科學家對地球大陸地殼最初的形成過程有了新見解

原文網址：www.sciencedaily.com/releases/2016/09/160919121941.htm

科學家對地球大陸地殼最初的形成過程有了新見解

科學家精確定出地球現存最古老的岩石單位於42億年前形成，這讓他們能解開地球地殼的最初歷史中一些仍然未解的基本問題。由阿爾伯塔大學主持的發現認為早期地球的表面大多由類似海洋地殼的物質包覆。

「這給了我們關於原始陸地是如何形成的重要資訊，」第一作者Jesse Reimink表示。「由於陸地的形成要追溯到很遠之前，因此我們必須盡量掌握任何一片能找到的證據。而我們並沒有多少資料點可以用來推測當時地球上發生了何種事物。」事實上，全世界只有三個地方擁有年代早於40億年的岩石或礦物，他們分別為來自魁北克北方的岩石、西澳產出的礦物顆粒以及這項研究的探討對象：位於加拿大西北地區的岩層。

雖然已知此地最古老的岩石形成於40億年前，Reimink的岩石上的特殊紋路含有保存良好的鋯石(zircon)顆粒，使科學家可以精準無誤地定出其形成年代。他們調查的樣品是由Reimink的指導博士Tom Chacko在加拿大黃刀鎮(Yellowknife)北邊約300公里的地區進行野外考察時發現。Reimink最近完成他在阿爾伯塔大學的博士學位，現今則於美國華盛頓的卡內基科學研究院進行博士後研究。

「封存於鋯石中的不僅有年代方面的資訊，還有這篇研究中我們運用到的眾多地球化學訊息，」Reimink繼續說道。「我們結合岩石和鋯石能得到的訊息遠比它們個別呈現出來的還要多。鋯石擁有的化學訊號以及紀錄下來的年代資訊並不會被之後的地質事件重置，而岩石本身則能保存鋯石顆粒未能記下的化學訊息。」

他解釋岩石本身的化學性質看起來跟今日於冰島形成的岩石有相似之處。冰島是海洋地殼跟大陸地殼之間的過渡帶。事實上，有些假說認為可以用冰島來類比大陸地殼最初是如何開始形成。

「我們分析岩石本身的諸多訊號來探討岩漿以何種方式入侵至周圍的岩石當中，」其中有道特別訊號記錄到在地殼中曾發生了岩漿同化作用(assimilation)。「當岩漿冷卻的同時會加熱並且熔化周圍岩石，而我們有證據顯示曾經發生這類事件。」

Reimink表示這塊岩石缺乏大陸地殼應有的訊號，跟預期中早期陸塊會呈現的樣貌相異，造成它帶來的問題比回答的還要更多。Reimink說身為一名地質學家的最大挑戰，便是當我們越深入追溯地球的過往，我們能獲得的證據量與品質就下降的越多。「地球一直以來都在持續地進行自身物質的循環，隨著地殼被扭曲變形或者熔化，過往的歷史也就從此化為烏有。」 Reimink如此強調。

「陸地矗立於水面之上而暴露在空氣當中，對地球大氣的化學性質以及生命的出現與否都具有重大意涵。地球的陸塊大小對地球深部(地核和地函)以及表面(大氣層與生物圈)發生的化學作用會有相當影響。在化學作用以及地質過程之間有持續發生的循環回饋作用。雖然整個圖像仍然有許多尚未知曉的部分，這項研究又再一次傳達出地球所有事物都是環環相扣的。」此研究刊登於十月號的《自然—地質科學》(Nature Geoscience)，題名為「No evidence for Hadean continental crust within Earth's oldest evolved rock unit」。

引用自：University of Alberta. "Scientists uncover insights into the formation of Earth’s oldest continental crust." ScienceDaily. ScienceDaily, 19 September 2016. 

原始論文J. R. Reimink, J. H. F. L. Davies, T. Chacko, R. A. Stern, L. M. Heaman, C. Sarkar, U. Schaltegger, R. A. Creaser, D. G. Pearson. No evidence for Hadean continental crust within Earth’s oldest evolved rock unit. Nature Geoscience, 2016; DOI: 10.1038/ngeo2786

Scientists uncover insights into the formation of Earth’s oldest continental crust

Addressing fundamental unknowns about the earliest history of Earth's crust, scientists have precisely dated the world's oldest rock unit at 4.02 billion years old. Driven by the University of Alberta, the findings suggest that early Earth was largely covered with an oceanic crust-like surface.

