沒有甲烷的世界：早期地球如何保持溫暖？

原文網址：www.sciencedaily.com/releases/2016/10/161007090659.htm

沒有甲烷的世界：早期地球如何保持溫暖？

在至少10億年前的遙遠過去，地球應當整個冰封但實際上卻並未發生。科學家認為他們曉得箇中原由，但NASA天體生物學研究所的「替代地球」小組進行的新模擬研究，卻罷除了這段久為接受情節中的主角位置。

雖然人類十分擔憂溫室氣體，但對18億至8億年前生活在海洋的微小生物來說，它們卻是不可或缺的。當時太陽比現今還要黯淡了百分之10到15，它傳遞的能量太過微弱使得單靠太陽並無法讓地球保持溫暖。因此地球勢必需要由可以困住熱量的氣體來混合成某種有效配方，讓海洋維持液態並使生命得以存活。

數十年以來，大氣科學家將甲烷推派為主要角色。他們的想法主要是認為在地球最初35億年，起初氧氣不存在而後續含量也微不足道的情況下，保溫能力是二氧化碳34倍的甲烷在這段期間多數時候應該具有絕對主導地位。(今日我們呼吸的空氣中氧氣佔了五分之一，這會使得甲烷在幾年內就被破壞殆盡。)

「從海洋生地化循環的角度可以合理解讀出甲烷有比氧氣還要危險的敵人。」此篇刊登於9月26日《美國國家科學院院刊》(Proceedings of the National Academy of Sciences)的論文第一作者，加州大學河濱分校的研究生Stephanie Olson說。她也是「替代地球」團隊的成員之一。「當海洋有硫酸鹽時，你不可能從中得到大量甲烷。」

直到大氣出現氧氣並引發陸上氧化造成的岩石風化作用之後，硫酸鹽才不太造成影響。黃鐵礦之類的礦物被分解時會產生硫酸鹽，隨後會跟著河水流向大海。 Olson表示雖然氧氣較少代表海洋硫酸鹽含量也較低，但只要濃度有現在海洋的1%，就足以將甲烷消滅殆盡。

Olson和她「替代地球」團隊的另外兩位共同作者，喬治亞理工學院地球和大氣科學助理教授 Chris Reinhard，以及加州大學河濱分校生物化學特聘教授Timothy Lyons的主張中，他們推測10億年前海洋硫酸鹽將大氣甲烷濃度限制在1至10 ppm(百萬分之一)，這跟先前某些模型呈現出大氣含有高達300 ppm的甲烷相比微小許多。

Olson說過往氣候模型以及它們估計出的大氣成分會有嚴重缺陷，是因為甲烷大部分來自於海洋特定種類細菌分解有機物質而產生，但這些模型卻忽略了海洋中發生的作用。

海洋硫酸鹽會以兩種方式對甲烷造成麻煩：首先硫酸鹽會直接摧毀甲烷，而限制甲烷從海洋逸散出來和累積在大氣中的量。另一方面硫酸鹽會減少甲烷產量，生物從還原硫酸鹽得到的能量多於製造甲烷時得到的，因此近乎所有海洋環境中的細菌都會優先消耗硫酸鹽而非製造甲烷。

此篇研究中的數值模型計算了從18億至8億年前這10億年間，海洋中硫酸鹽還原、甲烷產量以及其他各種生地化循環如何作用。這是目前為止運用在早期地球的模型中解析度最高者，它將海洋分成15000塊三維區域並分別計算各個地區發生的循環作用。相較而言，其他生地化模型是將全球海洋分割成二維網格且最多不超過5格。

「確實還沒有其他模型可以與之相提並論。」另一篇刊登於美國國家科學院院刊的相關研究第一作者Reinhard說。這篇論文從顯示硫酸鹽與甲烷之間存有致命關係的同一份模擬結果中，來敘述當時氧氣經歷了何種命運。

