地質學家發現第八大陸：西蘭大陸

原文網址：http://www.nature.com/news/geologists-spy-an-eighth-continent-zealandia-1.21503

地質學家發現第八大陸：西蘭大陸

研究人員主張這座幾乎沉於海中的世界應該跟非洲、澳洲和其他大陸齊名

原作：Alexandra Witze

在西南太平洋的波濤之下坐落著一塊大多區域不為人所見的陸地――稱為「西蘭大陸」――地質學家表示其有資格成為一座大陸。

紐西蘭、澳洲和新喀里多尼亞的科學家組成的團隊在三月/四月號的《今日美國地質學會》(GSA Today)中提出，這個廣達500萬平方公里，涵蓋紐西蘭以及新喀里多尼亞的區域，經由地球物理數據顯示其為一片完整的大陸地殼，且從地質角度上來說跟澳洲有所區別。

「如果可以拔掉全球海洋底下的塞子，那麼許久以前應該就會承認西蘭大陸的地位了。」主持研究團隊，紐西蘭達尼丁地質與核子科學研究所的地質學家Nick Mortimer說。

然而，國際上並沒有一個組織負責正式公告大陸是哪幾座，因此研究人員得冀望他們的同僚中有夠多人同樣認可這座陸塊。否則他們的提案只能流於理論層面的願景，無法從根本改寫每個孩子在地理課上都會學到的內容。

「研究結果促使我們重新思考在地質上大陸地塊的現有定義，應該可以適用的範圍有多廣。」紐西蘭地質與核子科學研究所下哈特分部的礦物地質學家Patricia Durance表示。

並非混雜而成

Mortimer和他的同事十多年來在演講、科普文章和書籍上推廣西蘭大陸存在的證據，這篇最新論文則是彙整了他們目前為止在學術上的研究成果。在此他們發表西蘭大陸大約是從1億年前開始從岡瓦那超大陸分裂出來。

這次分裂導致西蘭大陸獨立出來，卻也讓它的地殼受到拉張而變薄，造成西蘭大陸逐漸下沉，最終使其大部分區域皆位於水中。今日，西蘭大陸位在水面以上的部分僅占總面積的6%，像是紐西蘭和新喀里多尼亞。

Mortimer表示由地球重力場繪製而成的地圖清楚顯示西蘭大陸在地理上為一個整體，從澳洲東北外海一路往東南方延伸並經過紐西蘭諸島。海床岩石的採樣指出西蘭大陸是由密度低的大陸地殼組成，而非組成附近海底高原的暗色火山岩。西蘭大陸在結構上看起來相當完整，並不是由不同大陸地殼碎片拼湊而成。

目前對於「大陸」還沒有一個廣為接受的定義，地理學家和地質學家在此問題上也有所分歧 (在地理上，歐洲和亞洲被視為不同的大陸；但地質學家卻把它們視作同一塊歐亞大陸) 。「這篇文章帶來的主要效益之一是，吸引人們去關注像『大陸』如此基本的用語卻有許多反覆且不一致的使用方式。」加拿大安蒂崗尼希，聖弗朗西斯澤維爾大學的地質學家Brendan Murphy說。

要讓西蘭大陸跟歐亞、非洲、南極洲、澳洲、南美和北美齊名，成為大眾認可的名字勢必會面臨一場苦戰。「主張西蘭大陸是座新大陸跟集郵有幾分相似，」澳洲墨爾本，蒙納許大學的地質學家Peter Cawood說，「有這麼重要嗎？」

Mortimer表示無論如何，西蘭大陸的研究應能幫助生物地理學家更加瞭解紐西蘭的動植物特有種如何出現，也可以促進地質學家去理解大陸地殼的重塑過程。

Geologists spy an eighth continent: Zealandia

This mostly submerged world should be recognized alongside Africa, Australia and others, argue some researchers.

Beneath the waves in the southwest Pacific Ocean lies a mostly hidden realm — dubbed Zealandia — that deserves to be called a continent, geologists say.

