指出地函的碳來源為何的關鍵證物

原始網址：www.sciencedaily.com/releases/2016/11/161109132623.htm

指出地函的碳來源為何的關鍵證物

科學找到了關鍵證物可以指出從地函中發現的碳是否源自大陸地殼。這讓我們對地球表面地殼形成的歷史，以及不同地質年代的板塊移動速率有更深入的瞭解，進而探討地函的冷卻速率。

刊登在期刊《自然-地質科學》(Nature Geoscience)的新研究結果顯示在稱為碳酸岩(carbonatite)的火成岩當中，不同年代的岩石其硼同位素比例也不同。這項研究由聖母大學地球科學系和土木與環境工程學系的副教授Antonio Simonetti領導。

目前有三種理論來解釋從地函中發現的碳來源為何：其一認為這些碳是45.6億年前地球形成時便已存在的原始(primordial)成分；另一則認為它們是星體碰撞地球的產物；最後則是在板塊移動過程中，當一塊板塊下沉至另一塊之下而形成隱沒帶時，海洋或大陸板塊上的碳會於此循環回地函。

「我們最重要的發現是看到硼同位素的比例變化非常地大，代表在地函中，碳的來源會隨著地質時間經過而跟著改變。」Simonetti說。研究碳酸岩中的硼同位素比例，研究人員得以更精細地決定在地質歷史上的特定時刻應該適用哪一項理論。

「隱沒速率在過去45.6億年間不斷變化著。」 Simonetti說。「原先，在地球形成最初的20億年左右，地函溫度比今日高上許多。因此當隱沒作用開始發生時，由於地函溫度較高，造成下沉的板塊跟今日發生的隱沒作用相比並無法穿透至地球深處。在最近這20億年左右，地函變得較冷因此能讓隱沒板塊下沉至地函深部，並讓地殼循環進來的物質有機會儲存在地球深處，甚至有可能一路深達核幔邊界。」

這項探討地球歷史重要時刻產生的碳酸岩中，硼同位素組成性質的初步結果，讓Simonetti和他的團隊得以進一步去觀察這個性質的長期變化。這使得他們有機會對地殼形成如何隨時間變化有更清晰的瞭解，甚至可以追溯至數十億年以前的時光。

研究共同作者包括聖母大學土木、環境工程及地質學系的Samuel R.W. Hulett，石溪大學的E. Troy Rasbury，以及紐約市立大學皇后學院的N. Gary Hemming。

Key indicator of carbon sources in Earth's mantle

Scientists have found a key indicator in determining whether the presence of carbon, found in the Earth's mantle, is derived from continental crust -- a step toward better understanding the history of crustal formation on Earth's surface and the rate at which tectonic plates have moved throughout geologic time, which can be linked to the cooling of Earth's mantle.

Results of a new study published in the journal Nature Geoscience show evidence of varying ratios of boron isotopes in igneous rocks, known as carbonatites, of different ages. The research was led by Antonio Simonetti, associate professor in the Department of Civil and Environmental Engineering and Earth Sciences at the University of Notre Dame.

Three theories exist regarding the source of carbon found within the Earth's mantle: It is of primordial origin, formed during the creation of the planet 4.56 billion years ago; it is a result of planetary collision; or it had been present in marine environments or continental crust, and recycled back into the mantle in areas of subduction, where tectonic plates shifted, one diving beneath the other.

"Our most important finding is that the Boron isotope ratios are highly variable, indicating that the source of carbon within the mantle changed with geological time on Earth," Simonetti said. Studying the ratios of boron isotopes within carbonatites, researchers are closer to determining which hypothesis applies to specific moments in geological time.

"During the past 4.56 billion years, the subduction rate has varied," said Simonetti. "Early on, during the first 2 billion years or so, Earth's mantle was much hotter than it is today, so when subduction did occur, the diving plate did not penetrate as deep into the mantle as it does today because of the higher temperature. During the last 2 billion years or so, a cooler mantle has allowed the subducting plate to dive deeper into the mantle and provide the opportunity to store recycled crustal materials at greater depths, and possibly all the way down to the core-mantle boundary."

This preliminary investigation into the boron isotope compositions of carbonatites from significant periods in Earth's history allows Simonetti and his team to monitor long-term temporal variations -- creating a clearer picture of crustal formation over time, with the potential to go as far back as several billion years.

