科學家發現非洲之外最古老的現代人化石

原文網址：https://www.binghamton.edu/news/story/949/scientists-discover-oldest-known-modern-human-fossil-outside-of-africa

科學家發現非洲之外最古老的現代人化石

成員包括紐約州立大學―賓漢頓大學的Rolf Quam，由以色列臺拉維夫大學的Israel Hershkovitz領導的國際大型團隊發現了在非洲之外找到過的最古老現代人類化石。這項發現顯示現代人離開非洲的時間至少比之前認為的還要早了50000年。

「Misliya是項令人興奮的發現。」共同作者，賓漢頓大學的人類學教授Rolf Quam表示，「它呈現了迄今最清楚的證據，顯示我們祖先從非洲遷移出去的時間比過去認為的還要早上許多。這也意謂現代人可能有更長的時間跟其他古人族相遇並產生互動，使他們有更多機會產生文化以及生物層面上的交流。」

這副擁有數顆牙齒的上顎骨化石發現於以色列的Misliya洞窟，其為迦密山上數個史前考古洞穴場址之一。運用幾種不同技術來定年此處的考古材料和化石本身，顯示這具顎骨的年代大致介於17萬5000年至20萬年，將現代人從非洲遷出的時間至少往前推了5萬年。

研究人員依靠微型電腦斷層掃描和三維虛擬模型來分析這具化石遺骸，並跟其他從非洲、歐洲和亞洲發現的人族化石比對。

「雖然Misliya化石在解剖構造上的細節都完全符合現代人，卻也含有一些在尼安德塔人和其他人族身上同樣能見到的特徵。」賓漢頓大學的人類學副教授Quam說。「這項研究的難處之一便是要在Misliya化石上找出只能在現代人身上發現的特徵，它們是最為明確的訊號可以告訴我們Misliya化石究竟是什麼物種。」

考古證據顯示住在Misliya洞穴的居民具有獵捕大型動物的能力並可以控制用火，同時出土的還有一套舊石器時代早中期的石器組，跟在非洲發現，隨著最古老現代人一起發現的石器十分相似。

研究人員表示雖然在非洲曾發現更早的現代人化石，但如果想要瞭解我們所屬物種的演化歷程，現代人從非洲遷出的時間點和路徑是關鍵議題。中東地區是更新世時人族遷移的重要通道，在不同時期曾被現代人和尼安德塔人入主過。

相較於過往研究，Quam表示這項新發現讓我們有機會得知更早以前該區域的人口更迭或者現代人與當地族群的基因交流。最近從古代DNA得出的訊息指出現代人更早之前就已經離開非洲，大概是早於22萬年前，而從Misliya找到的證據確實也與之相符。最近幾個在亞洲發現的考古和化石證據也將現代人首次出現在這些區域的時間更往前推，間接暗示了現代人離開非洲的時間。

這篇文章「The earliest modern humans outside Africa」發表於期刊《科學》(Science)。

Scientists discover oldest known modern human fossil outside of Africa

A large international research team, led by Israel Hershkovitz from Tel Aviv University and including Rolf Quam from Binghamton University, State University of New York, has discovered the earliest modern human fossil ever found outside of Africa. The finding suggests that modern humans left the continent at least 50,000 years earlier than previously thought.

“Misliya is an exciting discovery,” says Rolf Quam, Binghamton University anthropology professor and a coauthor of the study. “It provides the clearest evidence yet that our ancestors first migrated out of Africa much earlier than we previously believed. It also means that modern humans were potentially meeting and interacting during a longer period of time with other archaic human groups, providing more opportunity for cultural and biological exchanges.”

The fossil, an upper jawbone with several teeth, was found at a site called Misliya Cave in Israel, one of several prehistoric cave sites located on Mount Carmel. Several dating techniques applied to archaeological materials and the fossil itself suggest the jawbone is between 175,000-200,000 years old, pushing back the modern human migration out of Africa by at least 50,000 years.

Researchers analyzed the fossil remains relying on microCT scans and 3D virtual models and compared it with other hominin fossils from Africa, Europe and Asia.

“While all of the anatomical details in the Misliya fossil are fully consistent with modern humans, some features are also found in Neandertals and other human groups,” said Quam, associate professor of anthropology at Binghamton. “One of the challenges in this study was identifying features in Misliya that are found only in modern humans. These are the features that provide the clearest signal of what species the Misliya fossil represents.”

The archaeological evidence reveals that the inhabitants of Misliya Cave were capable hunters of large game species, controlled the production of fire and were associated with an Early Middle Paleolithic stone tool kit, similar to that found with the earliest modern humans in Africa.

