來自陸地的碳在上次冰河期消退時具有重要地位

原文網址：www.sciencedaily.com/releases/2016/03/160314161245.htm

Carbon from land played a role during last deglaciation

來自陸地的碳在上次冰河期消退時具有重要地位

As the Earth emerged from its last ice age several thousand years ago, atmospheric carbon dioxide increased and further warmed the planet. Scientists have long speculated that the primary source of this CO2 was from the deep ocean around Antarctica, though it has been difficult to prove.

自地球於數萬年前進入末次冰河期起，大氣中的二氧化碳便不斷累積而進一步暖化地球。雖然相當難以證實，但科學家長期以來猜測這些二氧化碳的主要來源是南極周圍的深海。

A new study published this week in Proceedings of the National Academy of Sciences confirmed that the ocean played a significant role in the rise of atmospheric carbon dioxide, but also documents the signature of land-based carbon sources in Antarctic ice cores that contributed to abrupt increases in CO2.

刊登於本周《美國國家科學院學報》(Proceedings of the National Academy of Sciences, PNAS)的新研究證實海洋的確在大氣二氧化碳濃度的上升中具有重要地位，但同時也論述南極冰芯中的碳有來自陸地的跡象，顯示陸地在二氧化碳的突然上升中佔有一席之地。

"There wasn't a steady rate of rising carbon dioxide during the last deglaciation," said Edward Brook, an Oregon State University paleoclimatologist and co-author on the PNAS study. "It happened in fits and starts. With the new precise techniques we developed to fingerprint the sources, it is apparent that the early carbon largely came from the ocean, but we think the system got a jolt from an influx of land-based carbon a few times as the climate warmed."

「在末次冰消期( last deglaciation)時二氧化碳的上升速率並非固定。」此篇刊於PNAS的研究的共同作者，奧勒岡州立大學的古氣候學家 Edward Brook說。「這起事件發生地時快時慢。利用我們研發可以追查二氧化碳來源的精準新技術，結果顯示碳起初大都來自海洋，但我們認為隨著氣候逐漸變暖，這個系統會屢次受到來自陸地的碳影響而擺盪。」

The study was funded by the National Science Foundation with support from the Marsden Fund Council in New Zealand.

這起研究的資金來自國家科學基金會，並由紐西蘭的Marsden基金委員會提供協助。

The breakthrough came from the comparison of carbon isotope ratios in pristine samples of ice mined from the Taylor Glacier in Antarctica. Although such isotopic fingerprinting strategies have been attempted before, the key was detailed work both in the field and in the laboratory that improved the precision to read the record in fine detail.

這起突破性的研究來自於完整冰芯樣本中的碳同位素比對結果，其採自於南極的Taylor冰河。雖然早已有人試過這種同位素測蹤技術，但這起研究的關鍵是他們的野外和實驗室作業皆相當詳盡，使得他們的精確度更加提高而能夠徹頭徹尾地解讀這些紀錄。

The study found that during the initial rise in atmospheric CO2 -- from 17,600 years ago to 15,500 years ago -- the light isotope 12-C increased faster than the heavier isotopes, pointing to a release of carbon from the deep ocean. However, at about 16,300 years ago and 12,900 years ago, there were abrupt, century-scale perturbations in the carbon ratio that suggested rapid release of carbon from land sources such as plants and soils.

研究發現從17,600至15,500年前大氣二氧化碳濃度剛開始上升的階段，較輕的同位素碳-12上升得比較重的同位素還要快，顯示碳從深海中釋放了出來。然而在16,300和12,900年前，碳同位素卻突然有些長達數百年的擾動，代表來自陸地(像是植物和土壤)的碳在這段期間迅速排放至大氣。

Although the region of the CO2 source is not clear, the scientists say, at least one of the two events may come from the tropics because methane from tropical swamps rose at the same time.

