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.

科學家首度對地函的內部分界提出解釋

原文網址：www.sciencedaily.com/releases/2015/12/151210144707.htm

First explanations for boundary within Earth's mantle

科學家首度對地函的內部分界提出解釋

Observed physical transition hundreds of miles below Earth's surface

觀測顯示地表數百哩之下的物理性質發生轉變

Earth's mantle, the large zone of slow-flowing rock that lies between the crust and the planet's core, powers every earthquake and volcanic eruption on the planet's surface. Evidence suggests that the mantle behaves differently below 1 megameter (1,000 kilometers, or 621 miles) in depth, but so far seismologists have not been able to explain why this boundary exists.

地函位於地殼和地核之間，是一片由緩緩流動的岩石組成的廣大區域，並驅使了所有發生在地表的地震與火山噴發。證據顯示地函的行為在深於1000公里(621哩)後會跟淺處有所差異，但至今為止地震學家仍無法解釋為何會有這個分界存在。

Two new studies co-authored by University of Maryland geologists provide different, though not necessarily incompatible, explanations. One study suggests that the mantle below 1 megameter is more viscous--meaning it flows more slowly--than the section above the boundary. The other study proposes that the section below the boundary is denser--meaning its molecules are more tightly packed--than the section above it, due to a shift in rock composition.

兩篇最新的研究各自提出了一套解釋，但兩者之間未必互相牴觸。其中一篇認為地函在1000公里之下較上方更為黏滯，意即地函在這深度以下更加難以流動；另一篇則表示由於岩石成分改變，使得在此分界下方的密度較上方要高，代表地函物質在這深度以下分子的排列更加緊密。這兩篇研究的共同作者皆包括了馬里蘭大學的地質學家。

Taken together, the studies provide the first detailed look at why large-scale geologic features within the mantle behave differently on either side of the megameter divide. The papers were published on December 11, 2015, in the journals Science and Science Advances.

總和而言，這兩篇研究為科學家首度詳細驗證以深度1000公里為界，兩側地函會出現行為差異的大尺度地質構造是如何出現。它們將會於2015年12月11日分別刊登在期刊《科學》和《科學前緣》。

"The existence of the megameter boundary has been suspected and inferred for a while," said Vedran Lekic, an assistant professor of geology at UMD and co-author of the Science paper that addresses mantle viscosity. "These papers are the first published attempts at a detailed explanation and it's possible that both explanations are correct."

「早已有人猜測且提出證據在1000公里深處有道分界存在於地函當中。」馬里蘭大學地質科學系的助理教授Vedran Lekic說。他參與在內刊登於《科學》的研究引述了地函黏滯性理論。「這兩篇研究為科學家首次發表他們試著對此現象提出的詳細解釋，而這兩者都有可能都是對的。」

Although the mantle is mostly solid, it flows very slowly in the context of geologic time. Two main sources of evidence suggest the existence of the megameter boundary and thus inspired the current studies.

雖然地函絕大部分是由固體組成，但從地質時間尺度來看它仍會緩緩流動。有兩項主要證據顯示1000公里分界確實存在，並啟發了目前正在進行的諸多研究。

First, many huge slabs of ocean crust that have been dragged down, or subducted, into the mantle can still be seen in the deep Earth. These slabs slowly sink downward toward the bottom of the mantle. A large number of these slabs have stalled out and appear to float just above the megameter boundary, indicating a notable change in physical properties below the boundary.

首先，許多巨大的海洋板塊在被拖入(隱沒)至地函後，我們仍然能看到它們存在於地球深處。這些隱沒板塊會緩緩往地函底部下沉，但其中有許多會在停滯在1000公里分界上，這讓它們看起來像在此處漂浮。這種現象意味著在此分界之下地函的物理性質勢必發生了顯著改變。

Second, large plumes of hot rock rise from the deepest reaches of the mantle, and the outlines of these structures can be seen in the deep Earth as well. As the rock in these mantle plumes flows upward, many of the plumes are deflected sideways as they pass the megameter boundary. This, too, indicates a fundamental difference in physical properties on either side of the boundary.

其次，在地函最深處會有由熾熱岩石形成的地函柱往上湧動，而我們同樣能看到這種地球深處構造的外觀。當這些地函柱中的岩石往上流動，可以發現有許多地函柱在經過1000公里分界後流動方向會偏折。這一樣代表著分界兩側的物理性質一定有本質上的差異。

"Learning about the anatomy of the mantle tells us more about how the deep interior of Earth works and what mechanisms are behind mantle convection," said Nicholas Schmerr, an assistant professor of geology at UMD and co-author of the Science Advances paper that addresses mantle density and composition. "Mantle convection is the heat engine that drives plate tectonics at the surface and ultimately leads to things like volcanoes and earthquakes that affect people living on the surface."

「認識地函的基本性質可以告訴我們地球深處如何運作，以及地函的對流機制為何」馬里蘭大學地質科學系的助理教授Nicholas Schmerr說。他參與在內發表於《科學前緣》的論文則是以地函密度和成分差異來解釋1000公里分界。「地函對流是驅動地表板塊構造運動的動力來源，這最終會引發火山爆發和地震之類的種種事物而影響到生活在地表的人們。」

The physics of the deep Earth are complicated, so establishing the mantle's basic physical properties, such as density and viscosity, is an important step. Density refers to the packing of molecules within any substance (gas, liquid or solid), while viscosity is commonly described as the thickness of a fluid or semi-solid. Sometimes density and viscosity correlate with each other, while sometimes they are at odds. For example, honey is both more viscous and dense than water. Oil, on the other hand, is more viscous than water but less dense.

