岩漿化學中的臨界點決定了火山是否會猛烈爆發

原文網址：http://www.bristol.ac.uk/news/2017/december/chemical-tipping-point-of-magma.html

岩漿化學中的臨界點決定了火山是否會猛烈爆發

在活躍行星內部進行的作用於行星表面最壯觀的表現方式便是火山爆發。

在溢流式噴發中，熔岩流會緩緩地持續流出至地表；反之，爆裂式噴發則是一種劇烈的自然現象，可以將熾熱的物質噴發至大氣上空數公里處。

這兩種噴發類型的轉變也代表了最危險的自然災害之一。

過去數十年來，為了瞭解這樣的轉變背後機制究竟為何，而啟發了無數地球科學研究。

在一篇新研究中，由布里斯托大學地球科學院的Danilo Di Genova博士領導的國際科學家團隊，首度提出證據顯示岩漿化學中極度微小的差異，就能將世界各地的寧靜式噴發和爆裂式噴發明確區分開來。

此外，他們也指出岩漿在奈米尺度上的變化也能劇烈增加火山發生爆裂式噴發的可能性。

Di Genova博士表示：「我們在研究中呈現了新的實驗數據、熱力學模擬，並分析全球火山紀錄中有關成分方面的資料，這些證據結合起來顯示流紋岩質岩漿的流動特性中出現的急遽轉變，決定了一座火山的噴發方式是溢流式或爆裂式。」

「我們發現在流紋岩質岩漿中，些微的成分變化就能讓流動性質產生重大差異。成分變化的原因可能是結晶作用、同化作用、岩漿補充或者混合作用。」

「由成分引起的流動特性變化也可能是源自於岩漿本身性質的改變，像是溫度、壓力或者氧逸度。」

這可能會讓先前已經「關閉」的岩漿庫因為化學作用造成的液化而重新活動；或者降低脫氣作用的進行效率，使得岩漿因為化學作用而「固化」，造成它發生爆裂式噴發的可能性增加。

此外，研究也顯示最近在火山岩中發現含鐵奈米晶體迅速沉澱的現象，會使得熔岩結構中的鐵變少，並提供引起爆裂式噴發的氣泡形成時所需的成核中心，而增加岩漿發生爆裂式噴發的機率。

Chemical tipping point of magma determines explosive potential of volcanoes

Volcanic eruptions are the most spectacular expression of the processes acting in the interior of any active planet.

Effusive eruptions consist of a gentle and steady flow of lava on the surface, while explosive eruptions are violent phenomena that can eject hot materials up to several kilometres into the atmosphere.

The transition between these eruptions represents one of the most dangerous natural hazards.

Understanding the mechanisms governing such transition has inspired countless studies in Earth Sciences over the last decades.

In a new study led by Dr Danilo Di Genova, from the University of Bristol’s School of Earth Sciences, an international team of scientists provide evidence, for the first time, that a subtle tipping point of the chemistry of magmas clearly separates effusive from explosive eruptions worldwide.

Moreover, they demonstrate that variabilities at the nanoscale of magmas can dramatically increase the explosive potential of volcanoes.

Dr Di Genova said: “The new experimental data, thermodynamic modelling and analysis of compositional data from the global volcanic record we presented in our study provide combined evidence for a sudden discontinuity in the flow behaviour of rhyolitic magmas that guides whether a volcano erupts effusively or explosively.

“The identified flow-discontinuity can be crossed by small compositional changes in rhyolitic magmas and can be induced by crystallisation, assimilation, magma replenishment or mixing.

“Composition-induced flow behaviour variations may also originate from changes in magmas intrinsic parameters such as temperature, pressure or oxygen fugacity.”

These can result in revitalization of a previously “locked” magma chamber via chemical fluidification or may hinder efficient degassing and lead to increased explosive potential via chemical “stiffening” of a magma.

Furthermore, the study showed how the sudden precipitation of iron-bearing nanocrystals, which have been recently found in volcanic rocks, can increase the explosive potential of a magma via both depletion of iron in the melt structure and providing nucleation points for gas bubbles which drive explosive eruption. 

原始論文：D. Di Genova, S. Kolzenburg, S. Wiesmaier, E. Dallanave, D. R. Neuville, K. U. Hess, D. B. Dingwell. A compositional tipping point governing the mobilization and eruption style of rhyolitic magma. Nature, 2017; 552 (7684): 235 DOI: 10.1038/nature24488

引用自；University of Bristol. "Chemical tipping point of magma determines explosive potential of volcanoes." 

