關於地球氧氣的爭論有了新的看法

原文網址：http://www.leeds.ac.uk/news/article/4513/breathing_new_life_into_earths_oxygen_debate

關於地球氧氣的爭論有了新的看法

新的研究極力主張導致地球空氣可供呼吸的大型「氧化事件」可以自發性發生，而非生物或者板塊運動有所變革的結果。

里茲大學發表在期刊《科學》(Science)上的研究不只闡明了有關地球氧氣的歷史，也讓我們對氧氣在其他星球上的普及程度有新的看法。

原始地球的海洋與大氣當中都沒有氧氣，一直到了大概24億年前三次大型氧化事件的第一次發生時才開始有氧氣出現。地球氧氣為何會呈現「階段性」的增加？這是科學家長久以來不斷爭辯的議題。

里茲大學的研究人員在新的研究當中，對一個已經完備、有關海洋生物地球化學的概念模型進行修改，使其可以模擬整個地球歷史。結果發現模型本身就能製造出三次氧化事件。

大約30億年前，地球已經出現原始的光合作用微生物並且開始進行板塊運動。他們的發現主張只要具備這兩項要素，接下來要讓氧氣提升到複雜生物存活所需的濃度，只是時間早晚的問題。

這項新理論大幅提升了其他世界具有大量氧氣的可能性。

里茲大學地球與環境學院的博士後研究員Lewis Alcott是研究主要作者，他說：「這項研究深入檢驗了我們認知中地球變成具有大量氧氣，因而可以支持智慧生命的原因。」

「根據研究結果，不需要生物有所進展或者板塊運動的偶發事件出現好幾次――其實這也相當不可能――就能讓行星含有氧氣，因此這類行星可能比我們之前認為的還要普遍。」

第一次「大氧化事件」發生在大約24億年前的古元古代；下一個大規模氧化事件發生在8億年前左右的新元古代；最後一次則發生在4.5億年前附近的古生代，使得大氣中的氧氣提升至目前的水準。

耗費眾多能量的大型動物需要高濃度的氧氣，因此牠們在最後一個階段結束之後才快速演化出來，最終演化成恐龍和哺乳類。

目前兩種盛行的理論主張這些氧化事件的推手來自於以下兩者，或是其中之一的演變過程出現了重大躍進：第一個是生物，也就是演化逐漸創造出更加複雜的生命形式，牠們能夠透過「生物工程」的方法從根本提高氧化程度；另一個則是板塊運動，也就是火山活動的類型或者地殼的組成發生變化，造成氧氣增加。

新研究的重點改擺在全球磷、碳、氧循環過程中互相牽連的一系列負回饋作用，這些作用可以促使海洋和大氣氧濃度迅速發生變化，卻不需要板塊運動和生物，或者其中之一也發生階段式變化。

同為里茲大學地球與環境學院的教授Simon Poulton是研究共同作者，他說：「我們的模型顯示一旦演化出可以製造氧氣的微生物，地球的氧化程度就必然會提高至足以支持複雜生命的水準。」

作者用來模擬負回饋的模型「地球系統」僅靠著地表環境隨著時間經過，會逐漸從還原轉變成氧化狀態就重現了觀察中的三階段氧化模式。驅動這種轉變過程的作用是海洋裡的磷循環會受到氧濃度變化的影響，這反過來又會影響到需要磷的光合作用。

領導里茲大學生物地球化學模擬團隊的資深作者Benjamin Mills博士表示：「模擬呈現出地球表面隨時間逐漸氧化的過程，就會讓大氣和海洋當中出現三次顯著的氧化事件，這跟我們在地質紀錄中看到的相符。」

「我們的成果顯示在探討地球氧氣變化歷史的時候，基本上要先瞭解全球磷、碳、氧循環之間的關係。這有助於我們更加瞭解地球以外的其他行星，可以經由什麼過程而變得適合居住。」

Breathing new life into Earth's oxygen debate

New research strongly suggests the distinct "oxygenation events" which created Earth’s breathable atmosphere happened spontaneously, rather than as a consequence of biological or tectonic revolutions.

The Leeds study, published in the journal Science, not only shines a light on the history of oxygen on our planet, it gives new insight into the prevalence of oxygenated worlds other than our own.

The early Earth had no oxygen in its atmosphere, or oceans, until roughly 2.4 billion years ago when the first of three major oxygenation events occurred. The reasons for these "stepwise" increases of oxygen on Earth have been the subject of ongoing scientific debate.

In a new study, Leeds researchers modified a well-established conceptual model of marine biogeochemistry so that it could be run across the whole of Earth history, and found that it produced the three oxygenation events all by itself.

Their findings suggest that beyond early photosynthetic microbes and the initiation of plate tectonics – both of which were established by around three billion years ago – it was simply a matter of time before oxygen would reach the necessary level to support complex life.

This new theory drastically increases the possibility of high-oxygen worlds existing elsewhere.

Study lead author Lewis Alcott, a postgraduate researcher in the School of Earth and Environment at Leeds, said: “This research really tests our understanding of how the Earth became oxygen rich, and thus became able to support intelligent life.

“Based on this work, it seems that oxygenated planets may be much more common than previously thought, because they do not require multiple – and very unlikely – biological advances, or chance happenings of tectonics.”

The first “Great Oxidation Event” occurred during the Paleoproterozoic era – roughly 2.4 billion years ago. The subsequent wholesale oxygenation events occurred in the Neoproterozoic era around 800 million years ago and finally in the Paleozoic Era roughly 450 million years ago, when atmospheric oxygen rose to present day levels.

Large animals with high energy demands require high levels of oxygen, and evolved soon after the last of these steps, ultimately evolving into dinosaurs and mammals.

Currently, the two prevailing theories suggest the drivers of these oxygenation events were either major steps in biological revolutions – where the evolution of progressively more complex lifeforms essentially “bioengineered” oxygenation to higher levels; or tectonic revolutions – where oxygen rose due to shifts in the style of volcanism or make-up of the crust.

The new study instead highlights a set of feedbacks that exist between the global phosphorus, carbon and oxygen cycles, which are capable of driving rapid shifts in ocean and atmospheric oxygen levels without requiring any "stepwise" change in either tectonics or biology.

Study co-author Professor Simon Poulton, also from the School of Earth and Environment at Leeds, said: “Our model suggests that oxygenation of the Earth to a level that can sustain complex life was inevitable, once the microbes that produce oxygen had evolved.”

The authors' "Earth system" model of the feedbacks reproduces the observed three-step oxygenation pattern when driven solely by a gradual shift from reducing to oxidising surface conditions over time. The transitions are driven by the way the marine phosphorus cycle responds to changing oxygen levels, and how this impacts photosynthesis, which requires phosphorus.

Senior author Dr Benjamin Mills, who leads the biogeochemical modelling group at Leeds, said: “The model demonstrates that a gradual oxygenation of Earth’s surface over time should result in distinct oxygenation events in the atmosphere and oceans, comparable to those seen in the geological record.

“Our work shows that the relationship between the global phosphorus, carbon and oxygen cycles is fundamental to understanding the oxygenation history of the Earth. This could help us to better understand how a planet other than our own may become habitable.”

原始論文：Lewis J. Alcott, Benjamin J. W. Mills, Simon W. Poulton. Stepwise Earth oxygenation is an inherent property of global biogeochemical cycling. Science, 2019 DOI: 10.1126/science.aax6459

引用自：University of Leeds. "Breathing new life into the rise of oxygen debate."