氧氣對於生命來說至關重要，但是地球大氣氧濃度最初上升的原因以及確切的發生時間，70多年來對於科學家仍是難解的問題。
許多科學研究認為大約24億年前氧氣迅速增加，並在接下來的2億年以同樣的速度突然下降，這起事件被稱做「大氧化事件」(Great
Oxygenation Event , GOE))。
最近一項國際研究提出了不同的理論。該團隊由塔斯馬尼亞大學的地質學家主持，成員包括華盛頓卡內基研究所與多倫多大學的科學家。
研究人員提出大氣氧氣是在28億年前到18億年前以非常緩慢的速度增加，原因牽涉到超大陸循環中的大陸板塊碰撞，以及海洋裡藍綠菌的演化。
大氣氧氣在十億年間逐漸上升，並在大約19億年前到達顛峰，接近目前的氧濃度21%。接著氧氣在一段更長的時期中下降，稱做「無聊的十億年」。
研究表示地殼礦物的演變與氧氣增加有關，原因是新型的氧化物礦物唯有在氧氣增加之後才能出現。
隨著地質時間經過，海床上與岩石裡會形成不同的礦物。研究人員透過測量它們的氧化還原性質而得出這項新理論。
塔斯馬尼亞大學的地質學家Ross
Large表示，他們根據從許多種礦物與同位素測量出來的大量資料得到這項結果。
塔斯馬尼亞大學、多倫多大學和卡內基研究所組成的團隊建立一個龐大的資料庫，彙整了許多種礦物的化學性質。這些資料網羅了過去15年間的數萬筆分析結果。
「科技的迅速演變使我們可以獲得數以千計的分析結果，造成資料導向的研究在世界各地都有增加的趨勢，」Large教授如此解釋。
「在這項主題上，過去的研究大多仰賴有限的分析資料，輔以電腦模型來填補數據並試著預測出結果。這種方法通常會得到『線性的』解釋，因此忽略了地球在地質時間中會有的來回循環。」
Large教授表示氧氣初次增加的時候二氧化碳與甲烷也跟著減少，使得大氣與海洋的環境條件更適合生物。
「早於26億年前的古代太古宙海洋裡有很多含毒的元素，像是砷和汞，造成我們所知的生物很難生存，」Large教授表示。
「我們的研究顯示隨著氧氣增加，海洋化學也跟著改變，有毒的元素減少而對生命重要的元素則變得更多，像是磷、鉬和鋅，這刺激了演化過程發生變化。」
Large教授表示上述重大改變的起因為大陸漂移首次進行，這又和超大陸循環有關。超大陸循環描述了地球最大型的陸塊如何組合而成，維持一段時間，最終分裂的循環。
「在每一次超大陸循環的起始階段，板塊碰撞過程中的造山運動會造成侵蝕作用，使得養分流入海洋，促進生命生長並釋放氧氣到大氣當中，」Large教授解釋。
「我們提出兩階段的造山運動促使氧氣增加、新礦物產生並讓早期生命演化。第一次大約發生在28億年前，和凱諾蘭超大陸的形成有關；第二次則發生在大約21億年前，和哥倫比亞超大陸的形成有關。」
第三次氧氣循環大約始於十億年前，之後循環的頻率變快，從相隔二億年左右變成六千萬年。團隊先前的研究指出每次氧氣循環結束時都會發生大滅絕事件，但是很快演化速度又會爆炸性地增加。
跟某些人的主張不同，Large教授不認為我們正邁入另一場大滅絕。他說過往的大滅絕事件中，二氧化碳濃度都會超過4000
ppm，氧氣濃度則會大幅降到10%以下，甚至低到5%；相較於此，今日的二氧化碳濃度為300
ppm左右，氧氣濃度為21%。
他提出根據地球的循環步調，下次大滅絕還要再過將近三千萬年才會發生。
Ancient oxygen levels provide clues to
the timing of life and death on Earth
Oxygen is critical for life, but what
promoted the first rise in atmospheric oxygen on Earth and precisely when it
happened have been challenging scientists for the last 70 years.
