山脈成長會影響溫室效應

 原文網址：https://www.gfz-potsdam.de/en/media-and-communication/news/details/article/mountain-growth-influences-greenhouse-effect/

台灣是座極端的島嶼：大型地震與強烈颱風反覆侵襲這個地方，使得它的地貌因此改變，有時還會釀成重大災難。這讓台灣成為一個對地球科學來說再好不過的實驗室。舉例來說，侵蝕作用在台灣中央的速率是最南邊的數千倍。侵蝕速率的差異會影響岩石化學風化，而讓我們洞見地球以數百萬年為時間尺度來進行的碳循環。由德國地球科學中心的Aaron Bufe與Niels Hovius領導的研究團隊最近利用侵蝕速率的差異，探討岩石的抬升與侵蝕如何決定碳在吸收與排放之間的平衡。他們得到了驚人的結果：風化作用在侵蝕速率高的地方會釋放二氧化碳；而在侵蝕速率低的地方則會封存大氣裡的二氧化碳。這項研究發表在《自然―地球科學》(Nature Geoscience).

從岩床湧出來的泉水帶有因為風化而呈現黃褐色的液體，地點為台灣廬山。圖片來源：Kristen Cook, GFZ

在這項研究背後的原理皆關乎構造運動與化學作用。構造抬升和侵蝕作用會持續把新鮮的岩石材料從地底帶上來，特別是在迅速成長的山裡。岩石在地表接觸流動的酸性水體會遭到溶解或者改變成分。這種風化作用根據岩石種類的不同，對地球氣候產生的影響也有很大的差異。舉例來說，如果土壤中的碳酸跟矽酸鹽礦物接觸便會形成石灰岩沉澱(碳酸鈣，CaCO₃)，這可以把碳封在其中很長一段時間。如果是跟含硫礦物(像是黃鐵礦)與石灰岩接觸的話，就會發生相反的情況。黃鐵礦跟水和氧氣接觸的時候會形成硫酸，進而溶解碳酸鹽礦物並產生二氧化碳。科學家認為從數百萬年的時間尺度來看，這種造山運動和化學風化之間的關係對地球氣候有很大的影響。但是阿爾卑斯山或者喜馬拉雅山變高究竟對氣候有什麼樣的影響？是讓矽酸鹽的風化作用加速，使得氣候變冷？或者硫酸溶解石灰岩的作用更具優勢，造成大氣中的二氧化碳濃度上升，使得全球暖化隨之發生？

這項問題可以從台灣南部獲得解答。台灣所處的位置是個隱沒帶，在此有塊海洋板塊正滑進亞洲大陸下方，這種隱沒作用會讓山脈快速成長。雖然台灣中央的地勢數百萬年來都相當高聳，它的南端卻才剛從海裡升起而已。台灣南部的山嶺地勢較低，侵蝕速度也相對較緩；而較為北方的山脈則相當陡峭且高聳，新鮮的岩石迅速地被帶到地表而遭到風化。這裡的好處在於台灣南部的岩石和世界各地許多典型的年輕山脈一樣，大部份的成分都是矽酸鹽礦物，帶有一些碳酸鹽和黃鐵礦。

在這項研究中，研究人員從侵蝕速率不同的山區當中採集河水樣品。透過溶解在河川裡面的物質，研究人員可以估計風化作用中硫化物、碳酸鹽和矽酸鹽礦物所佔的比例。接著，他們利用這項結果估計風化反應釋出和封存的碳各有多少。第一作者Aaron Bufe寫道：「我們發現台灣最南部主要是以封存大氣二氧化碳的作用為主；但在較為北邊、侵蝕速率較快的山區，則是以碳酸鹽和硫化物的風化為主，因此會釋放出二氧化碳。」

所以說，山脈的風化會讓大氣中的二氧化碳增加？Aaron Bufe表示：「我們對於台灣所做的論述相較而言較為詳細：此處極為活躍的造山帶所導致的化學風化，總體來說似乎會增加大氣中的二氧化碳。但是，當沉積物從山脈沖刷下來並堆積在廣闊的沖積平原，像是喜馬拉雅山和阿爾卑斯山的山腳，情況可能就會有所改變。這些沉積物通常富含矽酸鹽，因此它們受到風化後會把二氧化碳封存起來。此外，造山運動帶到地表的岩石不只是含有黃鐵礦與碳酸鹽的沉積岩，還有岩漿凝固所形成的岩石種類，它們具有相當多的新鮮矽酸鹽因此會快速風化。想要完全了解風化作用對地球氣候整體的效應之前，研究人員還得登上好些山脈。」

