化石燃料的形成是大氣擁有氧氣的關鍵？

原文網址：www.sciencedaily.com/releases/2016/12/161230185406.htm

化石燃料的形成是大氣擁有氧氣的關鍵？

在動物演化的過程中，除了DNA之外，恐怕沒有比大氣中的氧氣還重要的事物了。

氧氣讓動物能進行化學反應，獲取儲存在碳水化合物，也就是食物中的能量。因此5億年前左右，動物在「寒武紀大爆發」(Cambrian explosion)事件中大量出現並演化，同時大氣氧含量出現高峰，也許並不是單純的巧合。

現在能看到的動物型態，多半是於寒武紀大爆發期間開始出現。

在綠色植物體內，光合作用會將二氧化碳分解成氧氣(排放到大氣中)以及碳(儲存在碳水化合物內)。

但當時光合作用早就已經存在了至少25億年之久。所以是什麼原因造成了氧氣含量於寒武紀突然出現高峰？

刊登於《地球和行星科學通訊》(Earth and Planetary Science Letters )二月號線上版的研究，將氧氣含量的提高歸因於沉積物埋藏量的迅速增加，這些沉積物具有大量富含碳的有機物質。共同作者，威斯康辛大學麥迪遜分校的地質科學教授Shanan Peters表示，關鍵在於瞭解沉積物可以將碳封存起來防止其被氧化。

如果沒有埋藏作用，地球表面死亡的植物殘骸會因為氧化作用而燃燒。它們體內原本來自大氣中的碳，就會跟氧氣結合而形成二氧化碳。因此，若要讓氧氣在大氣中持續累積，就必須要防止植物的有機物質受到氧化。

而這就是有機物質—也就是木炭、石油和天然氣的原始成分—透過地質作用被埋藏時所發生的。

為了證明這項理論，Peters和的博士後研究員Jon Husson從Macrostrat中發掘出一組全新的資料。Peters用了10年策劃並建立了Macrostrat，此資料庫彙整了北美各處的地質資訊。

他們根據沉積岩層提供的資訊，建立了同時顯示大氣氧含量和沉積物埋藏量變化的圖表，從中可以看出氧氣和沉積物之間有關聯性存在。它們都在23億年前有個較小的高峰，另一個較大的高峰則出現在5億年前。

「雖然這僅顯示出兩者之間有相關性，但我們可以宣稱地質作用和大氣氧含量的變化之間確實有因果關係存在。」Husson表示。「光合作用會把二氧化碳轉換成生質(biomass)並釋放氧氣到大氣當中。當你把沉積物儲存起來，也會連帶隔絕由光合作用產生的有機質。埋藏作用可以帶走地表的碳，防止它們從大氣帶走氧分子並結合在一起。」

Husson和Peters辨識出來沉積物大量埋藏的時間點，有些和今日仍在開採的大型化石燃料礦場的形成時間一致。這些礦場包括了德州形成於二疊紀石油蘊藏豐富的盆地，以及阿帕拉契地區晚石炭紀時形成的煤田。

「埋藏那些會成為化石燃料的沉積物，是高等動物生命出現在地球的關鍵。」Peters表示，並強調多細胞生物大多是誕生於寒武紀。

今日，燃燒化石燃料中數十億噸的碳正持續將大量氧氣從大氣中移除，這跟使大氣氧含量提升的方式恰好相反。因此，隨著大氣二氧化碳的濃度增加，氧氣含量便會下降。

Macrostrat中跟北美有關的資料代表了超過一世紀以來數千名地質學家的研究成果。目前的研究僅考慮到北美地區，這是因為涵蓋地球陸地表面其餘80%的綜合資料庫還尚未建立起來。

導致這兩次氧氣增加的沉積物加速埋藏事件，根本的地質成因仍然不清楚。「有許多概念可以用來解釋不同階段的氧氣濃度變化。」Husson承認。「我們猜測在板愧運動、地函熱傳導或對流中發生的基本變化可能具有一定作用，但現階段我們還未能給出解釋。」

Peters拿著一塊大約於4.5億年前形成，嵌有數隻三葉蟲的奧陶紀頁岩說：「為什麼大氣中會有氧氣？高中教科書给的答案是『光合作用』。但從威斯康辛州的地質學家Thomas Chrowder Chamberlin(1843-1928，曾擔任威斯康辛大學的校長)那時起，我們長久以來已經了解到氧氣的增加需要黑色頁岩這類的岩層形成，它們含有大量本該燃燒殆盡的碳。這些頁岩當中的有機碳是經由光合作用而固定住，將它們埋藏並保存在岩石當中才能真正將氧分子釋放出來。」

Husson表示此研究的創新之處，在於從涵蓋地球陸地20%的豐富資料庫中，可以找到這種關聯性存在的證據。

必須要持續地將碳埋藏起來才能使大氣氧含量不斷提高。Husson強調地表發生的許多作用，像是鐵氧化而產生的鐵鏽會消耗自由氧。「大氣有氧氣的秘訣在於將當時存在的一小部分生質移除並封存在沉積物當中。這就是化石燃料沉積時可以辦到的事情。」

Fossil fuel formation: Key to atmosphere’s oxygen?