The study was co-authored by Samuel R.W. Hulett in the Department of Civil and Environmental Engineering and Earth Sciences at Notre Dame, E. Troy Rasbury of Stony Brook University N. Gary Hemming of Queens College -- CUNY.

原始論文：Samuel R. W. Hulett, Antonio Simonetti, E. Troy Rasbury, N. Gary Hemming. Recycling of subducted crustal components into carbonatite melts revealed by boron isotopes. Nature Geoscience, 2016; DOI: 10.1038/ngeo2831

引用自：University of Notre Dame. "Key indicator of carbon sources in Earth's mantle." ScienceDaily. ScienceDaily, 9 November 2016. 

冰川融化使得阿爾卑斯山增高

原始網址：www.sciencedaily.com/releases/2016/11/161110085844.htm

冰川融化使得阿爾卑斯山增高

阿爾卑斯山以大概每年1~2釐米的速率穩定「長高」。同樣地，在過往被冰河覆蓋的北美以及斯堪地那維亞次大陸也正在經歷持續地向上成長過程。這是因為18,000年前的末次冰盛期(Last Glacial Maximum, LGM)結束時，冰河消融使得之前冰河施加於地表的龐大重量跟著減輕的緣故。雖然冰河對氣候變遷的反應相當迅速，然而直到今日，地殼還在回應相對而言突如其來的冰川融化。於末次冰盛期，阿爾卑斯山也被冰帽所覆蓋，範圍甚至一度遠達阿爾卑斯前緣地區。但此處的冰河覆蓋程度遠遠不及北美和斯堪地那維亞次大陸上發生的。這是科學家長久以來假設冰帽後退對今日阿爾卑斯山的穩定成長來說，並沒有多少重要性的原因。然而，德國地質研究中心(GFZ)的科學家Dirk Scherler和Taylor Schildgen參與在內的國際研究團隊，表示現在阿爾卑斯山的持續成長，有90%仍要歸因於末次冰盛期時形成的冰帽消失。

地球地殼發生垂直運動的主要因素為板塊移動造成的構造變形作用，另外還有火山作用以及水、冰、沉積物的減壓作用(unloading)。我們可以利用衛星及地面測站進行大地測量來得知地殼的運動。對於北美和斯堪地那維亞次大陸這種古老且構造穩定的陸塊，科學家很早便瞭解它們的垂直運動幾乎都是由所謂的「冰後回彈效應」(postglacial rebound effect)造成，也就是上方冰河融化使得下方地殼往上隆起。然而，在阿爾卑斯山這種年輕的造山帶中，會有許多複雜機制共同作用而互相影響。此處發生的作用有非洲板塊隱沒至歐亞板塊下方，同時亞德里亞板塊(Adriatic Plate，非洲板塊的次板塊)則在歐亞板塊下方以逆時針方向移動。另外，如同在斯堪地那維亞以及北美發生的，此處也具備由侵蝕和沉積物搬運，以及「冰河消退」造成的減壓作用。現今阿爾卑斯山持續抬升的因素究竟為何，已經爭論超過四分之一個世紀之久。

長久以來科學家推測抬升主要是由侵蝕作用，和以河川為主的搬運作用將沉積物運送至山脈前緣地區造成。這項研究比對了侵蝕、冰河減壓以及區域構造運動對阿爾卑斯山垂直運動的貢獻程度。科學家利用電腦模型輔以鑽井資料，顯示冰川消融之後，也就是冰盛期結束以後大多數侵蝕下來的物質仍沉積於造山帶內部。故阿爾卑斯山隆起的主因中，可以將此作用屏除在外。另一方面，模型則顯示如同斯堪地那維亞和北美地區，抬升訊號的最佳解釋是上次冰盛期結束時，阿爾卑斯山的冰河於短短3000年間消退了將近80%而造成的減壓補償運動。今日的抬升中僅約10%得肇因於沉積物的減壓作用。在某些區域，特別是奧地利某些地區，構造運動也是抬升的重要因素，可能是由亞德里亞次板塊的旋轉運動導致。利用他們的模型科學家顯示冰河的荷重高達將近62兆噸，但冰河期之後的沉積物減壓作用僅有4兆噸。

Thawing ice makes the Alps grow

The Alps are steadily "growing" by about one to two millimeters per year. Likewise, the formerly glaciated subcontinents of North America and Scandinavia are also undergoing constant upward movement. This is due to the fact that at the end of the Last Glacial Maximum (LGM) about 18,000 years ago the glaciers melted and with this the former heavy pressure on Earth's surface diminished. The ice reacted rapidly to climate change at that time whereas Earth's crust is still responding today to this relatively sudden melting of ice. During the LGM the Alps were also coated with an ice cap that temporarily reached far into the alpine foreland. The extent of glaciation was much smaller here than on the subcontinents of North America and Scandinavia. This is why it was assumed for a long time that the retreat of the ice cap back then did not play a significant role in the steady uplifting of the Alps today. However, an international team with the participation of the GFZ scientists Dirk Scherler and Taylor Schildgen have now been able to show that the loss of the LGM ice cap still accounts for 90 percent of today's uplifting of the Alps.

