經常用來追溯地球氧含量歷史演變的工具可能會給出偽陽性結果

原文網址：http://www.news.gatech.edu/2017/11/17/popular-tool-trace-earths-oxygen-history-can-give-false-positives

經常用來追溯地球氧含量歷史演變的工具可能會給出偽陽性結果

對於追尋地球大氣中的氧氣最初如何演變的研究人員來說，一項新研究可能會讓他們失望地大喊「真的嗎？」。根據此研究，一種檢驗古代岩層以得到氧氣含量的現行工具會產生偽陽性結果，而這種隨機性可能會讓科學家誤以為他們得到的發現十分驚人。

研究人員時常利用一種稱作鉻同位素系統的化學示蹤劑來檢驗沉積岩層，以得到岩石形成時大氣氧含量的相關線索，但一種稱作配位基的常見分子卻會使結果產生偏差。喬治亞理工學院的研究人員在實驗室證明許多種配位基產生的訊號跟氧分子產生的十分相似。

研究主要作者之一Chris Reinhard表示：「在某些地理位置和古代環境所測得的訊號值，產生原因可能跟周遭的氧氣多寡沒有任何關係。」雖然近期某些發現所用的估算方式可能會受此新研究影響，但不表示這項工具變得全然無用。

岩石的紀錄工具

「我們並非在嘗試徹底顛覆人們對這項工具的評價。」同為研究主要作者的Yuanzhi Tang表示。「我們是在瞭解它的潛在限制而讓它在特定情況下可以當作具有鑑別力的工具。」

Tang和Reinhard皆為喬治亞理工大學地球和大氣科學院的生物地球化學助理教授，他們團隊的研究成果刊登在2017年11月17日期刊《自然通訊》(Nature Communications)的論文之中。資助他們研究的單位包括NASA天體生物學研究所、NASA地外生物學計畫以及艾古隆研究所。

「從全球層級來看，鉻同位素系統仍然是指示各個年代大氣氧含量的良好指標。」Tang表示。「而我們在實驗室呈現的議題則攸關局部地區的個別樣本，特別是在大氣氧含量還很低的年代所形成的岩石。」

活躍的配位基

研究人員從和鉻有關的化學反應中證實若環境中的氧氣不多，配位基可能會取代氧氣成為相當活躍的反應物。因為配位基這種化學族的特徵跟氧氣類似，非常容易吸引電子對。

就像跟氧氣發生反應一樣，類似鉻的金屬與配位基發生反應之後可以更容易地遷移到世界各處。研究人員在此研究中的關注重點是有機配位基，也就是含有碳的配位基。

鉻的遷移能力會在沉積岩中留下訊號，成為今日科學家探討古代大氣氧含量時用的指標。而此篇研究的作者即著重於比較氧氣和有機配位基對鉻遷移能力的影響。

接著是鉻同位素系統運作方法的大略介紹，然後是有機配位基如何產生偽陽性結果。

載運鉻的高速列車

在地球這座巨大的化學實驗室中進行的化學反應涵蓋了各種環境條件，從極地的嚴寒至火山的熾熱，從海洋深處的極大壓力至大氣上層的毫無壓力。風吹水流像是繁忙而紊亂的輸送帶將不同物質帶到世界各處，其中有些會落腳在沉積物當中並在之後轉變成岩石。

鉻要搭上通往沉積物的高速列車通常需要氧化劑這張票卷，它可以提高鉻的溶解程度使得鉻更容易跟著水流移動，而大氣中的氧氣即為理想氧化劑。此研究提及的反應有點像是替含鉻化合物裝上浮筒一樣，而可以把氧轉手給鉻的錳氧化物也參與其中。

地球最初數十億年大氣處於幾乎無氧的狀態，但氧氣開始增加之後氧氣就變成了主要的氧化劑，尤其是在最近8億年。因此，古代岩層中含鉻沉積物的性質也成為當時大氣含有多少氧氣的重要指標。

今日的研究人員測量遠古岩層樣品中兩種鉻同位素的關係，以參透氧氣在整部地質歷史中的蹤跡。這兩種同位素分別是⁵²Cr(目前世上最多的鉻同位素)和⁵³Cr。

「將岩石磨碎後用酸溶解，接著就能用質譜儀測量樣品中⁵²Cr和⁵³Cr的比例。」Reinhard表示。「我們關注的便是它們之間的比例關係，此數值會受控於許多複雜的作用；但整體來說，海洋沉積岩中的⁵³Cr含量升高通常意謂大氣出現了氧氣。」