科學家說雖然這些二氧化碳的來源地區尚未明朗，但這兩起事件的其中之一可能出自熱帶，因為同一時間來自熱帶沼澤的甲烷量也提升了。

"One theory," Brooks said, "is that an influx of icebergs in the Northern Hemisphere at about 16,300 years ago -- from retreating ice sheets -- cooled the North Atlantic Ocean and pushed the tropical rain belt southward over Brazil, expanding the wetlands. Swamps in the Southern Hemisphere, in places like Brazil, may have become wetter and produced methane, while plants and soils in the Northern Hemisphere, in places like China, may have been hit by drought and produced CO2."

「有項理論認為在16,300年前因為冰棚退縮使得北半球冰山流入海洋的速率增加，造成北大西洋冷卻，並將熱帶雨帶推至巴西以南，濕地因而擴展。位處南半球，比如巴西的沼澤地或許會因此變得更加潮濕而製造更多甲烷；而在北半球，像是中國的植物和土壤卻可能因遭受乾旱而產生二氧化碳。」

During the next 4,000 years, the continued rise of atmospheric CO2 -- by about 40 parts per million -- was marked by small changes in the carbon-13 to carbon-12 ratio indicating additional sources of carbon from rising ocean temperatures. This CO2 source, analogous to the bubbles released from warming soda pop, may have added to the biological carbon sources.

在接下來4000年大氣二氧化碳濃度持續攀升約10億分之4的過程中，碳-13與碳-12的比例有幾處些微的變化，顯示有額外來源的碳因為海洋溫度升高而出現。這些碳的來源可以比喻成碳酸飲料加熱後冒出的氣泡，它們也許會加入那些來自生物的碳。

The application of this carbon isotope technique became possible because of a unique site along the margin of the Antarctic ice sheet where old ice that flowed from the interior is exposed at the surface of a large glacier -- Taylor Glacier -- named for a geologist on an early expedition to the frozen continent. Ice that normally would be a mile or more below the surface is available to easily sample in large quantities.

這種碳同位素技術得以運用的原因在於樣品來自得天獨厚的環境。這些樣品的採集地點為一處南極冰棚的邊緣地帶，這是一條大型冰川-Taylor冰川(以一位早期遠征此冰封大陸的地質學家命名)中的古老冰塊從內部流出至表面處，因此正常來說要在冰河表面下一英哩甚至更深處才能取得的冰可以在此輕易地大量採樣。

These large samples, laboriously cut from the exposed ice layers, allowed the precise measurements, the Oregon State researchers report.

據奧勒岡州立大學的研究人員描述，他們可以精確分析這些從出露的冰層上辛勤切割下的大量樣品。

"The isotope ratio technique gives us a sort of 'return address' for carbon dioxide," noted Thomas Bauska, a former Ph.D. student and post-doctoral researcher in OSU's College of Earth, Ocean, and Atmospheric Sciences, who was lead author on the PNAS study. "The technique is new, extremely precise and gives us one of the best windows into the Earth's past climate."

「同位素技術提供給我們的資訊有點像是二氧化碳的『寄件地址』。」此篇刊登於PNAS的研究的第一作者 Thomas Bauska如此解釋。他之前是奧勒岡州立大學地球、海洋暨大氣科學系的博士生及博士後研究員。「這項技術不但新穎且極度精確，還給了我們窺向地球過去氣候的絕佳管道。」

Bauska is now a post-doctoral researcher at the University of Cambridge in England.

Bauska現在是英國劍橋大學的博士後研究員。

That window into the past may provide hints at what may happen in the future under a new global warming regime, noted Alan Mix, an Oregon State oceanographer and co-author on the study. However, he cautioned, it isn't always simple to predict the future based on past events.

此篇研究的共同作者，奧勒岡州立大學的海洋學家 Alan Mix點出這個管道也許可以提供我們在新的全球暖化局勢下，未來可能會發生何種事物的蛛絲馬跡。然而，他也提醒以過去的事件來預測未來並非總是那麼單純。

"The rise of CO2 is a complicated beast, with different behaviors triggered at different times," Mix said. "Although the natural changes at the end of the ice age are not a direct analogy for the future, the rapid changes do provide a cautionary tale. Humanmade warming from CO2 pollution may trigger further release from 'natural sources,' and this could exacerbate greenhouse gases and warming."

