研究對於地球陸塊的出現原因提供了新的線索

 原文網址：https://www.si.edu/newsdesk/releases/study-presents-new-clues-about-rise-earths-continents

造成陸地出現的性質源自何方？新實驗顯示一種廣為接受的解釋其實不太可能

地球在太陽系的行星中是唯一適合居住的，原因之一是地球擁有陸塊。但驚人的是，對於地球表層這些巨大的裂片以及它們的特殊性質是如何產生這方面，我們卻所知不多。相較於海洋地殼，大陸地殼的鐵含量較低而氧化程度較高的問題，由Elizabeth Cottrell(史密森尼美國國立自然史博物館的岩石研究員暨地質學家)及主要作者Megan Holycross(之前為該博物館彼得•巴克獎學金與美國國家自然基金會獎學金補助的研究員，現為康乃爾大學的助理教授)進行的新研究測試了一項廣為接受的假說之後將其否決，也讓我們對於地殼有更深的理解。大陸地殼的成分中鐵含量稀少是主要的原因，造成地球表面許多地方可以高出海平面而形成陸地，使得今日所見的陸生生物得以出現。

本研究中一次實驗中的顯微鏡照片，裡面包含了玻璃質(橘色)、石榴子石(粉紅色)以及其他微小的礦物晶體。視野大小為410微米寬，跟一粒糖晶差不多。圖片來源：G. Macpherson and E. Cottrell, Smithsonian

地球如何保有磁場？

原文網址：https://carnegiescience.edu/news/how-does-earth-sustain-its-magnetic-field

地球如何保有磁場？

地球核心的化學組成如何塑造地球的歷史並讓地球成為生命可以存在的地方？

太陽風以及來源更遠的宇宙射線裡帶有的游離粒子會對生命造成危害。因此，如果沒有地球磁場使它們偏離地球，我們所知的生命就不可能存在。地球磁場是由外核裡的液態鐵的運動來持續產生，這種現象稱為「地球發電機」。

科學家發現地核會下起「鐵之雪」

原文網址：http://www.jsg.utexas.edu/news/2019/12/scientists-find-iron-snow-in-earths-core/

科學家發現地核會下起「鐵之雪」

地球內核的溫度相當熾熱且壓力十分巨大――而且還有雪堆覆蓋。這項新的研究成果有助於科學家更加了解影響整個地球的作用力。

上圖為新研究提出的地球內部簡圖。黑色和白色的地方代表含有鐵晶體，像是泥漿一般的層位。這道黏稠的層位在外核的部分(白色)會形成鐵晶體，他們會像「雪花」一樣飄落到內核頂部，堆積在此並且壓密(黑色)。這道已經壓密的層位在西半球(W)的內核頂端較厚，在東半球(E)的內核頂端則較薄。圖片來源：德州大學奧斯汀分校傑克遜地球科學院

誰先出現：複雜的生命形式？還是大氣中的大量氧氣？

誰先出現：複雜的生命形式？還是大氣中的大量氧氣？

By Robert Sanders

如果地球的海洋和大氣裡沒有大量氧氣，那我們和其他動物今天也不會在這裡。但是對於生物來說，要從型態簡單的單細胞生物轉變成我們今日所見的複雜生物，高濃度的氧氣有多重要？

一項由加州大學柏克萊分校的地球化學家進行的研究提出了新證據，指出高濃度的氧氣對於動物的起源來說並不是那麼重要。

研究人員發現地球海洋深處變成氧化狀態大概是發生在5.4億至4.2億年前。他們將此現象歸因於當時大氣的氧濃度已經提升到跟現今的大氣氧濃度21%差不多的程度。

他們推測氧濃度上升的時間點比動物出現的時間――7億至8億年前――還晚了數億年。

「我們對深海氧化的解釋為大氣氧濃度上升所造成。從地球歷史來看，這起事件發生的時間可說是相當近期。」加州大學柏克萊分校的地球與行星科學助理教授Daniel Stolper表示。「這項研究的重要之處在於它提供了新的證據，顯示需要氧氣來進行代謝作用的原始動物，或許可以在大氣氧濃度相比於現在低上許多的世界誕生。」