順道一提，這些鉻同位素為穩定同位素，不會發生放射性衰變。故此系統的運作方式和依據碳14衰變的放射性碳定年法不同。

化學裡的騙徒

Tang的團隊在實驗室利用少數幾種類型的有機配位基，顯示鉻和配位基反應過後產生的⁵³Cr/⁵²Cr訊號跟源自於氧氣―鉻化學反應的訊號十分相像。

Tang表示：「配位基同樣具有讓鉻遷移的能力。事實上，在某些岩石紀錄中，配位基或許會是操控鉻同位素訊號的重要因子。」

在地球大氣充滿氧氣許久之前，有機配位基可能就已經出現在世上許多地方了。而在化學反應發生過後數億年的今天，我們基本上是無法得知當時作用的究竟是氧氣還是配位基。

毫微之差

如果沒有考慮到配位基參與的反應，可能會因此誤判岩石紀錄中有關大氣氧含量的微小細節。事實上，這可能已經發生了。

就像分門別類古代動物骸骨和其他化石的古生物學家，地質學家也保有數量龐大且數位化的岩石檔案庫，並研究它們以更加瞭解地球古老的地質歷史。大約2009年開始，科學家開始以鉻同位素系統檢驗這些岩石的原始樣品，並將結果納入文獻紀錄當中。

「之後，某些歧異開始浮現出來。」Reinhard表示。「在氧氣還不該出現的時候，古代的土壤層卻呈現出氧氣存在的證據，而同一時期的其他樣品卻沒有出現該訊號。」

但面對這類特殊鉻訊號的某些研究人員卻認為他們可能偶然之間得到了基礎層面的發現，並推論出一套解釋認為當氧分子在全球各處還很稀少的時候，形成這些特殊岩層的局部點位可能具有含量十分驚人的氧氣。另外一些人則思索在遍及全球的證據出現許久之前，大氣氧濃度或許就已經開始攀升。

Reinhard表示：「這些證據記下的可能是其他化學作用，而非氧氣造成的反應。」

此研究或許警惕了我們要審慎看待鉻同位素的紀錄，特別是當它們看起來格外誘人的時候。

A popular tool to trace Earth’s oxygen history can give false positives

For researchers pursuing the primordial history of oxygen in Earth’s atmosphere, a new study might sour some “Eureka!” moments. A contemporary tool used to trace oxygen by examining ancient rock strata can produce false positives, according to the study, and the wayward results can mask as exhilarating discoveries.

Common molecules called ligands can bias the results of a popular chemical tracer called the chromium (Cr) isotope system, which is used to test sedimentary rock layers for clues about atmospheric oxygen levels during the epoch when the rock formed. Researchers at the Georgia Institute of Technology have demonstrated in the lab that many ligands could have created a signal very similar to that of molecular oxygen.

“There are some geographical locations and ancient situations where measurable signals could have been generated that had nothing to do with how much oxygen was around,” said Chris Reinhard, one of the study’s lead authors. Though the new research may impact how some recent findings are assessed, that doesn’t mean the tool isn’t useful overall.

Rock record tool

“We’re not trying to revolutionize the way the tool is viewed,” said Yuanzhi Tang, who co-led the study. “This is about understanding its possible limitations to make discerning use of it in particular cases.”

Tang and Reinhard, both assistant professors of biogeochemistry in Georgia Tech’s School of Earth and Atmospheric Sciences, published their team’s results in a study on November 17, 2017, in the journal Nature Communications. Their work was funded by the NASA Astrobiology Institute, the NASA Exobiology program, and the Agouron Institute.

“On a global level, the chromium isotope system is still a great indicator of atmospheric oxygen levels through the ages,” Tang said. “The issue we exposed in the lab is more local with isolated samples, especially during eras when there wasn’t much atmospheric oxygen.”

Leaping ligands

Without a dominant oxygen presence, ligands likely made a great reactive substitute, as the researchers demonstrated in reactions with chromium. Like oxygen, ligands strongly attract electron pairs, which is what characterizes them as a chemical group.

And like reactions with oxygen, reactions with ligands enable metals like chromium to move around more easily in the world. In this case, the researchers were interested in organic ligands, ligands that contain carbon. 

They were more apt to match oxygen’s mobility effect on chromium that made it end up as the signals in sedimentary rock that scientists, today, look for as a sign of ancient atmospheric oxygen.

Here’s roughly how the chromium isotope system works, followed by how organic ligands could make for false positives.

Chromium rollercoaster

The Earth is an enormous chemical laboratory performing reactions in conditions varying from arctic cold to volcanic heat, and from crushing ocean depths to no-pressure upper atmospheres. Winds and waves sweep around materials like turbulent conveyor belts, depositing some in sediments that later turn to stone.

Chromium’s ticket for the rollercoaster ride into sedimentary rock was usually an oxidizing agent that made it more soluble and better able to float, and atmospheric oxygen was an ideal oxidizer. The chemical reaction, which can be found in the study and involved manganese oxide handing off oxygens to chromium, would be a little like adding pontoons to chromium compounds.