Reinhard依據「替代地球」小組近日發表於期刊《科學》( Science)和《地質》( Geology)所顯示的獨立證據，注意到氧氣給予了甲烷另一次打擊。依據這兩篇論文的論述，直到氧氣於8億年前含量急遽攀升之前，較早岩石紀錄中的地化訊號顯示大氣含氧量非常低落，可能遠少於現今濃度的百分之一。

既然這兩種氣體彼此之間無法共處，較少氧氣對甲烷來說似乎是項好消息。然而當氧氣濃度降到如此稀薄，會產生另一項問題。

「臭氧層可以保護甲烷免受光化學反應破壞。要產生這道保護層，大氣中必需有自由氧分子存在，」Reinhard表示。當研究人員以預估的低氧濃度來運行模擬，在過程中從未有臭氧層產生，導致勉強從海洋散出的一絲甲烷還是難逃被光化學作用破壞的命運。

Lyons表示在缺乏甲烷的情況下，科學家會面臨新的嚴峻挑戰來確立溫室氣體大雜燴是如何調製，以解釋地球氣候和生命的眾多情節為何發生，包含長達10億年間地球無冰河形成的原因。了解其他暖化氣體，像是水氣、氮氧化物以及二氧化碳的確切組成，也可以幫助我們估計銀河系中其他與地球類似的數千億顆行星的適居程度。

「若我們在一顆系外行星上偵測到甲烷，由於其為一種生命跡象(biosignature)因此這顆行星會成為我們尋找生命的最佳候選對象之一。另外，在尋找火星生命的許多討論中也以甲烷為主軸。」 Lyons說，「然而，若外星文明在十億年前觀察我們的星球時，他們幾乎無法偵測到甲烷--儘管地球大部分歷史中很可能都有生物在不斷地產生甲烷。」

Methane muted: How did early Earth stay warm?

For at least a billion years of the distant past, planet Earth should have been frozen over but wasn't. Scientists thought they knew why, but a new modeling study from the Alternative Earths team of the NASA Astrobiology Institute has fired the lead actor in that long-accepted scenario.

Humans worry about greenhouse gases, but between 1.8 billion and 800 million years ago, microscopic ocean dwellers really needed them. The sun was 10 to 15 percent dimmer than it is today -- too weak to warm the planet on its own. Earth required a potent mix of heat-trapping gases to keep the oceans liquid and livable.

For decades, atmospheric scientists cast methane in the leading role. The thinking was that methane, with 34 times the heat-trapping capacity of carbon dioxide, could have reigned supreme for most of the first 3.5 billion years of Earth history, when oxygen was absent initially and little more than a whiff later on. (Nowadays oxygen is one-fifth of the air we breathe, and it destroys methane in a matter of years.)

"A proper accounting of biogeochemical cycles in the oceans reveals that methane has a much more powerful foe than oxygen," said Stephanie Olson, a graduate student at the University of California, Riverside, a member of the Alternative Earths team and lead author of the new study published September 26 in the Proceedings of the National Academy of Sciences. "You can't get significant methane out of the ocean once there is sulfate."

Sulfate wasn't a factor until oxygen appeared in the atmosphere and triggered oxidative weathering of rocks on land. The breakdown of minerals such as pyrite produces sulfate, which then flows down rivers to the oceans. Less oxygen means less sulfate, but even 1 percent of the modern abundance is sufficient to kill methane, Olson said.

Olson and her Alternative Earths coauthors, Chris Reinhard, an assistant professor of earth and atmospheric sciences at Georgia Tech University, and Timothy Lyons, a distinguished professor of biogeochemistry at UC Riverside, assert that during the billion years they assessed, sulfate in the ocean limited atmospheric methane to only 1 to 10 parts per million -- a tiny fraction of the copious 300 parts per million touted by some previous models.

The fatal flaw of those past climate models and their predictions for atmospheric , Olson said, is that they ignore what happens in the oceans, where most methane originates as specialized bacteria decompose organic matter.

