長期而言火山對於地球氣候具有安全閥一般的功用

 原文網址：https://www.southampton.ac.uk/news/2021/08/volcanic-arc-weathering.page

南安普敦大學的科學家發現在地質時間當中，廣大的火山長鏈兼具排放二氧化碳到大氣當中並把它們移除的功能，這讓地表的氣溫得以保持穩定。

現今位於俄羅斯堪察加半島的火山陸弧。圖片來源：Tom Gernon博士

研究人員和雪梨大學、澳洲國立大學、渥太華大學與里茲大學的同僚合作，探討過去四億年來固體地球、海洋、大氣的作用結合起來的影響。他們的發現發表在期刊《自然―地球科學》(Nature Geoscience)。

地表岩石在自然狀況下遭到分解與溶解的過程稱為化學風化。這項作用非常重要，因為化學風化的產物(鈣和鎂之類的元素)被河流沖進海洋之後，會形成把二氧化碳固定下來的礦物。經過長久的地質時間，這道回饋機制可以調節大氣中的二氧化碳含量，進而影響地球氣候。

「從這方面來看，地表風化作用的功能就像是地質學裡的恆溫器一樣，」主要作者Tom Gernon博士表示。他是南安普敦大學的地球科學副教授以及圖靈研究院的研究員。「但是之前已經證明很難確定其中的控制因素是什麼，因為地球是個相當複雜的系統。」

「地球上許多作用都互有關連，而且有些作用造成的影響要過了很久才會浮現出來，」共同作者，國立澳洲大學海洋與氣候變遷科學的教授Eelco Rohling表示。「因此想要了解地球系統出現的反應裡面，特定作用造成的影響相對來說有多少，一直以來都是棘手的問題。」

為了拆解如此複雜的關係，團隊納入機器學習演算法以及板塊運動的重建結果，建立出一種新型的「地球網路」。這讓團隊可以辨識地球系統當中主要的交互作用，以及它們隨著時間經過如何演變。

團隊發現過去四億年來決定風化作用強度最重要的因素為火山陸弧。火山陸弧是由一連串的火山組成，現今的例子像是南美洲的安地斯山脈與美國的喀斯開山脈。這些火山當中的一部份是目前地球上最高聳以及侵蝕速度最快的地形。由於組成火山的岩石相當破碎且容易發生化學反應，因此它們會快速風化接著被沖到海裡。

共同作者，南安普敦大學的地球化學教授Martin Palmer表示：「這些火山一方面會噴出大量二氧化碳使空氣中的二氧化碳含量增加；但同樣的火山另一方面卻能藉由迅速的風化反應來幫忙移除大氣中的二氧化碳，使得兩者之間達到平衡。」

這項研究質疑了一項科學家長久以來持有的觀念，認為地球氣候數億年來可以保持穩定，反映了海床和大陸內部的風化作用達成平衡。「有項說法認為從地質角度來看，陸塊和海床之間的拔河是影響地表風化的主要因素，但我們的數據並不支持，」Gernon博士主張。

「不幸的是，研究結果並非代表自然界可以讓我們免於氣候變遷的後果，」Gernon博士強調。「現今大氣二氧化碳的含量是過去三百萬年以來的新高，而人類排放的二氧化碳則是火山排放量的150倍左右。許久之前可能曾經保護過地球的陸弧，完全無法達到抵銷現今二氧化碳排放量所需的規模。」

不過團隊的發現還是提供了重要的見解，告訴人類社會該如何處理眼前的氣候危機。把岩石磨碎之後分散到陸地各處以加速化學反應的速率――這種以人工加速的岩石風化作用，或許可以成為安全移除大氣二氧化碳的重要方法。團隊的這項發現顯示這類計畫也許最適合用在陸弧環境中常見的岩石，也就是含有鈣、鉀、鈉的鈣鹼性火山岩。

「這絕對不是解決氣候危機的靈丹妙藥，我們需要立即遵照IPCC的減碳途徑來減少二氧化碳的排放量，也就是完全停止。我們估計長時間尺度下風化作用的回饋所得到的結果，或許可以用來設計大規模的風化速率提升計畫並評估其效用，但這只是對抗全球氣候變遷所需的行動中的一小步而已。」

 

Volcanoes acted as a safety valve for Earth’s long-term climate

Scientists at the University of Southampton have discovered that extensive chains of volcanoes have been responsible for both emitting and then removing atmospheric carbon dioxide (CO₂) over geological time. This stabilised temperatures at Earth’s surface.