Geophysical data suggest that a region spanning 5 million square kilometres, which includes New Zealand and New Caledonia, is a single, intact piece of continental crust and is geologically separate from Australia, a team of scientists from New Zealand, Australia and New Caledonia argue in the March/April issue of GSA Today¹. (see 'Hidden crust')

“If you could pull the plug on the world’s oceans, then Zealandia would probably long ago have been recognized as a continent,” says team leader Nick Mortimer, a geologist at GNS Science in Dunedin, New Zealand.

However, there is no international body in charge of designating official continents, and so the researchers must hope that enough of their colleagues agree to recognize the landmass. Otherwise, their proposal could remain more of a theoretical wish than a radical reshaping of what every child has to learn in geography class.

“The results are pushing us to rethink how broadly we can or should apply the established definition of geological continental landmasses,” says Patricia Durance, a mineral geologist at the GNS Science office in Lower Hutt, New Zealand.

Not a mash-up

Mortimer and his colleagues have been making the case for Zealandia for more than a decade, in talks, popular articles and books; the latest paper is their most technical synthesis yet. In it, they report that Zealandia began to peel away from the supercontinent of Gondwana starting about 100 million years ago.

The rift gave Zealandia its independence, but it also pulled and thinned the crust, causing the area to sink, and dooming most of it to a watery existence. Today, only about 6% of it remains above water, as New Zealand and New Caledonia.

Satellite maps made using Earth’s gravitational field clearly show that Zealandia is a coherent geographical feature stretching from near Australia’s northeastern coast well past the islands of New Zealand, Mortimer says. Sea-floor samples reveal that Zealandia consists of light continental crust and not the dark volcanic rocks that make up nearby underwater plateaus. The area seems to be structurally intact, rather than a mash-up of different continental-crust fragments.

There is no widely accepted definition of a continent, and geographers and geologists differ on the question. (Geographically, Europe and Asia are considered separate continents, whereas geologists consider them the single landmass of Eurasia.) “One of the main benefits of this article is that it draws attention to the arbitrary and inconsistent use of such a fundamental term as continent,” says Brendan Murphy, a geologist at St. Francis Xavier University in Antigonish, Canada.

Zealandia will face an uphill battle in garnering the same popular name recognition as Eurasia, Africa, Antarctica, Australia and North and South America. “Claiming that Zealandia is a continent is a bit like stamp collecting,” says Peter Cawood, a geologist at Monash University in Melbourne, Australia. “So what?”

Whatever it is called, Mortimer says, studies of Zealandia should help biogeographers to better understand how New Zealand’s endemic plants and animals arose — and give geologists a boost in learning how continental crust can be reshaped.

引用自：Nature doi:10.1038/nature.2017.21503

研究表示季風的變化造成中國古文明的消長

原文網址：www.sciencedaily.com/releases/2017/02/170207092837.htm

研究表示季風的變化造成中國古文明的消長

強降雨在全新世早期和中期往北移動了400公里

一則新研究的作者表示，每年在約有10億人居住的東亞地區降下雨水的夏季季風於遠古時期曾發生劇烈改變。當時夏季季風往北移動多達400公里，造成中國北域的雨量多出一倍。過去10,000年來，季風的改變很可能對中國的早期人類文明如何演變有相當影響。

拉蒙特―多爾蒂地球觀測所和西安中國科學院的學者，研究了位於中國西北內蒙古的一座封閉湖――洱海的古水位變化。他們發現全新世(始於11,700年前)早期和中期，此湖泊是現今的六倍大且深度多了60公尺。許多人類文明也在這段時期有所演變。

「我認為這項研究的重要之處，在於強調出季風在空間上的波動可以讓中國北方發生重大變化。」論文的主要作者，拉蒙特―多爾蒂地球觀測所的研究生Yonaton Goldsmith表示。「當季風強度變強，它會往北移動並造成中國北部變得一片翠綠。當季風強度減弱，它則會停留在中國南部而北方則陷入乾旱。這麼大的波動必定會劇烈改變中國北部的生態系。」