For billions of years, Earth’s atmosphere was nearly devoid of O, but after oxygen began increasing, especially in the last 800 million years,  it became the domineering oxidizer. And characteristics of chromium deposits in ancient layers of rock became a great indicator of how much O₂ was in the atmosphere.

Today, researchers test deep rock layer samples for the relation between two chromium isotopes, ⁵²Cr, by far the most common Cr isotope, and ⁵³Cr, to get a read on oxygen presence across geological eras.

“You powder the rock up; you dissolve it with acid, and then you measure the ratio of ⁵³Cr to ⁵²Cr in the material by using mass spectrometry,” Reinhard said. “It’s the ratio that matters, and it will be controlled by a range of complex processes, but generally speaking, elevated ⁵³Cr in ocean sediment rock tends to indicate oxygen in the atmosphere.”

By the way, these Cr isotopes are stable and don’t undergo radioactive decay, thus the system does not work the way radiocarbon dating does, which relies on the decay of carbon 14.

Chemical imposter

In the lab, with a small assortment of organic ligands, Tang’s group showed that reactions of chromium with ligands led to ⁵³Cr/⁵²Cr signals that closely mimicked those stemming from oxygen-chromium reactions.

“Ligands have the capability to mobilize chromium as well,” Tang said. “In fact, ligands might be a significant factor in controlling chromium isotope signals in certain rock records.”

Organic ligands were probably around long before Earth’s atmosphere filled up with O₂. And today, hundreds of millions of years after the reactions occurred, it’s basically impossible to find out if oxygen or ligands were at work.

Little discrepancies

If not accounted for, ligand reactions can distort small details in rock records about atmospheric oxygen, and they may have already.

Like paleontologists, who catalog ancient animal bones and other fossils, geologists keep massive, digitized archives of rock that they study to learn more about Earth’s ancient geological history. Scientists began testing physical samples of them with the Cr isotope system around 2009 and adding the results to the records.

“Since then, some discrepancies have turned up,” Reinhard said. “Ancient soil layers were showing evidence of oxygen when it probably shouldn’t have been there. Other samples from the same period weren’t showing that signal.”

But some researchers confronted with odd Cr signals have thought they had perhaps stumbled upon a radical find, and they developed explanations for how O2 may have been surprisingly bountiful on the lonesome spot where a particular rock layer formed, while molecular oxygen was scant on the rest of the globe. Others puzzled that atmospheric O₂ levels may have risen much earlier than overwhelmingly broad evidence has indicated.

“A lot of that could be chalked up to other chemical processes and not to interactions with oxygen,” Reinhard said.

The study may serve as a cautionary tale about how to view Cr isotope data, especially when they leap off the page. 

原始論文：Emily M. Saad, Xiangli Wang, Noah J. Planavsky, Christopher T. Reinhard, Yuanzhi Tang. Redox-independent chromium isotope fractionation induced by ligand-promoted dissolution. Nature Communications, 2017; 8 (1) DOI: 10.1038/s41467-017-01694-y

引用自：Georgia Institute of Technology. "A popular tool to trace Earth's oxygen history can give false positives." 

在地球深處相遇的水和鐵或許孕育了生命誕生的條件

原文網址：https://carnegiescience.edu/node/2262

在地球深處相遇的水和鐵或許孕育了生命誕生的條件

根據最近一組國際團隊刊登在《國家科學評論》(National Science Review)的研究，地核和地函邊界儲有大量氧的鐵，或許對地球歷史上的事件具有重大影響，像是超大陸分裂、地球大氣組成的劇烈改變、以及生命的誕生。

這組團隊包括了美國卡內基科學研究院、史丹佛大學、芝加哥大學，以及中國北京高壓科學研究中心的科學家。他們詳細探討在地球核幔邊界的極端溫度壓力下水和鐵的化學性質。

當板塊構造運動將含水礦物拖曳至地球深處而接觸鐵質地核，此處的極端環境會讓鐵抓住水分子中的氧原子，並將氫原子釋放出來。之後氫會往地表逸出，但氧則會困在二氧化鐵――這種只能在如此極端的高溫高壓下存在的化合物的結晶構造中。

結合理論計算結果以及重現核幔邊界環境的實驗室試驗，團隊確定利用鑽石高壓砧將材料暴露在介於平常大氣壓力950至100萬倍的壓力，以及超過1900℃的溫度下可以形成二氧化鐵。

主要作者Ho-kwang “Dave” Mao表示：「板塊構造運動會將隱沒板塊拖進地球內部深處。基於我們對隱沒板塊化學組成的認識，我們認為每年會有3億噸的水被帶到地球深處跟地核的鐵接觸，並產生大量二氧化鐵。」

