地球在數百萬年中逐漸發生的氣候變遷跟火山有重大關聯

原文網址：www.sciencedaily.com/releases/2016/04/160421150056.htm

Volcanoes tied to shifts in Earth's climate over millions of years

地球在數百萬年中逐漸發生的氣候變遷跟火山有重大關聯

A new study in the April 22 edition of Science reveals that volcanic activity associated with the plate-tectonic movement of continents may be responsible for climatic shifts from hot to cold over tens and hundreds of millions of years throughout much of Earth's history.

在地球歷史上的許多時期，都曾發生氣候於數千萬年至數億年間由酷熱逐漸轉變成嚴寒的現象。4月22日發表在《科學》(Science)期刊上的新研究顯示，這或許跟大陸板塊移動時伴隨產生的火山活動大有關係。

The study, led by researchers at The University of Texas at Austin Jackson School of Geosciences, addresses why the Earth has fluctuated from periods when the planet was covered in ice to times when even the polar regions were ice-free.

此篇由德克薩斯大學奧斯汀分校傑克遜地球科學院的研究人員領導的研究，提出了可能的成因來解釋這個問題：地球為何會在整顆星球被冰封的時期，與連極區都未見一絲冰霜的時期之間來回變化。

The study explores very long-term shifts in Earth's baseline climate, not short-term or human-induced climate change.

這篇研究探討了地球的基準氣候(baseline climate)在相當長的一段時間中如何變化，而非短期或者人為導致的氣候變遷。

Lead researcher Ryan McKenzie said the team found that periods when volcanoes along continental arcs were more active coincided with warmer, or greenhouse, conditions over the past 720 million years. Conversely, periods when continental arc volcanos were less active coincided with colder, or icehouse, conditions.

率領此研究的 Ryan McKenzie說研究團隊發現自過去的7億2000萬年以來，當陸弧(continental arc)中的火山活動較為活躍時，也是氣候較溫暖的時期，此時的地球處於溫室(greenhouse)環境。反之，當陸弧火山活動趨緩時，此時的氣候就較為寒冷，地球便處於冰室(icehouse)環境。

Continental volcanic arc systems such as the Andes Mountains are created at active continental margins where two tectonic plates meet and the oceanic plate descends under the continental plate, forming a subduction zone. When this happens, magma mixes with carbon trapped in the Earth's crust and releases carbon dioxide (CO2) gas into the atmosphere when volcanoes in the system erupt.

當兩個板塊之間彼此擠壓，海洋板塊下沉至大陸板塊之下而形成隱沒帶時，在上方的活動型大陸邊緣便會有陸弧系統生成，現今的安地斯山脈即為一例。此構造環境形成後，當陸弧系統中的火山爆發時，就會噴發出混有地殼內部的碳的岩漿，並釋放二氧化碳氣體至大氣中。

"Continental arc systems are plumbed through the Earth's crust and they tend to interact with carbon reservoir rock preserved beneath the surface," said McKenzie, who began the work as a postdoctoral researcher at the Jackson School and finished the study at Yale University.

「陸弧系統會貫穿地殼，因此它們有相當大的機會能跟保存在地表下方岩石中的碳儲存庫之間發生交互作用。」 McKenzie說。他在傑克遜地球科學院擔任博士後研究員時開始進行這項研究，之後在耶魯大學任職時完成。

Scientist have long known that the amount of carbon dioxide in the atmosphere influences the Earth's climate, McKenzie said. The unanswered question is what caused the fluctuations in CO2 observed in the geologic record. Other theories have suggested that geological forces such as mountain building have, at different times in the planet's history, introduced large amounts of new material to the Earth's surface, and weathering of that material has drawn CO2 out of the atmosphere. The new study points to the amount of CO2 being released into the atmosphere, rather than the amount removed from it, as the primary driver of Earth's climate.