While older fossils of modern humans have been found in Africa, the timing and routes of modern human migration out of Africa are key issues for understanding the evolution of our own species, said the researchers. The region of the Middle East represents a major corridor for hominin migrations during the Pleistocene and has been occupied at different times by both modern humans and Neandertals.

This new discovery opens the door to demographic replacement or genetic admixture with local populations earlier than previously thought, said Quam. Indeed, the evidence from Misliya is consistent with recent suggestions based on ancient DNA for an earlier migration, prior to 220,000 years ago, of modern humans out of Africa. Several recent archaeological and fossil discoveries in Asia are also pushing back the first appearance of modern humans in the region and, by implication, the migration out of Africa.

The article, “The earliest modern humans outside Africa,” was published in Science Magazine.

原始論文：Israel Hershkovitz, Gerhard W. Weber, Rolf Quam, Mathieu Duval, Rainer Grün, Leslie Kinsley, Avner Ayalon, Miryam Bar-Matthews, Helene Valladas, Norbert Mercier, Juan Luis Arsuaga, María Martinón-Torres, José María Bermúdez de Castro, Cinzia Fornai, Laura Martín-Francés, Rachel Sarig, Hila May, Viktoria A. Krenn, Viviane Slon, Laura Rodríguez, Rebeca García, Carlos Lorenzo, Jose Miguel Carretero, Amos Frumkin, Ruth Shahack-Gross, Daniella E. Bar-Yosef Mayer, Yaming Cui, Xinzhi Wu, Natan Peled, Iris Groman-Yaroslavski, Lior Weissbrod, Reuven Yeshurun, Alexander Tsatskin, Yossi Zaidner, Mina Weinstein-Evron. The earliest modern humans outside Africa. Science, 26 Jan 2018 456-459 DOI: 10.1126/science.aap8369

引用自：Binghamton University. "Scientists discover oldest known modern human fossil outside of Africa."

在一團混亂中分離的地核和地函

原文網址：https://carnegiescience.edu/node/2289

在一團混亂中分離的地核和地函

根據卡內基研究院和史密森尼學會的科學家團隊發表在《自然》(Nature)的新研究，從地函內部湧升至火山熱點的岩石熱柱所含有的證據顯示，地球形成時的那段歲月可能比我們之前認為的還要混亂許多。

科學家已經清楚知道地球是由環繞幼年太陽的物質加積而成。當地球最後成長至足夠大小時密度較高的鐵沉入地球內部而形成地核的原型，剩下富含矽酸鹽的地函漂在上方。

但卡內基研究院的Yingwei Fei，以及同時隸屬於卡內基研究院和史密森尼學會的Colin Jackson領導的團隊進行的新研究，卻主張地函和地核的分離過程並非很有秩序地進行。

「我們的發現指出雖然地核是從地函分離出來，但地函本身從來沒有完整混合過。」Jackson解釋，「這令人感到十分驚訝。在地球的成長過程中，經歷了早期太陽系的其他天體重重撞擊之後緊接著形成了地核。這些撞擊事件跟之後形成月球的超大撞擊事件頗為相似。過去，科學家大都認為這些撞擊事件產生的巨大能量可以徹底翻攪地函，使得地函成分被攪拌得相當均一。」

讓團隊得出他們假設的關鍵證據來自於夏威夷之類的火山熱點，他們在此找到了獨特且古老的鎢和氙同位素訊號。雖然一般相信地函柱是來自於地函最深的區域，但這些特殊的同位素訊號起源為何仍有很大的爭議。研究團隊認為答案就在氙的母元素――碘――在非常高的壓力之下的化學性質。

同位素是同一元素質子數相同但中子數不同的版本。元素的放射性同位素是不穩定的，比方說碘-129。為了變成穩定狀態，碘-129會衰變成氙-129。因此，在地函柱樣品中的氙同位素訊號和碘在地核跟地函分離時的行為有直接關聯。

Jackson、Fei和共同作者――卡內基研究院的Neil Bennett和Zhixue Du，以及史密森尼學會的Elizabeth Cottrell，利用鑽石高壓砧來重現地球的地核從地函分離時的極端環境，來測量碘在當時如何分散至金屬地核和矽酸鹽質地函。他們也證實了如果地函還在成長時最深處就已經分離出原型地核，則地函中這些區塊擁有的化學性質可以解釋現今觀察到鎢和氙同位素呈現的特殊訊號，代表直到今日這些區塊和地函其他區域始終沒有完全混和。

據Bennett所言：「我們發現碘具有一種重要性質，它在相當高的溫度和壓力之下會開始溶入地核當中。在此極端環境下，碘和鉿(一種會放射性衰變成氙和鎢的元素)對形成地核的金屬會呈現出相反的喜好。這種行為造成的特殊同位素訊號跟我們現今在熱點看到的如出一轍。」

