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.

地球在動物出現許久之前就已經擁有相當充足的氧氣

原文網址：www.sciencedaily.com/releases/2016/01/160104163200.htm

 

Enough oxygen on Earth long before animals rose

地球在動物出現許久之前就已經擁有相當充足的氧氣

Oxygen is crucial for the existence of animals on Earth. But, an increase in oxygen did not apparently lead to the rise of the first animals. New research shows that 1.4 billion years ago there was enough oxygen for animals -- and yet over 800 million years went by before the first animals appeared on Earth.

動物能生活在地球上氧氣扮演了至關重要的角色。然而，大氣氧含量的提升似乎並未促成第一批動物出現。新研究顯示雖然在14億年前大氣氧含量已經多到足以使動物生存，但還要再過8千萬年地球上首批動物才會誕生。

Animals evolved by about 600 million years ago, which was late in Earth's history. The late evolution of animals, and the fact that oxygen is central for animal respiration, has led to the widely promoted idea that animal evolution corresponded with a late a rise in atmospheric oxygen concentrations.

動物大約在6000萬年前演化出來，在地球史上算是相當近代的事件。動物於晚近才演化而成，加上氧氣是動物呼吸作用中的核心原料，使得一項廣為宣揚的想法認為動物的演化歷程可對應至大氣氧濃度於晚期才提升。

"But sufficient oxygen in itself does not seem to be enough for animals to rise. This is indicated by our studies," say postdoc Emma Hammarlund and Professor Don Canfield, Nordic Center for Earth Evolution, University of Southern Denmark.

「然而，我們的研究指出，僅靠充足的氧氣本身似乎並不足以讓動物出現。」南丹麥大學北歐地球演化中心的博士後研究員Emma Hammarlund和教授Don Canfield說。

Together with colleagues from the China National Petroleum Corporation and the University of Copenhagen, Hammarlund and Canfield have analyzed sediment samples from the Xiamaling Formation in China. Their analyses reveal that a deep ocean 1.4 billion years ago contained at least 4% of modern oxygen concentrations.

Hammarlund和Canfield跟中國石油天然氣集團以及哥本哈根大學合作，分析了來自中國下馬齡層的沉積物樣品。他們的分析結果顯示14億年前深海的氧濃度至少已達到現今氧濃度的4%。

The new study is published in the journal Proceedings of National Academy of Sciences.

此篇新研究刊登於《美國國家科學院院刊》(Proceedings of National Academy of Sciences)。

Usually it is very difficult to precisely determine past oxygen concentrations. The new study, however, combines several approaches to break new ground in understanding oxygen concentrations 1.4 billion years ago.

要精確分析過往的氧濃度通常是件十分困難的工作。然而，此篇研究結合了多種方法而開創出可以得知14億年前氧濃度的新方式。

The study uses trace metal distributions to show that the bottom waters where the Xiamaling Formation sediments deposited contain oxygen. The distribution of biomarkers, molecules derived from ancient organisms, demonstrate that waters of intermediate depth contain no oxygen. Therefore, the Xiamaling Formation deposited in an ancient oxygen-minimum zone, similar to (but also different) from those found off the present coasts of Chile and Peru.

此篇研究利用稀有金屬元素的分布模式來顯示下馬齡層沉積物沉積時的底層水含有氧氣。而生物標記(biomarker，古代有機體產生的分子)則顯現出中層水並未存有氧氣。因此下馬齡層的沉積環境為遠古時候的低氧帶(oxygen-minimum zone)，與現今在祕魯和智利外海發現的環境頗為相似(但仍有所差異)。

With this backdrop, the researchers used a simple ocean model to estimate the minimum concentrations to atmospheric oxygen required to reproduce the distribution of water-column oxygen in the Xiamaling Formation.

有了這項背景資料，研究人員便可以利用簡化的海洋模型來預估大氣中的氧濃度最低要有多少，才能重現下馬齡層沉積時水體中的氧含量分布情況。

"The water column had an oxygen concentration at least 4 % of present atmospheric levels (PAL). That should be sufficient for animals to exist and evolve," says Canfield.

「當時水體中的氧含量至少已經有現今量(present atmospheric levels，PAL)的4%，而這應該足以使動物誕生並開始演化。」Canfield說。

"Having determined the lowest concentration of oxygen in the air almost one and a half billion years ago is unique," says Hammarlund, adding:

「能夠確認將近15億年前大氣的最低氧含量是項十分獨特的成果。」Hammarlund說。他補充：

"Researchers know of simple animals, such as sponges and worms, that today are capable of managing with less than 4% PAL, even much less."

「研究人員知道現存的一些簡單生物，像是海棉與蠕蟲，可以生活在氧濃度小於PAL 4%，甚至是濃度低上許多的環境中。」

"Sponges probably resemble some of the first animals on Earth. If they manage with less than 4 % today's oxygen levels, it is likely that the first animals could do with these concentrations or less," says Canfield.

「海綿可能跟地球上最早出現的生物十分相似。如果牠們可以在氧含量小於4%現今氧濃度的環境中苟活，那麼首批生物也很可能可以在同樣或者更低的氧濃度下生存。」Canfield說。

The results differ from other studies and raise several questions, such as: Why then did animals rise so late in Earth's history?

這項結果跟其他的研究並不相同，且引起了某些疑問，比如說：那為什麼在地球史中動物這麼晚才出現？

"The sudden diversification of animals probably was a result of many factors. Maybe the oxygen rise had less to do with the animal revolution than we previously assumed," says Hammarlund.

「動物急遽多樣化可能是由許多因素造成。或許氧濃度的升高在動物演化中扮演的腳色比我們先前認為的還要渺小。」Hammarlund說。

引用自:University of Southern Denmark. "Enough oxygen on Earth long before animals rose." ScienceDaily. ScienceDaily, 4 January 2016.