"It gives us important information about how the early continents formed," says lead author Jesse Reimink. "Because it's so far back in time, we have to grasp at every piece of evidence we can. We have very few data points with which to evaluate what was happening on Earth at this time." In fact, only three locations worldwide exist with rocks or minerals older than 4 billion years old: one from Northern Quebec, mineral grains from Western Australia, and the rock formation from Canada's Northwest Territories examined in this new study.

While it is well known that the oldest rocks formed prior to 4 billion years ago, the unique twist on Reimink's rock is the presence of well-preserved grains of the mineral zircon, leaving no doubt about the date it formed. The sample in question was found during fieldwork by Reimink's PhD supervisor, Tom Chacko, in an area roughly 300 kilometres north of Yellowknife. Reimink recently completed his PhD at the University of Alberta before starting a post-doctoral fellowship at the Carnegie Institute for Science in Washington, D.C.

"Zircons lock in not only the age but also other geochemical information that we've exploited in this paper," Reimink continues. "Rocks and zircon together give us much more information than either on their own. Zircon retains its chemical signature and records age information that doesn't get reset by later geological events, while the rock itself records chemical information that the zircon grains don't."

He explains that the chemistry of the rock itself looks like rocks that are forming today in modern Iceland, which is transitional between oceanic and continental crust. In fact, Iceland has been hypothesized as an analog for how continental crusts started to form.

"We examined the rock itself to analyze those signatures to explore the way that the magma intrudes into the surrounding rock." One signature in particular recorded the assimilation step of magma from Earth's crust. "While the magma cooled, it simultaneously heated up and melted the rock around it, and we have evidence for that."

Reimink says that the lack of signatures of continental crust in this rock, different from what the early continents were expected to look like, leads to more questions than answers. Reimink says one of the biggest challenges as a geologist is that as we travel back in time on Earth, the quantity and quality of available evidence decreases. "Earth is constantly recycling itself, the crust is being deformed or melted, and pre-history is being erased," remarks Reimink.

"The presence of continents above water and exposed to the atmosphere has huge implications in atmospheric chemistry and the presence or absence of life. The amount of continents on Earth has a large chemical influence both on processes in the deep Earth (mantle and core) and at the Earth's surface (atmosphere and biosphere). There are constant feedback loops between chemistry and geology. Though there are still a lot of unknowns, this is just one example that everything on Earth is intertwined." "No evidence for Hadean continental crust within Earth's oldest evolved rock unit" appears in the October issue of Nature Geoscience.

發現地球最古老化石的說法挑起眾多研究人員的關注

原始網址：http://www.nature.com/news/claims-of-earth-s-oldest-fossils-tantalize-researchers-1.20506

發現地球最古老化石的說法挑起眾多研究人員的關注

By Alexandra Witze

格陵蘭岩石中的構造可能是由生活在37億年前的生物堆積而成

地質學家說他們挖掘出一些地球生命存在的最古老證據。雖然還尚未接受進一步的驗證，這項發現指出在星球年幼時生命很快就出現了。

在本周的期刊《自然》(Nature)當中，澳洲和英國的研究人員發表說他們從格陵蘭年代為37億年前的岩石中，發現了一種稱作「疊層石」(stromatolite)的層狀構造。疊層石是一種跟花椰菜有幾分相似的地質構造，當微生物抓住沉積物並往上疊而形成一層一層的圓丘時，就會形成疊層石。

但這項研究用到的岩石在數十億年的板塊地殼移動過程中，承受劇烈的擠壓及高溫而成為地球上扭曲變形得最嚴重的岩石之一。這樣的高溫高壓會造成岩石重新結晶，而抹去研究人員通常用來辨識疊層石的細小構造—故這項研究已經激起相當熱烈的爭議。

「在接受這篇研究的結果之前，我有14點需要他們妥善說明的問題及疑問。」西雅圖華盛頓大學的地質生物學家Roger Buick說。

這些岩石採自格陵蘭的 Isua，研究人員一直以來努力在這些岩石中尋找數十億年前有生命存在的可能跡象。過往對這些岩石化學性質的工作，像是1999年關於碳同位素的一篇論文指出這些岩石含有原始生物遺留下來的「生物指標」，但這些說法經過了數年仍然備受爭議。

重見天日的證據

融雪於今日帶來了新的證據。由澳洲臥龍崗大學的地質學家 Allen Nutman領導的研究團隊前往一處長年被雪堆掩蓋，直到最近才因氣溫上升融化而重見天日的岩石露頭。他們在此採集到一塊年代為37億年的岩石，並將它帶回澳洲進行深入研究。