這類極度富含氧的固態岩石可能會年復一年地持續堆積在地核上方，逐漸成長為陸塊大小的巨大岩體。如果有地質事件加熱這些二氧化鐵岩體，就可能會引發大規模的噴發事件，瞬間將大量氧氣釋放到地球表面。

作者提出的假說認為這類氧氣爆發事件可以將巨量氧氣釋放到地球大氣當中――多到足以使大氧化事件(Great Oxygenation Event)發生。發生於25億年前的大氧化事件使得地球大氣充滿氧氣，而讓我們所知以氧氣維生的生物就此崛起。

「這項新發現水在高溫高壓下的裂解反應，對地球內部深處到大氣的地球化學皆有影響。」Mao表示。「(它讓)我們需要重新審視過往發表的許多理論。」

When water met iron deep inside the earth, it might have created conditions for life

Reservoirs of oxygen-rich iron between the Earth’s core and mantle could have played a major role in Earth’s history, including the breakup of supercontinents, drastic changes in Earth’s atmospheric makeup, and the creation of life, according to recent work from an international research team published in National Science Review.

The team—which includes scientists from Carnegie, Stanford University, the Center for High Pressure Science and Technology Advanced Research in China, and the University of Chicago—probed the chemistry of iron and water under the extreme temperatures and pressures of the Earth’s core-mantle boundary.

When the action of plate tectonics draws water-containing minerals down deep enough to meet the Earth’s iron core, the extreme conditions cause the iron to grab oxygen atoms from the water molecules and set the hydrogen atoms free. The hydrogen escapes to the surface, but the oxygen gets trapped into crystalline iron dioxide, which can only exist under such intense pressures and temperatures.

Using theoretical calculations as well as laboratory experiments to recreate the environment of the core-mantle boundary, the team determined that iron dioxide can be created using a laser-heated diamond anvil cell to put materials under between about 950 and 1 million times normal atmospheric pressure and more than 3,500 degrees Fahrenheit.

“Based on our knowledge of the chemical makeup of the slabs that are drawn into the Earth’s deep interior by plate tectonics, we think 300 million tons of water could be carried down to meet iron in the core and generate massive iron dioxide rocks each year,” said lead author Ho-kwang “Dave” Mao.

These extremely oxygen-rich solid rocks may accumulate steadily year-by-year above the core, growing into gigantic, continent-like sizes. A geological event that heated up these iron dioxide rocks could cause a massive eruption, suddenly releasing a great deal of oxygen to the surface.

The authors hypothesize that such an oxygen explosion could put a tremendous amount of the gas into the Earth’s atmosphere—enough to cause the so-called Great Oxygenation Event, which occurred about 2.5 billion years ago and created our oxygen-rich atmosphere, conditions that kickstarted the rise oxygen-dependent life as we know it.

 “This newly discovered high-temperature and intense-pressure water-splitting reaction affects geochemistry from the deep interior to the atmosphere” said Mao. “Many previous theories need to be re-examined now.

原始論文：Ho-Kwang Mao, Qingyang Hu, Liuxiang Yang, Jin Liu, Duck Young Kim, Yue Meng, Li Zhang, Vitali B. Prakapenka, Wenge Yang, Wendy L. Mao. When water meets iron at Earth's core–mantle boundary. National Science Review, 2017; DOI: 10.1093/nsr/nwx109

引用自：Carnegie Institution for Science. " When water met iron deep inside the earth, it might have created conditions for life."

深海「陰影帶」如何困住世上最古老的海水

原文網址：http://www.su.se/english/research/research-news/how-a-shadow-zone-traps-the-world-s-oldest-ocean-water-1.356556

深海「陰影帶」如何困住世上最古老的海水

由一組國際團隊進行的新研究闡明世上最古老的海水，為何會滯留在北太平洋深度2公里處的陰影帶超過一千多年。

具體來說，這團海水最後一次跟空氣接觸時哥德人才剛入侵西羅馬帝國(大約為西元五世紀末，可對應至東晉末年)。研究提出海床的形狀對垂直流動造成的影響決定了古代海水會停留在海底多久。

「我們早就利用碳14定年得知世上最古老的海水位在北太平洋深處。但我們到現在仍難以理解為何這些相當古老的海水會聚集在深度大約2公里的位置。」主要作者，南威爾斯大學的Casimir de Lavergne博士表示。「我們發現印度洋和太平洋表面下方2公里處有個『陰影帶』，由於此處海水幾乎沒有任何垂直流動使得它們停留在此長達數個世紀。」

