Hannah Hickey
分析澳洲25億年前的岩石得到的最新結果顯示，當時火山爆發或許刺激了海洋微生物族群大量成長，進而產生大氣中的第一股氧氣。這項新的發現可能會改寫科學家關於地球早期大氣的現有想法，他們推測早期大氣的多數變化都是受控於地質或化學作用。
2004年，Roger Buick 身處澳洲西部的麥克雷山頁岩。附近鑽取出來的岩石顯示在24億年前的大氧化事件之前曾經有過一絲氧氣。新的分析得出稍早之前的岩石裡，元素汞含量有道火山噴發所形成的高峰，這場爆發或許也促成了單細胞生物大量生長，產生了短暫出現的一絲氧氣。圖片來源：Roger Buick/University of Washington
雖然這項研究的重點是地球早期的歷史，但也能讓我們更加了解地外生命，甚至是氣候變遷。研究由華盛頓大學、密西根大學與其他研究機構主持，成果發表在八月的《美國國家科學院院刊》(Proceedings
of the National Academy of Sciences)。
「過去數十年，可以愈來越清楚地看到無生命的固體地球和生命的演化過程之間，其實有諸多關聯，」研究第一作者，華盛頓大學地球與太空科學系的博士生Jana
Meixnerová表示。「但究竟是哪些關聯，促使地球上的生命演化成我們所知的這副模樣？」
在地球最早的那段日子大氣中並沒有氧氣，可以呼吸氧氣的生命體就算有也非常稀少。地球的大氣要到24億年前才開始持續擁有豐富的氧氣，或許是因為在那之前可以把二氧化碳和水轉換成氧氣，也就是行光合作用的生命體大幅增加的關係。
但這項研究的共同作者，亞歷桑納州立大學Ariel
Anbar在2007年分析澳洲西部的麥克雷山頁岩之後，發表在大氣持續擁有許多氧氣的五千萬到一億年之前，曾經有一絲氧氣短暫地出現大氣當中。之後其他研究也證實了在更加久遠的年代氧氣還有幾次短暫的高峰，不過都沒有解釋它們為何出現，又是為何消失。
Joel Blum是這項新研究的通訊作者，在他的領導之下，密西根大學的研究人員同樣採集了古老的麥克雷山頁岩，分析裡面從火山釋放出來的汞元素的濃度與中子數。大型火山爆發可以把汞蒸氣噴到高層大氣，它們在現代會飄散一到兩年之後才隨著雨水落到地表。新的分析顯示氧氣短暫上升的數百萬年之前，汞的濃度出現了高峰。
「在氧氣出現短暫高峰下方的岩石裡，我們非常肯定汞的豐度與同位素呈現出來的證據，最合理的解釋都是汞被火山爆發噴到大氣中，」共同作者，華盛頓大學地球與太空科學的教授Roger
Buick表示。
作者推論只要有火山噴發，必定會有熔岩與火山灰覆蓋到地面。接著這些富含營養素的岩石會在風雨當中逐漸風化而把磷釋放出來。它們被河水帶到附近的海岸地區之後具有施肥的作用，使得可以產生氧氣的藍綠菌與其他單細胞生命體大量滋長。
「雖然其他種類的營養素短期來說也能調控生物活動，但是長期而言磷是最重要的營養素之一，」Meixnerová表示。
今日，磷在生物材料與農業用肥中都非常豐富，但是在非常古老的年代卻是稀有資源，當時的主要來源為火山岩的風化產物。
「在太古代大氣造成的風化過程當中，新鮮的玄武岩可能會慢慢溶解，將磷這種生物必需的巨量營養素釋放到河水。它們流到海裡之後便能餵養居住在淺灘的微生物，使得生物生產力增加，而副產物之一可能就是氧氣出現短暫的高峰，」Meixnerová表示。
Buick說他們不知道這些火山以及熔岩平原的確切地點，但是現今的印度、加拿大以及其他地方有年代剛好在這附近的大型熔岩平原。
「我們的研究提出形成這些暫時出現的微量氧氣，最直接的原因是氧氣產量增加，而非岩石消耗的氧氣減少或者其他非生物作用，」Buick表示。「這項研究非常重要，因為氧氣出現改變了大氣的本質，是驅使大型複雜生命演化出來的最大原因。」
最後，研究人員表示這項研究提出了對於在行星表面演化出的任何生命來說，行星地質可能會有什麼樣的影響。了解這項關係有助於在搜尋宇宙間的生命時，找出適合居住的系外行星(太陽系之外的行星)。
論文其他作者包括之前為華盛頓大學天體生物學的研究生，現在任職於蘇格蘭聖安德魯斯大學的共同通訊作者Eva
Stüeken；之前為華盛頓大學的研究生，現在任職於加州理工學院的Michael
Kipp；以及密西根大學的Marcus
Johnson。研究經費來自NASA、由NASA資助的華盛頓大學虛擬行星實驗室，以及密西根大學授予Blum的MacArthur教授職位。
Volcanic eruptions may have spurred
first ‘whiffs’ of oxygen in Earth’s atmosphere
A new analysis of 2.5-billion-year-old
rocks from Australia finds that volcanic eruptions may have stimulated
population surges of marine microorganisms, creating the first puffs of oxygen
into the atmosphere. This would change existing stories of Earth’s early
atmosphere, which assumed that most changes in the early atmosphere were
controlled by geologic or chemical processes.