他們在這塊岩石當中發現了他們所稱的疊層石，還有其他指向遠古生命的證據。「不同類型的證據組合在一起使得這個說法十分讓人信服。」團隊成員之一 Martin Van Kranendonk表示。他是澳洲肯辛頓的新南威爾斯大學的地質學家。

據Van Kranendonk 和他的同仁所言，這些構造是高度僅1-4公分的凸起，其外型和內部層狀結構跟古代及現存的疊層石十分相似。從周圍岩石的質地可以看出這些構造是堆積在淺海海底而形成，這跟今日在巴哈馬群島和澳洲西部當地的疊層石形成環境一致。另外這些岩石當中也含有白雲石之類的碳酸鹽礦物，這在近代的疊層石中也很常見。

之前廣為接受的最古老生命證據是澳洲西部皮爾布拉(Pilbara)地區的疊層石群，而格陵蘭構造的年代跟其相比還早了2.2億年。「跟澳洲的岩石相比，這篇研究提出的證據整體來說都薄弱許多。」美國加州帕薩迪納，NASA噴射推進實驗室的天體生物學家Abigail Allwood說。「雖然如此，從之前什麼證據都未能找著的岩石中發現任何事物都是相當驚人的消息。這也是這篇論文值得關注的原因。」

令人起疑的結晶

研究古代疊層石的問題之一便是層狀構造可以經由許多跟生命無關的作用來形成。就像浴缸內側的環狀水垢一樣，礦物沉澱在海床時也可以形成看起來像是疊層石，實際上卻不然的層狀構造。

「這些構造頂多只能被分類成『假疊層石』(pseudostromatolite)。」澳洲伯斯的古生物顧問專家，研究古代疊層石已有多年時間的Kathleen Grey表示。「令人遺憾的是，我不認為這項證據足以支持這麼重大的主張。」

美國劍橋市麻省理工學院的地質生物學家Tanja Bosak補充，她想看看據稱為疊層石的這些結構內部或周圍是否有少量的有機物。比對這些岩石中不同類型的碳可能有助於顯示這些結構是否由生物形成。

Allwood強調說，至少格陵蘭的岩石能幫助天體生物學家準備如何研究有史以來第一批從火星帶回來的樣品，此為NASA預計於2020年執行的任務內容之一。這項新發表的疊層石研究或許可以當作測試案例，來讓科學家討論出要擁有何種條件才能成為令人信服的遠古生命證據。

「如果我們在火星上找到像這樣的東西，是否能給它貼上標籤並稱呼其為『生命』？」她提出疑問。「我不認為我們可以這麼做。」

原始論文：Allen P. Nutman, Vickie C. Bennett, Clark R. L. Friend, Martin J. Van Kranendonk, Allan R. Chivas. Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures. Nature, 2016

Claims of Earth's oldest fossils tantalize researchers

Rocks in Greenland contain structures that could have been laid down by living organisms 3.7 billion years ago.

Geologists say that they have unearthed some of the oldest known evidence for life on Earth. The discovery, yet to be confirmed, suggests that life arose quickly on the young planet.

In this week's Nature, Australian and British researchers report finding layered structures called stromatolites in 3.7-billion-year-old rocks from Greenland1. Stromatolites, which look a bit like geological cauliflowers, form when microbes trap sediment and build up layer after dome-shaped layer.

But the discovery involves some of the most physically tortured rocks on Earth, which have been squeezed and heated over billions of years as crustal plates shifted. The pressure and heat recrystallizes the rocks, erasing much of the fine-scale detail that researchers normally use to identify fossilized stromatolites — so the work is already triggering heated debate.

“I’ve got 14 queries and problems that need addressing before I’ll believe it,” says Roger Buick, a geobiologist at the University of Washington in Seattle.

The rocks hail from Isua, Greenland, where researchers have laboured to tease out potential signs of life dating back billions of years. Previous work on the rocks’ chemistry, such as a 1999 paper analysing carbon isotopes2, suggested that they contain ‘biomarker’ traces of early organisms. But various claims over the years have remained contentious.

Unearthing evidence

Now, melting snow has revealed new clues. A team led by Allen Nutman, a geologist at the University of Wollongong in Australia, visited a rock outcrop that had been buried under a perennial snow patch until warmer temperatures melted it away. They sawed out a chunk of 3.7-billion-year-old rock and took it back to Australia to study.

