遠古有機碳深深埋藏於地函之中

原文網址：www.sciencedaily.com/releases/2017/04/170425092325.htm

遠古有機碳深深埋藏於地函之中

岩石學實驗支持在地球「大氧化事件」中板塊運動的地位

萊斯大學的岩石學家重建了距地表60英里深處的高溫高壓環境，而發現了關於地球久遠以前的一起重要事件的線索。

他們的研究描述了化石碳――地球最古老單細胞生物的遺骸，是如何自24億年前左右開始進入地球內部深處並封於其中，此時也是大氣氧氣劇烈上升的時間點。這篇論文本周刊登於期刊《自然―地質科學》(Nature Geoscience)的線上版。

「這是一項很有趣的概念，為了讓更複雜的生物演化出來，最早的生命形式必須深埋於地函內部。」萊斯大學的地球科學教授Rajdeep Dasgupta表示，「要讓埋藏作用發生牽涉到兩種機制。首先，你需要某種形式的板塊運動，它可以將早期生命形式含有的碳帶回地球內部。第二，你需要適當的地球化學條件，才能把有機碳運至地球內部深處，而讓它們離開地表環境很長一段時間。」

在許多古老岩石中清楚呈現的「大氧化事件」(great oxidation event)係指大氣氧濃度急遽上升的現象，但其成因仍然尚無定論。由於地質學家熟知此事件，因此他們常將其簡稱為「GOE」。雖然如此，但科學家仍沒有共識是什麼造成了GOE。舉例來說，科學家知道地球最早的已知生命之一――單細胞的藍綠菌會捕捉大氣中的二氧化碳並釋放出氧氣。但是近期發現的化石將早期生命出現的時間點越來越往前推，科學家現在知道至少在GOE發生的5億年以前，藍綠菌就已經十分興盛。

「藍綠菌也許佔有一席之地，但是GOE實在太過劇烈――在這段期間氧氣濃度增加了多達10000倍――不可能單靠藍綠菌來解釋。」在萊斯大學進行博士論文研究的主要共同作者Megan Duncan表示，「必須要有一種機制來大量移除空氣中的還原碳，才能改變系統中的相對氧含量。」

要移除碳卻不移除氧需要某種特別的條件，因為這兩種元素傾向於跟彼此結合。它們共同組成了大氣的重要成分之一――二氧化碳，同時也形成了所有碳酸岩類。

矽酸鹽岩漿為隱沒的地殼岩石熔化後的產物，並會經由火山噴發回到地表。Dasgupta和Duncan發現在決定化石有機碳，也就是石墨會沉入地函或是透過火山作用上升並回到地表時，矽酸鹽岩漿的化學成分具有重要意義。

現為華盛頓卡內基研究院的研究學者Duncan表示，這篇研究是首度探討稱作流紋岩(rhyolite)的一種岩漿攜帶石墨的能力。此類岩漿通常形成於地函深處，並會帶給火山大量的碳。她說流紋岩攜帶石墨的能力相當重要，因為如果石墨很容易順著流紋岩岩漿流出而趁勢回到地表，就不會有足夠的碳埋藏在地底而能解釋GOE的成因。

「矽酸鹽的成分具有重要地位。」她說，「科學家之前已經探討了組成中含有許多鐵鎂，但矽含量稀少的岩漿攜帶碳的能力。但是流紋岩岩漿的化學成分則含有豐富的矽和鋁，而十分缺乏鈣、鎂和鐵。其重要之處在於鈣和鎂是陽離子，它們會改變溶於岩漿中的碳有多少。」

Dasgupta和Duncan發現流紋岩質岩漿即使是處在高溫條件下，能溶解的石墨還是相當稀少。

地球科學教授Dasgupta表示：「我們的動機之一是：如果以前的隱沒帶十分高溫並且會產生大量岩漿，它們是否會完全分解有機碳而將它們釋放回地表？」

「我們發現即使是在非常、非常高溫的環境中，也不會有太多石墨碳溶解在岩漿裡。」他表示，「因此，即使溫度相當高且有大量岩漿產生，在這種岩漿裡有機碳的溶解度也不高，結果來說碳還是被埋藏於地函當中。」

Dasgupta表示：「巧妙的是有了在POE前夕開始發生，地殼埋藏至地函深處的起始時間與預估進程，加上我們的實驗數據顯示出還原碳深埋的效率，我們可以模擬並預估整個GOE期間大氣氧濃度的提升過程。」

研究支持了2016年同樣是萊斯大學的岩石學家Cin-Ty Lee和其同僚於論文中的發現。他們提出板塊運動、陸地形成以及早期生命的出現是形成氧含量豐富的地球大氣的關鍵要素。

研究領域越來越著重於地外行星系統的Duncan表示，這項研究可以提供給科學家重要線索，讓他們知道當評估哪些地外行星適宜生命居住時，應該要觀察什麼條件。

Early organic carbon got deep burial in mantle

Petrology experiments support tectonic role in Earth's 'great oxidation event'

Rice University petrologists who recreated hot, high-pressure conditions from 60 miles below Earth's surface have found a new clue about a crucial event in the planet's deep past.