Seawater sulfate is a problem for methane in two ways: Sulfate destroys methane directly, which limits how much of the gas can escape the oceans and accumulate in the atmosphere. Sulfate also limits the production of methane. Life can extract more energy by reducing sulfate than it can by making methane, so sulfate consumption dominates over methane production in nearly all marine environments.

The numerical model used in this study calculated sulfate reduction, methane production, and a broad array of other biogeochemical cycles in the ocean for the billion years between 1.8 billion and 800 million years ago. This model, which divides the ocean into nearly 15,000 three-dimensional regions and calculates the cycles for each region, is by far the highest resolution model ever applied to the ancient Earth. By comparison, other biogeochemical models divide the entire ocean into a two-dimensional grid of no more than five regions.

"There really aren't any comparable models," says Reinhard, who was lead author on a related paper in Proceedings of the National Academy of Sciences that described the fate of oxygen during the same model runs that revealed sulfate's deadly relationship with methane.

Reinhard notes that oxygen dealt methane an additional blow, based on independent evidence published recently by the Alternative Earths team in the journals Science and Geology. These papers describe geochemical signatures in the rock record that track extremely low oxygen levels in the atmosphere, perhaps much less than 1 percent of modern values, up until about 800 million years ago, when they spiked dramatically.

Less oxygen seems like a good thing for methane, since they are incompatible gases, but with oxygen at such extremely low levels, another problem arises.

"Free oxygen [O₂] in the atmosphere is required to form a protective layer of ozone [O₃], which can shield methane from photochemical destruction," Reinhard said. When the researchers ran their model with the lower oxygen estimates, the ozone shield never formed, leaving the modest puffs of methane that escaped the oceans at the mercy of destructive photochemistry.

With methane demoted, scientists face a serious new challenge to determine the greenhouse cocktail that explains our planet's climate and life story, including a billion years devoid of glaciers, Lyons said. Knowing the right combination other warming agents, such as water vapor, nitrous oxide, and carbon dioxide, will also help us assess habitability of the hundreds of billions of other Earth-like planets estimated to reside in our galaxy.

"If we detect methane on an exoplanet, it is one of our best candidates as a biosignature, and methane dominates many conversations in the search for life on Mars," Lyons said. "Yet methane almost certainly would not have been detected by an alien civilization looking at our planet a billion years ago -- despite the likelihood of its biological production over most of Earth history."

原始論文：Stephanie L. Olson, Christopher T. Reinhard, Timothy W. Lyons. Limited role for methane in the mid-Proterozoic greenhouse. Proceedings of the National Academy of Sciences, 2016; 201608549 DOI: 10.1073/pnas.1608549113

引用自：University of California - Riverside. "Methane muted: How did early Earth stay warm?." ScienceDaily. ScienceDaily, 7 October 2016. 

古代的全球冷化事件造就了當代生態系

原文網址：www.sciencedaily.com/releases/2016/09/160930144424.htm

古代的全球冷化事件造就了當代生態系

大約7百萬年前，全球各處的地貌及生態系統開始發生了劇烈變化。非洲的副熱帶地區越趨乾燥，導致撒哈拉沙漠形成。雨林在南北美、非洲和亞洲的面積減少，直到今日仍被廣大的莽原和草原取代。

目前為止，通常以互不相關的構造運動，像是山脈隆起或海盆變化，各自造成局部地區的氣候變化來解釋上述事件。但在一項新研究中，研究人員發現這些環境變遷跟一起先前無文獻紀載的全球冷化事件正好同期。它的成因可能是因大氣二氧化碳含量銳減而造成。

由布朗大學的地質學家領導，發表在期刊《自然―地質科學》(Nature Geoscience)的研究，主要立論於一段新建立起來的全球海溫紀錄，其區間橫跨了過去1200萬年。紀錄顯示在700萬至540萬年前的中新世(Miocene)末，有段時期海洋表面溫度特別的低。中新世全球氣候據信是比現在還要溫暖許多，但在這篇研究中發現的寒冷時期，海洋表面溫度卻驟降到近似於現在的數值。