 

Mountain growth influences greenhouse effect

Taiwan is an island of extremes: severe earthquakes and typhoons repeatedly strike the region and change the landscape, sometimes catastrophically. This makes Taiwan a fantastic laboratory for geosciences. Erosion processes, for example, occur up to a thousand times faster in the center of the island than in its far south. This difference in erosion rates influences the chemical weathering of rocks and yields insights into the carbon cycle of our planet on a scale of millions of years. A group of researchers led by Aaron Bufe and Niels Hovius of the German Research Center for Geosciences (GFZ) has now taken advantage of the different erosion rates and investigated how uplift and erosion of rocks determine the balance of carbon emissions and uptake. The surprising result: at high erosion rates, weathering processes release carbon dioxide; at low erosion rates, they sequester carbon from the atmosphere. The study will be published in Nature Geoscience.

Behind all this are tectonic and chemical processes. In rapidly growing mountains in particular, tectonic uplift and erosion constantly bring fresh rock material up from underground. There it is exposed to circulating acidic water which dissolves or alters the rock. Depending on the type of rock, this weathering has very different effects on Earth's climate. For example, if carbonic acid from the soil comes into contact with silicate minerals, limestone (calcium-carbonate or CaCO₃) precipitates, in which the carbon is then bound for a very long time. In the case of a combination of sulfurous mineral, such as pyrite, and limestone, the opposite happens. The sulfuric acid that forms when pyrite comes into contact with water and oxygen dissolves carbonate minerals, thus producing CO₂. This relationship between mountain building and chemical weathering is thought to affect our planet's climate on a scale of millions of years. But how exactly does the growth of the Alps or the Himalayas affect climate? Does silicate weathering accelerate, causing the climate to cool? Or does the dissolution of limestone by sulfuric acid dominate, driving the concentration of atmospheric CO₂ up, with attendant global warming?

This question can be answered in southern Taiwan. Taiwan is located at a subduction zone, where an ocean plate slides under the Asian continent. This subduction causes rapid mountain growth. While the center of the island has been standing tall for several million years, the southern tip has just emerged from the sea. There, the mountains have low relief and they erode relatively slowly. Farther north, where the mountains are steep and tall, fresh rock is quickly brought to the Earth's surface to weather. Usefully, the rocks of southern Taiwan are typical of many young mountain ranges around the world, containing mostly silicate minerals with some carbonate and pyrite.

In their study, the researchers sampled rivers that collect water from these mountains at different erosion rates. From the material dissolved in the rivers, the researchers estimated the proportion of sulfide, carbonate, and silicate minerals in the weathering. These results allowed them to estimate the both the amount of CO₂ that is sequestered and the amount of CO₂ released by the weathering reactions. First author Aaron Bufe reports, "We found that in the southernmost part of Taiwan, atmospheric CO₂ sequestration dominates. However, farther north, where mountains are eroding faster, carbonate and sulfide weathering rates dominate and CO₂ is released."

So, does weathering of mountain ranges increase CO₂ in the atmosphere? Aaron Bufe says, "we can make relatively good statements about Taiwan. It appears that chemical weathering in this most active of mountain belts is a net emitter of CO₂ to the atmosphere due to chemical weathering. But, perhaps the story changes when sediments washed down from the mountains are trapped in vast alluvial plains; like at the foot of the Himalayas or the Alps. Those sediments are often rich in silicates, the weathering of which will sequester CO₂. In addition, mountain building brings not only sedimentary rocks with pyrite and carbonate to the Earth's surface, but also rock types that have formed from solidified magma and contain many fresh silicates that weather quickly. Researchers have some mountains to climb before we fully know the net effect of weathering on the Earth’s climate."

原始論文：Aaron Bufe, Niels Hovius, Robert Emberson, Jeremy K. C. Rugenstein, Albert Galy, Hima J. Hassenruck-Gudipati, Jui-Ming Chang. Co-variation of silicate, carbonate and sulfide weathering drives CO2 release with erosion. Nature Geoscience 14, 211–216 (2021). DOI: 10.1038/s41561-021-00714-3

引用自：GFZ German Research Centre for Geosciences. “Mountain growth influences greenhouse effect.”