新研究對地球大球為何充滿氧氣提出了解釋

原文網址：www.sciencedaily.com/releases/2016/05/160516151935.htm

New answer to why Earth's atmosphere became oxygenated

新研究對地球大球為何充滿氧氣提出了解釋

Earth scientists from Rice University, Yale University and the University of Tokyo are offering a new answer to the long-standing question of how our planet acquired its oxygenated atmosphere.

來自萊斯大學、耶魯大學和東京大學的地球科學家對這個由來以久的問題給出了新解答：地球大氣如何氧化？

Based on a new model that draws from research in diverse fields including petrology, geodynamics, volcanology and geochemistry, the team's findings were published online this week in Nature Geoscience. They suggest that the rise of oxygen in Earth's atmosphere was an inevitable consequence of the formation of continents in the presence of life and plate tectonics.

根據結合了多個領域的模型，包括岩石學、地球動力學、火山學以及地球化學，研究團隊將他們的發現刊登於本周《自然—地質科學》(Nature Geoscience)期刊的線上版。他們提出的說法認為在生物和板塊運動已經出現的情況下，陸地的形成必然會使地球大氣中的氧含量上升。

"It's really a very simple idea, but fully understanding it requires a good bit of background about how Earth works," said study lead author Cin-Ty Lee, professor of Earth science at Rice. "The analogy I most often use is the leaky bathtub. The level of water in a bathtub is controlled by the rate of water flowing in through the faucet and the efficiency by which water leaks out through the drain. Plants and certain types of bacteria produce oxygen as a byproduct of photosynthesis. This oxygen production is balanced by the sink: reaction of oxygen with iron and sulfur in Earth's crust and by back-reaction with organic carbon. For example, we breathe in oxygen and exhale carbon dioxide, essentially removing oxygen from the atmosphere. In short, the story of oxygen in our atmosphere comes down to understanding the sources and sinks, but the 3-billion-year narrative of how this actually unfolded is more complex."

「這是個相當簡單的概念，但要充分了解它確實需要對地球的運作方式有一定的背景知識。」第一作者，萊斯大學的地球科學教授Cin-Ty Lee說。「我最常使用的分析方式可以類比為一座漏水的浴缸。此浴缸中的水位高低受控於從水龍頭流進來的水流量，以及排水孔將水排出的效率。植物以及某些種類的藍綠菌在進行光合作用時排出的副產物之一即為氧氣。這些產氧作用會跟某些消耗機制相互抵消：地殼中的鐵和硫會跟氧氣反應，還有形成有機碳的逆反應。此種逆反應像是我們會呼吸氧氣並排出二氧化碳，這就會將氧氣從大氣中移除。簡單來說，了解我們大氣中氧氣的來源以及它們如何被消耗，便能得知大氣氧含量變動的歷史，但要闡明這段30億年的故事實際上是如何運作當然複雜許多。」

Lee co-authored the study with Laurence Yeung and Adrian Lenardic, both of Rice, and with Yale's Ryan McKenzie and the University of Tokyo's Yusuke Yokoyama. The authors' explanations are based on a new model that suggests how atmospheric oxygen was added to Earth's atmosphere at two key times: one about 2 billion years ago and another about 600 million years ago.

Lee和同樣來自萊斯大學的Laurence Yeung及Adrian Lenardic、耶魯大學的Ryan McKenzie和東京大學的 Yusuke Yokoyama共同進行了此篇研究。他們奠基於新模型的解釋呈現出大氣中的氧氣是如何在兩段關鍵時期進入到地球大氣層當中：其中一段發生在大約20億年前，而另一段則是在6億年前左右。

Today, some 20 percent of Earth's atmosphere is free molecular oxygen, or O2. Free oxygen is not bound to another element, as are the oxygen atoms in other atmospheric gases like carbon dioxide and sulfur dioxide. For much of Earth's 4.5-billion-year history, free oxygen was all but nonexistent in the atmosphere.