團隊的計算結果也預測鎢和氙的同位素訊號應該跟地函內部密度較高的區塊有關。

「就像是做餅乾的麵糰中加入的巧克力脆片，地函中這些密度較高的區塊非常難以攪拌開來。對於它們蘊含的古老鎢和氙同位素訊號來說，或許這是它們可以保存至今日的重要因素之一。」Jackson解釋。

「讓人更加興奮的是有越來越多地球物理證據顯示，緊鄰在地核上方的地函事實上有些密度較高的區域，它們被稱為超低速帶(ultralow velocity zones)或是大型低剪力波速群(large low shear velocity provinces)。而我們的研究成果和這些觀察可以緊密結合。」Fei補充，「我們在此研究中發展的方法也讓我們擁有新的機會能夠直接研究在地球深處發生的作用。」

本研究由國家科學基金會、卡內基科學研究院以及史密森尼學會資助。

Earth's core and mantle separated in a disorderly fashion

Plumes of hot rock surging upward from the Earth's mantle at volcanic hotspots contain evidence that the Earth's formative years may have been even more chaotic than previously thought, according to new work from a team of Carnegie and Smithsonian scientists published in Nature.

It is well understood that Earth formed from the accretion of matter surrounding the young Sun. Eventually the planet grew to such a size that denser iron metal sank inward, to form the beginnings of the Earth's core, leaving the silicate-rich mantle floating above.

But new work from a team led by Carnegie's Yingwei Fei and Carnegie and the Smithsonian's Colin Jackson argues that this mantle and core separation was not such an orderly process.

"Our findings suggest that as the core was extracted from the mantle, the mantle never fully mixed," Jackson explained. "This is surprising because core formation happened in the immediate wake of large impacts from other early Solar System objects that Earth experienced during its growth, similar to the giant impact event that later formed the Moon. Before now, it was widely thought that these very energetic impacts would have completely stirred the mantle, mixing all of its components into a uniform state."

The smoking gun that led the team to their hypothesis comes from unique and ancient tungsten and xenon isotopic signatures found at volcanic hotspots, such as Hawaii. Although it was believed that these plumes originated from the mantle's deepest regions, the origin of these unique isotopic signatures has been debated. The team believes that the answer lies in the chemical behavior of iodine, the parent element of xenon, at very high pressure.

Isotopes are versions of elements with the same number of protons, but different numbers of neutrons. Radioactive isotope of elements, such as iodine-129, are unstable. To gain stability, iodine-129 decays into xenon-129. Therefore, the xenon isotopic signatures in plume mantle samples are directly related to iodine's behavior during the period of core-mantle separation.

Using diamond anvil cells to recreate the extreme conditions under which Earth's core separated from its mantle, Jackson, Fei, and their colleagues -- Carnegie's Neil Bennett and Zhixue Du and Smithsonian's Elizabeth Cottrell -- determined how iodine was partitioning between metallic core and silicate mantle. They also demonstrated that if the nascent core separated from the deepest regions of the mantle while it was still growing, then these pockets of the mantle would possess the chemistry needed to explain the unique tungsten and xenon isotopic signatures, provided these pockets remained unmixed with the rest of the mantle all the way up through the present day.

According to Bennett: "The key behavior we identified was that iodine starts to dissolve into the core under very high pressures and temperatures. At these extreme conditions, iodine and hafnium, which decay radioactively to xenon and tungsten, display opposing preferences for core-forming metal. This behavior would lead to the same unique isotopic signatures now associated with hotspots."

Calculations from the team also predict that the tungsten and xenon isotopic signatures should be associated with dense pockets of the mantle.

"Like chocolate chips in cookie batter, these dense pockets of the mantle would be very difficult stir back in, and this may be a crucial aspect to the retention of their ancient tungsten and xenon isotopic signatures to the modern day," Jackson explained.

"Even more exciting is that there is increasing geophysical evidence that there actually are dense regions of mantle, resting just above the core -- called ultralow velocity zones and large low shear velocity provinces. This work ties together these observations," Fei added. "The methodology developed here also opens new opportunities for directly studying the deep Earth processes."

This work was supported by the National Science Foundation, the Carnegie Institution for Science, and the Smithsonian Institution.

原始論文：Colin R. M. Jackson, Neil R. Bennett, Zhixue Du, Elizabeth Cottrell & Yingwei Fei. Early episodes of high-pressure core formation preserved in plume mantle. Nature, 2018 DOI: 10.1038/nature25446

引用自：Carnegie Institution for Science. "Earth's core and mantle separated in a disorderly fashion."

