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.