科學家發現了地球如何得到氧氣的新線索

 

原文網址：https://news.uchicago.edu/story/uchicago-scientists-reveal-new-clues-how-earth-got-its-oxygen

科學家發現了地球如何得到氧氣的新線索

By Louise Lerner

在地球四十五億年的歲月當中，大部分時間都相當貧瘠、不適合生命居住，一直要到地球被氧氣包覆之後，多細胞生命才可以開始盡情發展。然而，地球是如何獲得這副具有氧氣的完美大氣？科學家仍在努力瞭解詳細的過程與原因。

覆蓋著地球的薄薄一層氧氣讓我們得以生存，但我們還是不曉得它究竟是怎麼形成的。芝加哥大學的新研究發現的線索，揭示了鐵在過程中的重要功能。圖片來源：NASA

「如果你仔細想想，就會發覺這是地球一生經歷過的變化中最重要的，但我們還是無法確定到底是怎麼發生的，」芝加哥大學地球物理學的教授Nicolas Dauphas表示。「任何可以讓我們靠近解答這項問題的進展都相當重要。」

在10月23日發表於《科學》(Science)的新研究中，芝加哥大學的研究生Andy Heard、Dauphas和他們的同事開創了一種新技術，使他們對於地球大氣的形成過程中，海洋中的鐵所扮演的腳色有了新的瞭解。這項發現不只揭露了地球歷史的更多面向，還能讓我們在搜尋其他恆星系統當中，是否有適合居住的行星時帶來新的方向。

科學家長久以來花費了大量心力來重建遠古地球的歷史進程。他們的方法是分析非常古老的岩石，這是因為其中的化學組成會根據它們形成時的環境條件而跟著變化。

「24億年前的大氧化事件造成了永久的變化。有趣的是，在這之前的歷史可以看到一些引人注目的證據，顯示曾經發生小型的氧氣激增事件，就像是地球正在為富含氧氣的大氣進行籌備工作。」這篇論文的第一作者Heard表示。「但是目前用的方法，精確程度還不足以讓我們挑出所需的資訊。」

簡而言之，一切都像個謎。

橋梁工程師和有車的人都知道，只要有水在附近，氧氣就會和鐵形成鏽斑。Heard說：「早年的地球海洋裡充滿了鐵，它們會把任何遊蕩的自由氧給吞得一乾二淨。」理論上來說，所有多出來的氧氣都會形成鐵鏽，使它們無法形成大氣的一部份。

Heard和Dauphas想要驗證一種解釋方法，可以回答儘管有這道顯而易見的限制，氧氣又要如何累積的問題。他們知道有些海洋裡的鐵實際上會和火山釋放的硫結合，形成黃鐵礦(較為所知的俗名是愚人金)，過程中會把氧氣釋放到大氣。問題在於哪個過程可以贏過對方。

為了驗證他們的解釋，Heard運用Dauphas的生命起源實驗室裡最尖端的儀器，發展出一套極為精密的新技術，可以測量鐵同位素的微小變化，進而找出鐵的反應路徑。除了和愛丁堡大學的國外專家合作之外，他也必須先對鐵形成黃鐵礦的反應路徑做出更加充分的瞭解。(Heard表示：「在製造硫化氫來進行這些實驗之前需要先通知你的同事，因為你會把實驗室弄得聞起來就和臭雞蛋一樣。」)接著這群科學家運用此技術，分析採集自澳洲和南非，年代為26億至23億年的岩石。

他們的分析結果顯示即使海洋應該會把大量的氧氣儲存在鐵鏽裡面，某些環境條件可以助長黃鐵礦大量形成，使得多餘的氧氣可以逃離水體而暫時存在於大氣當中。

Dauphas表示：「這是一項有許多變因的複雜問題，不過我們成功解決了其中之一。」

「在如此龐大的問題上取得的任何進展，對於我們這個領域來說都極為重要，」Heard說。「特別是在我們開始尋找系外行星的時候，必需先瞭解關於我們的地球如何變得適合生命居住的每分細節。」

