古代岩石呈現出地球早期歷史的蛛絲馬跡
研究人員發現氧氣濃度很早就已經升高的環境
Karin Valentine
氧氣的組成為氧分子，這種由植物製造的氣體對動物來說相當重要。幸好地球大氣和海洋中有豐富的氧氣，動物才能生存。不過，研究人員從地球歷史的研究中，了解地球46億年來的多數時間，氧氣相對來說都是十分稀少的。
麥克雷山頁岩位於澳洲西部，年代為25億年。研究人員分析鉈和鉬的同位素組成，得到的模式顯示當時有一大片古代海床埋有氧化錳礦物。代表25億年前此處的海洋從表層到海床都有氧氣，才能發生此現象。圖片來源：Chad
Ostrander, ASU
所以地球的氧氣是在什麼時候、什麼環境中開始累積起來？
研究人員研究古代的岩石，得知在25億年前至23億年前的某段時間，地球經歷了「大氧化事件」(簡稱為OGE)。在此事件中，大氣裡的氧氣首次開始累積起來，之後再也沒有消失過。
然而，經過這方面的大量研究之後，許多證據顯示大氧化事件前，就已經有些許氧氣出現在遠古地球的少數淺海地區。而最近發表於期刊《自然―地質科學》(Nature
Geoscience)的研究中，亞利桑那州立大學領導的研究團隊提出了有力證據，指出大氧化事件之前海洋曾發生大型氧化事件，涉及的規模和深度都超過了之前辨識出的事件。
團隊的研究對象為澳洲西部的麥克雷山頁岩，是25億年前的海洋沉積岩。
主要作者，亞利桑那州立大學地球與太空探索學院的Chadlin
Ostrander說：「之前的研究證實這組岩石是在大氧化事件發生前，一場突發的氧化事件中沉積而成，因此非常適合我們的研究」
頁岩是一種沉積岩，沉積於地球過往某個時刻的海床之上。有時候頁岩含有的化學訊息可以指示沉積當時古代海洋的環境。
Ostrander進行這項研究時先在乾淨的實驗室中溶解頁岩，分離出他們感興趣的元素，接著運用質譜儀來分析元素的同位素組成。協助完成實驗的共同作者包括美國麻州伍茲霍爾海洋研究所的Sune
Nielsen、美國佛羅里達州立大學的Jeremy
Owens、加拿大安大略省滑鐵盧大學的Brian
Kendall、亞利桑那州立大學地球與太空探索學院的科學家Gwyneth
Gordon和Stephen
Romaniello、亞利桑那州立大學地球與太空探索學院暨分子科學院的Ariel
Anbar。使用的儀器位於伍茲霍爾海洋研究所、佛羅里達州立大學和亞利桑那州立大學。實驗數據的蒐集過程耗時一年以上。
團隊利用質譜儀分析麥克雷山頁岩鉈和鉬的同位素組成，這是首度在一組頁岩樣品中同時分析鉈和鉬同位素系統的研究。他們得到的同位素分佈模式跟假設的一樣，顯示當時有一大片的海床埋有氧化錳礦物。代表25億年前此處的海洋從表層到海床都有氧氣，才能發生此現象。
這些發現讓科學家更加了解地球海洋的氧化歷史。大氧化事件之前，氧氣可能不只侷限在海洋表層的一小部分；更有可能的情況是海洋許多區域都有氧氣累積，而且可以到達海洋深處。甚至在某些地區，從海洋表層到海床都有氧氣累積。
「我們的發現迫使我們重新思考地球最初的氧化事件是什麼。」Ostrander表示，「許多線索顯示地球大氣是在距今25億年前之後的大氧化事件中才開始累積氧氣。但我們現在可以清楚看出地球最初的氧化事件背景是在海洋。早在大氣開始累積氧氣之前，海洋就已經這麼做了，而且根據我們的數據，海洋中累積的氧氣濃度還不低。」
「知道氧氣開始累積的時間地點之後，接下來便要找出原因。」共同作者，亞利桑那州立大學的教授Anbar表示，「我們認為大氣開始累積氧氣許久之前，海洋裡面有許多可以製造氧氣的細菌。至於發生了什麼變化使大氣開始累積氧氣，則是我們接下來要探討的問題。」
Ancient rocks
provide clues to Earth’s early history
Researchers discover the environments of O2's
early rise
Oxygen in the form of the oxygen molecule
(O2), produced by plants and vital for animals, is thankfully
abundant in Earth’s atmosphere and oceans. Researchers studying the history of
O2 on Earth, however, know that it was relatively scarce for much of
our planet’s 4.6 billion-year existence.