McKenzie說長久以來科學家就已經知道大氣中的二氧化碳含量會影響地球氣候。然而從地質紀錄中觀察到的二氧化碳含量變化究竟是由什麼原因所造成，仍尚未有人可以回答。某些理論認為在地球歷史的不同時期，像是造山運動之類的地質營力會將大量的新鮮岩石帶到地球表面，這些物質遭受風化作用時便會從大氣中吸收二氧化碳。但是，此篇新研究指出驅動地球氣候變化的動力是釋放至大氣中二氧化碳的多寡，而非被吸收的量有多少。

Using nearly 200 published studies and their own fieldwork and data, researchers created a global database to reconstruct the volcanic history of continental margins over the past 720 million years.

利用將近200篇已發表的研究，以及研究團隊自身的野外工作和資料，他們創建了一個資料庫可以用來重建過去7億2000萬年以來全球大陸邊緣的火山活動史。

"We studied sedimentary basins next to former volcanic arcs, which were eroded away over hundreds of millions of years," said co-author Brian Horton, a professor in the Jackson School's Department of Geological Sciences. "The distinguishing part of our study is that we looked at a very long geologic record -- 720 million years -- through multiple greenhouse-icehouse events."

「由於火山弧會在數億年的時光中被慢慢侵蝕殆盡，因此我們研究了過往位於它們附近的沉積盆地。」任職於傑克遜地球科學院的共同作者 Brian Horton說。「我們這篇研究裡最突出的一部份是我們觀察了地球在歷經多次溫室—冰室事件後而留下來的一段相當長期的地質紀錄，時間長達7億2000萬年。」

Specifically, researchers looked at the uranium-lead crystallization ages of the mineral zircon, which is largely created during continental volcanic arc activity. Zircon is less common in other types of volcanic settings, such as hot spots like Hawaii or island arc volcanoes such as the Marianas, so the mineral can be used to track continental arc volcanism. For the study, they looked at data for roughly 120,000 zircon grains from thousands of samples across the globe.

精確而言，研究人員探討的是鋯石礦物顆粒的鈾鉛結晶年代，這些礦物多半是在陸弧火山活動時產生出來的。鋯石很少在其他類型的火山構造環境中看到，像是夏威夷所處的熱點(hotspot)以及馬里亞納群島位於的火山島弧(island arc)，因 此這種礦物便能夠用來追蹤陸弧的火山活動。為了進行這項研究，研究人員從全世界蒐集到的數千個樣品裡，將近12萬顆的鋯石顆粒中探討他們需要的資訊。

"We're looking at changes in zircon production on various continents throughout Earth's history and seeing how the changes correspond with the various icehouse and greenhouse transitions," McKenzie said. "Ultimately, we find that during intervals of high zircon production we have greenhouse conditions, and as zircon production diminishes, we see a shift into our icehouse conditions."

「我們想要知道地球歷史上各個陸塊產生鋯石的速率是如何改變，並且觀察這些變化如何對應至溫室地球及冰室地球之間的多次轉換。」McKenzie說。「最後我們發現在鋯石產量較高的時期，地球便處於溫室環境；而隨著鋯石產量逐漸下滑，我們看到地球也逐漸轉變成冰室環境。」

The cooler icehouse periods tended to correlate with the assembly of the Earth's supercontinents, which was a time of diminished continental volcanism, Horton said. The warmer greenhouse periods correlated with continental breakup, a time of enhanced continental volcanism.

Horton說較為寒冷的冰室時期傾向對應至當地球陸地組合成超大陸的時期，此時陸地的火山活動較為平靜。而較為溫暖的溫室時期則對應至大陸分裂時，此時陸地的火山活動較為活躍。

Jackson School researchers Shannon Loomis and Daniel Stockli, Yale University's Noah Planavsky, and Rice University's Cin-Ty Lee also worked on the study. The research was funded by the National Science Foundation.

傑克遜地球科學院的研究人員 Shannon Loomis和Daniel Stockli、耶魯大學的 Noah Planavsky以及萊斯大學的 Cin-Ty Lee 也共同參與了此研究。研究由美國國家科學基金會贊助。

引用自：University of Texas at Austin. "Volcanoes tied to shifts in Earth's climate over millions of years." ScienceDaily. ScienceDaily, 21 April 2016.