「這是第一次有全球海洋表層溫度記錄將中新世晚期納入其中，而我們從中發現的溫度下降幅度連我們自己也感到相當驚訝。」領導此研究的布朗大學地球、環境與行星科學系的教授Timothy Herbert表示。「有了這段溫度變化，於此時開始發生的古生物學現象便開始顯得合理許多。」

這項新的海洋表面溫度紀錄源自於從世界各地17個不同地點採集的海洋沉積物樣品。當中含有一種浮游生物的遺骸，其細胞化學物質會依據溫度高低而產生變化。科學家測量這種對溫度敏感的分子含量多寡，便能重新建構溫度如何隨時間改變。

從南北半球各個海盆採取的樣品中都能偵測到在中新世晚期有段寒冷時期。冷化幅度越往兩極就越強烈，而往赤道則較緩和。Herbert說這種模式代表氣溫下降是由全球大氣發生變化所導致，最有可能的嫌疑犯便是二氧化碳(CO₂)。

「這起冷化事件具有兩半球對稱且在高緯度較為劇烈的特性―這些都是跟二氧化碳相關的溫度變化會留下的特徵。」Herbert說。「由於我們無法直接測量二氧化碳，因此不能證明其濃度有下降，但我們的確找到了二氧化碳濃度降低造成的間接證據。」

研究人員說氣溫降低在中新世末發生的副熱帶越趨乾燥現象中可能具有重大影響。「當地球變得較冷，尤其是海洋溫度降低，會造成大氣中的水氣減少。」Herbert表示，「隨著地球冷化，水循環的速率基本上也會減慢。」

如果大氣二氧化碳含量降低確實驅使了冷化現象發生，那麼這就能解釋為何在中新世晚期全球植被組成產生了劇烈改變，也就是南北美、亞洲和非洲副熱帶地區的森林轉變成草原或莽原。時至今日這些地區仍以此種生態系為主。在非洲，草原和莽原棲地出現可能跟早期人類祖先的演化有所關聯。

在這段時期開始佔據主流地位的草本植物為「四碳」植物(C4 plant)。它們行光合作用的路徑跟樹木和其他植物有些微不同，造成四碳路徑在低二氧化碳的環境條件中可以行使得更有效率。「如果當時二氧化碳濃度變得較低，就比較利於這些四碳植物生存。」Herbert說，「因此我們可以將植被轉變的原因和造成中新世末期冷化事件的可能成因連結至同一現象。」

Herbert表示現在還不清楚是什麼導致了二氧化碳濃度降低。有可能是當時發生了大規模地質現象而對碳循環產生重大影響。Herbert的實驗室目前正在深入探討這種可能性。

但可以確定的是在中新世末期全球氣溫發生了重大變化。

「現行觀點認為當時的全球氣候並無特別可看之處。」Herbert說，「但從研究結果看來，它其實比人們以往認為的還要有趣許多。」

Ancient global cooling gave rise to modern ecosystems

Around 7 million years ago, landscapes and ecosystems across the world began changing dramatically. Subtropical regions dried out and the Sahara Desert formed in Africa. Rain forests receded and were replaced by the vast savannas and grasslands that persist today in North and South America, Africa and Asia.

Up to now, these events have generally been explained by separate tectonic events -- the uplift of mountain ranges or the alteration of ocean basins -- causing discrete and local changes in climate. But in a new study, a team of researchers has shown that these environmental changes coincided with a previously undocumented period of global cooling, which was likely driven by a sharp reduction in atmospheric carbon dioxide.

The research, led by a Brown University geologist and published in Nature Geoscience, is based on a newly developed record of global sea surface temperatures spanning the past 12 million years. The record reveals a distinct period of cooler sea surface temperatures spanning 7 million to 5.4 million years ago, the end of the Miocene epoch. The global climate during the Miocene is known to have been much warmer than the present. During the cool period detected in this study, sea surface temperatures dropped to near modern levels.

