二十億年前的鹽岩顯示出古代大氣氧濃度的上升過程

原文網址：https://www.princeton.edu/news/2018/03/22/two-billion-year-old-salt-rock-reveals-rise-oxygen-ancient-atmosphere

二十億年前的鹽岩顯示出古代大氣氧濃度的上升過程

Catherine Zandonella

誠如我們所知，地球的生命能夠存活是因為大氣之中含有氧氣。科學家從一塊20億年前形成的海鹽當中，找到了新證據顯示地球大氣轉變成含氧狀態的過程。

科學家發現的證據顯示人類早期的革新行為在人類演化史中出現的時間可以更往前推

原文網址：https://newsdesk.si.edu/releases/scientists-discover-evidence-early-human-innovation-pushing-back-evolutionary-timeline

科學家發現的證據顯示人類早期的革新行為在人類演化史中出現的時間可以更往前推

根據三篇新研究，這類演化革新出現的證據可以追溯至某個時期，當時人類身處的環境動盪不安而難以預料

美國史密森尼國家自然歷史博物館的人類學家和國際團隊合作之下，發現大約320,000年前東非的早期人類已經開始會和遠方的人類族群進行貿易，還會使用顏料並運用比舊石器時代還要精密的工具。這些新發現的人類活動所在年代跟已知最早的智人化石大約同期，比之前在東非找到的類似證據還早了數萬年。上述這些行為是生活於中石器時代的人類具備的特徵，取代了之前數萬年來人類遵循的生活方式與技術。

科學家發現地震成像無法看見水

原文網址：http://news.mit.edu/2018/scientists-find-seismic-imaging-blind-water-0314

科學家發現地震成像無法看見水

這項發現可能使科學家需要重新詮釋地球內部的地震波速圖

Jennifer Chu

當地震來襲，附近的地震儀可以把地震造成的晃動以地震波的形式記錄下來。地震波除了可以透露出地震的震央在哪，科學家也能用地震波繪製地球內部構造的分布圖像，跟醫生可以利用電腦斷層掃瞄來得到身體內部的影像一樣。

於74000年前的古代超級火山爆發之後仍然繁盛的一群現代人

原文網址：ScienceDaily, 12 March 2018. www.sciencedaily.com/releases/2018/03/180312132956.htm

於74000年前的古代超級火山爆發之後仍然繁盛的一群現代人

想像非洲某年夏天從未到來：天空在白晝是一片灰濛，夜晚則閃耀著紅光；春花未開，冬日樹木枯萎；羚羊之類的大型哺乳類因挨餓而日漸消瘦，倚賴牠們維生的掠食者(肉食動物和獵人)也無法從牠們身上得到什麼養分。接著想像如此怵目驚心的景象年復一年地不斷重演，便是在74000年前左右的印尼多峇(Toba)超級火山爆發之後，地球上的生靈所處的景象。但在本周發表於期刊《自然》(Nature)的一篇論文中，科學家於南非海岸發現一群早期現代人在此事件下仍繁榮發展。

從鑽石得到的發現證實地球的海洋地殼會循環回地函

原文網址：https://www.folio.ca/diamond-discovery-proves-earths-oceanic-crust-gets-recycled/

從鑽石得到的發現證實地球的海洋地殼會循環回地函

首度在地表附近的鑽石中發現通常在地底700公里深才能找到的礦物。

By Jennifer-Anne Pascoe

植物葉片上變成化石的蠟質可以成為瞭解古代氣候的新工具

原文網址：www.sciencedaily.com/releases/2018/03/180302090955.htm

植物葉片上變成化石的蠟質可以成為瞭解古代氣候的新工具

一篇發表在《科學報告》(Scientific Reports)的新研究描述了一種估算古代大氣水含量的新方法：利用植物葉片上變成化石的蠟質。

於近代發生的火山活動跟地球誕生之後不久的事件有所關聯

原文網址：https://carnegiescience.edu/news/modern-volcanism-tied-events-occurring-soon-after-earth%E2%80%99s-birth