Vertical motions of the Earth's crust are mainly caused by tectonic deformation due to movements of tectonic plates, and by volcanism, and unloading of water, ice, and sediments. The movement of the crust can be measured by geodetic methods via satellites and ground stations. For old, tectonically stable continents like the subcontinents of North America and Scandinavia it has been known for a long time that vertical motion is almost exclusively caused by the so called postglacial "rebound effect" -- i.e. the upward motion of the crust due to the thawing of the glaciers. In young mountain belts such as the Alps, however, complex mechanisms come into play that mutually effect each other: The African Plate subducts below the Eurasian Plate, and the Adriatic Plate -- a sub-plate of the African Plate – moves counterclockwise below the Eurasian Plate. Furthermore, as in Scandinavia and North America, there is unloading due to erosion and sediment transport, and "deglaciation." The causes for today's uplift of the Alps has been a matter of debate for over a quarter of a century.

For a long time it was assumed that the uplift is primarily caused by erosion and sediment transport, mainly by rivers, towards the foreland. The new study compares by how much erosion, ice unloading, and local tectonics contribute to the vertical motion of the Alps. The scientists use models supported with drill core data to show that the better part of postglacially, and therewith after the end of the main glacial phase, eroded material was deposited within the orogen. Hence, this process can be excluded as a main cause for the alpine uplift. The models, however, show that, just like in Scandinavia and America, the uplift-signal is best explained with a relieving compensatory movement after the decline of the LGM-glaciers: Within only 3,000 years the glaciation of the Alps decreased by about 80 percent. Only about 10 percent of today's uplift can be attributed to sediment unloading. Locally, especially in parts of Austria, tectonic effects add to the uplift, likely caused by the circular motion of the Adriatic sub-plate. With their models the scientists are able to show that the glacial load weighed about 62,000 gigatonnes, while the postglacial sedimentary unloading only accounts for about 4,000 gigatonnes

原始論文：Jürgen Mey, Dirk Scherler, Andrew D. Wickert, David L. Egholm, Magdala Tesauro, Taylor F. Schildgen, Manfred R. Strecker. Glacial isostatic uplift of the European Alps. Nature Communications, 2016; 7: 13382 DOI: 10.1038/ncomms13382

引用自：GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre. "Thawing ice makes the Alps grow." ScienceDaily. ScienceDaily, 10 November 2016.

生命在陸地扎根的時間比過往認為的還要早了3億年

原文網址：www.sciencedaily.com/releases/2016/11/161108090052.htm

生命在陸地扎根的時間比過往認為的還要早了3億年

德國柏林、波茨坦和耶拿的科學家進行的研究認為生命早在32億年前就已經在陸地站穩腳跟了。由柏林自由大學的Sami Nabhan領導的團隊對南非巴柏頓綠岩帶的遠古岩層進行研究而得到了上述結論。

這些形成年代可追溯至35億年前的岩石是地球已知最古老岩石中的一員。科學家於其中定年為32.2億年的岩層裡，從硫化鐵礦物―黃鐵礦的微小顆粒中，發現了微生物活動的明顯痕跡。

這些痕跡同時紀錄在黃鐵礦的稀有元素分布以及硫同位素³⁴S和³²S的比例當中。

利用波茨坦大學2013年設置的儀器，科學家發現某些晶體核心部位³⁴S的比例跟周圍環狀部分有顯著差異，代表微生物代謝硫時該顆粒的外圍有參與其中，此作用稱為「生物成因分化作用」(biogenic fractionation)。GFZ(德國地質科學研究中心)二次離子質譜儀實驗室的Michael Wiedenbeck利用該機構的儀器，可以憑藉小於10億分之一公克重的樣品即能測出³⁴S和³²S的比例。