在海洋深度2.5公里以下的地方，崎嶇的海底地形和地熱供給之處使得水流湧升；而接近表面的海洋淺處則有受風力驅動的洋流；在此之間即為幾乎停滯的海水所處的陰影帶。本篇研究發表之前，模擬深海海流循環的模型並未將海床對底層水流動的限制考慮得相當周全。研究人員把它納入參數詳加模擬之後，他們發現底層水無法湧升至超過海洋表面下方2.5公里深的位置，使得深度2.5公里以上的地區直接跟其他地方孤立開來。雖然研究解開了這道謎題中的一部份，但他們的結果或許還具有更多意義。

「當此孤立的陰影帶困住海水數千年之久時，也困住了營養鹽和碳。這些物質對海洋的生產力有直接影響，進而可以從數百年的時間尺度上來調節氣候。」共同作者，斯德哥爾摩大學氣象學系的研究員Fabien Roquet博士表示。

這篇文章刊登於科學期刊《自然》(Nature)，題名為「Abyssal ocean overturning shaped by seafloor distribution」。

How a “shadow zone” traps the world’s oldest ocean water

New research from an international team has revealed why the oldest water in the ocean in the North Pacific has remained trapped in a shadow zone around 2km below the sea surface for over 1000 years.

To put it in context, the last time this water encountered the atmosphere the Goths had just invaded the Western Roman Empire. The research suggests the time the ancient water spent below the surface is a consequence of the shape of the ocean floor and its impact on vertical circulation.

 “Carbon-14 dating had already told us the most ancient water lied in the deep North Pacific. But until now we had struggled to understand why the very oldest waters huddle around the depth of 2km,” said lead author from the University of New South Wales, Dr Casimir de Lavergne. “What we have found is that at around 2km below the surface of the Indian and Pacific Oceans there is a ‘shadow zone’ with barely any vertical movement that suspends ocean water in an area for centuries.

The shadow zone is an area of almost stagnant water sitting between the rising currents caused by the rough topography and geothermal heat sources below 2.5km and the shallower wind driven currents closer to the surface. Before this research, models of deep ocean circulation did not accurately account for the constraint of the ocean floor on bottom waters. Once the researchers precisely factored it in they found the bottom water can not rise above 2.5km below the surface, leaving the region directly above isolated. While the researchers have unlocked one part of the puzzle their results also have the potential to tell us much more.

 “When this isolated shadow zone traps millennia old ocean water it also traps nutrients and carbon which have a direct impact on the capacity of the ocean to modify climate over centennial time scales,” said fellow author from Stockholm University, Dr Fabien Roquet, researcher at the Department of Meteorology.

The article "Abyssal ocean overturning shaped by seafloor distribution" is published in the scientific journal Nature.

原始論文：C. de Lavergne, G. Madec, F. Roquet, R. M. Holmes, T. J. McDougall. Abyssal ocean overturning shaped by seafloor distribution. Nature, 2017; 551 (7679): 181 DOI: 10.1038/nature24472

引用自：Stockholm University. "How a 'shadow zone' traps the world's oldest ocean water." 

恐龍教科書要改寫了嗎？言之甚早！

原文網址：http://www.bristol.ac.uk/news/2017/november/dinosaur-textbooks.html

恐龍教科書要改寫了嗎？言之甚早！

恐龍的分類看似太過艱澀，除了專家以外大概沒人會有興趣。

然而，實情絕非如此。最近劍橋大學的Matthew Baron和其同事對我們所知的恐龍主要支系關係，提出了相當激進的修訂版本，但今日刊出的一則評論文章也提出在我們重寫教科書之前要注意的事項。

每個孩童都學過恐龍可以分成兩大類：鳥臀目(像是劍龍、三角龍、禽龍和牠們的親戚)與蜥臀目(包括身為掠食者的獸腳亞目，像是霸王龍；以及具有長頸的蜥腳亞目，像是為人熟知的梁龍)。

Baron和他的同事提出了一種非常不同的分類。他們將鳥臀目和獸腳亞目合在一起成為新的類群，稱為Ornithoscelida；而蜥腳亞目則獨立成一類。

他們的證據看起來相當有說服力。他們辨識出18項僅由鳥臀目和獸腳亞目共享的特徵，並以此當作牠們擁有相同祖先的證據。

由研究早期恐龍的專家所組成的學會，近日重新評估Baron等人提出的數據能否支持他們的主張。此學會由巴西聖保羅大學的Max Langer領導，成員包括來自阿根廷、巴西、德國、英國和西班牙的專家。

他們今日發表在期刊《自然》(Nature)的結果，顯示要改寫恐龍教科書恐怕仍言之甚早。

作者在他們新的評估結果中，發現仍有證據支持把恐龍分成鳥臀目和蜥臀目的傳統劃分方式，同時指出儘管這些證據十分薄弱，但是以Ornithoscelida作為替代方案的可行性卻還要更低一些。