太古宙的軟糊地函

原文網址：https://www.nature.com/articles/s41561-018-0060-5

太古宙的軟糊地函

實驗數據顯示地球的地函比之前認為的更容易熔化，因此直到20億至30億年以前或許都一直處在軟糊糊的狀態。

地球在45.6億年前形成時是處於完全熔化的狀態，自此之後便持續冷卻下來。時至今日，大部分的地球已經完全固化。金屬鐵地核的最外圍仍然為液體――其為最初熔化的地球殘存下來的最後一部份；但包覆在地核外面，由岩石組成的矽質地函卻幾乎全為固體。地函在地球歷史的大半時期可能都是處於固態；或者，這不過是我們的想法。Andrault和其同事發表在《自然―地質科學》的論文中，呈現的高壓熔化試驗結果指出地函熔化的溫度可能比我們以為的還要低200到250℃。因此在20億至30億年前地球比現在還要高溫的時候，地函可能大部分都處於部分熔融的狀態。地函從早期的軟弱狀態轉變成固體時或許產生了某些重大變化，包括板塊構造運動的改變在內。

地球的溫度從地表至核心一路從近乎零度升高至攝氏數千度。但此地溫梯度並不是一條單純而平滑的曲線。化學成分的不均、相變以及熱傳輸方式的差異(傳導或對流)結合起來把地溫梯度彎曲成一條斜率不一且有多次躍變的曲線。隨著深度增加，固態地函開始熔化時的溫度――即固相線――也會跟著上升。固相線的斜率通常比地溫梯度還要和緩，在低壓時固態線會跟地溫梯度交叉而讓熔融發生。

確實，地球表面的火山爆發即為現今地函在某些地點還是會熔化的證據。今日的中洋脊標示出地函固相線跟地溫梯度有所交會的主要地點。在這些環繞全球的中洋脊所發生的熔融現象和岩漿活動產生了地球三分之二的地殼。不過，相較於整個地函來說發生熔化的只占一小部分，因為地函大部分區域的溫度還是低於固相線。

測量地函固相線的實驗是把代表地球內部成分的岩石樣品在不同壓力範圍下加熱。但這不是一件簡單的任務，數十年來實驗人員的研究結果有很大的出入而帶給他們相當艱鉅的挑戰。在多數實驗中，實驗人員是在實驗結束，樣品冷卻到室溫時立刻仔細檢查岩石是否有發生熔融。然而，要用這種方式找出極為少量的熔融物質不啻是在大海撈針。通常熔融物質在樣品冷卻時就已經結晶成固體，要把它們辨識出來幾乎是不可能的任務。

Andrault和其同事運用一種不同方式來測量地函的固相線：他們在極端高溫高壓的實驗環境下就地分析樣品是否有熔融產生。雖然就技術層面來說相當困難，但研究人員利用X光繞射和電導率成功測量出他們的樣品中不到整體體積1%的熔融物質，使得他們呈現的地函熔點或許是目前為止最貼近真實情況的。在較低的壓力下，他們的實驗結果跟過往的研究相當符合。然而，在壓力到達7 GPa以上，相當於地下大約200公里以下的時候，他們發現地函的固相線比之前的估計還要低200到250°C。

今日大部分地函所處的溫度較此新測定的固相線還低。但是早期地球的溫度比現在要高。雖然科學家還尚未確切界定出地球的冷卻速率，不過可以合理認為在20億至30億年前，地函大部分的溫度都在這條固相線之上，因此同時具有固態晶體和熔融成岩漿的液體而處在軟糊糊的狀態。地函大部分區域都相當軟弱會使得地函整體的黏滯性降低，造成地球火山活動的數量和類型有所改變。

許多地質現象指出地球在20億至30億年前的這段時期歷經了重大變化：地殼年齡分布在這段時期有個高峰，代表地殼的產量增加或者是有更多地殼被保存下來；有些種類的岩漿停止生成；大氣中的氧氣含量首度增加；同時板塊構造運動的訊號首度在地球各處出現。雖然驅動這些變化的因素仍尚未定論，但是地函的狀態從軟弱變成堅硬提供了一個簡潔且巧妙的合理解釋。精確來說，Andrault和其同事提出地函中的軟弱層位會讓地函跟上覆的岩石圈各自為政。隨著地球逐漸冷卻，軟弱層位凝固下來，地函和岩石圈板塊會形成連結，因而啟動隱沒作用、板塊構造運動，以及其他種種變化。

雖然Andrault和其同事運用的就地測量方法對於含量極少的熔融物質相當敏銳，但卻無法迅速定出究竟有多少熔融物質。未來研究還需要測出當溫度超過固相線時熔融物質的增加量有多快(即熔融速率)。過往研究提出熔融速率可能是高度非線性的，當溫度在固相線附近時熔融產生的速率相當慢，但溫度到達某一個閾值時產量就會急遽上升。在過往其他敏銳度較低的實驗方式中得到的固相線溫度較高，事實上可能就是閾值，此時熔融的體積百分比提升到1%以上。若是如此，則地函現今與過往的差異可能就只是因為少量的熔融物質所造成，這是否足以解釋20億至30億年前發生的巨大變化仍然不清楚。

Andrault和其同事利用高壓實驗顯示地函的固相線比之前預估的還要低，也暗示了地函在太古宙的多數時間都是處於部分熔融的狀態。地球在此時發生的重大變化，包括板塊構造運動的啟動在內，可能都是因為地函較晚固化所導致。

An Archaean mushy mantle

Stephen Parman

Experimental data reveal that Earth’s mantle melts more readily than previously thought, and may have remained mushy until two to three billion years ago.

