你在呼吸嗎？那得感謝火山、板塊運動和細菌
研究表示用一個說法就能解釋和可以用來呼吸的氧氣有關的數個謎團
By Jade Boyd
地球的大氣可以呼吸是生物存活的關鍵。新的研究認為大氣裡第一波氧氣的來源是板塊運動造成的一連串火山爆發。
距今25億年前左右地球大氣裡的氧濃度大幅增加，科學家稱為「大氧化事件」(Great
Oxidation Event, GOE)。在本周發表於《自然―地球科學》(Nature Geoscience)的研究中，萊斯大學的地球科學家提出了新的理論，有助於解釋大氧化事件的成因。
「這項研究特別的地方在於我們不只試著去解釋氧氣增加的原因。」主要作者，在加州大學河濱分校進行研究的NASA博士後研究員James
Eguchi表示。他在萊斯大學撰寫博士論文時進行了這項研究。「我們也試著解釋一些關係密切的地表地球化學變化，像是在氧化事件不久之後的碳酸岩紀錄中，可以觀察到碳同位素有所改變。我們嘗試把這些個別現象用一個機制來解釋，這牽涉到地球深處、板塊運動以及更多來自火山的二氧化碳。」
和Eguchi共同進行這項研究的包括Rajdeep
Dasgupta和Johnny
Seales，前者為萊斯大學地球環境與行星科學系的理論和實驗地球化學教授；後者為萊斯大學的研究生，負責協助運算模型及驗證理論。
由於光合作用過程中的廢物為氧氣，使得科學家長久以來認為大氧化事件時氧氣增加的原因可能就是光合作用。Dasgupta表示他們的新理論並沒有降低第一個光合作用生物――藍綠菌在大氧化事件中的地位。
「大多數人認為氧氣增加跟藍綠菌有關，這並沒有錯。」他說，「光合作用生物出現確實會把氧氣釋放出來。但最重要的問題是它們出現的時間點是否跟大氧化事件一致？證據顯示兩者並非同時發生。」
早在大氧化事件發生前的5億年，藍綠菌就已經生活在地球上了。雖然以前曾提出幾種理論來解釋為什麼要過了這麼久氧氣才出現在大氣當中，Dasgupta表示就他所知，這些理論都沒有一併解釋大氧化事件的1億年後，碳酸鹽礦物中開始出現的碳同位素變化。這個持續了數億年的現象地質學家稱為
Lomagundi事件。
在100顆碳原子中有1顆是同位素碳-13，其他99顆則是碳-12。這種1比99的比例清楚記錄在Lomagundi事件之前與之後的碳酸鹽中，但在事件當時形成的碳酸鹽中，碳13的比例卻高出了約10%。
Eguchi表示科學家長期以來認為Lomagundi事件中，和大氧化事件有關的藍綠菌大量增加是相當重要的因素。
「藍綠菌偏好利用碳-12勝於碳-13。」他說，「因此更多有機碳，也就是有更多藍綠菌出現之後，用來製造碳酸鹽的碳庫就會缺少碳-12。」
Eguchi表示科學家曾試著用此概念來解釋Lomagundi事件，但時間點卻是個問題。
「實際去看地質紀錄就可以發現碳-13對碳-12的比例增加，其實是在氧氣增加的一千多萬年後才開始發生。」他說，「因此要透過有機碳和碳酸鹽的比例變化，來解釋這兩個事件就變得十分困難。」
Eguchi、Dasgupta和Seales想出了下列情境來解釋所有的現象：
板塊活動劇烈增加形成了數以百計的火山，噴出二氧化碳到大氣當中。
接著氣候暖化和降雨增加使得風化作用增強，造成地球光禿禿地表上露出的岩石礦物受到化學作用而分解。
風化使逕流裡的礦物質增加，流到海裡之後促成藍綠菌和碳酸鹽大量形成。
這些無機碳和有機碳最終都會沉降到海床，隨著地球的隱沒帶把海洋板塊拉到大陸板塊之下，最後再次循環回到地函當中。
沉積物在地函中重新熔融的時候，碳酸鹽裡的無機碳會較早釋放出來，經由隱沒帶正上方的陸弧火山重新回到大氣。
含有極少碳-13的有機碳則會被拖入地球深處，過了數億年之後才會從夏威夷之類的熱點火山以二氧化碳的形式釋放出來。
「這是一種相當宏大的循環過程。」Eguchi表示，「我們認為藍綠菌在24億年前左右的數量變多，造成氧氣跟著增加。但是藍綠菌變多造成的影響會和碳酸鹽的增加達成平衡，使得碳-12和碳-13的比例並未改變。不過，碳酸鹽和有機碳隱沒到地球深部之後情況就不一樣了。此時地球化學開始發揮作用，造成這兩種不同形式的碳留在地函的時間不同。碳酸鹽更加容易釋放到岩漿當中，因而在短時間內就回到地表。Lomagundi事件便是開始於碳酸鹽富含碳-13的碳重回地表之時，並在許久之後富含碳-12的有機碳也回到地表，重新讓比例達成平衡的時候結束。」
Eguchi表示這項研究強調出地表生命的演化過程中，地球深處發生的作用可以造成重要影響。
「我們提出的二氧化碳釋放過程對於生命的繁榮來說相當重要。」他說，「這種過程確實試著連結了過去發生地球深部的作用如何影響到地表的生物。」
Dasgupta也是NASA「CLEVER
Planets」的計畫主持人，該計畫旨在探討遙遠的地外行星可能透過什麼方式來齊聚生命所需的要素。他說更加了解地球如何變得適合居住，對於研究遠方世界的適居性與演化過程來說相當重要。
「在地球歷史上，板塊運動對適居性來說似乎具有很大的影響，但這不必然代表要讓氧氣增加就一定需要板塊運動。」他說，「或許還有其他方法可以讓氧氣增加並維持住，探討這些可能性就是我們在CLEVER
Planets計畫中的一項目標。」
研究經費來自美國國家科學基金、NASA和深碳觀測計畫。
Breathing? Thank volcanoes, tectonics
and bacteria
Study points to one cause for several
mysteries linked to breathable oxygen
Earth’s breathable atmosphere is key for
life, and a new study suggests that the first burst of oxygen was added by a