「二氧化碳的上升是個相當錯綜複雜的過程，不同的時間點可能會有截然不同的反應發生。」 Mix說。「雖然上次冰河期末的自然變化並不能直接類推至將來，但這些迅速變化確實可成為借鏡。排放二氧化碳造成的人為暖化可能會進一步造成其他『自然來源』隨之釋放溫室氣體，而增加其濃度並加劇暖化。」

Other authors on the PNAS study include Daniel Baggenstos and Jeffrey Severinghaus, Scripps Institution of Oceanography; Shaun Marcott, University of Wisconsin-Madison; Vasillii Petrenko, University of Rochester; Hinrich Schaefer, National Institute of Water and Atmospheric Research in New Zealand; and James Lee, Oregon State University.

此篇PNAS的研究的其他作者還有斯克里普斯海洋研究所的 Daniel Baggenstos和Jeffrey Severinghaus；威斯康辛大學麥迪遜分校的 Shaun Marcott；羅徹斯特大學的 Vasillii Petrenko；紐西蘭國家水資源與大氣研究院的 Hinrich Schaefer及奧勒岡州立大學的 James Lee。

引用自：Oregon State University. "Carbon from land played a role during last deglaciation." ScienceDaily. ScienceDaily, 14 March 2016. 

新研究顯示人類對氣候的影響可追溯至1930年代

原文網址：www.sciencedaily.com/releases/2016/03/160308134917.htm

Human influence on climate dates back to 1930s, new research finds

新研究顯示人類對氣候的影響可追溯至1930年代

Humans have triggered the last 16 record-breaking hot years experienced on Earth (up to 2014), with our impact on the global climate going as far back as 1937, a new study finds.

新研究顯示人類導致了地球最近16個氣溫創新高的年份(至2014年為止)，而我們對全球氣候的影響最遠可追溯至1937年。

The study suggests that without human-induced climate change, recent hot summers and years would not have occurred. The researchers also found that this effect has been masked until recently in many areas of the world by the wide use of industrial aerosols, which have a cooling effect on temperatures.

此篇研究認為若沒有人類導致的氣候變遷，就不會發生近年來的熾熱夏季及年份。研究人員也發現這類影響在世上許多處可能因工業氣膠廣泛使用所造成的冷卻效應而被暫時掩飾，直到近幾年才展露出來。

"Everywhere we look, the climate change signal for extreme heat events is becoming stronger," said Andrew King, a climate extremes research fellow at the University of Melbourne, Australia and lead author of the study. "Recent record-breaking hot years globally were so much outside natural variability that they were almost impossible without global warming."

「不管我們觀察何處，極端高熱事件是氣候變遷的表徵可說是越來越明顯。」 此篇研究的第一作者Andrew King說，他是澳洲墨爾本大學極端氣候中心的研究員。「最近幾年發生在全球各處的破紀錄高溫遠遠超出了自然變化，其程度之大若無全球暖化幾乎不可能發生。」

The researchers examined weather events that exceeded the range of natural variability and used climate modelling to compare those events to a world without human-induced greenhouse gases. The study was accepted for publication yesterday in Geophysical Research Letters, a journal of the American Geophysical Union.

研究人員檢視這些超出自然變化範圍的氣候事件，並將這些事件以氣候模型來跟沒有人類排放溫室氣體的世界互相比較。這篇研究於昨日被美國地球物理學會的期刊《地球物理通訊》接受並刊登。

According to the new study, record-breaking hot years attributable to climate change globally are 1937, 1940, 1941, 1943-44, 1980-1981, 1987-1988, 1990, 1995, 1997-98, 2010 and 2014.

根據此研究，因全球氣候變遷導致的破紀錄高溫年份分別為 1937、1940、 1941、1943-44、1980-1981、1987-1988、1990 、1995、1997-98、2010和2014年。(為什麼這邊算起來只有15個)

"In Australia, our research shows the last six record-breaking hot years and last three record-breaking hot summers were made more likely by the human influence on the climate," King said. "We were able to see climate change even more clearly in Australia because of its position in the Southern Hemisphere in the middle of the ocean, far away from the cooling influence of high concentrations of industrial aerosols."

