Oxygen was present in the
atmosphere much earlier than previously assumed
氧氣在大氣中出現的時間遠較先前認為的早了許多
Reconstructing the emergence and evolution
of life on our planet is tightly linked to the questions as to when and to what
extent Earth's atmosphere became oxygenated. New geological studies based on
data from Western Greenland indicate that small levels of atmospheric oxygen
developed already 3.8 billion years ago, some 0.7-0.8 billion years earlier
than previously thought.
在重建我們星球上的生命何時出現與如何演化時，與這息息相關的問題是地球大氣層何時開始氧化及程度高低。於格陵蘭西部進行的地質學研究顯示，在38億年前大氣層中便已經有低含量的氧氣形成
Today, most researchers agree that the oxygenation of Earth's atmosphere happened in two major steps: the first during the so-called Great Oxidation Event about 2.5-2.4 billion years ago, and the second during the Late Neoproterozoic Era around 750 to 540 million years ago. The latter is thought to have been the cause for the emergence of animals during the so-called 'Cambrian explosion' around 540 to 520 million years ago.
今日大多數的研究人員同意地球大氣層的氧化主要以兩階段發生：第一次發生在25至24億年前，即一般所稱的「大氧化事件」(Great Oxidation Event)，而第二次則發生在7億5000萬至5億4500萬年前的新元古代(Neoproterozoic Era)晚期。後者被認為是動物於5億4000萬至5億2000萬年前時崛起，即「寒武紀大爆發」的成因。
An international team of researchers led by Professor Robert Frei from the Department of Geoscience and Natural Resource Management at the University of Copenhagen has just released a study indicating evidence for the presence of small concentrations of oxygen on Earth already 3.8 billion years ago. The researchers analysed Earth's oldest Banded Iron Formations (BIFs) from Western Greenland. BIFs are marine chemical sediments originally composed of alternating layers of silica and Fe-hydroxides and are widely used as geochemical archives. The reason for this is that they retain information on the composition and presence of oxygenation/reduction processes in ambient seawater and on the interaction of the atmosphere with Earth's surface.
由哥本哈根大學地質科學系和自然資源管理系的Robert Frei教授率領的國際團隊，近日發表的研究成果中有證據顯示早在38億年前就已經有微量的氧氣出現在地球上了。這些研究人員分析了位於格陵蘭西部，地球上最古老的帶狀鐵礦(Banded Iron Formations，BIFs)而得到這些成果。帶狀鐵礦是一種於海中形成，由矽質和鐵氧化物交疊而成的化學沉積物。它們通常被視為蘊含了大量地球化學訊息，這是因為它們保有形成當時周遭海水的化學成份，以及在其中進行的氧化還原作用，還有大氣如何與地表互相作用的相關訊息。
The research team used concentrations and isotope compositions, i.e. variations of the same elements with different atomic weight, of the elements chromium (Cr) and uranium (U) present in the BIFs. Chromium and uranium were used as these elements weather rapidly when continental landmasses are exposed to reactive oxygen species (ROS) such as oxygen (O2). After weathering, they are transported to the oceans by rivers, where they are deposited with chemical sediments and serve as geochemical signals of weathering by ROS.
研究團隊利用了BIFs當中鉻和鈾元素的濃度以及同位素(原子量不同的同一種元素)成分來進行研究。選用這兩種元素是因為當大陸陸塊暴露在像氧氣(O2)這類的活性氧化物(reactive oxygen species)中，這兩種元素會被迅速風化，接下來被河流帶到海洋當中。在此它們會隨著其他化學沉積物沉積下來，因而可以被視作由活性氧化物導致的風化作用留下的地球化學訊號。
The fact that the analyses of the BIF layers from Western Greenland show elements that require presence of oxygen in the atmosphere opens up for the possibility of evolution of the earliest primitive photosynthetic life forms as early as 3.8 billion years ago. As Robert Frei explains: "It is generally believed that the Early Earth was a completely anoxic, but our study shows that the surface of the Earth was exposed to a low oxygen atmosphere already this time. This has far reaching implications for how we investigate the pace of evolution of life and its biodiversity on our planet."
研究人員分析西格陵蘭BIF中需要大氣中有氧氣才會出現的元素，結果顯現最原始的光合生命有可能早在38億年前就已經出現了。Robert Frei解釋：「一般認為地球早期是處於完全缺氧狀態，但我們的研究顯示地球表面在當時就已經暴露在含有微量氧氣的空氣當中。這對我們星球生命的演化路程及生物多樣性的研究來說具有深遠的意義。」
引用自：Faculty of Science -
University of Copenhagen. "Oxygen was present in the atmosphere much
earlier than previously assumed." ScienceDaily. ScienceDaily, 16 February
2016.