For the development of animals, nothing -- with the exception of DNA -- may be more important than oxygen in the atmosphere.

Oxygen enables the chemical reactions that animals use to get energy from stored carbohydrates -- from food. So it may be no coincidence that animals appeared and evolved during the "Cambrian explosion," which coincided with a spike in atmospheric oxygen roughly 500 million years ago.

It was during the Cambrian explosion that most of the current animal designs appeared.

In green plants, photosynthesis separates carbon dioxide into molecular oxygen (which is released to the atmosphere), and carbon (which is stored in carbohydrates).

But photosynthesis had already been around for at least 2.5 billion years. So what accounted for the sudden spike in oxygen during the Cambrian?

A study now online in the February issue of Earth and Planetary Science Letters links the rise in oxygen to a rapid increase in the burial of sediment containing large amounts of carbon-rich organic matter. The key, says study co-author Shanan Peters, a professor of geoscience at the University of Wisconsin-Madison, is to recognize that sediment storage blocks the oxidation of carbon.

Without burial, this oxidation reaction causes dead plant material on Earth's surface to burn. That causes the carbon it contains, which originated in the atmosphere, to bond with oxygen to form carbon dioxide. And for oxygen to build up in our atmosphere, plant organic matter must be protected from oxidation.

And that's exactly what happens when organic matter -- the raw material of coal, oil and natural gas -- is buried through geologic processes.

To make this case, Peters and his postdoctoral fellow Jon Husson mined a unique data set called Macrostrat, an accumulation of geologic information on North America whose construction Peters has masterminded for 10 years.

The parallel graphs of oxygen in the atmosphere and sediment burial, based on the formation of sedimentary rock, indicate a relationship between oxygen and sediment. Both graphs show a smaller peak at 2.3 billion years ago and a larger one about 500 million years ago.

"It's a correlation, but our argument is that there are mechanistic connections between geology and the history of atmospheric oxygen," Husson says. "When you store sediment, it contains organic matter that was formed by photosynthesis, which converted carbon dioxide into biomass and released oxygen into the atmosphere. Burial removes the carbon from Earth's surface, preventing it from bonding molecular oxygen pulled from the atmosphere."

Some of the surges in sediment burial that Husson and Peters identified coincided with the formation of vast fields of fossil fuel that are still mined today, including the oil-rich Permian Basin in Texas and the Pennsylvania coal fields of Appalachia.

"Burying the sediments that became fossil fuels was the key to advanced animal life on Earth," Peters says, noting that multicellular life is largely a creation of the Cambrian.

Today, burning billions of tons of stored carbon in fossil fuels is removing large amounts of oxygen from the atmosphere, reversing the pattern that drove the rise in oxygen. And so the oxygen level in the atmosphere falls as the concentration of carbon dioxide rises.

The data about North America in Macrostrat reflects the work of thousands of geoscientists over more than a century. The current study only concerns North America, since comprehensive databases concerning the other 80 percent of Earth's continental surface do not yet exist.

The ultimate geological cause for the accelerated sediment storage that promoted the two surges in oxygen remains murky. "There are many ideas to explain the different phases of oxygen concentration," Husson concedes. "We suspect that deep-rooted changes in the movement of tectonic plates or conduction of heat or circulation in the mantle may be in play, but we don't have an explanation at this point."

Holding a chunk of trilobite-studded Ordovician shale that formed approximately 450 million years ago, Peters asks, "Why is there oxygen in the atmosphere? The high school explanation is 'photosynthesis.' But we've known for a long time, going all the way back to Wisconsin geologist (and University of Wisconsin president) Thomas Chrowder Chamberlin, that building up oxygen requires the formation of rocks like this black shale, which can be rich enough in carbon to actually burn. The organic carbon in this shale was fixed from the atmosphere by photosynthesis, and its burial and preservation in this rock liberated molecular oxygen."

What's new in the current study, Husson says, is the ability to document this relationship in a broad database that covers 20 percent of Earth's land surface.

Continual burial of carbon is needed to keep the atmosphere pumped up with oxygen. Many pathways on Earth's surface, Husson notes, like oxidation of iron -- rust -- consume free oxygen. "The secret to having oxygen in the atmosphere is to remove a tiny portion of the present biomass and sequester it in sedimentary deposits. That's what happened when fossil fuels were deposited."

原始論文：Jon M. Husson, Shanan E. Peters. Atmospheric oxygenation driven by unsteady growth of the continental sedimentary reservoir. Earth and Planetary Science Letters, 2017; 460: 68 DOI:10.1016/j.epsl.2016.12.012

引用自：University of Wisconsin-Madison. "Fossil fuel formation: Key to atmosphere’s oxygen?." ScienceDaily. ScienceDaily, 30 December 2016.