Earth was completely molten when it formed 4.56 billion years ago, and it has been cooling ever since. Today, most of the Earth has solidified. The outer part of the metallic iron core is still liquid — the last survivor of the originally molten Earth. But the rocky, silicate mantle that surrounds the core is largely solid. The mantle has probably been solid for most of Earth’s history. Or so we thought. Writing in Nature Geoscience, Andrault and colleagues present high-pressure melting experiments that indicate that the mantle can melt at temperatures 200 to 250 °C lower than previously thought. So, 2 to 3 billion years ago, when Earth was hotter than today, much of the mantle could have been partially molten. The transition from an early mushy mantle to a solid one may have generated several significant changes attributed to this time period, including the shift to plate tectonics.

Earth’s temperature increases from essentially zero at the surface to thousands of degrees Celsius at the centre. But this geothermal gradient is not a simple, smooth curve. Chemical heterogeneities, phase changes and differing modes of heat transport (conduction versus convection) combine to yield a geothermal gradient with varying slopes and numerous jumps. The temperature at which the solid mantle begins to melt — the solidus — also increases with depth. The slope is generally shallower than the geothermal gradient and, at low pressures, the solidus can cross the geothermal gradient, causing melting to occur.

Indeed, volcanic eruptions at Earth’s surface are evidence that the present-day mantle does melt in some places. Today, the mid-ocean ridges mark the primary location where the mantle solidus and geothermal gradient coincide. Melting and magmatism at these ridges — which encircle the globe — produces two-thirds of our planet’s crust. Still, the proportion of the total mantle that melts is small because most of it exists at temperatures below the solidus.

The mantle solidus is measured experimentally by heating rock samples representative of Earth’s interior over a range of pressures. But this is no easy task and has challenged experimentalists for decades, with studies producing widely disparate results. In most experiments, the presence of melt is determined by closely examining the sample after the experiment, once it has cooled to room temperature. However, finding tiny amounts of melt this way can be like searching for a needle in a haystack. Often the melts crystallize when the sample is cooled, making their identification nearly impossible.

Andrault and colleagues1 use a different approach to measure the mantle solidus. They analyse their samples for the existence of melt in situ at extreme high pressure and temperature. This is technically quite challenging, but the researchers use X-ray diffraction and electrical conductivity to detect less than 1 vol% melt in their samples, thus providing possibly the best constraints yet on the mantle’s melting point. At relatively low pressures, their experiments match previous work. However, at pressures above 7 GPa, which corresponds to a depth of about 200 km, they find that the mantle solidus is 200 to 250 °C lower than previous estimates.

Today, most of the mantle exists at temperatures below this newly determined solidus. But the early Earth was warmer. Although Earth’s exact cooling rate is not well constrained, it is plausible that 2 to 3 billion years ago, most of the mantle was above the solidus temperature and could have been mushy — composed of both magmatic melt and crystals. The existence of extensive mushy mantle would have lowered the viscosity of the mantle and changed the amount and types of volcanism on the planet.

Many geologic observations suggest this period 2 to 3 billion years ago was a time of great change on Earth: there was a spike in crustal ages, indicative of either increased crustal production or preservation, some lava types ceased production, oxygen rose in the atmosphere for the first time, and the first pervasive signs of plate tectonics appeared. What drove these changes is debated, but a transition from a mushy to a solid mantle provides a plausible and elegantly simple mechanism. Specifically, Andrault and colleagues propose that a mushy layer could have decoupled the mantle from the overlying lithosphere. As Earth cooled and the mush layer crystallized, the mantle and lithospheric plates could have become coupled, triggering the onset of subduction and plate tectonics, among other changes.

Although the in situ methods used by Andrault and colleagues are sensitive to the presence of exceedingly small amounts of melt, they do not readily quantify the amount of melt present. Future work will need to measure how quickly the amount of melt increases as temperature rises above the solidus (the melting rate). Previous work suggests that the melting rate can be highly non-linear, with very low rates of melt production at temperatures near the solidus and a dramatic increase in melt production only when a threshold temperature is reached. The higher-temperature solidi found by previous less-sensitive experimental approaches may actually be this threshold point, where the melt percent rises above about 1 vol%. If so, the difference between the modern and ancient mantle may be the presence of only a small amount of melt. Whether that would be enough to explain the large changes seen 2 to 3 billion years ago is not yet clear.