原文網址：https://www.sciencedaily.com/releases/2017/11/171130141045.htm

發現強烈地震初期產生的重力訊號可以用來定出其規模

在地震發生之後地球重力場幾乎立刻就會受到擾動，這種現象被儀器記錄到的時間會早於地震學家經常分析的地震波。在2017年12月1日發表在《科學》(Science)的研究中，由CNRS、IPGP和巴黎第七大學的研究人員組成的團隊成功觀測到這種跟重力有關的微弱訊號，並且解釋它們的來源。由於此訊號易受地震規模的影響，因此在快速辨識大地震是否發生時，它們或許能起到相當重要的作用。

想要更加瞭解地震的地震學家跟為了偵測重力波而發展精確重力觀測技術的物理學家合作之下促成了這項研究。地震會劇烈改變地球應力的平衡狀態而產生地震波，造成的後果可能相當具有破壞力。不過同樣的波動也會擾亂地球的重力場而放出不同的訊號。對於想要快速定量震動幅度的研究人員來說，這種訊號令他們相當有興趣，因為它們不像震波的行進速度僅有時速3至10公里，而是以光速來行進。因此距離震央1000公里遠的地震儀測站，或許有機會能在地震波到達兩分鐘之前就先偵測到此種訊號。

在2016年的一篇研究中初次呈現此種訊號，而此篇研究接續下去做出了更加詳細的探討。首先，在距2011年日本地震(規模9.1)震央周圍500至3000公里的範圍內，科學家確實從十個左右的地震儀紀錄中觀察到這類訊號。研究人員從此觀察結果接續證明訊號是源自於兩種效應。第一種效應是地震儀所在地點的重力變化會改變儀器質量的平衡分布。第二個效應則是以間接方式影響：由於地球任何一處的重力變化也會擾動原有的應力平衡，因此會產生新的地震波而被地震儀接收到。

研究人員探討上述兩種效應而證明跟重力有關的此類訊號相當容易受到地震規模影響，因此在強烈地震發生時有望利用它們來快速定量地震規模。研究人員未來面臨的挑戰是要想辦法將此種訊號運用在規模小於8至8.5的地震，因為在低於這個規模時，訊號相對於地球自然產生的地震波噪訊相比過於微弱，而要將兩者分離需要相當複雜的過程。所以研究人員已經開始著眼於數種技術，其中有些靈感源自於用來偵測重力波的儀器，計畫更進一步地對這些十分珍稀的訊號進行測量。

New early gravity signals to quantify the magnitude of strong earthquakes

After an earthquake, there is a disturbance in the field of gravity almost instantaneously. This could be recorded before the seismic waves that seismologists usually analyze. In a study published in Science on December 1, 2017, a team formed of researchers from CNRS, IPGP, the Université Paris Diderot and Caltech has managed to observe these weak signals related to gravity and to understand where they come from. Because they are sensitive to the magnitude of earthquakes, these signals may play an important role in the early identification of the occurrence of a major earthquake.

This work came out of the interaction between seismologists who wanted to better understand earthquakes and physicists who were developing fine gravity measurements with a view to detecting gravitational waves. Earthquakes change the equilibrium of forces on Earth brutally and emit seismic waves whose consequences may be devastating. But these same waves also disturb Earth's field of gravity, which emits a different signal. This is particularly interesting with a view to fast quantification of tremors because it moves at the speed of light, unlike tremor waves, which propagate at speeds between 3 and 10 km/s. So seismometers at a station located 1000 km from the epicenter may potentially detect this signal more than two minutes before the seismic waves arrive.

The work presented here, which follows on a 2016 study which demonstrated this signal for the first time, greatly increases its understanding. First, the scientists did indeed observe these signals on the data from about ten seismometers located between 500 and 3000 km from the epicenter of the 2011 Japanese earthquake (magnitude 9.1). From their observations, the researchers then demonstrated that these signals were due to two effects. The first is the gravity change that occurs at the location of the seismometer, which changes the equilibrium position of the instrument's mass. The second effect, which is indirect, is due to the gravity change everywhere on Earth, which disturbs the equilibrium of the forces and produces new seismic waves that will reach the seismometer.

Taking account of these two effects, the researchers have shown that this gravity-related signal is very sensitive to the earthquake's magnitude, which makes it a good candidate for rapidly quantifying the magnitude of strong earthquakes. The future challenge is to manage to exploit this signal for magnitudes below about 8 to 8.5, because below this threshold, the signal is too weak relative to the seismic noise emitted naturally by Earth, and dissociating it from this noise is complicated. So several technologies, including some inspired from instruments developed to detect gravitational waves, are being envisaged to take a new step forward in detection of these precious signals.

原始論文：Martin Vallée, Jean Paul Ampuero, Kévin Juhel, Pascal Bernard, Jean-Paul Montagner, Matteo Barsuglia. Observations and modeling of the elastogravity signals preceding direct seismic waves. Science, 2017; DOI: 10.1126/science.aao0746

引用自：CNRS. "New early gravity signals to quantify the magnitude of strong earthquakes." ScienceDaily. ScienceDaily, 30 November 2017. 

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."