大陸分裂、火山排放的碳與演化之間的關聯

原文網址：http://www.cam.ac.uk/research/news/link-identified-between-continental-breakup-volcanic-carbon-emissions-and-evolution

大陸分裂、火山排放的碳與演化之間的關聯

研究人員發現超大陸在數億年間的形成和分裂過程控制了火山排放的碳。發表在期刊《科學》(Science)的這項成果可能讓科學家需要重新解讀碳循環在地球歷史上的演變過程，而它又如何影響地球適居性的變化。

英國劍橋大學的研究人員利用世界各地超過80座火山已知的碳和氦元素測量結果，來定出這些氣體的來源。火山排出的碳跟氦可能是來自地球深處，或是近地表物質的重新循環。測量這些元素的化學記號可以界定出它們的來源。研究團隊在分析數據的時候發現火山噴出的碳大多數是近地表物質重新循環的產物，這跟先前的推測中，認為碳是來自地球內部深處的想法互相牴觸。研究第一作者Marie Edmonds博士表示：「這是解開地質學有關碳循環的謎題中相當重要的一片線索。」

碳在數百萬年之間會在地球深處和表面之間來回流動。像是石灰岩形成和動植物遺骸的埋藏與腐朽等作用可以將碳從地表移除，並讓地表大氣中的氧氣增加。火山是碳返回地表的一種方式，雖然它們的產量不及由人類活動造成碳排放量的百分之一。今日大部分從火山排放出來的碳都是由近地表物質循環而來，但是情況可能並非總是如此。

火山會沿著大型島弧或陸弧形成，它們位於兩座板塊碰撞，其中一方滑入另一方之下的區域，像是阿拉斯加和俄羅斯之間的阿留申群島、南美安地斯山脈、遍布義大利的火山以及西太平洋的馬里亞納群島。這些火山具有不同的化學記號：「島弧」火山噴發出來的碳較少是來自地函深處，而「陸弧」火山噴發出來的碳則更多是來自接近地表之處。

地球的陸地在數億年間會反覆地聚合在一起然後又分裂開來。陸地聚合在一起的時期，火山活動主要由島弧火山主導；在陸地分裂開來的時候，則由陸弧火山主導。這種反覆進行的過程會讓來到地表的碳含有的化學指紋在地質時間中出現系統性的變化，經由測量碳和氦的不同同位素我們可以得出這些變化。

科學家通常會在石灰岩中測量碳同位素的比例變化，也就是化學記號。先前研究人員認為影響石灰岩中碳的化學記號的唯一因素是大氣中含有多少氧氣。因為如此，過往研究人員會利用石灰岩中的碳同位素記號來解釋地表適居程度的變化歷程。但是，由劍橋大學研究團隊得出的成果指出火山在碳循環扮演的腳色比過往認知中還要吃重，因此之前的推論需要重新審視。

Edmonds表示：「這使得我們要從基本層面重新評估碳循環的演變過程。我們的結果指出如果火山噴出至地表的碳可以改變石灰岩的碳同位素組成，那科學家就需要全面重新解讀石灰岩的紀錄。」

其中一個很重要的例子是1億4400萬年至6500萬年前的白堊紀。科學家發現此時形成的石灰岩碳同位素比例有大幅增長的現象，過往將此解讀為大氣氧濃度上升的結果，而氧濃度的上升又被認為跟哺乳類在白堊紀晚期大為興盛有因果關係。然而，劍橋大學的研究成果提出此時石灰岩中的碳同位素比例提高，幾乎可以全歸因於地表火山噴發類型的改變。

共同作者，同樣來自劍橋大學地球科學系的Alexandra Turchyn博士表示：「氧濃度跟有機碳的埋藏之間的關係使得地球上我們所知的生物發生演化，但我們的地質紀錄顯示出科學家需要重新評估這種關聯。」

Link identified between continental breakup, volcanic carbon emissions and evolution

Researchers have found that the formation and breakup of supercontinents over hundreds of millions of years controls volcanic carbon emissions. The results, reported in the journal Science, could lead to a reinterpretation of how the carbon cycle has evolved over Earth’s history, and how this has impacted the evolution of Earth’s habitability.

The researchers, from the University of Cambridge, used existing measurements of carbon and helium from more than 80 volcanoes around the world in order to determine its origin. Carbon and helium coming out of volcanoes can either come from deep within the Earth or be recycled near the surface, and measuring the chemical fingerprint of these elements can pinpoint their source. When the team analysed the data, they found that most of the carbon coming out of volcanoes is recycled near the surface, in contrast with earlier assumptions that the carbon came from deep in the Earth’s interior. “This is an essential piece of geological carbon cycle puzzle,” said Dr Marie Edmonds, the senior author of the study.

Over millions of years, carbon cycles back and forth between Earth’s deep interior and its surface. Carbon is removed from the surface from processes such as the formation of limestone and the burial and decay of plants and animals, which allows atmospheric oxygen to grow at the surface. Volcanoes are one way that carbon is returned to the surface, although the amount they produce is less than a hundredth of the amount of carbon emissions caused by human activity. Today, the majority of carbon from volcanoes is recycled near the surface, but it is unlikely that this was always the case.

Volcanoes form along large island or continental arcs where tectonic plates collide and one plate slides under the other, such as the Aleutian Islands between Alaska and Russia, the Andes of South America, the volcanoes throughout Italy, and the Mariana Islands in the western Pacific. These volcanoes have different chemical fingerprints: the ‘island arc’ volcanoes emit less carbon which comes from deep in the mantle, while the ‘continental arc’ volcanoes emit far more carbon which comes from closer to the surface.

Over hundreds of millions of years, the Earth has cycled between periods of continents coming together and breaking apart. During periods when continents come together, volcanic activity was dominated by island arc volcanoes; and when continents break apart, continental volcano arcs dominate. This back and forth changes the chemical fingerprint of carbon coming to Earth’s surface systematically over geological time, and can be measured through the different isotopes of carbon and helium.

Variations in the isotope ratio, or chemical fingerprint, of carbon are commonly measured in limestone. Researchers had previously thought that the only thing that could change the carbon fingerprint in limestone was the production of atmospheric oxygen. As such, the carbon isotope fingerprint in limestone was used to interpret the evolution of habitability of Earth’s surface. The results of the Cambridge team suggest that volcanoes played a larger role in the carbon cycle than had previously been understood, and that earlier assumptions need to be reconsidered.

“This makes us fundamentally re-evaluate the evolution of the carbon cycle,” said Edmonds. “Our results suggest that the limestone record must be completely reinterpreted if the volcanic carbon coming to the surface can change its carbon isotope composition.”

A great example of this is in the Cretaceous Period, 144 to 65 million years ago. During this time period there was a major increase in the carbon isotope ratio found in limestone, which has been interpreted as an increase in atmospheric oxygen concentration. This increase in atmospheric oxygen was causally linked to the proliferation of mammals in the late Cretaceous. However, the results of the Cambridge team suggest that the increase in the carbon isotope ratio in the limestones could be almost entirely due to changes in the types of volcanoes at the surface.

“The link between oxygen levels and the burial of organic material allowed life on Earth as we know it to evolve, but our geological record of this link needs to be re-evaluated,” said co-author Dr Alexandra Turchyn, also from the Department of Earth Sciences.

原始論文：Emily Mason, Marie Edmonds, Alexandra V. Turchyn. Remobilization of crustal carbon may dominate volcanic arc emissions. Science, 2017. DOI: 10.1126/science.aan5049

引用自：University of Cambridge. “Link identified between continental breakup, volcanic carbon emissions and evolution.”