Andrault and colleagues use high-pressure experiments to show that the mantle solidus is lower than previously estimated, implying that it could have remained partially molten for much of the Archaean. Significant changes on Earth in this time period, including the initiation of plate subduction, may have resulted from this delayed solidification.

原始論文：Denis Andrault, Giacomo Pesce, Geeth Manthilake, Julien Monteux, Nathalie Bolfan-Casanova, Julien Chantel, Davide Novella, Nicolas Guignot, Andrew King, Jean-Paul Itié & Louis Hennet. Deep and persistent melt layer in the Archaean mantle. Nature Geoscience, 2018; doi:10.1038/s41561-017-0053-9

引用自：Stephen Parman. An Archaean mushy mantle. Nature Geoscience, 2018; doi:10.1038/s41561-018-0060-5

太古宙的氧氣綠洲

原文網址：https://www.nature.com/articles/s41561-018-0058-z

太古宙的氧氣綠洲

Maya L. Gomes

在大氣氧濃度呈兩階段上升的過程中，第一階段發生在太古宙結束之際。分析黃鐵礦的硫和鐵同位素，顯示太古宙中期的沿岸環境有個地方處於局部氧化的狀態。

大約在25億年前，地球大氣的氧濃度突然開始飆升。此次「大氧化事件」(Great Oxidation Event)造成地表的生態系從無氧為主轉變成有氧狀態，最終使地球演化出複雜的生命形式。藍綠菌是此次變化的推手之一，這種細菌演化出的能力可以把水當作電子供應者來獲得能量，並釋放出副產物――氧氣。但是科學家仍不清楚大氧化事件是在產氧光合作用演化出來之後立刻發生，或者是一段時間之後才因為其他演化革新或是環境因素而引發。Eickmann等人在《自然―地質科學》(Nature Geoscience)撰寫的論文中，呈現的證據指出大約在30億年前的太古宙中期，海洋近岸地區有座氧氣綠洲。意謂藍綠菌演化出產氧光合作用之後過一段時間大氣才開始出現大規模的氧化現象。

由於太古宙中期(32億至28億年前)位於大氧化事件發生之前，所以一般認為當時的大氣幾乎是處在無氧狀態。在無氧大氣下，光化學反應可以造成跟質量無關的硫同位素分餾。因此，老於24.5億年前的岩石中保有跟質量無關的硫同位素訊號而之後的岩石卻沒有，通常被視為大氧化事件時氧氣濃度上升的決定性證據。然而，跟質量無關的硫訊號卻在太古宙中期沉寂下來而跟前述現象形成矛盾。這可能是因為當時大氣光化學作用有所改變或者是被跟質量相關的硫同位素訊號淡化所導致。造成後者的可能嫌犯之一是微生物進行的硫酸鹽還原作用，它產生的硫會帶有跟質量相關的訊號。因此，有人認為大氣氧濃度上升加上海洋硫酸鹽增加，造成微生物硫酸鹽還原作用產生的硫增加，導致跟質量無關的硫同位素訊號在當時減弱。但是，以往缺乏證據可以指出太古宙中期有發生跟質量相關的分餾作用，使得這項解釋的說服力不足。

Eickmann和其同事決心要解決此矛盾。他們呈現的硫同位素數據來自南非沉積岩中的黃鐵礦，其形成於30億年前卡普瓦克拉通(Kaapvaal Craton)上受到潮汐影響的淺海環境。數據中的黃鐵礦有兩種不同類型：全岩樣品磨碎後取出的浸染狀黃鐵礦細粒，以及用微鑽孔得到的黃鐵礦結核樣品。浸染狀黃鐵礦細粒跟過往研究一致，並沒有確切證據顯示曾經發生過跟質量相關的硫同位素分餾作用。相較之下，黃鐵礦結核的硫同位素訊號卻指示了在富含硫酸鹽的環境下，由微生物進行的跟質量相關的硫同位素分餾作用。相當重要的一點是，兩種類型的黃鐵礦都含有跟質量無關的硫同位素訊號，表示它們是在幾乎無氧的大氣中沉積而成。因此，當時必定有一座維持高硫酸鹽濃度的氧氣綠洲，使得黃鐵礦結核擁有跟質量相關的硫同位素訊號。