於近代發生的火山活動跟地球誕生之後不久的事件有所關聯

印度洋上的留尼旺島是由火山熱點形成。根據卡內基科學研究所的Bradley Peters、Richard Carlson、Mary Horan，以及斯克里普斯海洋研究所的James Day發表在《自然》(Nature)的新成果，此熱點的高溫岩漿柱是從地下深處成分異常原始的地方湧升而成。

留尼旺島標示出一處曾在6600萬年前噴發的熱點當今的所在位置。該次噴發產生的洪流玄武岩形成了覆蓋印度大部分地區的德干高原，而恐龍的滅絕它可能也參與了一部分。相較於地表多數火山，一般認為洪流玄武岩和其他熱點產生的岩漿係源自於地球深部的不同位置。因此研究這些物質或許可以幫助科學家更加瞭解我們居住的地球是如何演變。

地球形成過程中的高熱使得當時的地球處於大規模的熔融狀態，造成地球分離成兩層――密度高的金屬往內沉至核心位置形成地核，留下浮於上方富含矽酸鹽的地函。

在地球接下來45億年的演變過程中，地函深部上湧、熔化，然後再次因為密度而分離，形成地殼並在整個過程中改變地球內部的化學成分。今日，地殼正沿著太平洋的邊界沉回地球內部，此種現象會讓地函緩緩流動而攪動這些物質，使得地殼特有的化學成分重新回到地球深處。

但是，地函並非全都如此過程所述的經過充分混和。有些年代較老的區塊至今仍然存在――就像未經充分攪拌的蛋糕麵糊中出現的麵粉團塊一樣。分析留尼旺島火山岩的化學成分，顯示它們的來源不同於地函今日其他混和良好的地方。

研究團隊利用新的同位素數據，顯示產生留尼旺島岩漿的區域是獨立於範圍較廣、充分混和的地函之外。這些獨立存在的區塊形成的時間早於地球歷史的最初十分之一。

同位素是一群質子數量相同，但是中子數量卻不同的元素。有時原子核含有的中子數量會讓同位素呈現不穩定的狀態。為了變成穩定狀態，這類同位素會經由放射性衰變的過程來釋放出帶有能量的粒子。同位素的質子和中子數量在過程中會有所變化，使它們轉變成另一種元素。這項新研究利用此種作用留下的線索來得到這些特殊地函區塊的年代以及歷史。

釤-146就是一種不穩定的(即「放射性」)同位素，它會衰變成釹-142，半衰期僅有1.03億年。雖然地球剛形成時帶有釤-146，但它很快地就會在地球的幼年期消失殆盡。這意謂釹-142可以詳細紀錄地球最早期的歷史，卻不會記下釤-146通通轉變成釹-142之後的地球歷史。在地球45億年的歷史當中，只有最初5億年在地函發生的成分變化，才會造成釹-142跟其他釹同位素含量的比例出現差異。

分析留尼旺島火山岩中釹-142跟釹-144的比例，配合在實驗室進行的模擬研究，指出地函的混和作用儘管已經進行數十億年之久，留尼旺島地函柱的岩漿可能是來自於地函一處保存良好的區塊，曾經歷地函最初發生的大規模熔融作用使成分有所改變。

研究團隊的發現也有助於科學家解釋正好位在地核和地核邊界的高密度區域起源為何。這類區域稱為大型低剪力波速群(large low shear velocity provinces, LLSVP)與超低速帶(ultralow velocity zones ,ULVZ)，當地震波經過深部地函的這些區域時波速通常會下降。它們也許是地球早期的熔融事件殘留下來的痕跡。

主要作者Peters表示：「這些熱點地函柱中保留下來的地函分化事件，可以讓我們更加瞭解早期地球的地球化學作用，並且解釋那些位在地函深處的高密度區域產生的謎樣地震波訊號。」

Modern volcanism tied to events occurring soon after earth’s birth

Plumes of hot magma from the volcanic hotspot that formed Réunion Island in the Indian Ocean rise from an unusually primitive source deep beneath the Earth’s surface, according to new work in Nature from Carnegie’s Bradley Peters, Richard Carlson, and Mary Horan along with James Day of the Scripps Institution of Oceanography.