這篇刊登於本周《美國國家科學院院報》(Proceedings of the National Academy of Sciences)的研究，也把季風的移動跟地球軌道的改變，以及氣候系統中的其他週期性變化連結在一起。研究有助於科學家瞭解季風如何受這些自然循環影響，以及今日發生的氣候變遷未來可能對季風造成的衝擊。

Goldsmith說目前還不清楚季風會如何對全球暖化做出回應。有種看法認為季風應該會變得更強，但區域研究卻顯示最近數十年來中國北部越趨乾燥。他說：「因此在我們將答案確定下來以前，此區域仍有許多研究尚待完成。」

洱海的位置接近於東亞季風所及區域的西北邊界，因此可以反映出季風往北移動時帶來的改變。研究人員探討洱海比現今大上許多時遺留下來的沉積物露頭，配合其他標記他們可以重建出湖面的變化歷程，以及數千年來雨量如何波動。

他們發現湖水的深度於123,000前年曾達到高值，另一次則在58,000年前左右，而11,000年至5000年前之間也發生過一次。這些事件跟季風界線往北移動最多達到400公里，而造成的降雨週期性增加一致。跟之前從中國北方與南方的洞穴沉積物中得到的測量結果相比，洱海的紀錄跟它們之間具有「高度相關性」。

研究人員發現在5500至5000年前，季風減弱使得中國北方整體雨量減少了約50%。他們猜測乾旱造成了此區的文明發生重大轉變。他們的敘述中寫道，兩個新石器時代早期的文明――中國北方的紅山文化以及中國中部的仰韶文化――都在5000年前滅絕。在中國中部，隨之出現的是政治以及社會階級更加分明且複雜的社會，像是龍山文化。而西藏高原東緣之前未受開發的區域也開始有人遷入。於此同時，以小河沿文化為代表，中國東北則經歷了人口大幅衰減。

作者寫道：「這些發現顯示氣候變遷會對人類社會造成重大影響，也強烈指出瞭解全球暖化對中國和全世界的降雨模式會造成何種影響確實有其必要。」

在其他文明的殞落中，雨量的強烈變化可能也扮演了重要腳色。由拉蒙特的科學家Brendan Buckley領導在數年前刊登的論文中，提出伴隨季風移動而發生的長期乾旱可能造成了柬埔寨吳哥窟的古暹粒文明於大約600年前滅絕。在古典馬雅文明的衰敗中，也認為乾旱具有推波助瀾的作用。雖然在此例中，另一則拉蒙特的研究認為是馬雅人興建城市和耕種而進行的除林行為讓自己陷入乾旱的處境。

這篇關於洱海的論文其他作者包括了拉蒙特―多爾蒂地球觀測所的Wallace S. Broecker、 Pratigya J. Polissar和Peter B. deMenocal；中國科學院地球環境研究所，黃土與第四紀國家重點實驗室的徐海、藍江湖、程鵬、周衛建和安芷生；以及以色列地質調查所的Naomi Porat。

Shifting monsoon altered early cultures in China, study says

Heavier rainfall moved 400 km north during early and middle Holocene

The annual summer monsoon that drops rain onto East Asia, an area with about a billion people, has shifted dramatically in the distant past, at times moving northward by as much as 400 kilometers and doubling rainfall in that northern reach. The monsoon's changes over the past 10,000 years likely altered the course of early human cultures in China, say the authors of a new study.

Researchers from the Lamont-Doherty Earth Observatory and the Chinese Academy of Sciences in Xi'an studied ancient water levels for Lake Dali, a closed-basin lake in Inner Mongolia in the northeast of China. They found that the lake was six times larger and water levels were 60 meters higher than present during the early and middle Holocene -- the period beginning about 11,700 years ago, and encompassing the development of human civilization.

"I think it is important to emphasize that these spatial fluctuations in the monsoon drive large changes in northern China," said Yonaton Goldsmith, a graduate student at Lamont-Doherty Earth Observatory and lead author of the paper. "When the monsoon is strong, it shifts northward and northern China becomes green. When the monsoon is weak, the monsoon stays in the south and northern China dries out. Such large fluctuations must have altered the ecosystems in northern China dramatically."