Max Langer表示：「我們學會花費許多心力親自檢查所有陸塊上許多種類的恐龍，以確定我們對牠們特徵的編纂方式正確無誤。」

「起初我們以為只會對Ornithoscelida這個想法提出些許質疑，但現在我會說必須要非常仔細地重新審視整個問題。」

Baron和其同事認為他們的想法也證明恐龍可能是源自於北半球，但重新分析之後證明了長久以來的看法：恐龍最有可能誕生於南半球，或許是位在南美洲。

布里斯托大學地球科學院的教授Mike Benton是重審學會的一員，他補充：「在科學上，若你想要推翻傳統觀點，就需要相當強力的證據才行。」

「我們在恐龍演化樹底端發現的證據對兩種可能分類方式都各有相當的支持。雖然Baron和其同僚有可能是對的，但我們仍主張應該暫時堅持傳統的鳥臀目―蜥臀目二分法，直到出現更有說服力的證據。」

同為重審學會的一員，ARAID的高級研究員暨曼徹斯特大學的名譽高級研究員Fabien Knoll也表示：「Baron和其同僚建構出的提案雖然驚人；然而，正如所有新的科學假說一樣，都必須要先經過科學家同儕的嚴格審視才行。」

「對這些證據提出的最合理解釋唯有評估支持和反對兩方的論述之後，最終才能決定要採用或者是揚棄這些解釋。我們的分析結果僅是重審過程中的第一步，顯示早期恐龍分類關係的主要理論沒有任何一方顯得特別牢靠。」

愛丁堡大學的Steve Brusatte是重審學會的一員，他說：「我們之前以為今年此時就已經可以得到恐龍演化樹的樣貌。」

「但目前我們還是無法確定這三種主要恐龍類群彼此之間的關係到底為何。某種程度上來說這很令人挫敗，但另一方面卻讓我們感到十分振奮，因為這代表我們需要繼續尋找新的化石來解開這道謎題。」

Time to rewrite the dinosaur textbooks? Not quite yet!

The classification of the dinosaurs might seem to be too obscure to excite anyone but the specialists.

However, this is not at all the case. Recently, Matthew Baron and colleagues from the University of Cambridge proposed a radical revision to our understanding of the major branches of dinosaurs, but in a critique published today some caution is proposed before we rewrite the textbooks.

Every child learns that dinosaurs fall into two major groups, the Ornithischia (bird-hipped dinosaurs; Stegosaurus, Triceratops, Iguanodon and their kin) and the Saurischia (lizard-hipped dinosaurs; the predatory theropods, such as Tyrannosaurus, and the long-necked sauropodomorphs, including such well-known forms as Diplodocus).

Baron and colleagues proposed a very different split, pairing the Ornithischia with the Theropoda, terming the new group the Ornithoscelida, and leaving the Sauropodomorpha on its own.

Their evidence seemed overwhelming, since they identified at least 18 unique characters shared by ornithischians and theropods, and used these as evidence that the two groups had shared a common ancestor.

An international consortium of specialists in early dinosaurs, led by Max Langer from the Universidade de São Paulo, Brazil, and including experts from Argentina, Brazil, Germany, Great Britain, and Spain has now re-evaluated the data provided by Baron et al. in support of their claim.

Their results, presented today in the journal Nature, show that it might still be too early to re-write the textbooks for dinosaurs.

In this new evaluation, the authors found support for the traditional model of an Ornithischia-Saurischia split of Dinosauria, but also noted that this support was very weak, and the alternative idea of Ornithoscelida is only slightly less likely.

Max Langer said: "This took a great deal of work by our consortium, checking many dinosaurs on all continents first-hand to make sure we coded their characters correctly.

"We thought at the start we might only cast some doubt on the idea of Ornithoscelida, but I'd say the whole question now has to be looked at again very carefully."

Baron and colleagues believed their data suggested that dinosaurs might have originated in the northern hemisphere, but the re-analysis confirms the long-held view that the most likely site of origin is the southern hemisphere, and probably South America.

Professor Mike Benton from the University of Bristol’s School of Earth Sciences, a member of the revising consortium, added: "In science, if you wish to overthrow the standard viewpoint, you need strong evidence.

"We found the evidence to be pretty balanced in favour of two possible arrangements at the base of the dinosaurian tree. Baron and colleagues might be correct, but we would argue that we should stick to the orthodox Saurischia–Ornithischia split for the moment until more convincing evidence emerges."

Fabien Knoll, ARAID Senior Researcher and Honorary Senior Research Fellow at the University of Manchester, a member of the revising consortium, added: "Baron and colleagues formulated a stimulating proposition. However, like every new scientific hypothesis, it has to be critically evaluated by the community of scientists.

