在大氧化事件獲得的成功以前，早期地球的氧氣濃度曾數度上升卻又下降
Peter Kelley
在距今大約24億年前發生的大氧化事件(Great
Oxidation Event)成功讓整個地球全面氧化，而由華盛頓大學進行的新研究顯示在此之前的數億年間，地球的氧氣濃度曾經提升卻又下跌不只一次。
西澳的Jeerinah組地層，由華盛頓大學領導的團隊在此發現氮同位素有突然的轉變。主要作者Matt
Koehler表示：「氮同位素訴說了一段關於海洋表層氧化的故事。這段含有氧氣的時期持續時間不超過5000萬年，範圍則涵蓋了一座寬達數百公里的洋盆。」圖片來源：Roger
Buick
一項新研究提出的證據指出地球在非常久遠以前有些微氧氣存在於大氣與廣大的海洋表層。這是第二筆且年代更加古老的證據顯示這種情形曾經發生，表示地球的氧化是在漫長歲月中不斷嘗試卻又失敗的複雜過程。
這項發現對於尋找地球以外的生命也有所啟發。在未來數年我們將會迎來更加有力的地面與太空望遠鏡，它們具備可以分析遙遠行星大氣的能力。這項成果有助於天文學家在利用望遠鏡尋找地外生命時避免剔除掉太多呈現「偽陰性」的行星――也就是適合生物居住的行星因為氧氣濃度低到無法被偵測，使得第一眼看來可能不適合生物居住。
論文主要作者，華盛頓大學地球和太空科學系的博士生Matt
Koehler表示：「直到大氧化事件之前，氧氣在海洋和大氣中的生成與消滅是一場長期抗戰，沒有明確跡象顯示最後誰能獲勝。」這篇新論文刊登於7月9日當周的《美國國家科學院院刊》(Proceedings of the
National Academy of Sciences)。
「這些短暫的氧化事件是在整場戰爭中，當平衡傾向對氧化較為有利時所出現的戰役結果。」
論文共同作者，華盛頓大學地球和太空科學系的教授Roger
Buick在2007年參與的國際科學團隊找到的證據顯示，約莫在大氧化事件發生的5億至10億年前，曾經有段時期地球上存在著些微氧氣。當時他們鑽探至澳洲西部麥克雷山頁岩層的沉積岩深處，並分析樣本中的稀有金屬鉬和錸――這兩種金屬的沉積取決於環境中的氧氣――而得到該結論。
由Koehler領導的團隊現在證明了地球過去還有另一段氧氣曾經短暫出現的時期，它比前次發現還早了大約1.5億年，也就是距今26.6億年前左右，而持續時間則不超過5000萬年。在這項研究中他們利用了可以指示氧氣的兩種不同指標：氮同位素和硒元素。同樣地，這兩種物質能用各自的方式指出氧氣曾經存在。
「我們在這篇論文中以高解析度的方法偵測出另一次有些微氧氣出現的短暫時期，」Koehler表示。「氮同位素訴說了一段關於海洋表層氧化的故事。這段含有氧氣的時期持續時間不超過5000萬年，範圍則涵蓋了一座寬達數百公里的洋盆。」
團隊分析的樣品為2012年Buick在西澳西北部另一處稱為Jeerinah組的地層所鑽探出來的樣品。
研究人員鑽取的兩個岩芯位置雖然相隔了大約300公里，但還是位於同一個沉積岩層。其中一組岩芯樣品為沉積在淺水的沉積物，另一組則是沉積在深水的沉積物。Buick表示分析這兩組連續堆積的岩層可以看出氮同位素逐漸發生變化，然後又回歸平淡。「它所代表的意義只能用環境裡出現了氧氣來解釋。它的出現實在令人感到訝異，而又如此突然。」
氮同位素可以顯示出某些海洋微生物的活動，像是利用氧氣形成硝酸鹽的微生物，以及其他利用這些硝酸鹽產生能量的微生物。從氮同位素得出的數據顯示了海洋表層的狀況，另一方面硒含量則指出古代地球的空氣中有氧氣存在。Koehler表示當時深海可能處於缺氧狀態。
團隊發現只有淺海鑽井中的樣品有大量的硒，意謂著硒來自於附近的陸地且並未到達深海。陸地上的硒存在於含硫礦物中。當大氣裡的氧濃度升高，氧化造成的風化作用(Buick表示：「也就是岩石生鏽」)會讓更多硒從陸地淋溶出來而輸送到海洋當中。
「然後硒會累積在海洋在沉積物裡面，」Koehler表示。「因此當我們在海洋沉積物中測量到硒的含量出現高峰時，就代表大氣中的氧氣有短暫增加的現象。」
Buick和Koehler表示這項發現也能用於偵測太陽系以外的行星之上是否有生命。
Buick表示：「一般認為氧氣是大氣中最明顯的生物跡象(biosignature)，但這項研究證明了行星的氧氣變成永久存在的過程中，可能會有幾個時期地表環境會有氧氣，但僅持續數百萬年之後就再次滑落成無氧狀態。」
「因此，如果無法在一顆行星的大氣層中偵測到氧氣，也不代表它不適合生命居住，甚至是缺乏行光合作用的生命。可能只是因為它的氧氣來源還不夠多，只能在短時間之內壓過消耗氧氣的因子而無法延長更久。」
「換句話說，缺乏氧氣在偵測生物時很容易就呈現「偽陰性」。」
Koehler補充：「你可能會看著一顆行星而找不到一丁點氧氣――但它實際上可能充滿了微生物。」
Oxygen levels on early Earth rose and
fell several times before the successful Great Oxidation Event
Earth’s
oxygen levels rose and fell more than once hundreds of millions of years before
the planetwide success of the Great Oxidation Event about 2.4 billion years
ago, new research from the University of Washington shows.