Réunion marks the present-day location of the hotspot that 66 million years ago erupted the Deccan Traps flood basalts, which cover most of India and may have contributed to the extinction of the dinosaurs. Flood basalts and other hotspot lavas are thought to originate from different portions of Earth’s deep interior than most volcanoes at Earth’s surface and studying this material may help scientists understand our home planet’s evolution.

The heat from Earth’s formation process caused extensive melting of the planet, leading Earth to separate into two layers when the denser iron metal sank inward toward the center, creating the core and leaving the silicate-rich mantle floating above. 

Over the subsequent 4.5 billion years of Earth’s evolution, deep portions of the mantle would rise upwards, melt, and then separate once again by density, creating Earth’s crust and changing the chemical composition of Earth’s interior in the process.  As crust sinks back into Earth’s interior—a phenomenon that’s occurring today along the boundary of the Pacific Ocean—the slow motion of Earth’s mantle works to stir these materials, along with their distinct chemistry, back into the deep Earth.

But not all of the mantle is as well-blended as this process would indicate. Some older patches still exist—like powdery pockets in a poorly mixed bowl of cake batter. Analysis of the chemical compositions of Réunion Island volcanic rocks indicate that their source material is different from other, better-mixed parts of the modern mantle.

Using new isotope data, the research team revealed that Réunion lavas originate from regions of the mantle that were isolated from the broader, well-blended mantle. These isolated pockets were formed within the first ten percent of Earth’s history.

Isotopes are elements that have the same number of protons, but a different number of neutrons. Sometimes, the number of neutrons present in the nucleus make an isotope unstable; to gain stability, the isotope will release energetic particles in the process of radioactive decay. This process alters its number of protons and neutrons and transforms it into a different element. This new study harnesses this process to provide a fingerprint for the age and history of distinct mantle pockets.

Samarium-146 is one such unstable, or radioactive, isotope with a half-life of only 103 million years. It decays to the isotope neodymium-142. Although samarium-146 was present when Earth formed, it became extinct very early in Earth’s infancy, meaning neodymium-142 provides a good record of Earth’s earliest history, but no record of the Earth from the period after all the samarium-146 transformed into neodymium-142. Differences in the abundances of neodymium-142 in comparison to other isotopes of neodymium could only have been generated by changes in the chemical composition of the mantle that occurred in the first 500 million years of Earth’s 4.5 billion-year history.

The ratio of neodymium-142 to neodymium-144 in Réunion volcanic rocks, together with the results of lab-based mimicry and modeling studies, indicate that despite billions of years of mantle mixing, Réunion plume magma likely originates from a preserved pocket of the mantle that experienced a compositional change caused by large-scale melting of the Earth’s earliest mantle.

The team’s findings could also help explain the origin of dense regions right at the boundary of the core and mantle called large low shear velocity provinces (LLSVPs) and ultralow velocity zones (ULVZs), reflecting the unusually slow speed of seismic waves as they travel through these regions of the deep mantle. Such regions may be relics of early melting events.

“The mantle differentiation event preserved in these hotspot plumes can both teach us about early Earth geochemical processes and explain the mysterious seismic signatures created by these dense deep-mantle zones,” said lead author Peters.

原始論文：Bradley J. Peters, Richard W. Carlson, James M. D. Day, Mary F. Horan. Hadean silicate differentiation preserved by anomalous 142Nd/144Nd ratios in the Réunion hotspot source. Nature, 2018; 555 (7694): 89 DOI: 10.1038/nature25754

引用自：Carnegie Institution for Science. "Modern volcanism tied to events occurring soon after Earth's birth: Primordial mantle pockets preserved under Réunion Island for billions of years." 

