原文網址：https://home.dartmouth.edu/news/2022/01/research-questions-whiff-oxygen-ancient-earth
By David Hirsch
根據最近發表在《科學前緣》(Science
Advances.)的研究，用來主張23億年前的地球大氧化事件之前就有「一絲氧氣」的證據，可能是在許久之後才傳入的化學訊號。
地球大概是出現在數十億年前，但是氧氣是在什麼時候進入大氣？(圖片來源：奧札克自然史博物館)
過往的研究發現在大氧化事件(研究人員稱為GOE)之前，氧氣就已經存在於大氣當中，但是新的結果反駁了那些發現，因此有可能改寫我們所知的地球歷史。
「如果先前發表的一系列研究結果中的一絲氧氣並不存在，科學界對於他們理解的地球前半段歷史，就需要加以批判並重新評估才行，」研究第一作者，達特茅斯學院的地球科學助理教授Sarah Slotznick表示。
研究指出原先用來判定在更早之前的地球歷史當中，大氣含有氧氣的化學數據可能是來自數億年後才發生的事件。
這項研究也進行了其他分析，結果再次確認早於23億年前地球大氣的特徵便是極低的氧含量。
「我們利用新工具來調查微量氧氣訊號的來源，」研究共同作者之一Jena Johnson表示。她是密西根大學地球與環境科學的助理教授。「我們發現沉積物堆積在海床之後發生的一連串變化，可能造成了那些代表氧氣的化學證據。」
氧化之始
科學家數十年來持續爭論地球大氣是在什麼時候首次出現足以測出來的氧氣。在過去一世紀，科學家逐漸建構出「大氧化事件」的概念，認為氧氣是在二十幾億年前開始增加，為複雜的細胞、動物乃至於人類的崛起奠下了基礎。
然而，近期有些對氧氣相關的化學訊號所進行的研究，卻提出更早之前氧氣曾短暫出現，他們稱為「一絲氧氣」。
2007年，兩篇獨立進行的研究發現了這類氧氣的證據。他們根據的樣品為25億年前的麥克雷山頁岩，來自於NASA天體生物學鑽探計畫2004年於西澳鑽取出來、受到大量研究的一根岩芯。
「他們在十幾年前得出那些結果時都相當震驚，」這篇新研究的共同作者之一Joseph Kirschvink表示。他是加州理工學院地質生物學的教授，也是東京工業大學地球生命科學研究所的成員。「從其他地質指標得到的大量證據主張大氧化事件前自由氧並不存在，但他們卻得到了看似矛盾的發現。」
研究的來龍去脈
2007年的研究依據的是鉬和硫對於氧化還原所提供的證據。由於科學家無法直接從岩石中測出大氣是否含有氧氣，因此常常用這兩種元素來代為驗證。結果對於地球生命的早期演化過程提出了基本層面的問題。
這項對於早期氧氣的觀測結果被其他研究團隊用來支持更早之前的發現，他們認為最早改革光合作用的生物之一――微小的藍綠菌將氧氣灌輸到遠古大氣，但是地球上的其他作用卻讓氧氣維持在低濃度。
2007年的研究成果以及對於生命起源和演化的啟發，在過去14年來已經被廣為接受，並且成為一連串其他研究論文的基礎。
新研究始於2009年，當時由加州理工大學主持的團隊開始規畫更多的分析項目。團隊用了十幾年的時間蒐集並分析數據――其中有些人甚至已經前往其他研究單位――他們最近發表的成果為正式發表的論文中，首次對早期一絲氧氣的發現直接提出反對意見的。
「如此古老的岩石所陳述的故事相當複雜，不只是這些泥巴在海底沉積時世界的樣貌，」研究共同作者Woodward Fischer表示。他是加州理工大學的地質生物學教授。「這些樣品還有沉積之後過了很久才產生的礦物，它們形成的時候古老的環境訊號會跟較年輕的混在一起，因而擾亂我們對於遠古地球環境的解讀。」
方法決定一切
2007年發現地球在完全氧化之前有一絲氧氣的研究論文，利用的技術為總體分析，這種方法是從麥克雷山頁岩各處取出樣品磨成粉後，再測量地球化學成分。新研究並非利用粉末來進行化學分析，而是運用一系列高解析度技術來檢視岩石樣品。
研究團隊進行這項新研究的時候，先把2004年的鑽井樣品放在平板光學掃描儀上紀錄影像，根據觀察結果再採集薄片樣品進行更多分析。他們透過許多方法來分析這些實體標本，像是同步光子X射線螢光光譜法，這讓團隊對於樣品的地質及化學性質，以及他們辨識出來各種作用的相對發生時間有另外的見解。
這篇研究論文表示：「綜合我們的觀測結果，我們認為指出『一絲氧氣』的總體化學資料，是在沉積過後的事件當中發展出來。」
新的分析顯示麥克雷山頁岩的組成是有機碳和火山灰。研究指出鉬一開始來自於火山，接著濃度才增加，該段期間在之前被認作是一絲氧氣的出現時間。在沉積物經過一連串化學和物理變化而成為岩石的過程中，產生的裂隙就像通道一般，讓岩石形成數億年之後某些成分相異的流體可以將氧化的訊號帶進來。
論文寫到：「我們觀察到大量玻璃質的火山碎屑以及凝灰岩夾層，加上最近有些觀點認為火山玻璃存有大量的鉬，使得『一絲氧氣』出現期間鉬的富集有新的解釋。」
回顧從前，指引未來
如果鉬並非來自氧氣造成陸地岩石風化的作用，並且之後在海裡集中，那它的出現就不能支持原先認為早期大氣含氧的發現。新研究運用的方法完全異於第一篇發現一絲氧氣的研究，結果也讓人懷疑那些跟著使用總體分析技術的研究。
「我們全新的高解析度數據清楚指出在所有的遠古紀錄當中，化學訊號代表的沉積背景必須要仔細考慮才行，」Johnson表示。
除了對麥克雷山頁岩的氧氣代用指標提出另一種解釋，團隊也確認大氧化事件之前的大氣氧濃度相當低。他們認為大氣氧濃度在突然改變前的大約1.5億年間，都是處於「可以忽略」的狀態。
這項發現也質疑了藍綠菌早就存在的說法；反之，它支持了其他假說，認為產氧光合作用生物是在大氧化事件不久前才演化出來。
「我們預期對我們的研究成果會產生興趣的不只是研究地球的科學家，還有探討其他星球的，」Slotznick表示。「我們希望成果可以讓我們在分析數十億年前的岩石裡的化學訊號這方面，刺激出更多想法並產生更多交流。」
貢獻這項研究的還有西澳大學暨中國地質大學的Birger Rasmussen、聖安德魯斯大學暨地質遺產研究所的Timothy D. Raub、SLAC國家加速器實驗室的Samuel Webb以及中國地質大學的Jian-Wei Zi(訾建威)。
New research questions ‘whiff of
oxygen’ in Earth’s early history