In it they found the purported stromatolites, along with other clues to ancient life. “It’s a combination of different types of evidence that makes the story so compelling,” says team member Martin Van Kranendonk, a geologist at the University of New South Wales in Kensington, Australia.

The structures are tiny bumps, just 1–4 centimetres tall, whose shape and internal layering strongly resemble ancient and modern stromatolites, Van Kranendonk and his colleagues say. The texture of the surrounding rocks suggests that they were laid down at the bottom of a shallow sea, much as stromatolites are today in places such as the Bahamas and western Australia. And the rocks contain carbonate minerals such as dolomite, which are also common in younger stromatolites.

The Greenland structures are about 220 million years older than the oldest widely accepted evidence for life, a set of stromatolites from the Pilbara region of western Australia3. “The evidence is a whole lot thinner than the rocks in Australia,” says Abigail Allwood, an astrobiologist at NASA’s Jet Propulsion Laboratory in Pasadena, California. “Having said that, it’s incredible that anything can be found in these rocks that are barely a ghost of what they were before. That’s why it’s worthy of attention.”

Doubts crystallize

Part of the problem with studying ancient stromatolites is that layered structures can form through processes that have nothing to do with life. Minerals precipitating out on the seafloor can leave layers, like rings on a bathtub, that look like stromatolites but aren't4.

“At most, these structures should be classified as pseudostromatolites,” says Kathleen Grey, a consulting palaeontologist in Perth, Australia, who has worked on ancient stromatolites. “Sadly, I don’t feel the evidence is convincing for such an important claim.”

Tanja Bosak, a geobiologist at the Massachusetts Institute of Technology in Cambridge, adds that she would like to see whether the proposed stromatolites have small amounts of organic matter in or near them. Comparing different types of carbon in the rock could help to reveal whether the structures are biological or not.

At a minimum, Allwood notes, the Greenland rocks should help astrobiologists as they prepare for the first ever samples to be returned from Mars, from a NASA mission slated to launch in 2020. The newly reported stromatolites may serve as a test case for scientists to argue about what constitutes convincing evidence of past life.

“If we found something like this on Mars would we stick a flag in it and call it life?” she asks. “I don’t think we would.”

在幼年地球上繁榮發展的生命：科學家發現年代為37億年的化石

原始網址：www.sciencedaily.com/releases/2016/08/160831172441.htm

在幼年地球上繁榮發展的生命：科學家發現年代為37億年的化石

在這項驚人的發現當中，來自澳洲的研究團隊在格陵蘭的偏遠地區發現了地球最古老的化石。這讓我們得知地球最久遠之前的歷史片段，顯示地球尚處年幼時生命便已經迅速出現在這顆星球上。

由澳洲臥龍崗大學的教授 Allen Nutman主持的研究團隊，在全世界最古老的沉積岩中發現了年代為37億年的疊層石(stromatolite)化石。此沉積岩分佈於格陵蘭冰蓋邊緣的Isua綠岩帶中。

刊登於期刊《自然》(Nature)上的論文中大略描述了這項發現，共同作者包括了澳洲國立大學的副教授Vickie Bennett、新南威爾斯大學的教授 Martin Van Kranendonk以及臥龍崗大學的教授 Allan Chivas。

Isua疊層石化石的發現讓我們對地球早期生命的多樣化程度有更長足的了解，研究人員說這也有助於我們認識火星生命的樣貌。臥龍崗大學地球與環境科學院的教授 Nutman說， 相較於之前發現於澳洲西部的最古老疊層石化石，Isua疊層石化石的年代還要早了2.2億年。

這項發現把化石紀錄出現的時間點回推至地球地質紀錄的原點附近，表示在地球歷史相當初期時就已經有生命跡象出現在地球上。 Isua疊層石位於長期積雪區近期因融化而露出的地帶。由於它們原先是在淺海地帶堆積而成，這成為了早期生命於何種環境繁榮生長的初步證據。

在地球歷史上很長一段時間，生命只不過是些單細胞生物。疊層石便是由這些微生物群落建構而成的碳酸鹽土丘。

「疊層石的重要之處不只是呈現出一種用肉眼就能見到的遠古生命證據，它還是一種複雜的生態系統。」Nutman教授說。

「這表示遠在37億年前微生物就已經發展出相當的多樣性了。發展出如此的生命多樣性顯示生命在地球誕生後的數億年內便已經出現了，這跟生物學家計算生物的基因編碼具有相當古老歷史的結果一致。」