The researchers, working with colleagues at the University of Sydney, Australian National University (ANU), University of Ottawa and University of Leeds, explored the combined impact of processes in the solid Earth, oceans and atmosphere over the past 400 million years. Their findings are published in the journal Nature Geoscience.

Natural break-down and dissolution of rocks at Earth’s surface is called chemical weathering. It is critically important because the products of weathering (elements like calcium and magnesium) are flushed via rivers to the oceans, where they form minerals that lock up CO₂. This feedback mechanism regulates atmospheric CO₂ levels, and in turn global climate, over geological time.

“In this respect, weathering of the Earth’s surface serves as a geological thermostat”, says lead author Dr Tom Gernon, Associate Professor in Earth Science at the University of Southampton, and a Fellow of the Turing Institute. “But the underlying controls have proven difficult to determine due to the complexity of the Earth system”.

“Many Earth processes are interlinked, and there are some major time lags between processes and their effects”, explains Eelco Rohling, Professor in Ocean and Climate Change at ANU and co-author of the study. “Understanding the relative influence of specific processes within the Earth system response has therefore been an intractable problem”.

To unravel the complexity, the team constructed a novel “Earth network”, incorporating machine-learning algorithms and plate tectonic reconstructions. This enabled them to identify the dominant interactions within the Earth system, and how they evolved through time.

The team found that continental volcanic arcs were the most important driver of weathering intensity over the past 400 million years. Today, continental arcs comprise chains of volcanoes in, for example, the Andes in South America, and the Cascades in the US. These volcanoes are some of the highest and fastest eroding features on Earth. Because the volcanic rocks are fragmented and chemically reactive, they are rapidly weathered and flushed into the oceans.

Martin Palmer, Professor of Geochemistry at the University of Southampton and co-author of the study, said: “It’s a balancing act. On one hand, these volcanoes pumped out large amounts of CO₂ that increased atmospheric CO₂ levels. On the other hand, these same volcanoes helped remove that carbon via rapid weathering reactions.”

The study casts doubt on a long-held concept that Earth’s climate stability over tens to hundreds of millions of years reflects a balance between weathering of the seafloor and continental interiors. “The idea of such a geological tug of war between the landmasses and the seafloor as a dominant driver of Earth surface weathering is not supported by the data,” Dr Gernon states.

“Unfortunately, the results do not mean that nature will save us from climate change”, stresses Dr Gernon. “Today, atmospheric CO₂ levels are higher than at any time in the past 3 million years, and human driven emissions are about 150 times larger than volcanic CO₂ emissions. The continental arcs that appear to have saved the planet in the deep past are simply not present at the scale needed to help counteract present-day CO₂ emissions”.

But the team’s findings still provide critical insights into how society might manage the current climate crisis. Artificially enhanced rock weathering - where rocks are pulverised and spread across land to speed up chemical reaction rates - could play a key role in safely removing CO₂ from the atmosphere. The team’s findings suggest that such schemes may be deployed optimally by using calc-alkaline volcanic materials (those containing calcium, potassium and sodium), like those found in continental arc environments.

“This is by no means a silver bullet solution to the climate crisis - we urgently need to reduce CO₂ emissions in line with IPCC mitigation pathways, full stop. Our assessment of weathering feedbacks over long timescales may help in designing and evaluating large-scale enhanced weathering schemes, which is just one of the steps needed to counteract global climate change”, Dr Gernon concludes.

原始論文：Thomas M. Gernon, Thea K. Hincks, Andrew S. Merdith, Eelco J. Rohling, Martin R. Palmer, Gavin L. Foster, Clément P. Bataille, R. Dietmar Müller. Global chemical weathering dominated by continental arcs since the mid-Palaeozoic. Nature Geoscience, 2021; DOI: 10.1038/s41561-021-00806-0

引用自：University of Southampton. "Volcanoes acted as a safety valve for Earth’s long-term climate."