他和博士後研究員Brenhin Keller會將他們的發現發表在期刊《自然》的論文中，並於1月3日先發布線上版。Keller同時隸屬於柏克萊地質年代學中心。

地球氧氣的歷史變化

氧氣在地球歷史中占有一席之地，不只是因為氧氣對於呼吸它們的生物來說至關重要，氧氣還很容易跟其他化合物發生反應――通常十分劇烈，而形成像是鐵鏽、植物燃燒、天然氣爆炸等現象。

然而，要追蹤大氣和海洋的氧氣濃度在地球超過45億年的歷史中是如何變化，卻不是一件簡單的事。多數科學家相信在地球形成的最初20億年，大氣或海洋幾乎沒有任何氧氣。但在大約25億到23億年前，大氣氧含量首次有提升的跡象。這次事件造成的地質現象十分顯著：暴露在地表的岩石突然變紅。這是因為岩石內部的鐵跟氧氣反應形成了鐵氧化物，就跟金屬鐵因生銹而變紅的過程類似。

地球科學家已經算出當時大氣的氧含量首度超過今日的十萬分之一(0.001%)，但是仍不足以使深海氧化，故深海大都還是處於缺氧狀態。

到了4億年前，首度出現含有木炭化石的沉積物，代表大氣二氧化碳含量已經高到足以讓野火發生。這需要現今大氣氧含量的50%到70%，也足以讓深海氧化。但科學家仍不確定大氣氧含量在25億年前到4億年前這段時期究竟是如何變化，因此這項議題仍有諸多探討空間。

「由於氧氣在許多地球化學和生物作用中都佔據了中心地位，因此科學家一直很想釐清25億年至4億年前大氣氧濃度的歷史進程。比方說，有些人對於動物為什麼會在那個時間點出現的解釋，是因為當時大氣氧含量首度接近我們現今所處的高濃度狀態。」Stolper表示，「這個解釋成立的前提是兩者之間有因果關係存在；也就是大氣氧含量升高至接近現今的水平，是驅動我們需要氧氣的祖先得以演化出來的環境因素。」

另一方面，有些研究人員認為兩個事件的關係並不大。要解決這項爭議的一個重要依據是精確定出大氣氧含量提升至接近現今水平的時間。但是，之前科學家預估氧氣升高的時間是發生在8億至4億年前，橫跨了動物誕生的那段時間。

氧氣含量的第二次變化發生在什麼時候？

Stolper和Keller希望他們可以定出地球歷史上一個重要的轉捩點：當大氣氧濃度高到足以使深海氧化的發生時間――當時大氣氧濃度相當於今日的10%至50%。他們的方法是觀察由海底火山爆發形成的火成岩中鐵的氧化狀態。當熔化的岩石從洋脊流出，會形成一大片的熔岩流和狀似枕頭的玄武岩。關鍵在於火山爆發之後海水會流經這些岩石。今日，由於流經岩石的海水含有氧氣，因此玄武岩中的鐵會被氧化。但在深海還缺乏氧氣的時候，科學家預估這些玄武岩中鐵的氧化狀態在噴發之後就不會發生什麼變化。

Stolper表示：「我們的想法是研究這類玄武岩中鐵的氧化態如何隨著歷史變化，並且看看我們能否精確指出鐵是什麼時候開始呈現出受到氧化的跡象，藉此找出深海首度含有大量溶氧的時間。」

為了進行這項研究，他們彙整了已經發表的研究中對古代海底玄武岩的鐵氧化態所進行的測量結果，數量超過1000筆。他們發現在5.4億年至2.4億年前以後，玄武岩中鐵的氧化態相較於岩漿本身才有明顯氧化的跡象，他們將此歸因於大氣氧含量已經提升至跟現今差不多的程度。該時間點比動物出現的時間還晚了數億年，跟某些大氣和海洋氧濃度歷史變化的研究相符，但不是全部。

「這項成果顯示大氣氧含量增加到足以使深海氧化，而形成類似於我們今日所見的世界，對於動物的出現來說並非必要條件。」Stolper表示，「此外，海底玄武岩紀錄也提供了一種新的定量方式，可以讓我們看見數億至數十億年前深海的地球化學條件。」

Which came first: complex life or high atmospheric oxygen?