為了進一步探討太古宙中期淺海環境氧化還原條件的不均情形以及生物地球化學循環，Eickmann和其同事也分析了鐵同位素。他們指出相較於元古宙中期從熱泉進入深海的鐵，黃鐵礦結核的鐵缺乏重同位素。來自熱泉的鐵在上湧過程中重同位素會逐漸減少，這是因為氧化反應以及後續的沉澱作用所造成(經由微生物參與的作用或者有自由氧的情況下發生的非生物作用)。Eickmann等人假設剩餘的鐵到達近岸環境時都已經完全氧化，因此含有較輕的鐵同位素訊號。而這些鐵之後在沉積物內部被還原時，就成為了黃鐵礦結核的原料。

要把硫和鐵同位素訊號解釋成局部地區處於氧化環境，所仰賴的前提是黃鐵礦形成於成岩作用的早期，因此仍保有當時近岸環境的資訊。作者提出地球化學和組構證據來佐證這些黃鐵礦結核跟其他太古宙的黃鐵礦結核研究類似，都顯示了原生環境的訊號。如果此篇研究中的黃鐵礦結核是由沉積物裡的浸染狀黃鐵礦在成岩階段早期溶解而形成，解開兩者之間的硫同位素訊號為何不一樣，或許可以提供額外資訊讓科學家瞭解元古宙中期海洋氧化還原條件不均的情形。

Eickmann和其同事雖然讓我們更加瞭解太古宙中期為何跟質量無關的硫同位素訊號會沉寂下來，但是氧氣綠洲的解釋本質上來說仍然是出現在局部地區的現象。從這4億年之間堆積的沉積物中得到的數據，都可以發現硫同位素訊號有沉寂下來的情形。因此，它和氧氣綠洲這種局部現象之間的關連仍然是有待解決的難題。此外，在太古宙中期之後的太古宙晚期，如果陸上黃鐵礦持續跟氧氣作用而風化，使得海洋繼續累積硫酸鹽，那麼跟質量無關的硫同位素訊號應該會越來越淡。但實際上，太古宙晚期跟質量無關的硫同位素訊號卻比較強。這或許暗示了在大氧化事件發生之前，太古宙晚期的氧氣濃度曾一度下降過。或者是硫循環內部還有別種未知作用，能改變大氣的光化學反應而強化跟質量無關的硫同位素訊號。

太古宙結束之際為何大氣氧濃度會快速上升仍然是個謎團。已經出現的藍綠菌也許有什麼生物學上的革新，使得它們可以進行更大規模的產氧光合作用，造成大氣中的自由氧快速累積。或者是環境變遷，比方說全球被冰河覆蓋，也有可能改變生物地球化學循環的狀態，使得平衡往大氣氧濃度較高的那側傾斜。不管是透過哪種方式，這些理論都還需要岩石紀錄來驗證。而Eickmann和其同事提供了相當重要的一片線索，在未來解決大氧化事件的謎題時勢必能派上用場。

太古宙硫同位素的紀錄有許多細微差異。Eickmann和其同事提出的地球化學證據雖然並未完全剔除藍綠菌對大氧化事件的重要性，但確實進一步指出在大氧化事件發生許久之前，會釋放氧氣的藍綠菌就已經出現在地球且相當活躍了。

An Archaean oxygen oasis

The first of two stepwise increases in atmospheric oxygen occurred at the end of the Archaean eon. Analyses of sulfur and iron isotopes in pyrite reveal a near-shore environment that hosted locally oxygenated conditions in the Mesoarchaean era.

About 2.5 billion years ago, oxygen concentrations in the Earth’s atmosphere rose sharply. This Great Oxidation Event1^,2 caused a transition from dominantly anaerobic to aerobic surface ecosystems, and ultimately led to the evolution of complex life on Earth. The agents of change were cyanobacteria, a type of bacteria that evolved the ability to gain energy using water as an electron donor, and release oxygen as a by-product. But it is not clear whether the Great Oxidation Event immediately followed the evolution of oxygenic photosynthesis or was initiated later by another evolutionary innovation or environmental driver. Writing in Nature Geoscience, Eickmann et al.3 present geochemical evidence for the existence of a near-shore marine oxygen oasis about 3 billion years ago, in the Mesoarchaean era. This suggests a delay between the evolution of oxygenic photosynthesis by cyanobacteria and the later widespread oxygenation of the atmosphere.

As the Mesoarchaean (3.2–2.8 billion years ago) falls before the Great Oxidation Event, atmospheric conditions are thought to have been mostly anoxic. Under an anoxic atmosphere, photochemical reactions can fractionate sulfur isotopes independent of mass. Thus, the change from mass-independent sulfur isotopic signatures preserved in older rocks to their absence in rocks younger than 2.45 billion years old is considered to be conclusive proof of the rise of oxygen at the Great Oxidation Event1. However, the mass-independent sulfur signal is muted during the Mesoarchaean, presenting a conundrum. This muted signal could be due to differences in atmospheric photochemistry4or dilution of the signal by sulfur with a mass-dependent signal. A likely culprit for the latter is the contribution of sulfur with a mass-dependent signal from microbial sulfate reduction. This requires high sulfate levels generated by enhanced oxidative weathering on land under an oxygenated atmosphere. But lack of evidence for mass-dependent fractionations in the Mesoarchaean has previously hampered the interpretation that mass-independent signatures were dampened by inputs from microbial sulfate reduction under elevated atmospheric oxygen and marine sulfate levels.