隨著望遠鏡掃視天空，尋找其他行星並有了數以千計的發現之後，科學家要進一步探討哪些行星可能存在生命時，勢必得先把範圍縮小。透過更深入地研究地球變得適合生命居住的過程，科學家可以在其他行星之上尋找類似作用的證據。

Heard說：「對此我很喜歡的說法是，在氧氣出現以前的地球，是我們在瞭解系外行星時最棒的實驗室。」

 

Scientists reveal new clues into how Earth got its oxygen

For much of Earth’s four and a half billion years, the planet was barren and inhospitable; it wasn’t until the world acquired its blanket of oxygen that multicellular life could really get going. But scientists are still trying to understand exactly how—and why—our planet got this beautifully oxygenated atmosphere.

“If you think about it, this is the most important change that our planet experienced in its lifetime, and we are still not sure exactly how this happened,” said Nicolas Dauphas, the Louis Block Professor of Geophysical Sciences at the University of Chicago. “Any progress you can make toward answering this question is really important.”

In a new study published Oct. 23 in Science, UChicago graduate student Andy Heard, Dauphas and their colleagues used a pioneering technique to uncover new information about the role of oceanic iron in the rise of Earth’s atmosphere. The findings reveal more about Earth’s history, and can even shed light on the search for habitable planets in other star systems.

Scientists have painstakingly recreated a timeline of the ancient Earth by analyzing very ancient rocks; the chemical makeup of such rocks changes according to the conditions they formed under.

“The interesting thing about it is that prior to the permanent Great Oxygenation Event that happened 2.4 billion years ago, you see evidence in the timeline for these tantalizing little bursts of oxygen, where it looks like Earth was trying to set the stage for this atmosphere,” said Heard, the first author on the paper. “But the existing methods weren’t precise enough to tease out the information we needed.”

It all comes down to a puzzle.

As bridge engineers and car owners know, if there’s water around, oxygen and iron will form rust. “In the early days, the oceans were full of iron, which could have gobbled up any free oxygen that was hanging around,” Heard said. Theoretically, the formation of rust should consume any excess oxygen, leaving none to form an atmosphere.

Heard and Dauphas wanted to test a way to explain how oxygen could have accumulated despite this apparent problem: they knew that some of the iron in the oceans was actually combining with sulfur coming out of volcanoes to form pyrite (better known as fool’s gold). That process actually releases oxygen into the atmosphere. The question was which of these processes “wins.”

To test this, Heard used state-of-the-art facilities in Dauphas’ Origins Lab to develop a rigorous new technique to measure tiny variations in iron isotopes in order to find out which route the iron was taking. Collaborating with world experts at the University of Edinburgh, he also had to flesh out a fuller understanding of how the iron-to-pyrite pathway works. (“In order to make sulfide and run these experiments, you need understanding colleagues, because you make labs smell like rotten eggs,” Heard said.) Then, the scientists used the technique to analyze 2.6 to 2.3 billion-year-old rocks from Australia and South Africa.

Their analysis showed that, even in oceans that should have tucked away a lot of oxygen into rust, certain conditions could have fostered the formation of enough pyrite to allow oxygen to escape the water and potentially form an atmosphere.

“It’s a complicated problem with many moving parts, but we’ve been able to solve one part of it,” said Dauphas. 

“Progress on a problem this enormous is really valuable to the community,” Heard said. “Especially as we’re starting to look for exoplanets, we really need to understand every detail about how our own earth became habitable.”

As telescopes scan the skies for other planets and find thousands, scientists will need to narrow down which to explore further for potential life. By learning more about the way that Earth became habitable, they can look for evidence of similar processes on other planets.

“The way I like to think about it is, Earth before the rise of oxygen is the best laboratory we have for understanding exoplanets,” said Heard.

原始論文：Andy W. Heard, Nicolas Dauphas, Romain Guilbaud, Olivier J. Rouxel, Ian B. Butler, Nicole X. Nie, Andrey Bekker. Triple iron isotope constraints on the role of ocean iron sinks in early atmospheric oxygenation. Science, 2020. DOI: 10.1126/science.aaz8821

引用自：University of Chicago. “Scientists reveal new clues into how Earth got its oxygen.”