The evidence comes from a new study that indicates a
second and much earlier “whiff” of oxygen in Earth’s distant past — in the
atmosphere and on the surface of a large stretch of ocean — showing that the
oxygenation of the Earth was a complex process of repeated trying and failing
over a vast stretch of time.
The finding also may have implications in the search
for life beyond Earth. Coming years will bring powerful new ground- and
space-based telescopes able to analyze the atmospheres of distant planets. This
work could help keep astronomers from unduly ruling out “false negatives,” or
inhabited planets that may not at first appear to be so due to undetectable
oxygen levels.
“The production and destruction of oxygen in the
ocean and atmosphere over time was a war with no evidence of a clear winner,
until the Great Oxidation Event,” said Matt Koehler, a UW doctoral student in
Earth and space sciences and lead author of a new paper published the week of
July 9 in the Proceedings of the National
Academy of Sciences.
“These transient oxygenation events were battles in the
war, when the balance tipped more in favor of oxygenation.”
In 2007, co-author Roger Buick, UW professor of Earth
and space sciences, was part of an international team of scientists that found
evidence of an episode — a “whiff” — of oxygen some 50 million to 100 million
years before the Great Oxidation Event. This they learned by drilling deep into
sedimentary rock of the Mount McRae Shale in Western Australia and analyzing
the samples for the trace metals molybdenum and rhenium, accumulation of which
is dependent on oxygen in the environment.
Now, a team led by Koehler has confirmed a second
such appearance of oxygen in Earth’s past, this time roughly 150 million years
earlier — or about 2.66 billion years ago — and lasting for less than 50
million years. For this work they used two different proxies for oxygen —
nitrogen isotopes and the element selenium — substances that, each in its way,
also tell of the presence of oxygen.
“What we have in this paper is another detection, at
high resolution, of a transient whiff of oxygen,” said Koehler. “Nitrogen
isotopes tell a story about oxygenation of the surface ocean, and this
oxygenation spans hundreds of kilometers across a marine basin and lasts for
somewhere less than 50 million years.”
The team analyzed drill samples taken by Buick in 2012
at another site in the northwestern part of Western Australia called the
Jeerinah Formation.
The researchers drilled two cores about 300
kilometers apart but through the same sedimentary rocks — one core samples
sediments deposited in shallower waters, and the other samples sediments from
deeper waters. Analyzing successive layers in the rocks years shows, Buick
said, a “stepwise” change in nitrogen isotopes “and then back again to zero.
This can only be interpreted as meaning that there is oxygen in the
environment. It’s really cool — and it’s sudden.”
The nitrogen isotopes reveal the activity of certain
marine microorganisms that use oxygen to form nitrate, and other microorganisms
that use this nitrate for energy. The data collected from nitrogen isotopes
sample the surface of the ocean, while selenium suggests oxygen in the air of
ancient Earth. Koehler said the deep ocean was likely anoxic, or without
oxygen, at the time.
The team found plentiful selenium in the shallow hole
only, meaning that it came from the nearby land, not making it to deeper water.
Selenium is held in sulfur minerals on land; higher atmospheric oxygen would
cause more selenium to be leached from the land through oxidative weathering —
“the rusting of rocks,” Buick said — and transported to sea.
“That selenium then accumulates in ocean sediments,”
Koehler said. “So when we measure a spike in selenium abundances in ocean
sediments, it could mean there was a temporary increase in atmospheric oxygen.”
The finding, Buick and Koehler said, also has
relevance for detecting life on exoplanets, or those beyond the solar system.
“One of the strongest atmospheric biosignatures is
thought to be oxygen, but this study confirms that during a planet’s transition
to becoming permanently oxygenated, its surface environments may be oxic for
intervals of only a few million years and then slip back into anoxia,” Buick
said.
“So, if you fail to detect oxygen in a planet’s
atmosphere, that doesn’t mean that the planet is uninhabited or even that it
lacks photosynthetic life. Merely that it hasn’t built up enough sources of
oxygen to overwhelm the ‘sinks’ for any longer than a short interval.
“In other words, lack of oxygen can easily be a
‘false negative’ for life.”
Koehler added: “You could be looking at a planet and
not see any oxygen — but it could be teeming with microbial life.”
原始論文：Matthew C.
Koehler, Roger Buick, Michael A. Kipp, Eva E. Stüeken, Jonathan Zaloumis. Transient
surface ocean oxygenation recorded in the ∼2.66-Ga Jeerinah Formation, Australia. Proceedings
of the National Academy of Sciences, 2018; 201720820 DOI: 10.1073/pnas.1720820115
引用自：University of Washington. "Oxygen levels
on early Earth rose, fell several times before great oxidation
even."