現今地球大氣中約有20%是游離氧分子( free molecular oxygen)，也稱作 O₂。游離氧不像其他大氣氣體中的氧原子，譬如二氧化碳和二氧化硫，它們並未跟其他元素產生鏈結。但在地球45億年的歷史中，多半時期大氣當中幾乎不存在游離氧。

"It was not missing because it is rare," Lee said. "Oxygen is actually one of the most abundant elements on rocky planets like Mars, Venus and Earth. However, it is one of the most chemically reactive elements. It forms strong chemical bonds with many other elements, and as a result, it tends to remain locked away in oxides that are forever entombed in the bowels of the planet -- in the form of rocks. In this sense, Earth is no exception to the other planets; almost all of Earth's oxygen still remains locked away in its deep rocky interior."

「雖然游離氧很稀少但還不至於消失無蹤。」Lee說。「事實上，在火星、金星和地球這類的石質行星上，氧可說是含量最多的元素。然而，由於氧是最容易發生化學反應的元素之一，因此它們很容易就跟其他元素產生強力的化學鍵，這使得它們很容易被封鎖在氧化物當中，並且永遠埋藏在行星的臟腑當中—也就是岩石內部。從這個角度來看，與其他的行星相比地球並非例外，地球的氧幾乎仍封存在地球深處的岩石內部。」

Lee and colleagues showed that around 2.5 billion years ago, the composition of Earth's continental crust changed fundamentally. Lee said the period, which coincided with the first rise in atmospheric oxygen, was also marked by the appearance of abundant mineral grains known as zircons.

Lee和他的同僚們顯示大約在25億年前，地球大陸地殼的成分在本質上發生了改變。Lee說那段時期跟地球大氣中的氧氣首度上升剛好同步，另外還有一個相當明顯的特徵，那時出現了許多鋯石(zircon)礦物的顆粒。

"The presence of zircons is telling," he said. "Zircons crystallize out of molten rocks with special compositions, and their appearance signifies a profound change from silica-poor to silica-rich volcanism. The relevance to atmospheric composition is that silica-rich rocks have far less iron and sulfur than silica-poor rocks, and iron and sulfur react with oxygen and form a sink for oxygen.

「鋯石的出現述說了一切。」他說。「鋯石只會從特定成分的熔岩中結晶出來。它們的現身明顯透露出岩漿從缺少二氧化矽轉變成相當豐富，這是一項十分深遠的改變。大氣成分也因此連帶出現了變化，富含二氧化矽的岩石跟缺少二氧化矽的岩石相比，鐵和硫的含量低了許多，而這兩者正好會跟氧氣反應並吸收氧氣。」

"Based on this, we believe the first rise in oxygen may have been due to a substantial reduction in the efficiency of the oxygen sink," Lee said. "In the bathtub analogy, this is equivalent to partially plugging the drain."

「根據於此，我們深信氧氣濃度首次上升或許可歸因於氧氣被吸收的速率發生了明顯下降。」Lee說。「在浴缸的比喻當中，這就相當於排水孔有一部分被塞住了。」

Lee said the study suggests that the second rise in atmospheric oxygen was related to a change in production -- analogous to turning up the flow from the faucet.。

Lee說這項研究認為大氣氧含量第二次上升則跟產生速率的變化相關—這可類比成將水龍頭的出水量轉得更大。

"The bathtub analogy is simple and elegant, but there's an added complication that must be taken into account," he said. "That is because oxygen production is ultimately tied to the global carbon cycle -- the cycling of carbon between Earth, the biosphere, the atmosphere and oceans."

「浴缸比喻確實相當簡單且優雅，但實際上仍得考慮到另一項更加複雜的作用。」他說。「這是因為氧氣的產生歸根究柢跟全球碳循環有密切的關係。碳循環是碳在地球生物圈、大氣層與海洋之間的循環流動。」

Lee said the model showed that Earth's carbon cycle has never been at a steady state because carbon slowly leaks out as carbon dioxide from Earth's deep interior to the surface through volcanic activity. Carbon dioxide is one of the key ingredients for photosynthesis.