"It is only by weighing the arguments for and against it that it will eventually be adopted or disregarded as the most reasonable explanation of the evidence. We have just provided the first of these re-examinations, which shows that none of the major hypotheses of early dinosaur relationships is particularly secure."

Steve Brusatte of the University of Edinburgh, a member of the consortium, said: "Up until this year, we thought we had the dinosaur family tree figured out.

"But right now, we just can’t be certain how the three major groups of dinosaurs are related to each other. In one sense it’s frustrating, but in another, it’s exciting because it means that we need to keep finding new fossils to solve this mystery."

原始論文：Max C. Langer, Martín D. Ezcurra, Oliver W. M. Rauhut, Michael J. Benton, Fabien Knoll, Blair W. McPhee, Fernando E. Novas, Diego Pol, Stephen L. Brusatte. Untangling the dinosaur family tree. Nature, 2017; 551 (7678): E1 DOI: 10.1038/nature24011

引用自：University of Bristol. "Time to rewrite the dinosaur textbooks? Not quite yet!."

地核內部的攪動或許刺激了大地震發生

原文網址：http://www.sciencemag.org/news/2017/10/sloshing-earth-s-core-may-spike-major-earthquakes

地核內部的攪動或許刺激了大地震發生

Paul Voosen

世界從未停止轉動，不過它的腳步經常會慢下來。科學家繪製長達數十年的圖表顯示地球每一天的長度會有些微變動：這天多了一毫秒，那天則少了一毫秒。上周於西雅圖舉行的美國地質年會上，兩名地球物理學家主張這些細微的變化可能足以影響大地震發生的時間點――因此或許能用來幫助預測大地震。

據科羅拉多大學(CU)波德分校的Roger Bilham和蒙大拿大學的Rebecca Bendick所言，過去100年內地球變慢的時期，和全球規模7以上的大地震數目增加的時期，兩者之間有驚人的對應關係。這類高峰期的地震比平常多上二到五次，其有用之處在於它是地球開始變慢很久一段時間之後才發生。發表此研究成果的Bilham表示：「令人驚訝的是地球可以在地震發生5年之前就向我們發出預警。」

多數地震學家同意「地震預測」是個相當棘手的議題。對於他們發現的模式可能成因為何，Bilham和Bendick到目前為止也只有模糊且難以證實的想法，但其他研究人員表示此發現確實新穎到令人無法忽視。「他們發現的相關性令人印象深刻，值得深入研究。」同為科羅拉多大學的地質學家Peter Molnar說。

這項研究一開始是在尋找地震發生時間的同步性(synchrony)。許多個別的震盪單元，可以是螢火蟲、心臟肌肉或是節拍器，最後可以因為某種類型的串擾(cross-talk)――也就是共同受到某些影響，使它們的震盪步調變得一致。產生地震的斷層會反覆累積應力並猛烈釋放出來，此特質讓Bendick可以輕易把斷層想成她所描述的「雜音十分眾多，品質相當低劣的震盪器」。她和Bilham深入探討有關數據，利用的資料為過去100年來唯一完整的地震目錄：其中僅包含規模7以上的地震。

在八月刊登於《地球物理通訊》( Geophysical Research Letters )的研究成果中，他們發表了兩種模式：第一種是大地震的發生時間似乎會成群聚集，但空間上並沒有此關係。第二種是大地震的數目似乎每隔32年就會達到高峰。地震也許能透過某種方式互通有無，或者地球因為受到某種外力輕輕擠壓而產生破裂。

研究人員探索這類可以影響全球的力量，最後他們發現此模式跟日長度變化一致。雖然聖嬰現象之類的天氣模式，可以在一年或數年之中讓日長度反覆增減一毫秒；然而，以好幾十年為週期的數毫秒變化，尤其是每三十年左右因地球轉速減緩而形成的低鋒，卻可以跟地震群聚的趨勢完美地對齊。「當然，這看起來有些異想天開。」Bendick表示。但她或許是對的。當日長度以數十年發生變化時，地球磁場也會出現短暫的波動。研究人員認為外核液態鐵的流動方式發生變化或許是這兩種現象的成因，但他們無法確定到底發生了什麼作用，或許是熔融的外核有部分被上方地函給卡住了。這可能會改變液態金屬的流動方式使得磁場發生變化，同時在地函和地核之間傳遞足夠動量而影響到日長度。

地震學家以往不認為深埋在地殼下方2900公里的地核會是地震發生之處，但Bilham在西雅圖的演講中認為它應當如此。他說地核「其實跟我們非常接近。它和我們的距離比這裡(西雅圖)距紐約還近。」