Eickmann and colleagues3 aim to resolve this conundrum. They present sulfur isotope data from pyrites in sediments from South Africa that were deposited 3 billion years ago in a tide-influenced, shallow-marine environment on the Kaapvaal Craton. The data come from two distinct types of pyrites: finely disseminated pyrite from powdered whole-rock samples and pyrite nodules that were sampled by micro-drilling. The finely disseminated pyrites show no conclusive evidence of mass-dependent sulfur isotope fractionation, in line with previous studies. In contrast, sulfur isotope signatures from the pyrite nodules are indicative of mass-dependent sulfur fractionation by microbial sulfate reduction in a sulfate-rich environment. Importantly, both types of pyrite carry mass-independent sulfur isotope signals indicating that they were deposited under a largely anoxic atmosphere. Thus, a localized oxygen oasis must have maintained sufficiently high sulfate levels to impart the mass-dependent sulfur isotope signals in the pyrite nodules.

To further explore redox heterogeneity and biogeochemical cycling in this Mesoarchaean shallow-marine environment, Eickmann and colleagues also analyse iron isotopes. They show that the pyrite nodules are depleted in the heavy iron isotope relative to iron that entered the Mesoarchaean deep sea from hydrothermal vents. The hydrothermally sourced iron became progressively depleted during upwelling owing to oxidation and subsequent precipitation, either by microbially mediated reactions or abiotic reactions in the presence of free oxygen. Eickmann et al. hypothesize that the residual iron reaching the near-shore environment was completely oxidized, inheriting the light isotope signal, and was the source of iron for pyrite nodule formation after subsequent reduction in the sediment.

The interpretation that the sulfur and iron isotopes indicate locally oxidized conditions relies on the assumption that the pyrite nodules were formed during early diagenesis and thus preserve information about the near-shore environment. The authors present geochemical and textural evidence in support of a primary environmental signal, similar to other Archaean pyrite nodule studies5. Understanding why the nodule pyrites in this study have different sulfur isotope signals from the disseminated pyrites if they formed from dissolution of disseminated pyrite in the sediments during early diagenesis6 may provide additional information about redox heterogeneity in Mesoarchaean oceans.

Although Eickmann and colleagues advance our understanding of the muted mass-independent signal in the Mesoarchaean, the oxygen oasis interpretation is, by nature, a localized condition. Thus, a challenge remains to explain how these local conditions might be related to the muted sulfur signature that is found in all the data generated from sediments deposited over approximately 400 million years. Further, after the Mesoarchaean — in the Neoarchaean — the mass-independent sulfur signal should continue to be increasingly diluted as sulfate accumulates in the ocean through oxidative weathering of terrestrial pyrite. But, in fact, the mass-independent signal is stronger in the Neoarchaean4. This could imply that the oxygen levels decreased in the Neoarchaean before the Great Oxidation Event. Alternatively, there may be additional processes, not yet recognized, of sulfur cycling associated with changing atmospheric photochemistry that strengthens the mass-independent signal.

The cause of the rapid rise of atmospheric oxygen at the end of the Archaean remains enigmatic. Biological innovations by existing cyanobacteria could have led to an expansion of their oxygenic photosynthetic activity and the accumulation of free oxygen in the atmosphere7. Or environmental changes, such as a global glaciation, could have led to a state change in biogeochemical cycling that tipped the scale towards higher atmospheric oxygen levels8. Either way, these hypotheses need to be tested in the rock record, and Eickmann and colleagues contribute an important piece of evidence that will be used to solve the mystery of the Great Oxidation Event.

There are many nuances in the sulfur isotope record of the Archaean. Although not necessarily absolving the cyanobacteria of responsibility, Eickmann and colleagues3provide further geochemical evidence to suggest that oxygen releasing cyanobacteria were present and active long before the Great Oxidation Event.

原始論文：Benjamin Eickmann, Axel Hofmann, Martin Wille, Thi Hao Bui, Boswell A. Wing & Ronny Schoenberg. Isotopic evidence for oxygenated Mesoarchaean shallow oceans. Nature Geoscience, 2018; Doi:10.1038/s41561-017-0036-x

引用自：Maya L. Gomes. An Archaean oxygen oasis. Nature Geoscience, 2018.