共同研究人員，國立澳洲大學的副教授Vickie Bennett說這項研究給予了有關地球歷史的新觀點。

「這項發現徹底改變了對行星適居性的研究。」副教授Bennett 說。

「這是我們首度擁有維持早期生命的環境條件為何的岩石紀錄，而不僅只是臆測遠古環境的可能樣貌。我們的研究將會為幼年行星的化學循環和岩石-水-微生物之間的關係帶出最新觀點。」

新南威爾斯大學澳洲天體生物學中心(Nutman教授為準會員)的主任，Martin Van Kranendonk教授說這項突破性的發現指出37億年前火星的環境仍然濕潤時，其上也可能會有類似的生命結構。

「格陵蘭最新出露的露頭中含有的構造及地球化學性質特徵，在較年輕的岩石中皆可以用來指示其是否為生物來源。」 Van Kranendonk教授說。

「這項發現為地球生物留存下來的最古老證據設立了新的基準。它指出生命在地球上其實出現地相當迅速，也支持在火星上同樣古老的岩石中尋找生命是可行的。」

這項研究由澳洲科學團隊與英國研究人員合作來進行，並由澳洲國科會提供資金。

Life thrived on young Earth: scientists discover 3.7-billion-year-old fossils

In an extraordinary find, a team of Australian researchers have uncovered the world's oldest fossils in a remote area of Greenland, capturing the earliest history of the planet and demonstrating that life on Earth emerged rapidly in the planet's early years.

Led by the University of Wollongong's (UOW) Professor Allen Nutman, the team discovered 3.7-billion-year-old stromatolite fossils in the world's oldest sedimentary rocks, in the Isua Greenstone Belt along the edge of Greenland's icecap.

The findings are outlined in a study published in Nature, with co-authors Associate Professor Vickie Bennett from The Australian National University (ANU), the University of New South Wales' (UNSW) Professor Martin Van Kranendonk, and Professor Allan Chivas, from UOW.

The discovery of the Isua stromatolite fossils provides a greater understanding of early diversity of life on Earth and researchers said could have implications for our understanding of life on Mars. Professor Nutman, from UOW's School of Earth and Environmental Sciences, said the Isua stromatolite fossils predated the world's previous oldest stromatolite fossils -- which were found in Western Australia -- by 220 million years.

The discovery pushes back the fossil record to near the start of Earth's geological record and points to evidence of life on Earth very early in its history. The Isua stromatolites, which were exposed by the recent melting of a perennial snow patch, were laid down in shallow sea, providing the first evidence of an environment in which early life thrived.

For much of Earth's history, life was just single cells, and stromatolite fossils are mounds of carbonate constructed by these communities of microbes.

"The significance of stromatolites is that not only do they provide obvious evidence of ancient life that is visible with the naked eye, but that they are complex ecosystems," Professor Nutman said.

"This indicates that as long as 3.7 billion years ago microbial life was already diverse. This diversity shows that life emerged within the first few hundred millions years of Earth's existence, which is in keeping with biologists' calculations showing the great antiquity of life's genetic code."

Co-lead investigator Associate Professor Vickie Bennett, from ANU, said this study provided a new perspective into the history of Earth.

"This discovery turns the study of planetary habitability on its head," Associate Professor Bennett said.

"Rather than speculating about potential early environments, for the first time we have rocks that we know record the conditions and environments that sustained early life. Our research will provide new insights into chemical cycles and rock-water-microbe interactions on a young planet."

Professor Martin Van Kranendonk, Director of the Australian Centre for Astrobiology at UNSW, of which Professor Nutman is also an Associate Member, said it was a groundbreaking find that could point to similar life structures on Mars, which 3.7 billion years ago was a damp environment.

"The structures and geochemistry from newly exposed outcrops in Greenland display all of the features used in younger rocks to argue for a biological origin," Professor Van Kranendonk said.

"This discovery represents a new benchmark for the oldest preserved evidence of life on Earth. It points to a rapid emergence of life on Earth and supports the search for life in similarly ancient rocks on Mars."

The investigation, conducted by the Australian science team in collaboration with a UK partner, was funded by a grant from the Australian Research Council.

引用自：University of Wollongong. "Life thrived on young Earth: scientists discover 3.7-billion-year-old fossils: Remarkable find by team of Australian researchers points to earliest existence of diverse life on Earth." ScienceDaily. ScienceDaily, 31 August 2016. 

原始論文：Allen P. Nutman, Vickie C. Bennett, Clark R. L. Friend, Martin J. Van Kranendonk, Allan R. Chivas. Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures. Nature, 2016; DOI:10.1038/nature19355