caused worldwide, we pushed this approach a step forward, adding an ecological dimension to the study to reveal how these climatic fluctuations severely affected ecosystems.”

Co-author Dr Philip Mannion, from University College London, added: “In this study we add a modelling approach to key geological and climate data that shows the devastating effect of the asteroid impact on global habitats. Essentially, it produces a blue screen of death for dinosaurs.”

Although volcanoes release Sun-blocking gases and particles, they also release carbon dioxide, a greenhouse gas. In the short term after an eruption, the Sun-blockers have a larger effect, causing a ‘volcanic winter’. However, in the longer term these particles and gases drop out of the atmosphere, while carbon dioxide stays around and builds up, warming the planet.

After the initial drastic global winter caused by the asteroid, the team’s model suggests that in the longer term, volcanic warming could have helped restore many habitats, helping new life that evolved after the disaster to thrive.

Dr Chiarenza said: “We provide new evidence to suggest that the volcanic eruptions happening around the same time might have reduced the effects on the environment caused by the impact, particularly in quickening the rise of temperatures after the impact winter. This volcanic-induced warming helped boost the survival and recovery of the animals and plants that made through the extinction, with many groups expanding in its immediate aftermath, including birds and mammals.”

原始論文：Alfio Alessandro Chiarenza, Alexander Farnsworth, Philip D. Mannion, Daniel J. Lunt, Paul J. Valdes, Joanna V. Morgan, and Peter A. Allison. Asteroid impact, not volcanism, caused the end-Cretaceous dinosaur extinction. PNAS, 2020 DOI: 10.1073/pnas.2006087117

引用自：Imperial College London. "Asteroid impact, not volcanoes, made the Earth uninhabitable for dinosaurs."