"This is the first time the late Miocene has been put in a context of global sea surface temperatures, and we were surprised to see the amount of cooling we found," said Timothy Herbert, professor in the Department of Earth, Environmental and Planetary Sciences at Brown, who led the study. "In light of this temperature change, the paleobiological observations from this period start to make a lot more sense."

The new record of sea surface temperatures was derived from ocean sediment sampled at 17 different sites around the world. The sediment preserves the remains of a plankton species that varies cellular chemistry with temperature. By measuring amounts of those temperature-sensitive molecules, scientists can recreate temperature through time.

The late Miocene cool period was detected at every site sampled, in both hemispheres and in every ocean basin on the planet. The cooling was strongest toward the poles and more moderate toward the equator. That pattern, Herbert says, suggests a global atmospheric cause for the temperature decline. The most likely suspect is carbon dioxide (CO₂).

"The hemispheric symmetry and the fact that cooling is much greater at the high latitudes -- these are the fingerprints of CO₂-related temperature change," Herbert said. "We haven't proven that it was a decline in CO₂ because we're not measuring it directly, but we're making a circumstantial case for a reduction in CO₂."

The cooler temperatures would likely have played a role in the drying of the subtropics in the late Miocene, the researchers say. "A cooler world -- particularly a cooler ocean -- would have decreased moisture in the atmosphere," Herbert said. "The hydrological cycle basically slows down with cooling."

And if the cooling was indeed driven by a reduction in atmospheric CO₂, it could explain a critical shift in global vegetation that occurred during the late Miocene: the transition from forests to grassland and savanna in the subtropical regions of North and South America, Asia and Africa. These ecosystems are still present today. In Africa, these are the habitats associated with the evolution of our early human ancestors.

Many of the grassy plant species that began thriving during this period are "C4" plants. These species use a slightly different photosynthetic pathway than trees and other plants. The C4 pathway is more efficient in low CO₂ environments. "It could be that that if CO₂ declined, these C4 species were favored," Herbert said. "So we can associate that shift in vegetation with the same thing that we suspect was driving the late Miocene cooling."

It isn't clear at this point what might have driven a reduction in CO₂ during this period, Herbert says. It could be that there were large-scale geological changes occurring at this time that affected the carbon cycle. Herbert's lab is looking into that possibility now.

But what is clear is that there was a significant global shift in global temperatures during the late Miocene.

"The prevailing view was that this wasn't a particularly exciting time in terms of global climate," Herbert said. "It turns out to be more interesting than people thought."

原始論文：Timothy D. Herbert, Kira T. Lawrence, Alexandrina Tzanova, Laura Cleaveland Peterson, Rocio Caballero-Gill, Christopher S. Kelly. Late Miocene global cooling and the rise of modern ecosystems. Nature Geoscience, 2016; DOI:10.1038/ngeo2813

引用自：Brown University. "Ancient global cooling gave rise to modern ecosystems." ScienceDaily. ScienceDaily, 30 September 2016. 

從陸地侵蝕而來的物質使古代海洋中的生命得以生存

原文網址：www.sciencedaily.com/releases/2016/09/160926221617.htm

從陸地侵蝕而來的物質使古代海洋中的生命得以生存

隨著科學家陸續找到30億年前海洋已經有生命存在的證據，這些遠古化石卻也造成了一種矛盾。包括於如此早期的海洋中生活的單細胞細菌在內，生物都需要穩定供給磷才能生存，但是「除非有從陸地上侵蝕而來的磷，否則很難說明它們的來源。」威斯康辛大學麥迪遜分校地質科學系的Aaron Satkoski表示。「因此要解釋我們在如此久遠的時代中找到的化石是相當困難的。」