岩石成分、晶體形狀以及在野外可見的層狀構造皆指出他們研究的岩石序列源自於古代土壤剖面，此「古土壤」(paleosol)形成於32.2億年前一處河流氾濫平原。

在他們的解釋中，此研究蒐集到的野外調查數據顯示當時有條辮狀河系攜帶的沉積物含有硫化鐵晶體。接著，在位置處於乾溼環境交替出現的土壤中生長的微生物，會讓這些黃鐵礦上增生一圈新的晶體。

根據這項證據，刊登於期刊《地質》(Geology )的論文中科學家總結說他們找到了十分早期陸地上便有生物活動的鐵證。相較於前人文獻，此篇研究將陸地生命出現的最早證據往前推進了約3億年。

Life took hold on land 300 million years earlier than thought

Life took hold on land at least as early as 3.2 billion years ago, suggests a study by scientists from Berlin, Potsdam and Jena (Germany). The team led by Sami Nabhan of the Freie Universität Berlin studied ancient rock formations from South Africa's Barberton greenstone belt.

These rocks are some of the oldest known on Earth, with their formation dating back to 3.5 billion years. In a layer that has been dated at 3.22 billion years old, tiny grains of the iron sulfide mineral pyrite were discovered that show telltale signs of microbial activity.

These signs are recorded both in trace element distributions as well as in the ratio between the sulfur isotopes 34S and 32S in the pyrite.

Using instrumentation installed in Potsdam in 2013, the scientists showed that the fraction of 34S in the core of some crystals differ characteristically from that of the same crystal's rim, indicating that the exterior of the grain involved a processing of sulfur by microbes, so-called biogenic fractionation. The determination of the 34S/32S ratio, using sample masses less than one billionth of a gram, was carried out at the GFZ German Research Centre for Geosciences by Michael Wiedenbeck of the GFZ's secondary ion mass spectrometry (SIMS) lab.

The composition of the rock, the shape of the crystals, and the layering visible in the field all indicate that the studied rock sequence was derived from an ancient soil profile; this so-called paleosol developed on a river flood plain 3.22 billion years ago.

Field data collected during this study imply that a braided river system transported the sediment containing the iron sulfide crystals. It is interpreted that microbes living in the soil, at a level that was continually shifting between wet and dry conditions, subsequently produced the rim overgrowths on the pyrite crystals.

Based on this evidence, the scientists conclude in their publication in the journal Geology that they found evidence for biological activity on land at this very early date. Their research pushes back the date for the oldest evidence of life on land to some 300 million years earlier than previously documented.

原始論文：Sami Nabhan, Michael Wiedenbeck, Ralf Milke, Christoph Heubeck. Biogenic overgrowth on detrital pyrite in ca. 3.2 Ga Archean paleosols. Geology, 2016; 44 (9): 763 DOI: 10.1130/G38090.1

引用自：GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre. "Life took hold on land 300 million years earlier than thought." ScienceDaily. ScienceDaily, 8 November 2016. 

為什麼地球每十萬年就會歷經一次冰河期？

原文網址：www.sciencedaily.com/releases/2016/10/161026081537.htm

為什麼地球每十萬年就會歷經一次冰河期？

卡迪夫大學的專家對地球為何開始每十萬年就會進入及退出冰河期的現象提出了解釋。

這項被暱稱為「十萬年謎題」的神秘現象大約發生在過去一百萬年，並產生了曾覆蓋北美大陸、歐洲和亞洲的廣大冰層。目前為止，科學家仍未能解釋其發生的原因。

科學家可以預測地球季節如何變化，而最寒冷的夏季會以四萬年為間距發生。因此，對他們來說，過去地球冰河期以同樣地間距發生是相當合理的。然而，在一百萬年前左右，於「中更新世轉型期」(Mid-Pleistocene Transition)這個時間點，冰河期的間距從四萬年一次轉變成了十萬年一次。

今日刊登於期刊《地質》(Geology )的新研究提出海洋可能是導致此轉變的推手，精確來說跟海洋從大氣吸收二氧化碳的方式有關。

研究團隊分析海床上微小化石的化學成分，發現在以十萬年為規律的區間，冰河期時深海儲存的二氧化碳量變多了。

這代表當時有更多二氧化碳從大氣抽離並儲存至海洋，造成地球溫度更加降低而讓廣大冰層可以吞沒整個北半球。

此篇研究的第一作者，地球和海洋科學院的教授Carrie Lear說：「我們可以想像海洋會吸入並呼出二氧化碳。因此冰層擴大時，表示海洋從大氣吸入了二氧化碳，造成整個地球變冷；當冰層縮小，則代表海洋呼出二氧化碳，使大氣二氧化碳變多進而讓地球氣溫升高。」