The study, appearing this week in the Proceedings of the National Academy of Sciences, also ties the shifting monsoon to changes in Earth's orbit and other periodic changes in the climate system. The study should help scientists understand how the monsoon is affected by those natural cycles, and how a changing climate today might influence the monsoon in the future.

Goldsmith said it's still unclear how the monsoon will react to global warming. One view is that the monsoon should grow stronger, but the area studied has been drying out over recent decades, he said, "so there is still a lot that needs to be done in that region before we can get definitive answers."

Dali Lake is located near the northwestern limit of the East Asian monsoon, and so would reflect the changes brought about when the monsoon shifted north. The researchers studied outcrops of sediments left behind when the lake was far larger, and used those and other markers to construct a timeline of lake levels, and the fluctuation of rainfall over millennia.

They found that the lake reached peak levels around 123,000 years ago, again around 58,000 years ago, and once more between 11,000 and 5,500 years ago. They tie the periodic increases in rainfall to the range of the monsoon shifting north by as much as 400 kilometers. The lake record is "highly correlated" with measurements taken earlier from cave deposits in both northern and southern China.

Between 5,500 and 5,000 years ago, the monsoon weakened and rainfall over northern China decreased by 50 percent, the researchers found. They speculate that this drying triggered a major cultural transition in the region. As they describe it, two early Neolithic societies, the Hongshan culture in North China and the Yangshao culture in central China, collapsed around 5,000 years ago. In central China, the following period saw the rise of more stratified and socially and politically complex societies, including the Longshan culture. Previously unoccupied areas on the eastern margin of the Tibetan plateau were populated. Meanwhile, northeast China experienced a sharp population decline, represented by the Xiaoheyan culture.

"These findings show that climate change can have dramatic effects on human societies and highlight the necessity to understand the effect of global warming on rainfall patterns in China and all over the world," the authors write.

Intense variations in rainfall may have played a role in the collapse of other civilizations. A study led by Lamont scientist Brendan Buckley, published several years ago, suggested that extended drought coupled with changes in the monsoon could have doomed Cambodia's ancient Khmer civilization at Angkor nearly 600 years ago. Drought is thought to have played a role in the decline of the Classic Maya civilization, too, though in that case, another Lamont study suggests that the Maya themselves contributed to the drought by clearing forests for cities and crops.

The Lake Dali paper's other authors are Wallace S. Broecker, Pratigya J. Polissar and Peter B. deMenocal of Lamont-Doherty; Hai Xu, Jianghu Lan, Peng Cheng, Weijian Zhou and Zhisheng An of the State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences; and Naomi Porat of the Geological Survey of Israel.

This work was supported by a Gary Comer Science and Education Foundation grant to Yonaton Goldsmith and Pratigya J. Polissar; Columbia's Center for Climate and Life; the National Basic Research Program of China Grant 2013CB955900; the External Cooperation Program of Bureau of International Cooperation, Chinese Academy of Sciences Grant 132B61KYSB20130003; and Lamont-Doherty Earth Observatory Contribution no. 8084.

原始論文：Yonaton Goldsmith, Wallace S. Broecker, Hai Xu, Pratigya J. Polissar, Peter B. deMenocal, Naomi Porat, Jianghu Lan, Peng Cheng, Weijian Zhou, Zhisheng An. Northward extent of East Asian monsoon covaries with intensity on orbital and millennial timescales. Proceedings of the National Academy of Sciences, 2017; 201616708 DOI: 10.1073/pnas.1616708114

引用自：Lamont-Doherty Earth Observatory, Columbia University. "Shifting monsoon altered early cultures in China, study says: Heavier rainfall moved 400 km north during early and middle Holocene." ScienceDaily. ScienceDaily, 7 February 2017.