Satkoski是對此遙遠年代的海洋化學進行研究的第一作者。他說地質科學界一般預期當時的地球是顆海洋行星，僅有少數陸地矗立於海浪之上，甚至根本就沒有。「回到1960年代，這時人們基於諸多因素而宣稱當時地球幾乎沒有任何陸地，因此風化作用不足以影響海洋化學。然而，並沒有多少顯示30億年前情況如何的實際數據來支持上述說法。」

Satkoski說發現超過30億年前由細菌遺骸形成的化石改變了這番情節。「但如果當時海洋中已有生命存在，就得發生某種程度的陸地風化作用，才能供應支持生物存活所需的磷。」

當今海洋化學成分的主要影響因子為來自熱泉的流體(循環至地殼而產生的熱水)，以及地表風化作用(從陸地侵蝕下來的物質經由河水運輸至海洋)。

為了估計32.6億年前兩種因素個別影響多少，地球化學教授Clark Johnson和Satkoski從南非採集形成於當時的樣品，並且比較兩種不同形式的重晶石礦物中同位素的差異。其中膠結在一起的顆粒是在水中分別形成，之後沉澱至海床時才彼此黏合。

而立方體或者片狀的重晶石則是直接於海床形成。Johnson、Satkoski及其同僚Brian Beard推測顆粒狀重晶石反映了海水化學成分，因此也可看出是否有任何從陸地侵蝕而來的物質。另一方面，片狀重晶石則代表了海水化學跟熱泉流體混和後的結果。這篇研究的關鍵為精確測量兩種重晶石的同位素組成。所謂同位素是指化學性質相同但質量不同的原子。

著極其微小―但仍相當顯著的差異，意味結果顯示它們的同位素比例之間有著粒狀重晶石確實是由陸地侵蝕而來的物質生成。也就是說，32.6億年前就已經有一定程度的侵蝕作用發生了。

這項甫發表於《地球和行星科學通訊》(Earth and Planetary Science Letters)的論文將確實有大規模陸地侵蝕作用發生的時間點提前了4億年。

「雖然對於當時地球表面有多少比例是陸地不過是猜測，但我們認為可能有高達今日陸地面積的三分之二。」Johnson表示。他是威斯康辛大學NASA天體生物學研究所的負責人。「過往有些研究估計當時完全沒有一塊陸地存在。」

「科學家在思考海洋化學時總是會把焦點放在熱泉流體上，但卻很少有數據可以用來研究它的影響，」Johnson說。「我們正嘗試將某些數據納入整個過程當中。」

這些關於陸地的研究跟來自火成岩的證據一致。從高熱融化狀態岩石中得到的證據指出，那段時期地表已經足夠堅硬而足以支撐山脈形成，使得侵蝕作用可以發生。「既然我們對當時情況擁有更趨完整的理解，整個故事也會更加完備。」Satkoski說。

這項結果也跟氣候方面的數據互相吻合，因為大氣中二氧化碳濃度上升會導致陸地風化作用更加強烈。Satkoski說雖然太陽當時相對來說溫度較低，但海洋卻未凍結。「這代表當時大氣中有更多溫室氣體。因為二氧化碳會形成碳酸並產生酸雨而加速化學風化，造成當時氣候較溫暖且風化作用更為劇烈。」

大陸出現也代表在如此遙遠的年代就已經開始有大範圍板塊構造運動緩緩發生。「普遍認為當時形成陸地的板塊構造運動尚未發生，使得地球上沒有多少陸地。」Satkoski表示，「我們呈現的證據卻持有相反意見。」Johnson說總括而言，他們的成果將各方面的證據充分統整起來。「我們正慢慢得出一個理論可以同時解釋生命源起、海洋營養源來自何方以及地球為何並未結凍。雖然這些現象彼此之間狀似互相切合，但僅僅20年前我們對早期地球的理解卻跟此天差地遠。」

Life in ancient oceans enabled by erosion from land

As scientists continue finding evidence for life in the ocean more than 3 billion years ago, those ancient fossils pose a paradox. Organisms, including the single-celled bacteria living in the ocean at that early date, need a steady supply of phosphorus, but "it's very hard to account for this phosphorus unless it is eroding from the continents," says Aaron Satkoski, a scientist in the geoscience department at the University of Wisconsin-Madison. "So that makes it really hard to explain the fossils we see at this early era."