「我們認為澳洲最近六個破紀錄的高溫年份和最近三個破紀錄的高溫夏季很可能是因為人類對氣候的影響而導致。」King說。「我們在澳洲可以更清楚地看到氣候變遷的影響，這是因為它位處南半球的海洋中央，距高濃度工業氣膠造成的冷卻效應距離甚遠。」

Aerosols in high concentrations reflect more heat into space, thereby cooling temperatures. However, when those aerosols are removed from the atmosphere, warming returns rapidly. The researchers observed this impact when they looked at five different regions: Central England, Central Europe, the central United States, East Asia and Australia.

高濃度的氣膠能反射更多的熱量回太空，因而可以降低氣溫。然而，一旦將這些氣膠從大氣中排除，氣溫就會迅速回暖。研究人員檢視了以下五個不同地區而觀察到上述效應：英格蘭中部、歐洲中部、美國中部、亞洲東部和澳洲。

There were cooling periods, likely caused by aerosols, in Central England, the central United States, Central Europe and East Asia during the 1970s before accelerated warming returned, and aerosol concentrations also delayed the emergence of a clear human-caused climate change signal in all regions studied except Australia, according to the study.

根據此篇研究，在1970年代英格蘭中部、歐洲中部、美國中部和東亞在暖化重回而加劇之前，都可能有因為氣膠而導致的冷化時期。另外，除了澳洲之外，其他所有地區都因為氣膠濃度較高，而造成人為導致的氣候變遷的明確跡象延後出現。

"In regards to a human-induced climate change signal, Australia was the canary in the coal mine for the rest of the world," King said.

「就人類導致的氣候變遷跡象而言，澳洲對世上其餘地區而言就像是煤礦坑裡的金絲雀。」King說。

引用自：American Geophysical Union. "Human influence on climate dates back to 1930s, new research finds." ScienceDaily. ScienceDaily, 8 March 2016. 

「遠古之根」：科學家辨認出已知最早的陸地居民化石

原文網址：www.sciencedaily.com/releases/2016/03/160302082245.htm

'A load of old rot': Fossil of oldest known land-dweller identified

「遠古之根」：科學家辨認出已知最早的陸地居民化石

A fossil dating from 440 million years ago is not only the oldest example of a fossilised fungus, but is also the oldest fossil of any land-dwelling organism yet found. The organism, and others like it, played a key role in laying the groundwork for more complex plants, and later animals, to exist on land by kick-starting the process of rot and soil formation, which is vital to all life on land.

這具4億4千萬年前的化石不只是現存樣品中最古老的真菌化石，同時也是迄今發現的陸居生物化石中年代最久遠者。這種生物和它的同類開啟了陸地上的生物腐朽及土壤形成作用，這對所有陸生生物來說都至關重要。因此在讓後世的高階植物及動物隨後得以在陸地安身立命的基礎建設上，它們扮演了關鍵性的一環。

This early pioneer, known as Tortotubus, displays a structure similar to one found in some modern fungi, which likely enabled it to store and transport nutrients through the process of decomposition. Although it cannot be said to be the first organism to have lived on land, it is the oldest fossil of a terrestrial organism yet found. The results are published in the Botanical Journal of the Linnean Society.

這種稱作Tortotubus的遠古拓荒者身上有一種構造跟某些現代真菌的十分相似，可能可以幫助它們在進行分解作用時儲存和運輸養分。雖然無法斷定它們就是在陸地上生活的第一種生物，但卻是目前為止所能找到的最古老陸生生物化石。這項研究結果刊登於《林奈學會植物學雜誌》(Botanical Journal of the Linnean Society)。

"During the period when this organism existed, life was almost entirely restricted to the oceans: nothing more complex than simple mossy and lichen-like plants had yet evolved on the land," said the paper's author Dr Martin Smith, who conducted the work while at the University of Cambridge's Department of Earth Sciences, and is now based at Durham University. "But before there could be flowering plants or trees, or the animals that depend on them, the processes of rot and soil formation needed to be established."

