誰先出現：複雜的生命形式？還是大氣中的大量氧氣？

誰先出現：複雜的生命形式？還是大氣中的大量氧氣？

By Robert Sanders

如果地球的海洋和大氣裡沒有大量氧氣，那我們和其他動物今天也不會在這裡。但是對於生物來說，要從型態簡單的單細胞生物轉變成我們今日所見的複雜生物，高濃度的氧氣有多重要？

一項由加州大學柏克萊分校的地球化學家進行的研究提出了新證據，指出高濃度的氧氣對於動物的起源來說並不是那麼重要。

研究人員發現地球海洋深處變成氧化狀態大概是發生在5.4億至4.2億年前。他們將此現象歸因於當時大氣的氧濃度已經提升到跟現今的大氣氧濃度21%差不多的程度。

他們推測氧濃度上升的時間點比動物出現的時間――7億至8億年前――還晚了數億年。

「我們對深海氧化的解釋為大氣氧濃度上升所造成。從地球歷史來看，這起事件發生的時間可說是相當近期。」加州大學柏克萊分校的地球與行星科學助理教授Daniel Stolper表示。「這項研究的重要之處在於它提供了新的證據，顯示需要氧氣來進行代謝作用的原始動物，或許可以在大氣氧濃度相比於現在低上許多的世界誕生。」

他和博士後研究員Brenhin Keller會將他們的發現發表在期刊《自然》的論文中，並於1月3日先發布線上版。Keller同時隸屬於柏克萊地質年代學中心。

地球氧氣的歷史變化

氧氣在地球歷史中占有一席之地，不只是因為氧氣對於呼吸它們的生物來說至關重要，氧氣還很容易跟其他化合物發生反應――通常十分劇烈，而形成像是鐵鏽、植物燃燒、天然氣爆炸等現象。

然而，要追蹤大氣和海洋的氧氣濃度在地球超過45億年的歷史中是如何變化，卻不是一件簡單的事。多數科學家相信在地球形成的最初20億年，大氣或海洋幾乎沒有任何氧氣。但在大約25億到23億年前，大氣氧含量首次有提升的跡象。這次事件造成的地質現象十分顯著：暴露在地表的岩石突然變紅。這是因為岩石內部的鐵跟氧氣反應形成了鐵氧化物，就跟金屬鐵因生銹而變紅的過程類似。

地球科學家已經算出當時大氣的氧含量首度超過今日的十萬分之一(0.001%)，但是仍不足以使深海氧化，故深海大都還是處於缺氧狀態。

到了4億年前，首度出現含有木炭化石的沉積物，代表大氣二氧化碳含量已經高到足以讓野火發生。這需要現今大氣氧含量的50%到70%，也足以讓深海氧化。但科學家仍不確定大氣氧含量在25億年前到4億年前這段時期究竟是如何變化，因此這項議題仍有諸多探討空間。

「由於氧氣在許多地球化學和生物作用中都佔據了中心地位，因此科學家一直很想釐清25億年至4億年前大氣氧濃度的歷史進程。比方說，有些人對於動物為什麼會在那個時間點出現的解釋，是因為當時大氣氧含量首度接近我們現今所處的高濃度狀態。」Stolper表示，「這個解釋成立的前提是兩者之間有因果關係存在；也就是大氣氧含量升高至接近現今的水平，是驅動我們需要氧氣的祖先得以演化出來的環境因素。」

另一方面，有些研究人員認為兩個事件的關係並不大。要解決這項爭議的一個重要依據是精確定出大氣氧含量提升至接近現今水平的時間。但是，之前科學家預估氧氣升高的時間是發生在8億至4億年前，橫跨了動物誕生的那段時間。

氧氣含量的第二次變化發生在什麼時候？

Stolper和Keller希望他們可以定出地球歷史上一個重要的轉捩點：當大氣氧濃度高到足以使深海氧化的發生時間――當時大氣氧濃度相當於今日的10%至50%。他們的方法是觀察由海底火山爆發形成的火成岩中鐵的氧化狀態。當熔化的岩石從洋脊流出，會形成一大片的熔岩流和狀似枕頭的玄武岩。關鍵在於火山爆發之後海水會流經這些岩石。今日，由於流經岩石的海水含有氧氣，因此玄武岩中的鐵會被氧化。但在深海還缺乏氧氣的時候，科學家預估這些玄武岩中鐵的氧化狀態在噴發之後就不會發生什麼變化。

Stolper表示：「我們的想法是研究這類玄武岩中鐵的氧化態如何隨著歷史變化，並且看看我們能否精確指出鐵是什麼時候開始呈現出受到氧化的跡象，藉此找出深海首度含有大量溶氧的時間。」

為了進行這項研究，他們彙整了已經發表的研究中對古代海底玄武岩的鐵氧化態所進行的測量結果，數量超過1000筆。他們發現在5.4億年至2.4億年前以後，玄武岩中鐵的氧化態相較於岩漿本身才有明顯氧化的跡象，他們將此歸因於大氣氧含量已經提升至跟現今差不多的程度。該時間點比動物出現的時間還晚了數億年，跟某些大氣和海洋氧濃度歷史變化的研究相符，但不是全部。

「這項成果顯示大氣氧含量增加到足以使深海氧化，而形成類似於我們今日所見的世界，對於動物的出現來說並非必要條件。」Stolper表示，「此外，海底玄武岩紀錄也提供了一種新的定量方式，可以讓我們看見數億至數十億年前深海的地球化學條件。」

Which came first: complex life or high atmospheric oxygen?