在地球深處進行的物理作用相當複雜，因此確立地函的基本物理性質，像是密度和黏滯性，對於我們要了解它會是相當重要的一步。任何物質(氣體、液體、固體)的密度代表了它內部分子排列的緊密程度，而黏滯性則通常用來描述液體或半液體的濃稠程度。有時密度與黏滯性會有關聯，但有時卻會互相衝突。比方說，蜂蜜跟水相比，它的黏滯性與密度都較高；另一方面，石油雖然比水更為濃稠，然而它的密度卻較低。

In their study, Schmerr, lead author Maxim Ballmer (Tokyo Institute of Technology and the University of Hawaii at Manoa) and two colleagues used a computer model of a simplified Earth. Each run of the model began with a slightly different chemical composition--and thus a different range of densities--in the mantle at various depths. The researchers then used the model to investigate how slabs of ocean crust would behave as they travel down toward the lower mantle.

在Schmerr、Maxim Ballmer(第一作者，任職於東京工業大學和夏威夷大學馬諾分校)以及另外兩名科學家同僚的研究中，他們利用了簡化的地球模型。每一次的模擬開始時，他們都會先微調地函不同深度的化學成分，因此密度也會跟著改變。接著研究人員利用模型來探討海洋隱沒板塊在往下潛至下部地函的過程中，會展現出何種行為模式。

In the real world, slabs are observed to behave in one of three ways: The slabs either stall at around 600 kilometers, stall out at the megameter boundary, or continue sinking all the way to the lower mantle. Of the many scenarios for mantle chemical composition the researchers tested, one most closely resembled the real world and included the possibility that slabs can stall at the megameter boundary. This scenario included an increased amount of dense, silicon-rich basalt rock in the lower mantle, below the megameter boundary.

現實世界中，隱沒板塊呈現出的行為模式會是下列三者其一：堆積在600公里深附近、停滯在1000公里分界、或者一路下沉至下部地函。研究人員反覆以不同地函成分測試，其中有一種呈現出來的情境與真實世界世界最為相似，且包含了隱沒板塊能停留在1000公里分界的可能性。在此情境中，1000公里分界之下的下部地函會擁有更多密度較高、富含矽的玄武岩質岩石。

Lekic, lead author Max Rudolph (Portland State University) and another colleague took a different approach, starting instead with whole-Earth satellite measurements. The team then subtracted surface features--such as mountain ranges and valleys--to better see slight differences in Earth's basic shape caused by local differences in gravity. (Imagine a slightly misshapen basketball with its outer cover removed.)

在Lekic、Max Rudolph(研究第一作者，任職於波特蘭州立大學)和另一名科學家同僚的研究中，他們則採取了不同方法。研究團隊先取得衛星對整個地球的觀測影像，接著他們減去地表特徵，像是山脈和深谷造成的影響，以更詳細的觀察因為各地重力不同，而對地球原本的外形造成的輕微起伏(想像一個外皮剝掉後稍稍變形的籃球)。

The team mapped these slight differences in Earth's idealized shape onto known shapes and locations of mantle plumes and integrated the data into a model that helped them relate the idealized shape to differences in viscosity between the layers of the mantle. Their results pointed to less viscous, more free-flowing mantle rock above the megameter boundary, transitioning to highly viscous rock below the boundary. Their results help to explain why mantle plumes are frequently deflected sideways as they extend upward beyond the megameter boundary.

研究團隊得到這些與地球理想外形的細微差異後，再將其疊合在標有已知地函柱的形狀和位置圖上。接著研究團隊把這些資料整合至模型當中，以幫助他們瞭解地球理想外型和現實的些微出入，跟地函不同層的黏滯性差異之間有何關聯。他們的結果指出原本黏滯性較低，更容易流動的地函岩石，在經過1000公里分界後會變成黏滯性較高的岩石。他們的結論有助於解釋為何地函柱往上流動經過1000公里分界後，時常會往他處偏折。

"While explaining one mystery--the behavior of rising plumes and sinking slabs--our results lead to a new conundrum," Lekic said. "What causes the rocks below the megameter boundary to become more resistant to flow? There are no obvious candidates for what is causing this change, so there is a potential for learning something fundamentally new about the materials that make up Earth."

「在解開為何上湧地函柱和隱沒板塊會有這些行為的謎題時，我們的結果卻又製造了一道新難題。」Lekic說。「是什麼造成1000公里分界下方的岩石更難以流動？目前並無有力的假說可以解答是什麼造成了這種改變，但也因此在解決這項謎題的過程中，我們可能相當有機會可以學習到一些攸關地球組成本質的新事物。」

Lekic and Schmerr plan to collaborate to see if the results of both studies are consistent with one another--in effect, whether the lower mantle is both dense and viscous, like honey, when compared with the mantle above the megameter boundary.

Lekic和Schmerr計畫要合作以觀察他們倆個團隊的研究成果是否可以彼此相容，實際上就是要確認下部地函與1000公里分界之上的地函互相比較，是否前者真的比後者更加緻密且濃稠，就像蜂蜜和水的關係一樣。

"This work can tell us a lot about where Earth has been and where it is going, in terms of heat and tectonics," Schmerr said. "When we look around our solar system, we see lots of planets at various stages of evolution. But Earth is unique, so learning what is going on deep inside its mantle is very important."

「這項工作可以告訴我們許多事物，讓我們了解地球的熱力學和構造運動一直以來是如何運作，而未來的走向又是如何。」Schmerr說。「當我們環顧我們所處的太陽系，我們可以看到處在不同演化階段的各式星體。但地球是獨一無二的，因此了解它的地函深處運作方式是件至關重要的事。」

引用自：University of Maryland. "First explanations for boundary within Earth's mantle: Observed physical transition hundreds of miles below Earth's surface." ScienceDaily. ScienceDaily, 10 December 2015.