在地球的遙遠過去，適合生命發展的條件也許曾經出現卻又消失

原文網址：www.sciencedaily.com/releases/2017/01/170117163939.htm

在地球的遙遠過去，適合生命發展的條件也許曾經出現卻又消失

由華盛頓大學領導的新研究發現，在生命真正穩定下來的十幾億年之前，地球海洋曾發展出適合複雜生命生長的環境，但之後卻逐漸消失。

這項研究以硒元素為工具來測量許久之前的氧氣含量，或許也能幫助尋找地球之外的生命訊號。

刊登於1月18日的《美國國家科學院院刊》(Proceedings of the National Academy of Sciences)的這篇論文，主要作者華盛頓大學地球與太空科學系的博士生Michael Kipp分析了沉積岩中硒元素的同位素比例，來測量20至24億年前地球大氣中氧含量的多寡。

Kipp於華盛頓大學的共同作者包括先前在地球與太空科學系擔任博士後研究員，現在則任教於蘇格蘭聖安德魯斯大學的Eva Stüeken，以及同時參與華盛頓大學天體生物計畫的教授Roger Buick。另一位共同作者是加州大學河濱分校的Andrey Bekker。研究人員表示這項研究有助於證實Bekker首創的理論。

Buick說：「複雜細胞存在的化石證據或許可追溯至17.5億年前。但是最早的化石並非代表曾經存活過的最古老生物，因為要成為化石而保存下來的機率是非常低的。」

「這項研究顯示在有化石紀錄之前，環境中已經有足夠的氧氣讓複雜細胞得以演化出來，甚至在生態系中具有重要地位。」他補充，「這不代表複雜細胞必定發生過上述現象，但它們的確可以發生。」

Kipp和Stüeken利用華盛頓大學同位素地球化學實驗室的質譜儀，來分析特定時期的頁岩(一種沉積岩)中微量的硒。他們藉此可以得知硒是否受到氧氣的出現，也就是氧化作用的影響而改變。氧化硒的化合物會在形成過後受到還原，而在岩石中留下同位素比例改變的紀錄。同時，當環境中含有許多氧氣時，硒的含量也會增加。

Buick表示之前認為地球氧氣含量變化的歷史是「沒有，接著有一些，然後變成相當多。但現在看起來氧氣在長達2.5億年左右的一段時期中曾經相當地多，之後又降回去。」

氧氣曾長時間保持在高水平具有相當重要的意義。Kipp強調：「僅管在此前後或許曾短暫出現偶爾可以支持這些生命的環境，但要讓它們繼續演化並成為生態系中的重要一份子，就得讓氧氣長時間維持在一定含量。」

Stüeken表示之前已經有人猜測曾發生氧氣增加的情形，但卻不清楚它的分布範圍有多廣。這項研究更加清楚地呈現出氧氣大量「溢出」的情形會是如何：「在大氣和海洋表層當中具有相當大量的氧氣，但在深海卻完全不是如此。」

是什麼原因造成氧氣含量戲劇性地增加，卻以同樣劇烈的速度減少？

「這是一則重要卻難以回答的問題。」Stüeken表示。「我們不知道該現象為什麼發生，也不知道為什麼結束。」

Buick說：「這在地球歷史上是一段前所未見的時期。如果你去觀察硒同位素隨時間變化的情形，此區間真的相當特殊。跟前後比較起來，一切都顯得如此不同。」

研究人員利用硒——由希臘文的月亮來命名的元素——作為探測久遠之前氧含量的有力工具，他們表示此方法也能幫助尋找地球以外的氧氣，甚至可能藉此找到生物。

他們指出未來世代的太空望遠鏡可以給予天文學家遠方行星的大氣成分。其中某些行星大小可能與地球相仿，或許大氣還含有相當可觀的氧氣。

「找出在地球遙遠的過去曾有段時間氧氣含量跟現今相差不遠，但是當時棲息的生物卻大相逕庭，意味著一個世界從遠端探測出具有豐富氧氣，不一定就保證也擁有複雜的生物圈。」Kipp表示。

Buick總結說：「這種新方法可以測量一個行星過去歷史當中的氧氣含量，由此可以了解此處是否有複雜的生物演化出來，且持續夠久而能夠演化出智慧生物。」

 

Conditions right for complex life may have come and gone in Earth's distant past

Conditions suitable to support complex life may have developed in Earth's oceans -- and then faded -- more than a billion years before life truly took hold, a new University of Washington-led study has found.