「藉由研究海床上的微小生物化石，我們發現冰層以十萬年為周期前進後退時，於冷期海洋吸入了更多二氧化碳，代表同一時期留在大氣中的二氧化碳變得較少。」

身為光合作用生物中的主要成員，海洋藻類在移除大氣二氧化碳的作用中扮演了重要角色。

經由稱做湧升流(upwelling)的作用，深海海水上湧回到海表時二氧化碳也會跟著回到大氣。但是，當有大量海冰出現時，海冰會阻斷海洋呼出二氧化碳，使得冰層更加擴張而冰河期也會延長。

「若我們想像海洋正在吸進呼出二氧化碳，大量冰層的出現就像有顆巨大糖球將其噎住。它好比是個鍋蓋封住了海洋表層。」 Lear教授繼續說道。

地球氣候現今正處於兩次冰河期之間的溫暖間隙。上次冰河期約在11000年前結束。從那時開始，氣溫和海平面便持續升高而冰帽也退回兩極。除了這些自然循環之外，人類排放的二氧化碳也對氣候暖化造成了深遠影響。

Why does our planet experience an ice age every 100,000 years?

Experts from Cardiff University have offered up an explanation as to why our planet began to move in and out of ice ages every 100,000 years.

This mysterious phenomena, dubbed the '100,000 year problem', has been occurring for the past million years or so and leads to vast ice sheets covering North America, Europe and Asia. Up until now, scientists have been unable to explain why this happens.

Our planet's ice ages used to occur at intervals of every 40,000 years, which made sense to scientists as the Earth's seasons vary in a predictable way, with colder summers occurring at these intervals. However there was a point, about a million years ago, called the 'Mid-Pleistocene Transition', in which the ice age intervals changed from every 40,000 years to every 100,000 years.

New research published today in the journal Geology has suggested the oceans may be responsible for this change, specifically in the way that they suck carbon dioxide (CO₂) out of the atmosphere.

By studying the chemical make-up of tiny fossils on the ocean floor, the team discovered that there was more CO₂ stored in the deep ocean during the ice age periods at regular intervals every 100,000 years.

This suggests that extra carbon dioxide was being pulled from the atmosphere and into the oceans at this time, subsequently lowering the temperature on Earth and enabling vast ice sheets to engulf the Northern Hemisphere.

Lead author of the research Professor Carrie Lear, from the School of Earth and Ocean Sciences, said: "We can think of the oceans as inhaling and exhaling carbon dioxide, so when the ice sheets are larger, the oceans have inhaled carbon dioxide from the atmosphere, making the planet colder. When the ice sheets are small, the oceans have exhaled carbon dioxide, so there is more in the atmosphere which makes the planet warmer.

"By looking at the fossils of tiny creatures on the ocean floor, we showed that when ice sheets were advancing and retreating every 100,000 years the oceans were inhaling more carbon dioxide in the cold periods, suggesting that there was less left in the atmosphere."

Marine algae play a key role in removing CO₂ from the atmosphere as it is an essential ingredient of photosynthesis.

CO₂ is put back into the atmosphere when deep ocean water rises to the surface through a process called upwelling, but when a vast amount of sea ice is present this prevents the CO₂ from being exhaled, which could make the ice sheets bigger and prolong the ice age.

"If we think of the oceans inhaling and exhaling carbon dioxide, the presence of vast amounts of ice is like a giant gobstopper. It's like a lid on the surface of the ocean," Prof Lear continued.

The Earth's climate is currently in a warm spell between glacial periods. The last ice age ended about 11,000 years ago. Since then, temperatures and sea levels have risen, and ice caps have retreated back to the poles. In addition to these natural cycles, humanmade carbon emissions are also having an effect by warming the climate.

原始論文：Caroline H. Lear, Katharina Billups, Rosalind E.M. Rickaby, Liselotte Diester-Haass, Elaine M. Mawbey, Sindia M. Sosdian. Breathing more deeply: Deep ocean carbon storage during the mid-Pleistocene climate transition.Geology, 2016; G38636.1 DOI: 10.1130/G38636.1

引用自：Cardiff University. "Why does our planet experience an ice age every 100,000 years?." ScienceDaily. ScienceDaily, 26 October 2016.