Molnar表示呈現固態的地殼和地函，與液態地核轉速之間的些微差異，或許會轉換成一股作用力，能用某種方式微調地震使它們同步發生。由於地球在赤道的自轉速度為每秒460公尺，考慮到速度如此之快，這樣的想法確實有合理之處。他接著說：「當然，也許這根本就是一派胡言」。不過，加州大學柏克萊分校的地球物理學家Michael Manga表示，呈現出他們兩者之間有某種關聯的證據相當有力。他說：「我一直在研究由季節變化，像是融雪而造成的地震。他(Bilham)呈現的關聯性比我之前看過的好上許多。」

洛杉磯南加州大學的地質學家James Dolan說：「無論如何，我們在5年之內就會知道其對錯，因為地球自轉從4年多前已開始進入另一個減速周期。地球平均一年會有17到20場大型地震，相較於今年目前為止的地震數目比平均還少了四場，從明年開始地球應該就會比平均多發生5場。如果此模式維持不變，那它將會對地震預警重下新的定義。」

Sloshing of Earth’s core may spike major earthquakes

The world doesn’t stop spinning. But every so often, it slows down. For decades, scientists have charted tiny fluctuations in the length of Earth’s day: Gain a millisecond here, lose a millisecond there. Last week at the annual meeting of the Geological Society of America here, two geophysicists argued that these minute changes could be enough to influence the timing of major earthquakes—and potentially help forecast them.

During the past 100 years, Earth’s slowdowns have correlated surprisingly well with periods with a global increase in magnitude-7 and larger earthquakes, according to Roger Bilham of the University of Colorado (CU) in Boulder and Rebecca Bendick at the University of Montana in Missoula. Usefully, the spike, which adds two to five more quakes than typical, happens well after the slow-down begins. “The Earth offers us a 5-years heads up on future earthquakes, which is remarkable,” says Bilham, who presented the work.

Most seismologists agree that earthquake prediction is a minefield. And so far, Bilham and Bendick have only fuzzy, hard-to-test ideas about what might cause the pattern they found. But the finding is too provocative to ignore, other researchers say. “The correlation they’ve found is remarkable, and deserves investigation,” says Peter Molnar, a geologist also at CU.

The research started as a search for synchrony in earthquake timing. Individual oscillators, be they fireflies, heart muscles, or metronomes, can end up vibrating in synchrony as a result of some kind of cross-talk—or some common influence. To Bendick, it didn’t seem a far jump to consider the faults that cause earthquakes, with their cyclical buildup of strain and violent discharge, as “really noisy, really crummy oscillators,” she says. She and Bilham dove into the data, using the only complete earthquake catalog for the past 100 years: magnitude-7 and larger earthquakes.

In work published in August in Geophysical Research Letters they reported two patterns: First, major quakes appeared to cluster in time—although not in space. And second, the number of large earthquakes seemed to peak at 32-year intervals. The earthquakes could be somehow talking to each other, or an external force could be nudging the earth into rupture.

Exploring such global forces, the researchers eventually discovered the match with the length of day. Although weather patterns such as El Nino can drive day length to vary back and forth by a millisecond over a year or more, a periodic, decades-long fluctuation of several milliseconds—in particular, its point of peak slow down about every three decades or so—lined up with the quake trend perfectly. "Of course that seems sort of crazy," Bendick says. But maybe it isn’t. When day length changes over decades, Earth’s magnetic field also develops a temporary ripple. Researchers think slight changes in the flow of the molten iron of the outer core may be responsible for both effects. Just what happens is uncertain—perhaps a bit of the molten outer core sticks to the mantle above. That might change the flow of the liquid metal, altering the magnetic field, and transfer enough momentum between the mantle and the core to affect day length.

Seismologists aren’t used to thinking about the planet’s core, buried 2900 kilometers beneath the crust where quakes happen. But they should, Bilham said during his talk here. The core is “quite close to us. It’s closer than New York from here,” he said.

At the equator, Earth spins 460 meters per second. Given this high velocity, it’s not absurd to think that a slight mismatch in speed between the solid crust and mantle and the liquid core could translate into a force somehow nudging quakes into synchrony, Molnar says. Of course, he adds, “It might be nonsense.” But the evidence for some kind of link is compelling, says geophysicist Michael Manga of the University of California, Berkeley. “I’ve worked on earthquakes triggered by seasonal variation, melting snow. His correlation is much better than what I’m used to seeing.”

One way or another, says James Dolan, a geologist at the University of Southern California in Los Angeles, “we’re going to know in 5 years.” That’s because Earth’s rotation began a periodic slow-down 4-plus years ago. Beginning next year, Earth should expect five more major earthquakes a year than average—between 17 to 20 quakes, compared with the anomalously low four so far this year. If the pattern holds, it will put a new spin on earthquake forecasting.