The findings, based on using the element selenium as a tool to measure oxygen in the distant past, may also benefit the search for signs of life beyond Earth.

In a paper published Jan. 18 in the Proceedings of the National Academy of Sciences, lead author Michael Kipp, a UW doctoral student in Earth and Space Sciences, analyzed isotopic ratios of the element selenium in sedimentary rocks to measure the presence of oxygen in Earth's atmosphere between 2 and 2.4 billion years ago.

Kipp's UW coauthors are former Earth and space sciences postdoctoral researcher Eva Stüeken -- now a faculty member at the University of St. Andrews in Scotland -- and professor Roger Buick, who is also a faculty member with the UW Astrobiology Program. Their other coauthor is Andrey Bekker of the University of California, Riverside, whose original hypothesis this work helps confirm, the researchers said.

"There is fossil evidence of complex cells that go back maybe 1 ¾ billion years," said Buick. "But the oldest fossil is not necessarily the oldest one that ever lived -- because the chances of getting preserved as a fossil are pretty low.

"This research shows that there was enough oxygen in the environment to have allowed complex cells to have evolved, and to have become ecologically important, before there was fossil evidence." He added, "That doesn't mean that they did -- but they could have."

Kipp and Stüeken learned this by analyzing selenium traces in pieces of sedimentary shale from the particular time periods using mass spectrometry in the UW Isotope Geochemistry Lab, to discover if selenium had been changed by the presence of oxygen, or oxidized. Oxidized selenium compounds can then get reduced, causing a shift in the isotopic ratios which gets recorded in the rocks. The abundance of selenium also increases in the rocks when lots of oxygen is present.

Buick said it was previously thought that oxygen on Earth had a history of "none, then some, then a lot. But what it looks like now is, there was a period of a quarter of a billion years or so where oxygen came quite high, and then sunk back down again."

The oxygen's persistence over a long stretch of time is an important factor, Kipp stressed: "Whereas before and after maybe there were transient environments that could have occasionally supported these organisms, to get them to evolve and be a substantial part of the ecosystem, you need oxygen to persist for a long time."

Stüeken said such an oxygen increase has been guessed at previously, but it was unclear how widespread it was. This research creates a clearer picture of what this oxygen "overshoot" looked like: "That it was moderately significant in the atmosphere and surface ocean -- but not at all in the deep ocean."

What caused oxygen levels to soar this way only to crash just as dramatically?

"That's the million-dollar question," Stüeken said. "It's unknown why it happened, and why it ended."

"It is an unprecedented time in Earth's history," Buick said. "If you look at the selenium isotope record through time, it's a unique interval. If you look before and after, everything's different."

The use of selenium -- named after the Greek word for moon -- as an effective tool to probe oxygen levels in deep time could also be helpful in the search for oxygen -- and so perhaps life -- beyond Earth, the researchers said.

Future generations of space-based telescopes, they note, will give astronomers information about the atmospheric composition of distant planets. Some of these could be approximately Earth-sized and potentially have appreciable atmospheric oxygen.

"The recognition of an interval in Earth's distant past that may have had near-modern oxygen levels, but far different biological inhabitants, could mean that the remote detection of an oxygen-rich world is not necessarily proof of a complex biosphere," Kipp said.

Buick concluded, "This is a new way of measuring oxygen in a planet's historical past, to see whether complex life could have evolved there and persisted long enough to evolve into intelligent beings."

原始論文：Michael A. Kipp, Eva E. Stüeken, Andrey Bekker, and Roger Buick. Selenium isotopes record extensive marine suboxia during the Great Oxidation Event. Proceedings of the National Academy of Sciences, January 2017 DOI:10.1073/pnas.1615867114

引用自：University of Washington. "Conditions right for complex life may have come and gone in Earth's distant past." ScienceDaily. ScienceDaily, 17 January 2017.