「在這種生物存活的年代，幾乎所有的生命都侷限在海洋當中。陸地上尚未演化出比簡單的苔癬和地衣類更複雜的生物。」本篇論文的作者 Martin Smith教授說。目前就職於杜倫大學的他於劍橋大學地球科學系任職時進行了此篇研究。「在開花植物和樹木，還有以它們維生的動物出現之前，(陸地上)必須要先建立起一套分解作用和土壤形成作用。」

Working with a range of tiny microfossils from Sweden and Scotland, each shorter than a human hair is wide, Smith attempted to reconstruct the method of growth for two different types of fossils that were first identified in the 1980s. These fossils had once been thought to represent parts of two different organisms, but by identifying other fossils with 'in-between' forms, Smith was able to show that the fossils actually represented parts of a single organism at different stages of growth. By reconstructing how the organism grew, he was able to show that the fossils represent mycelium -- the root-like filaments that fungi use to extract nutrients from soil.

Smith研究了一群來自瑞典和蘇格蘭的極端微小化石，當中的每一具寬度都比毛髮還要細小。他企圖重建兩種在1980年代首度辨認出來的化石的生長模式。過去一度認為這兩種化石分屬於兩種不同生物的結構，但在辨識出一些跟兩者皆頗為相似的其他化石後，Smith得以下結論說這些化石其實是同一生物在不同生長階段的身體部位。在重建了這種生物的生長模式後，他認為這些化石代表了菌絲，一種真菌用來從土壤中吸收養分的根狀纖毛構造。

It's difficult to pinpoint exactly when life first migrated from the seas to the land, since useful features in the fossil record that could help identify the earliest land colonisers are rare, but it is generally agreed that the transition started early in the Palaeozoic era, between 500 and 450 million years ago. But before any complex forms of life could live on land, there needed to be nutrients there to support them. Fungi played a key role in the move to land, since by kick-starting the rotting process, a layer of fertile soil could eventually be built up, enabling plants with root systems to establish themselves, which in turn could support animal life.

由於化石紀錄中可以用來辨認出最早移居至陸地的生物為何者的證據相當稀少，因此很難精確指出生命首度從海洋遷往陸地的時間點。但學者一般認同在古生代早期生命開始往陸地邁進，約在5億至4億5000萬年前。而在任何複雜的生命形式能在陸地生活之前，陸上必須要先有養分才能夠餵養它們。在爬上陸地的過程當中，真菌扮演了關鍵腳色，這是因為它們能使分解作用開始運作，最終可以形成一層肥沃的土壤。具備根系的植物得以依此落腳維生，連帶使動物能夠生長。

Fungi play a vital role in the nitrogen cycle, in which nitrates in the soil are taken up by plant roots and passed along food chain into animals. Decomposing fungi convert nitrogen-containing compounds in plant and animal waste and remains back into nitrates, which are incorporated into the soil and can again be taken up by plants. These early fungi started the process by getting nitrogen and oxygen into the soil.

真菌在氮循環中為不可或缺的一環。在此循環當中，土壤裡的硝酸鹽會被植物根部汲取，並隨著食物鏈轉移至動物身上。能行使分解作用的真菌會將含氮的動植物排泄物和遺骸轉化回硝酸鹽，當它們重新回到土壤後便能再次被植物利用。這些早期的真菌藉由將獲取土壤中的氮和氧而開啟了這整個過程。

Smith found that Tortotubus had a cord-like structure, similar to that of some modern fungi, in which the main filament sends out primary and secondary branches that stick back onto the main filament, eventually enveloping it. This cord-like structure is often seen in land-based organisms, allowing them to spread out and colonise surfaces. In modern fungi, the structure is associated with the decomposition of matter, allowing a fungus colony to move nutrients to where they are needed -- a useful adaptation in an environment where nutrients are scarce and unevenly distributed.

Smith發現 Tortotubus具有跟某些現代真菌相當類似的繩索狀構造。這是種由主菌絲發散出的分支及更細的分支回貼在主菌絲身上，最終將其層層包覆而形成的構造。這種繩索狀構造通常見於陸生生物，這有助於它們附著在固體表面並往外擴散。在現生的真菌身上，此構造通常跟物質的分解有關。真菌菌落可以利用它們將養分移送至有需求的部分，這在營養稀少且不均勻分布的環境中是種相當實用的適應構造。

In contrast with early plants, which lacked roots and therefore had limited interaction with activity beneath the surface, fungi played an important role in stabilising sediment, encouraging weathering and forming soils.