We and all other animals wouldn’t be here today if our planet didn’t have a lot of oxygen in its atmosphere and oceans. But how crucial were high oxygen levels to the transition from simple, single-celled life forms to the complexity we see today?

A study by UC Berkeley geochemists presents new evidence that high levels of oxygen were not critical to the origin of animals.

The researchers found that the transition to a world with an oxygenated deep ocean occurred between 540 and 420 million years ago. They attribute this to an increase in atmospheric O₂ to levels comparable to the 21 percent oxygen in the atmosphere today.

This inferred rise comes hundreds of millions of years after the origination of animals, which occurred between 700 and 800 million years ago.

“The oxygenation of the deep ocean and our interpretation of this as the result of a rise in atmospheric O₂ was a pretty late event in the context of Earth history,” said Daniel Stolper, an assistant professor of earth and planetary science at UC Berkeley. “This is significant because it provides new evidence that the origination of early animals, which required O₂ for their metabolisms, may have gone on in a world with an atmosphere that had relatively low oxygen levels compared to today.”

He and postdoctoral fellow Brenhin Keller will report their findings in a paper posted online Jan. 3 in advance of publication in the journal Nature. Keller is also affiliated with the Berkeley Geochronology Center.

The history of Earth’s oxygen

Oxygen has played a key role in the history of Earth, not only because of its importance for organisms that breathe oxygen, but because of its tendency to react, often violently, with other compounds to, for example, make iron rust, plants burn and natural gas explode.

Tracking the concentration of oxygen in the ocean and atmosphere over Earth’s 4.5-billion-year history, however, isn’t easy. For the first 2 billion years, most scientists believe very little oxygen was present in the atmosphere or ocean. But about 2.5-2.3 billion years ago, atmospheric oxygen levels first increased. The geologic effects of this are evident: rocks on land exposed to the atmosphere suddenly began turning red as the iron in them reacted with oxygen to form iron oxides similar to how iron metal rusts.

Earth scientists have calculated that around this time, atmospheric oxygen levels first exceeded about a hundred thousandth of today’s level (0.001 percent), but remained too low to oxygenate the deep ocean, which stayed largely anoxic.

By 400 million years ago, fossil charcoal deposits first appear, an indication that atmospheric O₂ levels were high enough to support wildfires, which require about 50 to 70 percent of modern oxygen levels, and oxygenate the deep ocean. How atmospheric oxygen levels varied between 2,500 and 400 million years ago is less certain and remains a subject of debate.

“Filling in the history of atmospheric oxygen levels from about 2.5 billion to 400 million years ago has been of great interest given O₂’s central role in numerous geochemical and biological processes. For example, one explanation for why animals show up when they do is because that is about when oxygen levels first approached the high atmospheric concentrations seen today,” Stolper said. “This explanation requires that the two are causally linked such that the change to near-modern atmospheric O₂ levels was an environmental driver for the evolution of our oxygen-requiring predecessors.”

In contrast, some researchers think the two events are largely unrelated. Critical to helping to resolve this debate is pinpointing when atmospheric oxygen levels rose to near modern levels. But past estimates of when this oxygenation occurred range from 800 to 400 million years ago, straddling the period during which animals originated.

When did oxygen levels change for a second time?

