地球大氣裡的氧濃度永久上升徹底改變了地球的特質，使其隨後變得適合生命居住。但是一項新研究認為這起事件比過去認為得還晚發生。
這組研究團隊由里茲大學領導，成員包括加州大學河濱分校、哈佛大學、南丹麥大學和聖安德魯斯大學的研究人員。結果也提供了解釋來說明為什麼這段期間，也發生了幾次對地球來說影響最為劇烈的氣候事件，它們造成當時的地球曾屢次整個覆蓋在冰層之下。
大氣裡首次明顯含有氧氣約莫是在24億3000萬年前，這也標示了大氧化事件(Great Oxidation Event)――地球歷史上一段關鍵時期的開始。
雖然大氧化事件提升的氧氣濃度仍然遠遠不如今日，但是已經讓地球表面的化學成分發生巨變，並且為生物之後的演化途徑設定好了方向，最終使地球充滿了形形色色的動物。
透過分析大氧化事件期間沉積在海中，今日位在南非的岩石，研究人員發現早期大氣裡的氧氣無法長時間存在，要到很久之後氧氣才成為地球大氣一直具備的特徵。
里茲大學地球與環境學院的Simon Poulton教授是這篇研究的主持人。
他說：「大氧化事件徹底改變了地球的環境與適居性。過去認為氧化的早期階段發生在24億3000萬到23億2000萬年前。」
「然而，我們的研究顯示事實上在長達兩億年的時間當中，大氣的氧化狀況都非常不穩定，這讓大氣永久氧化的時間比過去認為的還要晚了大約一億年。」
他們的研究發表在期刊《自然》(Nature)，內容也提出大氣氧濃度跟溫室氣體濃度兩者之間的波動有直接關聯。
加州大學河濱分校Andrey Bekker教授是研究共同作者，他說：「這項發現有助於解釋為什麼大氧化事件時也發生了四次覆蓋範圍相當大的冰河期，其中幾次可能把地球整個封在冰層底下長達數百萬年。」
「過往在探討大氣氧化初期和極度不穩定的氣候之間的關聯時，一項主要的問題是氧氣永久提升的時間點。我們的新數據顯示事實上一直要到當時最後一次大型冰河期之後，氧氣才獲得了永久性的提升。」
研究團隊把這段時期重新命名為「大氧化序幕」，其開創了為期15億年的氣候與環境穩定期，最後在前寒武紀末氧氣有了第二次的大量提升，同時氣候也變得不穩定。
哈佛大學的David Johnston教授是共同作者，他說：「大氣氧濃度提升是地球適合居住的關鍵因素。」
「釐清大氣氧化的歷史最終能讓我們了解氧氣的濃度是如何提高到足以讓動物演化出來。」
「大氧化序幕是大氣氧濃度首次提升至一定的水準，這是歷史上非常重要的一步。」
Poulton教授補充：「大氣氧化是控制地球是否適合居住的最重要因素，如果我們不曉得大氣永久氧化發生的確切時間點，那我們就不可能了解它的成因與後果。而我們現在終於掌握了這片重要的拼圖。」
Extra 100 million years before Earth
saw permanent oxygen rise
The permanent rise of oxygen in the Earth's atmosphere, which
fundamentally changed the subsequent nature of Earth's habitability, occurred
much later than thought, according to new research.
And the study, from an
international team led by the University of Leeds and including researchers
from the University of California-Riverside, Harvard University, the University
of Southern Denmark and the University of St Andrews, also provides an
explanation for some of the most extreme climate episodes to have affected the
Earth, when the planet was repeatedly covered with ice.
The first time oxygen was
significantly present in the atmosphere was about 2.43 billion years ago, and
this marks the start of the Great Oxidation Event - a pivotal period in Earth's
history.
Although the Great
Oxidation Event led to oxygen levels that were still much lower than today, it
dramatically changed the chemical composition of the planet's surface and set
the stage for the subsequent course of biological evolution on Earth, which
ultimately led to a planet teaming with animal life.
By analysing rocks from
South Africa, which were deposited in the ocean at the time of the Great
Oxidation Event, the researchers discovered that early atmospheric oxygenation
was short-lived, and oxygen did not become a permanent feature of the
atmosphere until much later.
Professor Simon Poulton, of
Leeds' School of Earth and Environment, led the research.
He said: "The Great
Oxidation Event fundamentally changed Earth's environment and habitability.
This early period of oxygenation was thought to have occurred between about
2.43 and 2.32 billion years ago.
"However, our research
shows that, in fact, oxygenation of the atmosphere was highly unstable over a
period of about 200 million years, with permanent atmospheric oxygenation
occurring about 100 million years later that previously thought."
Their findings, published
in the journal Nature, also suggest a direct link between
fluctuations in atmospheric oxygen concentration and greenhouse gas
concentrations.
Professor Andrey Bekker of
the University of California-Riverside, who co-authored the study, said:
"These findings help explain four widespread glaciations that occurred
coincident with the Great Oxidation Event, some of which were likely to have
covered the whole of the Earth in ice for millions of years.
"Our new data show
that the permanent rise of oxygen actually occurred after the final major
glaciation of the period and not before it, which had previously been a major
puzzle in our understanding of links between early atmospheric oxygenation and
intense climatic instability."
The research team has
re-labelled this period the Great Oxidation Episode. It ushered in a 1.5
billion year period of subsequent climatic and environmental stability, which
remained until a second major period of rising oxygen and climate instability
at the end of the Precambrian period.
Professor David Johnston, a
co-author from Harvard University said: "The rise of atmospheric oxygen
was a key factor in Earth's habitability.
"Unravelling the
history of atmospheric oxygenation ultimately allows us to understand how
oxygen rose to levels that were sufficient to allow the evolution of animals.
"The Great Oxidation
Episode, when atmospheric oxygen first rose to appreciable levels, represents a
pivotal step in this history."
Professor Poulton added:
"We cannot begin to understand the causes and consequences of atmospheric
oxygenation, the most significant control on Earth's habitability, if we do not
know when permanent atmospheric oxygenation actually occurred. Now at last we
have that piece of the puzzle."
原始論文：Simon W. Poulton, Andrey Bekker, Vivien M. Cumming, Aubrey L.
Zerkle, Donald E. Canfield & David T. Johnston. A 200-million-year delay in permanent atmospheric oxygenation. Nature, 2021. DOI:10.1038/s41586-021-03393-7
引用自：University
of Leeds. “Extra 100 million years before Earth saw permanent oxygen rise.”
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