大陸內部的構造活動或許不像地質學家認為的如此穩定

原文網址：https://news.illinois.edu/view/6367/614015

大陸內部的構造活動或許不像地質學家認為的如此穩定

 Lois Yoksoulian

南美和非洲內陸的構造活動相當穩定，但由伊利諾大學領導的團隊卻在此處下方辨識出意料之外的地球物理訊號。數據顯示在地球最淺層的穩定區塊發生的地質活動，也許比過往認為得還要接近現代。這項刊登在《自然―地質科學》(Nature Geoscience)的發現挑戰了當今某些關於板塊構造的主流理論。

地球最古老的岩石位處大陸內部，距活動頻繁、岩石重新循環回地球內部之處―――板塊邊界十分遙遠。這些堅硬、上浮且根基深入地球內部的地體稱作穩定地塊，它們漂在地球表面已有數十億年之久，看似完全不受打擾。偶爾，它們彼此之間會在稱為超大陸循環的舞蹈中順著自身邊界相聚然後分開。

「通常我們認為穩定陸塊為低溫、穩定且地勢低平的地區。」研究共同作者，伊利諾大學的地質學教授Lijun Liu表示。「低溫是因為此處岩石距下方高溫的地函相當遙遠；穩定則是此處地殼長久以來都未受到斷層或者變形作用的影響；地勢低平是因為它們一直待在地表，歷經數十億年的侵蝕而導致。」

然而，穩定陸塊中卻有某些地方不符合這些規則。

研究主要作者，伊利諾大學的研究生Jiashun Hu表示：「舉例來說，在南美和非洲穩定陸塊的內部就有一些地勢高起的地區。」

研究人員利用伊利諾大學國家超級電腦應用中心的超級電腦「藍水」來處理地球物理數據，期望能夠更加瞭解這些地勢高起的地區。地質學家一直以來認為穩定陸塊深厚的根基因為組成礦物密度較低的關係，使得它們可以浮在底下的高溫地函之上。但是Liu表示新數據指出南美和非洲穩定陸塊(它們過去曾結合起來成為盤古超大陸的一部份)內部地勢高起的地區其下方的低溫地函呈現出分層構造，且下層的密度在過去比今日還要高。

密度差可能是由於一種稱為「地函脫層作用」(mantle delamination)而造成的結果。研究人員表示當地函熱柱帶來的高溫岩漿和穩定陸塊的地殼作用之後，地殼下方密度較高的下層地函會從密度較輕的上層地函脫落下來。

Liu表示：「我們運用幾種不同類型的地震成像數據，可以看到我們認為是脫落下來的地函斷塊沉到高溫、黏稠的深部地函之中。」

「脫層作用發生之後，由於來自上方物質的冷卻作用，使得穩定陸塊根基附近的物質重新長回去。而這些新生物質的組成可能比之前在此處的物質密度還要低。」 伊利諾大學的地質學教授Craig Lundstrom表示，「這些物質會帶來額外的浮力。由上浮所產生的推力可能形成了這些地勢特別高凸的區域。」

研究人員表示，團隊經由跨領域研究開始對地球構造運動的歷史得到相當合理――儘管也十分複雜的最新版本故事。

「非洲和南美的高凸地形不過是整個故事的一部份。」Hu表示，「有許多地質現象的所在位置，像是熱點的軌跡、大陸內部的火山活動、地表隆起與侵蝕，還有地震成像在穩地陸塊根基中呈現出的變形，似乎全都跟我們提出的脫層事件有深切的關聯，意味著它們之間可能存在著因果關係。」

也有證據支持在地球歷史上的其他時間，穩定地塊和地函柱之間曾在別處發生交互作用。

「岩石證據顯示在之前的超大陸循環中曾經發生抬升和侵蝕事件。」伊利諾大學地球、社會與環境學院的主任暨地質學教授Stephen Marshak表示，「有個相關研究討論到類似事件，也就是陸地的抬升可能和穩定陸塊岩石圈的脫層有關。陸地抬升造成大規模侵蝕作用發生，使得前寒武紀基岩跟古生代的沉積岩地層的交界面成為『大不整合面』(Great Unconformity)。」

