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.”

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."