Evidence arguing for a "whiff of oxygen" before the
Earth's Great Oxygenation Event 2.3 billion years ago are chemical signatures
that were probably introduced at a much later time, according to research
published in Science
Advances.
The result rewinds previous
research findings that atmospheric oxygen existed prior to the so-called Great
Oxygenation Event-known to researchers as "GOE"- and has the
potential to rewrite what is known of the planet's past.
"Without the whiff of
oxygen reported by a series of earlier studies, the scientific community needs
to critically reevaluate its understanding of the first half of Earth's
history," said Sarah Slotznick, an assistant professor of earth sciences
at Dartmouth and first author of the study.
The study indicates that
the chemical data originally determined to suggest atmospheric oxygen earlier
in Earth's history may have been introduced by events hundreds of millions of
years later.
Additional analysis
conducted as part of the study reconfirms that Earth's atmosphere featured exceedingly
low oxygen levels prior to 2.3 billion years ago.
"We used new tools to
investigate the origins of the signals of trace oxygen," said Jena
Johnson, an assistant professor of earth and environmental sciences at the
University of Michigan and co-author of the study. "We found that a series
of changes after the sediments were deposited on the seafloor were likely
responsible for the chemical evidence of oxygen."
The Initiation of
Oxygenation
For decades, scientists
have debated when measurable levels of oxygen first appeared in Earth's
atmosphere. The idea of the Great Oxygenation Event has developed over the last
century and is thought to be when oxygen levels began to increase over 2
billion years ago, paving the way for the rise of complex cells, animals, and
eventually humans.
More recently, however,
research on chemical signals correlated to oxygen has suggested earlier
transient appearances of oxygen, known as "whiffs."
In 2007, two parallel studies
found evidence of such a whiff of oxygen based on samples of the
2.5-billion-year-old Mount McRae Shale, part of a heavily studied 2004 drill
core collected in Western Australia by the NASA Astrobiology Drilling Program.
"When the results came
out a decade ago, they were startling," said Joseph Kirschvink, professor
of geobiology at Caltech, a member of the Earth-Life Science Institute at the
Tokyo Institute of Technology, and a co-author of the new study. "The
findings seemed to contradict abundant evidence from other geological
indicators that argued against the presence of free oxygen before the Great
Oxygenation Event."