spate of volcanic eruptions brought about by tectonics.
The study by geoscientists at Rice University offers
a new theory to help explain the appearance of significant concentrations of
oxygen in Earth’s atmosphere about 2.5 billion years ago, something scientists
call the Great Oxidation Event (GOE). The research appears this week in Nature Geoscience.
“What makes this unique is that it’s not just trying
to explain the rise of oxygen,” said study lead author James Eguchi, a NASA
postdoctoral fellow at the University of California, Riverside who conducted
the work for his Ph.D. dissertation at Rice. “It’s also trying to explain some
closely associated surface geochemistry, a change in the composition of carbon
isotopes, that is observed in the carbonate rock record a relatively short time
after the oxidation event. We’re trying explain each of those with a single
mechanism that involves the deep Earth interior, tectonics and enhanced
degassing of carbon dioxide from volcanoes.”
Eguchi’s co-authors are Rajdeep Dasgupta, an
experimental and theoretical geochemist and professor in Rice’s Department of
Earth, Environmental and Planetary Sciences, and Johnny Seales, a Rice graduate
student who helped with the model calculations that validated the new theory.
Scientists have long pointed to photosynthesis — a
process that produces waste oxygen — as a likely source for increased oxygen
during the GOE. Dasgupta said the new theory doesn’t discount the role that the
first photosynthetic organisms, cyanobacteria, played in the GOE.
“Most people think the rise of oxygen was linked to
cyanobacteria, and they are not wrong,” he said. “The emergence of
photosynthetic organisms could release oxygen. But the most important question
is whether the timing of that emergence lines up with the timing of the Great
Oxidation Event. As it turns out, they do not.”
Cyanobacteria were alive on Earth as much as 500
million years before the GOE. While a number of theories have been offered to
explain why it might have taken that long for oxygen to show up in the
atmosphere, Dasgupta said he’s not aware of any that have simultaneously tried
to explain a marked change in the ratio of carbon isotopes in carbonate
minerals that began about 100 million years after the GOE. Geologists refer to
this as the Lomagundi Event, and it lasted several hundred million years.