原始植物由於缺少根部，因此它們難以跟地表之下的環境有所互動；相較起來，真菌在穩定環境、促進風化作用和形成土壤的過程中，皆具有更加重要的地位。

"What we see in this fossil is complex fungal 'behaviour' in some of the earliest terrestrial ecosystems -- contributing to soil formation and kick-starting the process of rotting on land," said Smith. A question, however, is what was there for Tortotubus to decompose. According to Smith, it's likely that there were bacteria or algae on land during this period, but these organisms are rarely found as fossils.

「我們從這些化石中看到在某些最原始的陸地生態系系統中，已經有相當複雜的真菌作用在進行著。它們的貢獻使土壤形成，並開啟了陸上的分解作用。」 Smith說。然而，有個問題是： Tortotubus究竟在陸地上分解著什麼物質。據 Smith說，在那段時期陸上可能已經有細菌或藻類存活，但這些生物很難以化石的形式發現。

Additionally, the pattern of growth in Tortotubus echoes that of the mushroom-forming fungi, although unambiguous evidence of mushrooms has yet to be found in the Palaeozoic fossil record. "This fossil provides a hint that mushroom-forming fungi may have colonised the land before the first animals left the oceans," said Smith. "It fills an important gap in the evolution of life on land."

此外，Tortotubus的生長模式也反映出當時可能已有會形成蕈菇的真菌了，雖然迄今我們仍尚未在古生代化石紀錄中確切發現蕈菇存在的證據。「這些化石暗示著會形成蕈菇的真菌定居在陸地的時間，也許比第一隻動物離開海洋還要來得早。」 Smith說。「這填補了陸地生命演化史上的一大空缺。」

The research was supported by Clare College, Cambridge.

本研究由劍橋大學克萊爾學院贊助。

引用自：University of Cambridge. "'A load of old rot': Fossil of oldest known land-dweller identified." ScienceDaily. ScienceDaily, 2 March 2016. 

氧氣在大氣中出現的時間遠較先前認為的早了許多

原文網址：www.sciencedaily.com/releases/2016/02/160216143057.htm

 

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.

新研究聚焦於板塊構造運動的起源之時

原文網址：www.sciencedaily.com/releases/2016/01/160121150103.htm

 

New study zeros in on plate tectonics' start date

新研究聚焦於板塊構造運動的起源之時

Analysis of trace elements places the onset of plate tectonics about 3 billion years ago

分析稀有元素的結果將板塊構造運動的起始時間定在約莫30億年前

Earth has some special features that set it apart from its close cousins in the solar system, including large oceans of liquid water and a rich atmosphere with just the right ingredients to support life as we know it. Earth is also the only planet that has an active outer layer made of large tectonic plates that grind together and dip beneath each other, giving rise to mountains, volcanoes, earthquakes and large continents of land.

地球擁有某些特色使得她跟其他太陽系內關係最近的親戚們截然不同，眾所皆知這包括了由液態水組成的廣袤海洋，以及成分恰到好處而能供養生命的豐厚大氣層。地球也是唯一擁有由大型板塊組成活躍外層的行星，這些板塊彼此之間的摩擦和傾沒，形成了山脈、火山、地震和巨大的陸塊。

Geologists have long debated when these processes, collectively known as plate tectonics, first got underway. Some scientists propose that the process began as early as 4.5 billion years ago, shortly after Earth's formation. Others suggest a much more recent start within the last 800 million years. A study from the University of Maryland provides new geochemical evidence for a middle ground between these two extremes: An analysis of trace element ratios that correlate to magnesium content suggests that plate tectonics began about 3 billion years ago. The results appear in the January 22, 2016 issue of the journal Science.

