2018年3月29日 星期四

二十億年前的鹽岩顯示出古代大氣氧濃度的上升過程


二十億年前的鹽岩顯示出古代大氣氧濃度的上升過程
Catherine Zandonella
誠如我們所知,地球的生命能夠存活是因為大氣之中含有氧氣。科學家從一塊20億年前形成的海鹽當中,找到了新證據顯示地球大氣轉變成含氧狀態的過程。

包括普林斯頓大學在內的國際學術團隊進行的研究,發現在23億年前發生的「大氧化事件」(Great Oxidation Event)期間,氧氣的上升幅度比過往研究指稱的還要大上許多。
「這比較像是強力消防水柱而非涓涓細流。」論文第一作者,普林斯頓大學地質科學系的博士後研究員Clara Blättler表示。本研究於322日星期四發表在期刊《科學》的線上版。「當時氧氣的產量發生了重大變化。」
研究人員於俄羅斯西北方的卡累利阿地區,一座1.2英里深的鑽井取得由鹽類結晶組成的岩石,其中含有氧氣大幅上升的證據。當古代的海水蒸發之後會留下這些鹽類結晶,使它們成為可以指出地球20億年以前大氣和海洋成分的線索,這對地質學家來說是前所未見的。
氧氣產量增加的關鍵證據來自研究人員發現這些礦床含有的硫酸鹽(一種海水成分)高得驚人,它是硫跟氧氣反應而成的產物。
「這是目前為止最強力的證據,顯示古代海水沉澱出這些礦物的時候含有高濃度的硫酸鹽。根據我們的估計,當時的硫酸鹽濃度至少達到現今海洋的30%。」本研究的主要作者之一Aivo Lepland表示。他是挪威地質調查所的研究人員,同時也是塔林理工大學的地質專家。「這個數值比過去認為的還要高出許多,也代表我們需要仔細重新思考20億年前,地球的大氣―海洋系統發生的氧化事件規模有多大。」
就如我們所知道的,氧氣大約佔空氣組成的20%,並且是生命賴以維生的要素。地質證據指出氧氣是在24億至23億年前開始在地球的大氣當中嶄露頭角。
光合作用的過程會吸收二氧化碳然後排出氧氣。隨著能夠進行光合作用的藍綠菌逐漸增加,大氣中的氧氣也持續累積。然而,在此篇研究之前,地質學家一直不確定氧氣的累積過程是經歷數百萬年緩慢發展的事件,或者進行速度要快上許多。
Blättler表示:「要測試這些想法非常困難。因為我們並沒有來自於那個年代的證據,可以告訴我們當時大氣的成分。」
近期發現的晶體提供了相關證據。採自俄羅斯的這些鹽類晶體比過往發現的鹽層還要古老十幾億年。該鹽層含有岩鹽,其化學成分就跟食鹽一樣,也就是氯化鈉;除此之外,還含有由鈣、鎂和鉀組成的鹽類。
一般情況下這類礦物因為易溶於水,隨著時間經過會逐漸被水流帶走。但在此例中,這些鹽層卻以特別良好的狀態被保存在地球深處。挪威地質調查局的地質學家和俄羅斯彼得羅扎沃茨克的卡累利阿研究中心合作之下,從奧涅加湖西岸的奧涅加參數鑽井中取出了這些鹽晶。
普林斯頓大學的地質科學助理教授John Higgins表示:「在拼湊大氧化事件之後發生的歷史變化時,這些樣品出眾的品質使它們別具價值。」他和其他共同作者呈現了這些樣品的地球化學分析成果。
「這是一種非常特別的地質沉積物,」Higgins表示。「大氧化事件發生時有許多化學訊號也跟著上升或下降,它們代表的是氧氣產量發生重大改變,或者只是剛好跨過某個門檻,一直以來都有許多爭議。這篇論文的重點在於證據顯示地球在這整段時期發生的氧化作用,涉及了氧氣產量的大幅增加。」
Blättler表示研究將驅使科學家發展新的模型,來解釋大氧化事件之後是什麼造成了大氣中的氧氣持續累積。「當時陸地和海洋的回饋作用之間的循環可能出現了重大變化,或者微生物產生的氧氣大量增加。不管是哪一種,它們的改變之劇烈可能都比我們之前認為得還要劇烈許多。」