Their study describes how fossilized carbon -- the remains of Earth's earliest single-celled creatures -- could have been subsumed and locked deep in Earth's interior starting around 2.4 billion years ago -- a time when atmospheric oxygen rose dramatically. The paper appears online this week in the journal Nature Geoscience.

"It's an interesting concept, but in order for complex life to evolve, the earliest form of life needed to be deeply buried in the planet's mantle," said Rajdeep Dasgupta, a professor of Earth science at Rice. "The mechanism for that burial comes in two parts. First, you need some form of plate tectonics, a mechanism to carry the carbon remains of early life-forms back into Earth. Second, you need the correct geochemistry so that organic carbon can be carried deeply into Earth's interior and thereby removed from the surface environment for a long time."

At issue is what caused the "great oxidation event," a steep increase in atmospheric oxygen that is well-documented in countless ancient rocks. The event is so well-known to geologists that they often simply refer to it as the "GOE." But despite this familiarity, there's no scientific consensus about what caused the GOE. For example, scientists know Earth's earliest known life, single-celled cyanobacteria, drew down carbon dioxide from the atmosphere and released oxygen. But the appearance of early life has been pushed further and further into the past with recent fossil discoveries, and scientists now know that cyanobacteria were prevalent at least 500 million years before the GOE.

"Cyanobacteria may have played a role, but the GOE was so dramatic -- oxygen concentration increased as much as 10,000 times -- that cyanobacteria by themselves could not account for it," said lead co-author Megan Duncan, who conducted the research for her Ph.D. dissertation at Rice. "There also has to be a mechanism to remove a significant amount of reduced carbon from the biosphere, and thereby shift the relative concentration of oxygen within the system," she said.

Removing carbon without removing oxygen requires special circumstances because the two elements are prone to bind with one another. They form one of the key components of the atmosphere -- carbon dioxide -- as well as all types of carbonate rocks.

Dasgupta and Duncan found that the chemical composition of the "silicate melt" -- subducting crustal rock that melts and rises back to the surface through volcanic eruptions -- plays a crucial role in determining whether fossilized organic carbon, or graphite, sinks into the mantle or rises back to the surface through volcanism.

Duncan, now a research scientist at the Carnegie Institution in Washington, D.C., said the study is the first to examine the graphite-carrying capacity of a type of melt known as rhyolite, which is commonly produced deep in the mantle and carries significant amounts of carbon to the volcanoes. She said the graphite-carrying capacity of rhyolitic rock is crucial because if graphite is prone to hitching a ride back to the surface via extraction of rhyolitic melt, it would not have been buried in sufficient quantities to account for the GOE.

"Silicate composition plays an important role," she said. "Scientists have previously looked at carbon-carrying capacities in compositions that were much more magnesium-rich and silicon-poor. But the compositions of these rhyolitic melts are high in silicon and aluminum and have very little calcium, magnesium and iron. That matters because calcium and magnesium are cations, and they change the amount of carbon you can dissolve."

Dasgupta and Duncan found that rhyolitic melts could dissolve very little graphite, even when very hot.

"That was one of our motivations," said Dasgupta, professor of Earth science. "If subduction zones in the past were very hot and produced a substantial amount of melt, could they completely destabilize organic carbon and release it back to the surface?

"What we showed was that even at very, very high temperatures, not much of this graphitic carbon dissolves in the melt," he said. "So even though the temperature is high and you produce a lot of melt, this organic carbon is not very soluble in that melt, and the carbon gets buried in the mantle as a result.

"What is neat is that with the onset and the expected tempo of crustal burial into the deep mantle starting just prior to the GOE, and with our experimental data on the efficiency of deep burial of reduced carbon, we could model the expected rise of atmospheric oxygen across the GOE," Dasgupta said.

The research supports the findings of a 2016 paper by fellow Rice petrologist Cin-Ty Lee and colleagues that suggested that plate tectonics, continent formation and the appearance of early life were key factors in the development of an oxygen-rich atmosphere on Earth.

Duncan, who increasingly focuses on exoplanetary systems, said the research could provide important clues about what scientists should look for when evaluating which exoplanets could support life.

原始論文：Megan S. Duncan, Rajdeep Dasgupta. Rise of Earth’s atmospheric oxygen controlled by efficient subduction of organic carbon. Nature Geoscience, 2017; DOI: 10.1038/ngeo2939

引用自：Rice University. "Early organic carbon got deep burial in mantle: Petrology experiments support tectonic role in Earth's 'great oxidation event'." ScienceDaily. ScienceDaily, 25 April 2017.