Stolper and Keller hoped to pinpoint a key milestone in Earth’s history: when oxygen levels became high enough – about 10 to 50 percent of today’s level – to oxygenate the deep ocean. Their approach is based on looking at the oxidation state of iron in igneous rocks formed undersea (referred to as “submarine”) volcanic eruptions, which produce “pillows” and massive flows of basalt as the molten rock extrudes from ocean ridges. Critically, after eruption, seawater circulates through the rocks. Today, these circulating fluids contain oxygen and oxidize the iron in basalts. But in a world with deep-oceans devoid of O₂, they expected little change in the oxidation state of iron in the basalts after eruption.

“Our idea was to study the history of the oxidation state of iron in these basalts and see if we could pinpoint when the iron began to show signs of oxidation and thus when the deep ocean first started to contain appreciable amounts of dissolved O₂,” Stolper said.

To do this, they compiled more than 1,000 published measurements of the oxidation state of iron from ancient submarine basalts. They found that the basaltic iron only becomes significantly oxidized relative to magmatic values between about 540 and 420 million years ago, hundreds of millions of years after the origination of animals. They attribute this change to the rise in atmospheric O₂ levels to near modern levels. This finding is consistent with some but not all histories of atmospheric and oceanic O₂ concentrations.

“This work indicates that an increase in atmospheric O₂ to levels sufficient to oxygenate the deep ocean and create a world similar to that seen today was not necessary for the emergence of animals,” Stolper said. “Additionally, the submarine basalt record provides a new, quantitative window into the geochemical state of the deep ocean hundreds of millions to billions of years ago.”

原始論文：Daniel A. Stolper & C. Brenhin Keller. A record of deep-ocean dissolved O2 from the oxidation state of iron in submarine basalts, Nature (2018). DOI:10.1038/nature25009

引用自：University of California – Berkeley. “Which came first: complex life or high atmospheric oxygen?”

原文網址：http://news.berkeley.edu/2018/01/03/which-came-first-complex-life-or-high-atmospheric-oxygen/

研究解釋地震波在地球深處觀測到的神祕訊號

原文網址：https://carnegiescience.edu/node/2264

研究解釋地震波在地球深處觀測到的神祕訊號

新研究探討了在地球深處的極端條件下氧和鐵的化學性質，結果或許可以解釋震測研究中一項歷時已久稱作「超低速帶」(ultralow velocity zones)的謎題。刊登於《自然》(Nature)的這項發現或許會深深影響到我們對地球地質歷史的認知，包括那些改變了地球生命發展的軌跡的事件――比方說發生在24億年前的大氧化事件。

下部地函和地核的交界位在地表以下2900公里處，坐落於附近的超低速帶(ultralow velocity zones,  UVZ)因其特殊的地震波訊號而為科學家所知。雖然這個區域的所在位置深到研究人員無法直接用肉眼觀察，但他們可以用儀器測量地震產生的地震波在此區域的傳遞情形，而得到地球內部構造如何變化的圖像；就像是醫療專業人員可以運用超音波儀器來看到我們身體的內部情況一樣。

科學家透過觀察地震波經過超低速帶時出現的減速現象，而看見超低速帶存在於沿著核幔邊界分布的某些地區之中。但是，知道超低速帶的存在並無法解釋它們的形成原理。

然而，最近研究鐵和氧在地球深處環境下的化學性質而得到的發現，對這項歷時已久的謎題提供了解答。

板塊構造活動會把某些含水礦物拖到地球深處。研究證明在極端的溫度壓力下，這些水會裂解而釋出氫，使得剩下的氧可以跟來自地核的金屬鐵結合，形成一種十分特殊的高壓礦物――過氧化鐵。

由卡內基的研究員Ho-kwang “Dave” Mao領導的研究團隊認為每年有多達3億噸的水會被帶到地球內部，並在深處產生巨大的二氧化鐵儲庫，其造成了在核幔邊界使地震波減速的超低速帶。

為了測試這個想法，研究團隊在美國阿貢國家實驗室運用雷射加熱的鑽石砧，模仿地球深處的溫度壓力條件來製造過氧化鐵樣品，接著利用精密儀器探討地震波在樣品中的傳遞情形。他們發現在平常的地函岩石中摻入40到50%的過氧化鐵所形成的混合物，地震波在其中傳遞產生的訊號跟神秘的超低速帶的訊號相同。