Two-billion-year-old salt rock reveals rise of oxygen in ancient atmosphere
A 2-billion-year-old chunk of sea salt provides new evidence for the transformation of Earth’s atmosphere into an oxygenated environment capable of supporting life as we know it.
The study by an international team of institutions including Princeton University found that the rise in oxygen that occurred about 2.3 billion years ago, known as the Great Oxidation Event, was much more substantial than previously indicated.
“Instead of a trickle, it was more like a firehose,” said Clara Blättler, a postdoctoral research fellow in the Department of Geosciences at Princeton and first author on the study, which was published online by the journal Science on Thursday, March 22. “It was a major change in the production of oxygen.”
The evidence for the profound upswing in oxygen comes from crystalized salt rocks extracted from a 1.2-mile-deep hole in the region of Karelia in northwest Russia. These salt crystals were left behind when ancient seawater evaporated, and they give geologists unprecedented clues to the composition of the oceans and atmosphere on Earth more than 2 billion years ago.
The key indication of the increase in oxygen production came from finding that the mineral deposits contained a surprisingly large amount of a component of seawater known as sulfate, which was created when sulfur reacted with oxygen.
“This is the strongest ever evidence that the ancient seawater from which those minerals precipitated had high sulfate concentrations reaching at least 30 percent of present-day oceanic sulfate as our estimations indicate,” said Aivo Lepland, a researcher at the Geological Survey of Norway, a geology specialist at Tallinn University of Technology, and senior author on the study. “This is much higher than previously thought and will require considerable rethinking of the magnitude of oxygenation of Earth’s 2-billion year old atmosphere-ocean system.”
Oxygen makes up about 20 percent of air and is essential for life as we know it. According to geological evidence, oxygen began to show up in the Earth’s atmosphere between 2.4 and 2.3 billion years ago.
Until the new study, however, geologists were uncertain whether this buildup in oxygen — caused by the growth of cyanobacteria capable of photosynthesis, which involves taking in carbon dioxide and giving off oxygen — was a slow event that took millions of years or a more rapid event.
 “It has been hard to test these ideas because we didn’t have evidence from that era to tell us about the composition of the atmosphere,” Blättler said.
The recently discovered crystals provide that evidence. The salt crystals collected in Russia are over a billion years older than any previously discovered salt deposits. The deposits contain halite, which is called rock salt and is chemically identical to table salt or sodium chloride, as well as other salts of calcium, magnesium and potassium.
Normally these minerals dissolve easily and would be washed away over time, but in this case they were exceptionally well preserved deep within the Earth. Geologists from the Geological Survey of Norway in collaboration with the Karelian Research Center in Petrozavodsk, Russia, recovered the salts from a drilling site called the Onega Parametric Hole (OPH) on the western shores of Lake Onega.
The unique qualities of the sample make them very valuable in piecing together the history of what happened after the Great Oxidation Event, said John Higgins, assistant professor of geosciences at Princeton, who provided interpretation of the geochemical analysis along with other co-authors.
“This is a pretty special class of geologic deposits,” Higgins said. “There has been a lot of debate as to whether the Great Oxidation Event, which is tied to increase and decrease in various chemical signals, represents a big change in oxygen production, or just a threshold that was crossed. The bottom line is that this paper provides evidence that the oxygenation of the Earth across this time period involved a lot of oxygen production.”
The research will spur the development of new models to explain what happened after the Great Oxidation Event to cause the accumulation of oxygen in the atmosphere, Blättler said. “There may have been important changes in feedback cycles on land or in the oceans, or a large increase in oxygen production by microbes, but either way it was much more dramatic than we had an understanding of before.”
原始論文:C. L. Blättler, M. W. Claire, A. R. Prave, K. Kirsimäe, J.A. Higgins, P. V. Medvedev, A. E. Romashkin, D. V. Rychanchik, A. L. Zerkle, K. Paiste, T. Kreitsmann, I. L. Millar, J. A. Hayles, H. Bao, A. V. Turchyn, M. R. Warke, A. Lepland. Two-billion-year-old evaporites capture Earth’s great oxidationScience, 2018; eaar2687 DOI: 10.1126/science.aar2687
引用自:Princeton University. "Two-billion-year-old salt rock reveals rise of oxygen in ancient atmosphere." 

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