Lee說模型顯示地球的碳循環從未達到穩定的狀態，這是因為經由火山活動碳會持續以二氧化碳的形式從地球深處漏往地表，而二氧化碳又是光合作用的關鍵原料之一。

"On long, geologic timescales, carbon is removed from the atmosphere by the production of condensed forms of carbon, such as organic carbon and minerals called carbonate," he said. "For most of Earth's history, most of this carbon has been deposited not in the deep ocean but rather on the margins of continents. The implications are profound because carbon deposited on continents does not return to Earth's deep interior. Instead, it amplifies carbon inputs into the atmosphere when the continents are subsequently perturbed by volcanism."

「就長久的地質時間尺度來看，將碳沉降成固態型式的作用，像是產生有機碳或稱作碳酸鹽類的礦物，都會將碳從大氣中移除。」他說。「在地球歷史上的大部分期間，這些碳並未沉積在深海而是在大陸邊緣。這具有相當重要的意涵，因為沉積在大陸的碳並不會返回地球深處；相反地，當大陸之後受到火山作用擾動時，這會增加更多進入大氣中的碳。」

Lee said the team's model showed that volcanic activity and other geologic inputs of carbon into the atmosphere may have increased with time, and because oxygen production is tied to carbon production, oxygen production also must increase. The model showed that the second rise in atmospheric oxygen had to occur late in Earth's history.

Lee說團隊的模型顯示火山活動以及其他會將碳輸送至大氣的地質作用可能會隨著時間經過而更加活躍，且由於氧氣增加跟碳的產生有密切關係，因此氧氣產量必定會隨之提升。模型顯示大氣氧含量第二次提昇會發生在地球史上較為近代的時間。

"Exactly when is model-dependent, but what is clear is that the formation of continental crust naturally leads to two rises in atmospheric oxygen, just as we see in the fossil record," Lee said.

「雖然確切的發生時間是由模型給出，但從化石紀錄中我們可以很明顯地看到大陸地殼的形成會順理成章地導致大氣層氧含量提升兩次。」Lee說。

Exactly what caused the composition of the crust to change during the first oxygenation event remains a mystery, but Lee said the team believes it may have been related to the onset of plate tectonics, where Earth's surface, for the first time, became mobile enough to sink back down into Earth's deep interior.

第一次氧化事件時，地殼成分變化的確切成因仍然是個謎團，但Lee說他們的團隊認為這可能跟板塊運動開始運作有關。這是地球歷史上地表首度具有足夠的動力可以下沉回地球深處。

Lee said the team's new model is not without controversy. For example, the model predicts that production of carbon dioxide must increase with time, a finding that goes against the conventional wisdom that carbon fluxes and atmospheric carbon dioxide levels have steadily decreased over the last 4 billion years.

Lee說他們團隊的新模型也並非完全沒有爭議。舉例來說，該模型預測二氧化碳的產量會隨著時間增加，但這項發現卻跟一般常識認為的相反，過往認為碳進入大氣層的速率以及大氣二氧化碳的含量在過去40億年來會隨著時間穩定下降。

"The change in flux described by our model happens over extremely long time periods, and it would be a mistake to think that these processes that are bringing about any of the atmospheric changes are occurring due to anthropomorphic climate change," he said. "However, our work does suggest that Earth scientists and astrobiologists may need to revisit what we think we know about Earth's early history."

「我們模型中描述的速率變化是在極長一段時間中慢慢發生的，因此若認為導致這些大氣變化的作用中有任何一項是起因於人為導致的氣候變遷，那完全是項謬誤。」他說。「然而，我們的成果確實告訴地球科學家和天體生物學家，也許有必要重新思量我們對地球早期歷史所知的一切。」

引用自：Rice University. "New answer to why Earth's atmosphere became oxygenated." ScienceDaily. ScienceDaily, 16 May 2016.