One in a hundred carbon atoms are the isotope
carbon-13, and the other 99 are carbon-12. This 1-to-99 ratio is well
documented in carbonates that formed before and after Lomagundi, but those
formed during the event have about 10% more carbon-13.
Eguchi said the explosion in cyanobacteria associated
with the GOE has long been viewed as playing a role in Lomagundi.
“Cyanobacteria prefer to take carbon-12 relative to
carbon-13,” he said. “So when you start producing more organic carbon, or
cyanobacteria, then the reservoir from which the carbonates are being produced
is depleted in carbon-12.”
Eguchi said people tried using this to explain
Lomagundi, but timing was again a problem.
“When you actually look at the geologic record, the
increase in the carbon-13-to-carbon-12 ratio actually occurs up to 10s of
millions of years after oxygen rose,” he said. “So then it becomes difficult to
explain these two events through a change in the ratio of organic carbon to
carbonate.”
The scenario Eguchi, Dasgupta and Seales arrived at to
explain all of these factors is:
A dramatic increase in tectonic activity led to the
formation of hundreds of volcanoes that spewed carbon dioxide into the
atmosphere.
The climate warmed, increasing rainfall, which in
turn increased “weathering,” the chemical breakdown of rocky minerals on
Earth’s barren continents.
Weathering produced a mineral-rich runoff that poured
into the oceans, supporting a boom in both cyanobacteria and carbonates.
The organic and inorganic carbon from these wound up
on the seafloor and was eventually recycled back into Earth’s mantle at
subduction zones, where oceanic plates are dragged beneath continents.
When sediments remelted into the mantle, inorganic
carbon, hosted in carbonates, tended to be released early, re-entering the
atmosphere through arc volcanoes directly above subduction zones.
Organic carbon, which contained very little
carbon-13, was drawn deep into the mantle and emerged hundreds of millions of
years later as carbon dioxide from island hotspot volcanoes like Hawaii.
“It’s kind of a big cyclic process,” Eguchi said. “We
do think the amount of cyanobacteria increased around 2.4 billion years ago. So
that would drive our oxygen increase. But the increase of cyanobacteria is
balanced by the increase of carbonates. So that carbon-12-to-carbon-13 ratio
doesn’t change until both the carbonates and organic carbon, from
cyanobacteria, get subducted deep into the Earth. When they do, geochemistry
comes into play, causing these two forms of carbon to reside in the mantle for
different periods of time. Carbonates are much more easily released in magmas
and are released back to the surface at a very short period. Lomagundi starts
when the first carbon-13-enriched carbon from carbonates returns to the
surface, and it ends when the carbon-12-enriched organic carbon returns much
later, rebalancing the ratio.”
Eguchi said the study emphasizes the importance of
the role that deep Earth processes can play in the evolution of life at the
surface.
“We’re proposing that carbon dioxide emissions were
very important to this proliferation of life,” he said. “It’s really trying to
tie in how these deeper processes have affected surface life on our planet in
the past.”
Dasgupta is also the principal investigator on a
NASA-funded effort called CLEVER Planets that is exploring how life-essential
elements might come together on distant exoplanets. He said better
understanding how Earth became habitable is important for studying habitability
and its evolution on distant worlds.
“It looks like Earth’s history is calling for
tectonics to play a big role in habitability, but that doesn’t necessarily mean
that tectonics is absolutely necessary for oxygen build up,” he said. “There
might be other ways of building and sustaining oxygen, and exploring those is
one of the things we’re trying to do in CLEVER Planets.”
The research was supported by the National Science
Foundation, NASA and the Deep Carbon Observatory.
原始論文：James
Eguchi, Johnny Seales, Rajdeep Dasgupta. Great Oxidation and Lomagundi events
linked by deep cycling and enhanced degassing of carbon. Nature Geoscience,
2019; DOI: 10.1038/s41561-019-0492-6
引用自：Rice
University. "Breathing? Thank volcanoes, tectonics and bacteria
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