地質學家長期以來爭論著這些合稱為板塊構造的作用於何時開始。有些科學家提出45億年前地球形成不久之後便有這些作用了。其他人則提倡較為近代的開始時間，約莫小於8億年前。馬里蘭大學的研究則以新的地球化學證據而在這兩端之間取了較為中庸的說法：分析與鎂含量相關的稀有元素比例，結果顯示板塊構造運動約起始於30億年前。這項結果刊登在2016年1月22日發行的期刊《科學》之中。

"By linking crustal composition and plate tectonics, we have provided first-order geochemical evidence for the onset of plate tectonics, which is a fundamental Earth science question," said Ming Tang, a graduate student in geology at UMD and lead author of the study. "Because plate tectonics is necessary for the building of continents, this work also represents a further step in understanding when and how Earth's continents formed."

「藉由連接地殼成分和板塊構造運動之間的關係，我們提供了地球化學方面的初步證據可以指引板塊運動啟動的時刻。這在地球科學之中可是個相當重要的問題。」這篇研究的第一作者，馬里蘭大學的研究生Ming Tang說。「由於板塊構造運動是陸地形成過程中不可或缺的要素，因此這件工作同樣能讓我們更進一步了解地球的陸地是在何時及如何形成。」

The study zeros in on one key characteristic of Earth's crust that sets it apart geochemically from other terrestrial planets in the solar system. Compared with Mars, Mercury, Venus and even our own moon, Earth's continental crust contains less magnesium. Early in its history, however, Earth's crust more closely resembled its cousins, with a higher proportion of magnesium.

此篇研究聚焦於地球地殼的某一項重要特性，這項特性使得地球跟其他太陽系的類地行星在地球化學特性上有所區隔。與火星、水星、金星甚至是和我們的月亮相比，地球的大陸地殼含有較少的鎂。然而，在地球歷史的早期，地球地殼和他的表親們較現今更為相似，有著比較高比例的鎂。

At some point, Earth's crust evolved to contain more granite, a magnesium-poor rock that forms the basis of Earth's continents. Many geoscientists agree that the start of plate tectonics drove this transition by dragging water underneath the crust, which is a necessary step to make granite.

於某個時間點，地球的地殼演變成含有較多的花崗岩，這種岩石是構成地球陸地的基礎，其鎂含量相當少。許多地質學家同意板塊構造運動發生時連帶將海水拖入地殼下方，這道形成花崗岩的關鍵步驟使得地殼成分發生轉變。

"You can't have continents without granite, and you can't have granite without taking water deep into the Earth," said Roberta Rudnick, former chair of the Department of Geology at UMD and senior author on the study. Rudnick, who is now a professor of earth sciences at the University of California, Santa Barbara, conducted this research while at UMD. "So at some point plate tectonics began and started bringing lots of water down into the mantle. The big question is when did that happen?"

「沒有花崗岩就沒有陸地，而沒有將水分帶入地球深處就不會形成花崗岩。」此研究的第二作者Roberta Rudnick說。他於馬里蘭大學地質科學系就任系主任時指導了這篇研究，現今則在加州大學聖芭芭拉分校地球科學系擔任教授一職。「因此板塊構造運動在某個時刻開始運作，並攜帶大量海水到下方的地函。而最重要的問題是：這究竟是在什麼時候發生的？」

A logical approach would be to look at the magnesium content in ancient rocks formed across a wide span of time, to determine when this transition toward low-magnesium crustal rocks began. However, this has proven difficult because the direct evidence--magnesium--has a pesky habit of washing away into the ocean once rocks are exposed to the surface.

一種合理的作法是分別檢驗過往一大段時期中不同年代岩石的鎂含量，來確立地殼岩石的成分何時開始轉變成鎂含量較低的狀態。然而，這個做法已被證明出來相當困難，因為達成這個目標所需的直接證據，也就是鎂，有個十分麻煩的特性：一旦岩石露出地表，其中的鎂很容易就會被水沖到海裡。

Tang, Rudnick and Kang Chen, a graduate student at China University of Geosciences on a one and a half-year research visit to UMD, sidestepped this problem by looking at trace elements that are not soluble in water. These elements--nickel, cobalt, chromium and zinc--stay behind long after most of the magnesium has washed away. The researchers found that the ratios of these elements hold the key: higher ratios of nickel to cobalt and chromium to zinc both correlate to higher magnesium content in the original rock.

