By Brian Maffly
距今大約25億年前，地球大氣的自由氧(O2)含量首次開始累積到有具體的影響，而為我們這顆不斷演進的星球，能有複雜生命出現奠定了基礎。
科學家將此現象稱為「大氧化事件」(Great
Oxidation Event，簡稱GOE)。雖然乍聽之下簡單明瞭，但是猶他大學的地球化學家在新的研究中顯示，地球氧氣最初累積的過程絕非如此直觀。
這起「事件」持續了至少2億年。地質與地球物理系的助理教授Chadlin Ostrander表示在本研究之前，追蹤海中的氧氣累積過程是件相當困難的事。
該研究6月12日發表在期刊《自然》。「近期證據指出氧氣最初在地球大氣上升的過程是動態的，這種斷斷續續的起步階段可能要到距今22億年前才結束，」研究主要作者Ostrander表示。「我們的資料證實了這項假說，甚至進一步地顯示海洋氧氣上升的過程也是動態的。」
Ostrander領導的國際團隊經費來源為NASA的太空生物學計畫。他們的研究對象主要為南非特蘭斯瓦超群(Transvaal
Supergroup)之內在海洋形成的頁岩，成果讓他們對於這個地球歷史上關鍵階段的動態海洋氧化過程有了全新觀點。透過分析穩定鉈同位素以及對氧化還原敏感的元素，他們證實了海洋氧含量的波動與大氣氧氣的變化一致。
此時期鋪陳了我們所知的生命得以演化出來的條件，因此在地球歷史上相當關鍵，而這些發現有助於我們深入瞭解決定這段時期地球氧氣含量的複雜過程。
「地球最早的生命形式可能是在海洋誕生並演化，但我們真的不太清楚當時海洋裡面的狀況，」Ostrander表示。他去年離開麻州的伍茲霍爾海洋研究所，成為猶他大學的教師。「因此，在研究早期生命的時候，瞭解海洋的氧含量以及隨時間的演變過程可能比瞭解大氣更加重要。」
共同作者，英國里茲大學的Simon
Poulton和加州大學河濱分校的Andrey
Bekker先前的成果構成了本研究的基礎。在2021年的文章當中，他們的研究團隊發現全球氧化過程開始大約2億年之後，氧氣才永遠成為大氣中的一份子，比過往認為的還要慢了很多。
大氣缺氧的決定性證據是出現了在GOE之前的沉積紀錄中，與質量無關的稀有硫同位素訊號。地球上只有極為少數的作用可以產生這種硫同位素訊號，就目前所知，如果它們在岩石紀錄當中保留下來，幾乎可以肯定當時的大氣並沒有氧氣。
在地球誕生至今的前半段歲月，大部分的時間大氣和海洋都缺乏氧氣。雖然海洋裡的藍綠菌在大氧化事件之前就在製造這些氣體，但初期的氧氣似乎很快就會和露出地表的礦物與火山氣體發生反應而分解。Poulton、Bekker和他們的同事發現大氧化事件期間這種稀有的硫同位素訊號反覆地消失出現，代表氧氣含量曾經上升下降許多次——也就是這並非一起單一事件。
「氧氣開始製造時地球還沒有做好氧化的準備。地球的生物、地質和化學性質都還需要一些時間才能演變到利於氧化進行，」Ostrander表示。「這就像是個蹺蹺板，如果你在製造氧氣的同時還有許多事物在消耗氧氣，那就不會發生任何變化。我們還在試著找出什麼時候這種局勢才被徹底打破，使得地球大氣再也回不去沒有氧氣的狀態。」
今日氧氣在大氣中的重量占比為21%，僅次於氮氣。但是在大氧化事件後的數億年之間，氧氣在大氣中仍只佔了非常小的一部分而已。
為了追溯大氧化事件期間海洋有多少氧氣，研究團隊藉助Ostrander在穩定鉈同位素的專長。
同位素是屬於同一種元素，但中子數不同的原子，這也讓它們的重量有輕微差異。研究特定一種元素的同位素之間的比例，已經幫助考古學、地球化學以及其他許多領域得到諸多發現。
質譜儀的進步使得科學家可以探討週期表越來越下方的元素，精確分析它們的同位素比例，像是鉈。對於Ostrander和團隊來說幸運的是，鉈的同位素比例對於埋藏在海床的氧化錳相當敏感，而此過程需要海水有氧氣才能發生。團隊探討了在海床形成的頁岩裡的鉈同位素，最近這些岩層已經顯示其中的稀有硫同位素可以用來追蹤大氧化事件期間大氣的氧氣波動。
Ostrander和團隊發現這些頁岩有個地方值得注意：它們富集質量較輕的鉈同位素(203Tl)。這種模式的最佳解釋方法是海床埋有氧化錳，也就是海水中要有氧氣累積。203Tl富集的樣本也缺乏稀有硫同位素的訊號，代表同時間的大氣並非處於缺氧狀態；更好的是，203Tl富集的現象，在稀有硫同位素訊號再次出現的時候也會跟著消失。他們也用另外一種較常用來追蹤古代氧氣變化的工具——對氧化還原敏感的元素富集模式——來佐證上述發現。
「當硫同位素顯示大氣氧化的時候，鉈同位素指出海洋也是處在氧化狀態；而硫同位素顯示大氣再次落入缺氧狀態時，鉈同位素指出海洋也處在相同的情形，」Ostrander表示。「因此大氣海洋的氧化與缺氧是一同發生的，對於研究興趣為古代地球的科學家來說，這是十分新穎且有趣的資訊。」
What the geologic record reveals about
how the oceans were oxygenated
About 2.5 billion years ago, free oxygen,
or O2, first started accumulating to meaningful levels in Earth’s
atmosphere, setting the stage for the rise of complex life on our evolving
planet.