對研究團隊來說，此發現最令人感到興奮的其中一個層面是地球內部深處可能有儲存大量氧氣的場所，如果它會週期性地往地表釋放出氧氣就能夠劇烈改變地球早期大氣的成分。這或許可以解釋地質紀錄中大約24億年前大氣中的氧濃度為何會突然迅速上升。

「發現地球內部具有儲存氧氣的巨大場所代表的意義相當深遠。」Mao解釋。「我們現在應該好好思索偶發性的氧氣爆發事件跟地球歷史上的其他重要事件有何關聯，像是帶狀鐵礦、雪球地球、大滅絕、洪流玄武岩以及超大陸分裂。」

Mysterious deep-Earth seismic signature explained

New research on oxygen and iron chemistry under the extreme conditions found deep inside the Earth could explain a longstanding seismic mystery called ultralow velocity zones. Published in Nature, the findings could have far-reaching implications on our understanding of Earth’s geologic history, including life-altering events such as the Great Oxygenation Event, which occurred 2.4 billion years ago.

Sitting at the boundary between the lower mantle and the core, 1,800 miles beneath Earth’s surface, ultralow velocity zones (UVZ) are known to scientists because of their unusual seismic signatures. Although this region is far too deep for researchers to ever observe directly, instruments that can measure the propagation of seismic waves caused by earthquakes allow them to visualize changes in Earth’s interior structure; similar to how ultrasound measurements let medical professionals look inside of our bodies

These seismic measurements enabled scientists to visualize these ultralow velocity zones in some regions along the core-mantle boundary, by observing the slowing down of seismic waves passing through them. But knowing UVZs exist didn’t explain what caused them.

However, recent findings about iron and oxygen chemistry under deep-Earth conditions provide an answer to this longstanding mystery.

It turns out that water contained in some minerals that get pulled down into the Earth due to plate tectonic activity could, under extreme pressures and temperatures, split up—liberating hydrogen and enabling the residual oxygen to combine with iron metal from the core to create a novel high-pressure mineral, iron peroxide.

Led by Carnegie’s Ho-kwang “Dave” Mao, the research team believes that as much as 300 million tons of water could be carried down into Earth’s interior every year and generate deep, massive reservoirs of iron dioxide, which could be the source of the ultralow velocity zones that slow down seismic waves at the core-mantle boundary.

To test this idea, the team used sophisticated tools at Argonne National Laboratory to examine the propagation of seismic waves through samples of iron peroxide that were created under deep-Earth-mimicking pressure and temperature conditions employing a laser-heated diamond anvil cell. They found that a mixture of normal mantle rock with 40 to 50 percent iron peroxide had the same seismic signature as the enigmatic ultralow velocity zones.

For the research team, one of the most-exciting aspects of this finding is the potential of a reservoir of oxygen deep in the planet’s interior, which if periodically released to the Earth’s surface could significantly alter the Earth’s early atmosphere, potentially explaining the dramatic increase in atmospheric oxygen that occurred about 2.4 billion years ago according to the geologic record.

“Finding the existence of a giant internal oxygen reservoir has many far-reaching implications,” Mao explained. “Now we should reconsider the consequences of sporadic oxygen outbursts and their correlations to other major events in the Earth’s history, such as the banded-iron formation, snowball Earth, mass extinctions, flood basalts, and supercontinent rifts.”

原始論文：Jin Liu, Qingyang Hu, Duck Young Kim, Zhongqing Wu, Wenzhong Wang, Yuming Xiao, Paul Chow, Yue Meng, Vitali B. Prakapenka, Ho-Kwang Mao, Wendy L. Mao. Hydrogen-bearing iron peroxide and the origin of ultralow-velocity zones. Nature 551, 494–497 DOI: 10.1038/nature24461

引用自：Carnegie Institution for Science. “Mysterious deep-Earth seismic signature explained.”

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."