Most scientific research suggests oxygen rose rapidly
about 2.4 billion years ago and then fell just as abruptly over the next 200
million years – this event is called the Great Oxygenation Event (GOE).
A new international study led by a team of geologists
from the University of Tasmania in collaboration with scientists from the
Carnegie Institute in Washington and the University of Toronto offers an
alternative theory.
The researchers propose that the rise of atmospheric
oxygen was a very slow process between 2.8 and 1.8 million years ago related to
the collision of continental plates during supercontinent cycles and the
evolution of cyanobacteria in our oceans.
Atmospheric oxygen rose over a period of a billion
years, with a peak close to present day levels of 21 per cent oxygen around 1.9
billion years ago. Oxygen then declined for a further period known as the
boring billion.
The research demonstrated that the evolution of
minerals in the Earth’s crust correlate with the rise of oxygen due to the
presence of new oxidised metal species that only became available because of
the rise in oxygen.
The new theory uses measurements on the redox
chemistry of minerals that form in rocks and on the seafloor through geological
time.
University of Tasmania geologist Professor Ross Large
said their results are based on a wealth of data from a range of minerals and
isotopes.
The teams at the University of Tasmania, Toronto and
Carnegie institute have built massive databases on the chemistry of a wide
range of minerals, involving tens of thousands of analyses collected over the
last 15 years.
“The world-wide trend toward data-driven research is increasing
because our technology is rapidly changing, enabling thousands of analyses to
be acquired,” Professor Large explained.
“Much previous research on this topic has depended on
limited analyses, supported by computer models to fill-in the data and attempt
to predict outcomes. This has commonly led to ‘straight line’ interpretations
that have ignored the Earth’s up-and-down cycles through geological time.”
Professor Large says the first rise in oxygen was
accompanied by a decline in carbon dioxide and methane, producing ocean and
atmosphere conditions more amenable to life.
“The old Archean oceans prior to 2.6 billion years
ago were enriched in toxic elements such as arsenic and mercury and very
inhospitable to life as we know it,” Professor Large said.
“Our research shows that with increasing oxygen the
chemistry of the ocean changed, toxic elements declined and elements important
to life such as phosphorus, molybdenum and zinc became more available to
stimulate evolutionary change.”
Professor Large said these major changes were brought
on by the first development of continental drift related to the supercontinent
cycles, which describe the assembly, duration and fragmentation of the largest
land masses on Earth.
“Mountain building during collision of plates in the
first phase of each supercontinent cycle led to erosion of nutrients to the oceans,
stimulating life and release of oxygen to the atmosphere,” Professor Large
explained.
“We propose that two phases of mountain building
helped drive the rise in oxygen, production of new minerals and evolution of
early life. The first occurred around 2.8 billion years ago with the formation
of the supercontinent Kenorland, and the second around 2.1 billion years ago
which formed the supercontinent Nuna.”
The third oxygen cycle started about a billion years
ago, and from then on the cycles increased in frequency from about 200 million
years apart down to 60 million years apart. Previous research by the team has
shown that each oxygen cycle ended with a mass extinction but was rapidly
followed by an explosion in evolution.
Contrary to some suggestions, Professor Large does
not consider we are heading into another mass extinction. He said that past
mass extinctions involved carbon dioxide rising to greater than 4000 parts per
million (ppm), compared to about 300 ppm today, and oxygen dropping well below
10 per cent and possibly as low as 5 per cent, compared with 21 per cent today.
He suggests that, based on Earth cycles, the next
mass extinction is about 30 million years away.
原始論文：Ross
R. Large, Robert M. Hazen, Shaunna M. Morrison, Dan D Gregory, Jeffrey
A.Steadman, Indrani Mukherjee. Evidence
that the GOE was a prolonged event with a peak around 1900 Ma. Geosystems and Geoenvironment, 2022; 1 (2)
https://doi.org/10.1016/j.geogeo.2022.100036
引用自：University
of Tasmania. “Ancient oxygen levels provide clues to the timing of life and
death on Earth”
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