Scientists refer to this phenomenon as the Great
Oxidation Event, or GOE for short. But the initial accumulation of O2
on Earth was not nearly as straightforward as that moniker suggests, according
to new research led by a University of Utah geochemist.
This “event” lasted at least 200 million years. And
tracking the accumulation of O2 in the oceans has been very
difficult until now, said Chadlin Ostrander, an assistant professor in the
Department of Geology & Geophysics.
“Emerging data suggest that the initial rise of O2
in Earth’s atmosphere was dynamic, unfolding in fits-and-starts until perhaps
2.2. billion years ago,” said Ostrander, lead author on the study published
June 12 in the journal Nature. “Our
data validate this hypothesis, even going one step further by extending these
dynamics to the ocean.”
His international research team, supported by the
NASA Exobiology program, focused on marine shales from South Africa’s Transvaal
Supergroup, yielding insights into the dynamics of ocean oxygenation during
this crucial period in Earth’s history. By analyzing stable thallium (Tl)
isotope ratios and redox-sensitive elements, they uncovered evidence of
fluctuations in marine O2 levels that coincided with changes in
atmospheric oxygen.
These findings help advance the understanding of the
complex processes that shaped Earth’s O2 levels during a critical
period in the planet’s history that paved the way for the evolution of life as
we know it.
“We really don’t know what was going on in the
oceans, where Earth’s earliest lifeforms likely originated and evolved,” said
Ostrander, who joined the U faculty last year from the Woods Hole Oceanographic
Institution in Massachusetts. “So knowing the O2 content of the
oceans and how that evolved with time is probably more important for early life
than the atmosphere.”
The research builds on the work of Ostrander’s
co-authors Simon Poulton of the University of Leeds in the U.K. and Andrey
Bekker of the University of California, Riverside. In a 2021 study, their team
of scientists discovered that O2 did not become a permanent part of
the atmosphere until about 200 million years after the global oxygenation
process began, much later than previously thought.
The “smoking gun” evidence of an anoxic atmosphere is
the presence of rare, mass-independent sulfur isotope signatures in sedimentary
records before the GOE. Very few processes on Earth can generate these sulfur
isotope signatures, and from what is known their preservation in the rock
record almost certainly requires an absence of atmospheric O2.
For the first half of Earth’s existence, its atmosphere
and oceans were largely devoid of O2. This gas was being produced by
cyanobacteria in the ocean before the GOE, it seems, but in these early days,
the O2 was rapidly destroyed in reactions with exposed minerals and
volcanic gasses. Poulton, Bekker and colleagues discovered that the rare sulfur
isotope signatures disappear but then reappear, suggesting multiple O2 rises
and falls in the atmosphere during the GOE. This was no single “event.”
“Earth wasn’t ready to be oxygenated when oxygen
started to be produced. Earth needed time to evolve biologically, geologically
and chemically to be conducive to oxygenation,” Ostrander said. “It’s like a
teeter-totter. You have oxygen production, but you have so much oxygen
destruction, that nothing’s happening. We’re still trying to figure out when
we’ve completely tipped the scales and Earth could not go backward to an anoxic
atmosphere.”
Today, O2 accounts for 21% of the
atmosphere, by weight, second only to nitrogen. But following the GOE, oxygen
remained a very small component of the atmosphere for hundreds of millions of
years.
To track the presence of O2 in the ocean
during the GOE, the research team relied on Ostrander’s expertise with stable
thallium isotopes.
Isotopes are atoms of the same element that have an
unequal number of neutrons, giving them slightly different weights. Ratios of a
particular element’s isotopes have powered discoveries in archaeology,
geochemistry and many other fields.
Advances in mass spectrometry have enabled scientists
to accurately analyze isotope ratios for elements farther and farther down the
Periodic Table, such as thallium. Luckily for Ostrander and his team, thallium
isotope ratios are sensitive to manganese oxide burial on the seafloor, a
process that requires O2 in seawater. The team examined thallium
isotopes in the same marine shales recently shown to track atmospheric O2
fluctuations during the GOE with rare sulfur isotopes.
In the shales, Ostrander and his team found
noticeable enrichments in the lighter-mass thallium isotope (203Tl),
a pattern best explained by seafloor manganese oxide burial, and hence
accumulation of O2 in seawater. These enrichments were found in the
same samples lacking the rare sulfur isotope signatures, and hence when the
atmosphere was no longer anoxic. The icing on the cake: the 203Tl
enrichments disappear when the rare sulfur isotope signatures return. These
findings were corroborated by redox-sensitive element enrichments, a more
classical tool for tracking changes in ancient O2.
“When sulfur isotopes say the atmosphere became
oxygenated, thallium isotopes say that the oceans became oxygenated. And when
the sulfur isotopes say the atmosphere flipped back to anoxic again, the
thallium isotopes say the same for the ocean,” Ostrander said. “So the
atmosphere and ocean were becoming oxygenated and deoxygenated together. This
is new and cool information for those interested in ancient Earth.”
原始論文：Chadlin
Ostrander, Andy Heard, Yunchao Shu, Andrey Bekker, Simon Poulton, Kasper Olesen,
Sune Nielsen. Onset of coupled
atmosphere–ocean oxygenation 2.3 billion years ago. Nature, 2024. DOI: 10.1038/s41586-024-07551-5
引用自：” What the geologic record reveals about
how the oceans were oxygenated.” The University of Utah.
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