Tang、Rudnick和Kang Chen(於馬里蘭大學進行為期一年半學術交流的中國地質大學研究生)探討不溶於水的稀有元素而迴避了上述問題。這些元素，包含鎳、鈷、鉻和鋅，在大多數的鎂被沖刷殆盡後仍能留存在岩石中很長一段時間。研究人員發現這些元素彼此之間的比例就是關鍵所在：當岩石的鎳鈷比和鉻鋅比較高時，意味著岩石的最初鎂含量也較高。

"To our knowledge, we are the first to discover this correlation and use this approach," Tang said. "Because the ratios of these trace elements correlate to magnesium, they serve as a very reliable 'fingerprint' of past magnesium content."

「就我們所知，我們是第一個發現這種相關性並將之利用的團隊。」Tang說。「由於這些稀有元素之間的比例跟鎂的含量有關，因此可將其視作過去鎂含量留下的清晰「指紋」。」

Tang and his coauthors compiled trace element data taken from a variety of ancient rocks that formed in the Archean eon, a time period between 4 and 2.5 billion years ago, and used it to determine the magnesium content in the rocks when they were first formed. They used these data to construct a computer model of the early Earth's geochemical composition. This model accounted for how magnesium (specifically, magnesium oxide) content in the crust changed over time.

Tang和他的同僚彙整了於太古宙(40億年前至25億年前)形成的各類岩石中的稀有元素含量，並利用這些資料來判定這些岩石形成當時的鎂含量。接著再用這些資料建構出早期地球化學成分的電腦模型。這個模型可以呈現地殼中鎂的含量(特別是氧化鎂)如何隨著時間變化。

The results suggest that 3 billion years ago, the Earth's crust had roughly 11 percent magnesium oxide by weight. Within a half billion years, that number had dropped to about 4 percent, which is very close to the 2 or 3 percent magnesium oxide seen in today's crust. This suggested that plate tectonics began about 3 billion years ago, giving rise to the continents we see today.

結果顯示30億年前，地球地殼的總重中約有百分之11是氧化鎂。但在短短5億年之內，這個數值便驟降到百分之4左右，跟現今地殼中氧化鎂只占百分之2到3的情況十分類似。這顯示板塊構造運動大概在30億年前開始，並形成了今日我們所見的陸地。

"It's really kind of a radical idea, to suggest that continental crust in Archean had that much magnesium," said Rudnick, pointing out that Tang was the first to work out the correlation between trace element ratios and magnesium. "Ming's discovery is powerful because he found that trace insoluble elements correlate with a major element, allowing us to address a long-standing question in Earth history."

「聲稱太古宙的大陸地殼有更多的鎂其實是種相當激進的說法。」Rudnick說。他並且指出Tang是第一個發現稀有元素比例和鎂含量之間有關係的人。「Ming的發現強而有力之處在於他找到了非可溶性的稀有元素跟主要元素之間有所關連，而讓我們可以解答地球歷史中懸宕已久的未解之謎。」

"Because the evolution of continental crust is linked to many major geological processes on Earth, this work may provide a basis for a variety of future studies of Earth history," Tang said. "For example, weathering of this magnesium-rich crust may have affected the chemistry of the ancient ocean, where life on Earth evolved. As for the onset of plate tectonics, I don't think this study will close the argument, but it certainly adds a compelling new dimension to the discussion."

「因為地球大陸地殼的演變跟許多重大地球化學作用都有牽扯，這項成果或許也能做為未來諸多地球歷史相關研究的基礎。」Tang說。「舉例來說，古代海洋是地球生命演化而成之處，而它的化學性質可能會被富含鎂的地殼受到的風化作用影響。至於板塊構造運動的起源問題，我不認為這起研究會終結有關這方面的爭論，但肯定會在這場激辯中佔有新的一席之地。」

引用自：University of Maryland. "New study zeros in on plate tectonics' start date: Analysis of trace elements places the onset of plate tectonics about 3 billion years ago." ScienceDaily. ScienceDaily, 21 January 2016.