A Research Origin
Story
The 2007 studies were based
on evidence of oxidation and reduction of molybdenum and sulfur, two elements
that are widely used to test for the presence of atmospheric oxygen since it
cannot be measured directly in rock. The findings raised fundamental questions
about the early evolution of life on Earth.
The observation of early
oxygen was taken by some research groups to support earlier findings that
microscopic cyanobacteria -- early innovators in photosynthesis -- pumped
oxygen into the ancient atmosphere but that other Earth processes kept oxygen
levels low.
The 2007 studies, including
their implications about the origin of life and its evolution, have been widely
accepted and have served as the basis for a series of other research papers
over the last 14 years.
The new study dates back to
2009, when a Caltech-led team began efforts to conduct additional analysis. The
team, some of whom have since moved to other institutions, took over a decade
to collect and analyze data, resulting now in the first published study that
directly refutes the finding of a whiff of early oxygen.
"Rocks this old tell a
complicated story that goes beyond what the world was like when the mud was
deposited," said Woodward Fischer, a professor of geobiology at Caltech
and co-author of the study. "These samples also contain minerals that
formed long after their deposition when ancient environmental signals were
mixed with younger ones, confusing interpretations of the conditions on ancient
Earth."
A Matter of Approach
The 2007 research papers
that found the whiff of oxygen prior to Earth's full oxygenation used bulk
analysis techniques featuring geochemical assessments of powdered samples
sourced from throughout the Mount McRae Shale. Rather than conducting a
chemical analysis on powder, the new research inspected specimens of the rock
using a series of high-resolution techniques.
For the new study, the
research team recorded images of the 2004 drill core on a flatbed optical
scanner. Based on those observations, they then collected thin samples for
additional analyses. The suite of approaches used on the physical specimens,
including synchrotron-based X-ray fluorescence spectroscopy, gave the team
additional insight into the geology and chemistry of the samples as well as the
relative timing of processes that were identified.
According to the research
paper: "Our collective observations suggest that the bulk chemical
datasets pointing toward a 'whiff' of oxygen developed during post-depositional
events."
The new analysis shows that
the Mount McRae Shale formed from organic carbon and volcanic dust. The research
indicates that molybdenum came from volcanoes and subsequently concentrated
during what has been previously characterized as the whiff interval. During a
series of chemical and physical changes that turned these sediments into rock,
fracturing created pathways for several distinct fluids to carry in signals of
oxidation hundreds of millions of years after the rocks formed.
"Our observations of
abundant pyroclastic glass shards and intercalated tuff beds, paired with the
recent insight that volcanic glass is a major host of [molybdenum], offers a
new explanation for the [molybdenum] enrichments in the 'whiff' interval,"
the paper says.
Looking Back to Point
a Way Forward
If the molybdenum was not
from oxygen-based weathering of rocks on land and concentration in the ocean,
its presence does not support the original finding of early atmospheric oxygen.
By using a totally different methodology than that used in the first studies
that found a whiff of oxygen, the new research also calls into question
research that followed from those studies using the same style of bulk
techniques.
"Our new,
high-resolution data clearly indicates that the sedimentary context of chemical
signals has to be carefully considered in all ancient records," said
Johnson.
In addition to providing an
alternate explanation for oxygen proxies that were found in the Mount McRae
Shale, the team confirmed that the level of atmospheric oxygen at the time
before the Great Oxygenation Event was very low, calling it
"negligible" in the approximate period 150 million years before the
abrupt change.
The findings call into
question the early existence of cyanobacteria, instead supporting other
hypotheses that oxygen-generating photosynthesis evolved only shortly before
the Great Oxygenation Event.
"We expect that our
research will generate interest both from those studying Earth and those
looking beyond at other planets," said Slotznick. "We hope that it
stimulates further conversation and thought about how we analyze chemical
signatures in rocks that are billions of years old."
Birger Rasmussen, of the
University of Western Australia and China University of Geosciences; Timothy D.
Raub, of the University of St Andrews and the Geoheritage Research Institute;
Samuel Webb, of SLAC National Accelerator Laboratory; and Jian-Wei Zi, of the
China University of Geosciences, all contributed to the study.
原始論文：Sarah P.
Slotznick, Jena E. Johnson, Birger Rasmussen, Timothy D. Raub, Samuel M. Webb,
Jian-Wei Zi, Joseph L. Kirschvink, Woodward W. Fischer. Reexamination
of 2.5-Ga “whiff” of oxygen interval points to anoxic ocean before GOE. Science
Advances, 2022; 8 (1) DOI: 10.1126/sciadv.abj7190
引用自：Dartmouth College. "New research questions
‘whiff of oxygen’ in Earth’s early history.”
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