Satkoski, who is first author of a new report on ocean chemistry from this remote period, says the conventional wisdom of geology has envisioned an oceanic planet, with little or no land above the waves. "Starting back in the 1960s, for various reasons people claimed there was very little continental mass, and so there wasn't enough weathering to affect the chemistry of the ocean. But there wasn't much real data from more than 3 billion years ago to support that."

Discoveries of fossil remains of bacteria from over 3 billion years ago have changed that picture, says Satkoski. "But if there was life in the ocean, you need some amount of continental weathering taking place to deliver phosphorus so the organisms can live."

The major influences on ocean chemistry today are hydrothermal flow (hot water that has circulated through the crust) and surface weathering (the river transport of material eroded from land into the ocean).

To evaluate each influence 3.26 billion years ago, geoscience Professor Clark Johnson and Satkoski collected samples from South Africa and compared isotopes in two forms of a rock called barite. The cemented granules had formed in the water, then fused after dropping to the ocean floor.

A solid, or bladed, type of barite had formed at the ocean floor. Johnson, Satkoski and colleague Brian Beard assumed that the granular rock would reflect ocean water chemistry, and therefore any eroded, continental material. The bladed barite would represent a mix of ocean chemistry and hydrothermal flow. The study hinged on precise measurements of isotopes -- atoms that are chemically identical but that have different masses.

The result was a nearly infinitesimal -- but still significant -- difference in the isotope ratios, signifying that the granular barite indeed was derived from sediment eroded from land. In other words, a significant amount of erosion was taking place 3.26 billion years ago.

Their report, just published online by Earth and Planetary Science Letters, pushes back the first solid date for large-scale continental erosion by 400 million years.

"It's a guess how much of the planet's surface was land, but it could be as high as two-thirds of the area of today's continents," says Johnson, who leads the NASA Astrobiology Institute at the University of Wisconsin. "Some previous estimates had no continents at all."

"When people were thinking about ocean chemistry, it was always centered on hydrothermal flow, but there was little data," Johnson says. "We are trying to put some data into the equation."

The finding about continents jibes with evidence from igneous rocks -- those sourced in hot, molten rock -- which indicated that the surface became rigid enough to support mountain belts, which would have eroded, during this period. "Now that we have a more complete picture, the story becomes more coherent," Satkoski says.

The result also meshes with climate data, as intense continental weathering could result from an increase in carbon dioxide in the atmosphere. Although the sun was relatively cold at that time, the oceans were not frozen, Satkoski says. "That suggests there was more greenhouse gas in the atmosphere, which would produce a warmer climate combined with increased weathering, because carbon dioxide creates carbonic acid and acid rain, which speeds chemical weathering."

The presence of continents also indicates that the broad, slow movements of plate tectonics had started at this distant time. "Conventional wisdom says Earth had few continents because it did not have plate tectonics, which is how continents are made," Satkoski says. "Our evidence says the opposite." Overall, the result provides a satisfying unification of diverse streams of evidence, Johnson says. "We are moving toward an explanation for the presence of life, and the nutrients in the ocean, and why Earth was not frozen. They seem to fit together, but this is a very different picture of the early Earth than we had just 20 years ago.

原始論文：Aaron M. Satkoski, Donald R. Lowe, Brian L. Beard, Max L. Coleman, Clark M. Johnson. A high continental weathering flux into Paleoarchean seawater revealed by strontium isotope analysis of 3.26 Ga barite. Earth and Planetary Science Letters, 2016; 454: 28 DOI:10.1016/j.epsl.2016.08.032

引用自：University of Wisconsin-Madison. "Life in ancient oceans enabled by erosion from land." ScienceDaily. ScienceDaily, 26 September 2016.