We and all other animals wouldn’t be here today if our planet didn’t have a lot of oxygen in its atmosphere and oceans. But how crucial were high oxygen levels to the transition from simple, single-celled life forms to the complexity we see today?

A study by UC Berkeley geochemists presents new evidence that high levels of oxygen were not critical to the origin of animals.

The researchers found that the transition to a world with an oxygenated deep ocean occurred between 540 and 420 million years ago. They attribute this to an increase in atmospheric O₂ to levels comparable to the 21 percent oxygen in the atmosphere today.

This inferred rise comes hundreds of millions of years after the origination of animals, which occurred between 700 and 800 million years ago.

“The oxygenation of the deep ocean and our interpretation of this as the result of a rise in atmospheric O₂ was a pretty late event in the context of Earth history,” said Daniel Stolper, an assistant professor of earth and planetary science at UC Berkeley. “This is significant because it provides new evidence that the origination of early animals, which required O₂ for their metabolisms, may have gone on in a world with an atmosphere that had relatively low oxygen levels compared to today.”

He and postdoctoral fellow Brenhin Keller will report their findings in a paper posted online Jan. 3 in advance of publication in the journal Nature. Keller is also affiliated with the Berkeley Geochronology Center.

The history of Earth’s oxygen

Oxygen has played a key role in the history of Earth, not only because of its importance for organisms that breathe oxygen, but because of its tendency to react, often violently, with other compounds to, for example, make iron rust, plants burn and natural gas explode.

Tracking the concentration of oxygen in the ocean and atmosphere over Earth’s 4.5-billion-year history, however, isn’t easy. For the first 2 billion years, most scientists believe very little oxygen was present in the atmosphere or ocean. But about 2.5-2.3 billion years ago, atmospheric oxygen levels first increased. The geologic effects of this are evident: rocks on land exposed to the atmosphere suddenly began turning red as the iron in them reacted with oxygen to form iron oxides similar to how iron metal rusts.

Earth scientists have calculated that around this time, atmospheric oxygen levels first exceeded about a hundred thousandth of today’s level (0.001 percent), but remained too low to oxygenate the deep ocean, which stayed largely anoxic.

By 400 million years ago, fossil charcoal deposits first appear, an indication that atmospheric O₂ levels were high enough to support wildfires, which require about 50 to 70 percent of modern oxygen levels, and oxygenate the deep ocean. How atmospheric oxygen levels varied between 2,500 and 400 million years ago is less certain and remains a subject of debate.

“Filling in the history of atmospheric oxygen levels from about 2.5 billion to 400 million years ago has been of great interest given O₂’s central role in numerous geochemical and biological processes. For example, one explanation for why animals show up when they do is because that is about when oxygen levels first approached the high atmospheric concentrations seen today,” Stolper said. “This explanation requires that the two are causally linked such that the change to near-modern atmospheric O₂ levels was an environmental driver for the evolution of our oxygen-requiring predecessors.”

In contrast, some researchers think the two events are largely unrelated. Critical to helping to resolve this debate is pinpointing when atmospheric oxygen levels rose to near modern levels. But past estimates of when this oxygenation occurred range from 800 to 400 million years ago, straddling the period during which animals originated.

When did oxygen levels change for a second time?

Stolper and Keller hoped to pinpoint a key milestone in Earth’s history: when oxygen levels became high enough – about 10 to 50 percent of today’s level – to oxygenate the deep ocean. Their approach is based on looking at the oxidation state of iron in igneous rocks formed undersea (referred to as “submarine”) volcanic eruptions, which produce “pillows” and massive flows of basalt as the molten rock extrudes from ocean ridges. Critically, after eruption, seawater circulates through the rocks. Today, these circulating fluids contain oxygen and oxidize the iron in basalts. But in a world with deep-oceans devoid of O₂, they expected little change in the oxidation state of iron in the basalts after eruption.

“Our idea was to study the history of the oxidation state of iron in these basalts and see if we could pinpoint when the iron began to show signs of oxidation and thus when the deep ocean first started to contain appreciable amounts of dissolved O₂,” Stolper said.

To do this, they compiled more than 1,000 published measurements of the oxidation state of iron from ancient submarine basalts. They found that the basaltic iron only becomes significantly oxidized relative to magmatic values between about 540 and 420 million years ago, hundreds of millions of years after the origination of animals. They attribute this change to the rise in atmospheric O₂ levels to near modern levels. This finding is consistent with some but not all histories of atmospheric and oceanic O₂ concentrations.

“This work indicates that an increase in atmospheric O₂ to levels sufficient to oxygenate the deep ocean and create a world similar to that seen today was not necessary for the emergence of animals,” Stolper said. “Additionally, the submarine basalt record provides a new, quantitative window into the geochemical state of the deep ocean hundreds of millions to billions of years ago.”

原始論文：Daniel A. Stolper & C. Brenhin Keller. A record of deep-ocean dissolved O2 from the oxidation state of iron in submarine basalts, Nature (2018). DOI:10.1038/nature25009

引用自：University of California – Berkeley. “Which came first: complex life or high atmospheric oxygen?”

原文網址：http://news.berkeley.edu/2018/01/03/which-came-first-complex-life-or-high-atmospheric-oxygen/