So when, and in what environments, did O2
begin to build up on Earth?
By studying ancient rocks, researchers have
determined that sometime between 2.5 and 2.3 billion years ago, Earth underwent
what scientists call the “Great Oxidation Event” or “GOE” for short. O2
first accumulated in Earth’s atmosphere at this time and has been present ever
since.
Through numerous studies in this field of research,
however, evidence has emerged that there were minor amounts of O2 in
small areas of Earth’s ancient shallow oceans before the GOE. And in a study
published recently in the journal Nature
Geoscience, a research team led by scientists at Arizona State University
has provided compelling evidence for significant ocean oxygenation before the
GOE, on a larger scale and to greater depths than previously recognized.
For this study, the team targeted a set of 2.5
billion-year-old marine sedimentary rocks from Western Australia known as the
Mt. McRae Shale.
“These rocks were perfect for our study because they
were shown previously to have been deposited during an anomalous oxygenation
episode before the Great Oxidation Event,” said lead author Chadlin Ostrander,
of ASU’s School of Earth and Space Exploration.
Shales are sedimentary rocks that were, at some time
in Earth’s past, deposited on the sea floor of ancient oceans. In some cases, these
shales contain the chemical fingerprints of the ancient oceans they were
deposited in.
For this research, Ostrander dissolved shale samples
and separated elements of interest in a clean lab, then measured isotopic
compositions on a mass spectrometer. This process was completed with the help
of co-authors Sune Nielsen at Woods Hole Oceanographic Institution
(Massachusetts); Jeremy Owens at Florida State University; Brian Kendall at the
University of Waterloo (Ontario, Canada); scientists Gwyneth Gordon and Stephen
Romaniello of ASU’s School of Earth and Space Exploration; and Ariel Anbar of
ASU’s School of Earth and Space Exploration and School of Molecular Sciences.
Data collection took over a year and utilized facilities at Woods Hole
Oceanographic Institution, Florida State University and ASU.
Using mass spectrometers, the team measured the
thallium and molybdenum isotope compositions of the Mt. McRae Shale. This was
the first time both isotope systems had been measured in the same set of shale
samples. As hypothesized, a predictable thallium and molybdenum isotope pattern
emerged, indicating that manganese oxide minerals were being buried in the sea floor
over large regions of the ancient ocean. For this burial to occur, O2
needed to have been present all the way down to the sea floor 2.5 billion years
ago.
These findings improve scientists’ understanding of
Earth’s ocean oxygenation history. Accumulation of O2 was probably
not restricted to small portions of the surface ocean prior to the GOE. More
likely, O2 accumulation extended over large regions of the ocean and
extended far into the ocean’s depths. In some of these areas, O2
accumulation seems to have even extended all the way down to the sea floor.
“Our discovery forces us to rethink the initial
oxygenation of Earth,” Ostrander said. “Many lines of evidence suggest that O2
started to accumulate in Earth’s atmosphere after about 2.5 billion years ago
during the GOE. However, it is now apparent that Earth’s initial oxygenation is
a story rooted in the ocean. O2 probably accumulated in Earth’s
oceans — to significant levels, according to our data — well before doing so in
the atmosphere.”
“Now that we know when and where O2 began
to build up, the next question is why,” said ASU President’s Professor and
co-author Anbar. “We think that bacteria that produce O2 were
thriving in the oceans long before O2 began to build up in the
atmosphere. What changed to cause that buildup? That’s what we’re working on
next.”
原始論文：Chadlin M.
Ostrander, Sune G. Nielsen, Jeremy D. Owens, Brian Kendall, Gwyneth W. Gordon,
Stephen J. Romaniello, Ariel D. Anbar. Fully oxygenated water columns
over continental shelves before the Great Oxidation Event. Nature
Geoscience, 2019; DOI: 10.1038/s41561-019-0309-7
引用自：Arizona State University. "Ancient rocks
provide clues to Earth's early history.”
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