Though focused on Earth’s early history, the research
also has implications for extraterrestrial life and even climate change. The
study led by the University of Washington, the University of Michigan and other
institutions was published in August in the Proceedings
of the National Academy of Sciences.
“What has started to become obvious in the past few
decades is there actually are quite a number of connections between the solid,
nonliving Earth and the evolution of life,” said first author Jana Meixnerová,
a UW doctoral student in Earth and space sciences. “But what are the specific
connections that facilitated the evolution of life on Earth as we know it?”
In its earliest days, Earth had no oxygen in its
atmosphere and few, if any, oxygen-breathing lifeforms. Earth’s atmosphere
became permanently oxygen-rich about 2.4 billion years ago, likely after an
explosion of lifeforms that photosynthesize, transforming carbon dioxide and
water into oxygen.
But in 2007, co-author Ariel Anbar at Arizona State
University analyzed rocks from the Mount McRae Shale in Western Australia,
reporting a short-term whiff of oxygen about 50 to 100 million years before it
became a permanent fixture in the atmosphere. More recent research has
confirmed other, earlier short-term oxygen spikes, but hasn’t explained their
rise and fall.
In the new study, researchers at the University of
Michigan, led by co-corresponding author Joel Blum, analyzed the same ancient
rocks for the concentration and number of neutrons in the element mercury,
emitted by volcanic eruptions. Large volcanic eruptions blast mercury gas into
the upper atmosphere, where today it circulates for a year or two before
raining out onto Earth’s surface. The new analysis shows a spike in mercury a
few million years before the temporary rise in oxygen.
“Sure enough, in the rock below the transient spike in
oxygen we found evidence of mercury, both in its abundance and isotopes, that
would most reasonably be explained by volcanic eruptions into the atmosphere,”
said co-author Roger Buick, a UW professor of Earth and Space Sciences.
Where there were volcanic emissions, the authors
reason, there must have been lava and volcanic ash fields. And those
nutrient-rich rocks would have weathered in the wind and rain, releasing
phosphorus into rivers that could fertilize nearby coastal areas, allowing
oxygen-producing cyanobacteria and other single-celled lifeforms to flourish.
“There are other nutrients that modulate biological
activity on short timescales, but phosphorus is the one that is most important
on long timescales,” Meixnerová said.
Today, phosphorus is plentiful in biological material
and in agricultural fertilizer. But in very ancient times, weathering of
volcanic rocks would have been the main source for this scarce resource.
“During weathering under the Archaean atmosphere, the
fresh basaltic rock would have slowly dissolved, releasing the essential
macro-nutrient phosphorus into the rivers. That would have fed microbes that
were living in the shallow coastal zones and triggered increased biological
productivity that would have created, as a byproduct, an oxygen spike,”
Meixnerová said.
The precise location of those volcanoes and lava
fields is unknown, but large lava fields of about the right age exist in
modern-day India, Canada and elsewhere, Buick said.
“Our study suggests that for these transient whiffs of
oxygen, the immediate trigger was an increase in oxygen production, rather than
a decrease in oxygen consumption by rocks or other nonliving processes,” Buick
said. “It’s important because the presence of oxygen in the atmosphere is
fundamental – it’s the biggest driver for the evolution of large, complex
life.”
Ultimately, researchers say the study suggests how a
planet’s geology might affect any life evolving on its surface, an
understanding that aids in identifying habitable exoplanets, or planets outside
our solar system, in the search for life in the universe.
Other authors of the paper are co-corresponding author
Eva Stüeken, a former UW astrobiology graduate student now at the University of
St. Andrews in Scotland; Michael Kipp, a former UW graduate student now at the
California Institute of Technology; and Marcus Johnson at the University of
Michigan. The study was funded by NASA, the NASA-funded UW Virtual Planetary
Laboratory team and the MacArthur Professorship to Blum at the University of
Michigan.
原始論文：Jana
Meixnerová, Joel D. Blum, Marcus W. Johnson, Eva E. Stüeken, Michael A. Kipp,
Ariel D. Anbar, Roger Buick. Mercury abundance and isotopic composition
indicate subaerial volcanism prior to the end-Archean “whiff” of oxygen. Proceedings
of the National Academy of Sciences, 2021; 118 (33): e2107511118 DOI: 10.1073/pnas.2107511118
引用自：University of Washington. "Volcanic
eruptions may have spurred first 'whiffs' of oxygen in Earth's
atmosphere."
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