至今仍不清楚穩定地塊和地函柱之間的作用，可能會以何種方式影響被認為是地質活動不活躍的地區今日所發生的地震和火山活動。不過這項研究清楚表明地質學家對於所謂的穩定陸塊或許有了新的一層看法。

Continental interiors may not be as tectonically stable as geologists think

 A University of Illinois-led team has identified unexpected geophysical signals underneath tectonically stable interiors of South America and Africa. The data suggest that geologic activity within stable portions of Earth’s uppermost layer may have occurred more recently than previously believed. The findings, published in Nature Geoscience, challenge some of today’s leading theories regarding plate tectonics.

The most ancient rocks on Earth are located within continental interiors, far from active tectonic boundaries where rocks recycle back into the planet’s interior. These strong, buoyant and deeply rooted blocks of Earth, called cratons, have been drifting on the surface for billions of years, seemingly undisturbed. They occasionally join and break apart along their edges in a dance called the supercontinent cycle.

“We usually think of cratons as being cold, stable and low-elevation,” said professor of geology and study co-author Lijun Liu. “Cold because the rocks are far above the hot mantle layers, stable because their crusts have not been disturbed significantly by faulting or deformation, and their low elevation is because they have been sitting there, eroding down for billions of years.”

However, there are places where cratons don’t follow these rules.

“For example, there are regions of high topography within the cratons of South America and Africa,” said graduate student and lead author Jiashun Hu.

The researchers processed geophysical data with the Blue Waters supercomputer at the National Center for Supercomputing Applications at Illinois hoping to better understand these high-elevation regions. The thick roots of cratons have been thought to be buoyant due to their low-density mineral content, allowing them to float on top of the hot underlying mantle. However, the new data indicate that the cold mantle that lies below these regions in South America and Africa – once joined as part of the supercontinent Pangea – has a layered structure and that the lower layer was more dense in the past than it is today, Liu said.

This density difference could be the result of a process called mantle delamination. During delamination, the denser lower mantle layer peels away from the buoyant upper layer under the crust of the craton after interacting with hot magma from mantle plumes, the researchers said.

“From several types of seismic imaging data, we can see what we think are delaminated mantle slabs sinking into the hot, viscous deep mantle,” Liu said. 

“The material that subsequently grows back at the roots of the cratons after delamination, due to cooling from above, is probably compositionally much less dense than what was there before,” said geology professor Craig Lundstrom. “That adds buoyancy, and that force from buoyancy could be what forms the anomalously high topography.”

This multidisciplinary study is beginning to give the team a very logical – albeit complicated – update on the story of Earth’s tectonic history, the researchers said.

“The high topography of Africa and South America is only part of the story,” Hu said. “There are many geologic phenomena such as the location of hotspot trajectories, continental volcanism, surface uplift and erosion, as well as seismically imaged deformation within the craton roots that all seem to correlate well with the proposed delamination event, implying a potential causal relationship.”

There is also evidence to support other locations of craton-plume interaction during other times in Earth’s history.

“The rock record shows that uplift and erosion events have taken place during previous supercontinent cycles,” said geology professor and School of Earth, Society and Environment director Stephen Marshak. “A related study discusses what might be a similar event, namely continental uplift possibly related to delamination of cratonic lithosphere that caused the period of global erosion resulting in the Great Unconformity, which is the contact between Precambrian basement rock and Paleozoic sedimentary strata.”

For now, it is not clear if and how craton-plume interaction may affect modern-day earthquake activity and volcanism in areas thought of as geologically inactive. However, the study marks new thinking in how geologists may understand the so-called stable cratons.

原始論文：Jiashun Hu, Lijun Liu, Manuele Faccenda, Quan Zhou, Karen M. Fischer, Stephen Marshak & Craig Lundstrom. Modification of the Western Gondwana craton by plume–lithosphere interaction. Nature Geoscience, 2018; doi: 10.1038/s41561-018-0064-1

引用自：University of Illinois Urbana-Champaign. “Continental interiors may not be as tectonically stable as geologists think”