岩石風化與氣候：地勢較低的山脈為最大的碳匯

 原文網址：https://www.lmu.de/en/newsroom/news-overview/news/rock-weathering-and-climate-low-relief-mountain-ranges-are-largest-carbon-sinks.html

侵蝕與風化如何影響地球數百萬年來的碳預算

地球表面的平均溫度數億年來的波動幾乎不超過20℃，而有助於生命存活在地球上。要維持如此穩定的溫度，地球勢必擁有一種「恆溫裝置」能以地質時間尺度來調節大氣中的二氧化碳濃度，進而影響全球溫度。在此恆溫裝置當中，岩石的侵蝕與風化作用是相當重要的一環。由慕尼黑大學的地質學家Aaron Bufe跟德國地球科學研究中心的Niels Hovius主持的研究團隊，最近模擬了這些作用對於大氣二氧化碳的影響。他們得到了令人驚訝的結果：透過風化反應捕捉到最多二氧化碳的地方為地勢較低、侵蝕速率一般的山脈，而非侵蝕速率最快的山區。

風化作用發生在岩石與水和空氣接觸的地方。「矽酸鹽風化的時候會從大氣中移除二氧化碳，之後以碳酸鈣的形式沉澱下來。相較之下，其他礦物相的風化作用，比方說碳酸鹽、硫化物或是岩石中的有機碳，則會釋放出二氧化碳。這些反應一般來說比矽酸鹽的風化速率快上許多，」Hovius表示。「結果便是造山作用對碳循環產生的影響是相當複雜的。」

Aaron Bufe正在觀察岩石受到的風化作用。 來自：C. Trepmann

風化作用模型顯示出共有的機制

為了解決這道複雜的問題，研究人員運用風化作用模型來分析硫化物、碳酸鹽與矽酸鹽的風化通量。他們挑選了幾個侵蝕速率差別很大的區域來進行研究，像是台灣、紐西蘭……等地。「我們發現每個地點都有類似的行為，顯示出其中有共同的機制，」Bufe表示。

進一步的模擬顯示侵蝕與二氧化碳通量的關係並非線性：風化作用捕捉到的二氧化碳會在侵蝕速率為每年0.1 mm的時候達到高峰；侵蝕速率更高或更低的情況下，二氧化碳的封存量會變少，甚至會釋放二氧化碳到大氣。「台灣或喜馬拉雅山這些高侵蝕速率的地方會讓風化作用變成二氧化碳的來源，因為在侵蝕速率到達某個程度後，矽酸鹽的風化作用不會繼續增強，但是碳酸鹽和硫化物的風化作用卻可以繼續提高，」Bufe解釋。

在侵蝕速率一般、大約每年為0.1mm的地貌當中，大部分都十分缺乏快速風化的碳酸鹽與硫化物，而矽酸鹽礦物則相當豐富並受到充分的風化。當侵蝕速率慢到不足每年0.1mm，只有極為少數的礦物可以遭到風化。因此最大的二氧化碳碳匯是那些高度較低、侵蝕速率接近最適值的山脈，像是德國的黑森林或奧勒岡海岸山脈。「因此從地質時間尺度來看，地球的『恆溫裝置』會把溫度設定在哪，很大一部份取決於全球的侵蝕速率如何分布，」Bufe表示。為了更詳細地了解侵蝕速率對於地球氣候系統的影響，Bufe認為未來的研究應該把有機碳的儲集過程以及氾濫平原的風化作用也考慮進去才行。

 

Rock weathering and climate: low-relief mountain ranges are largest carbon sinks

How erosion and weathering affect the CO₂ budget over millions of years

For many hundreds of millions of years, the average temperature at the surface of the Earth has varied by not much more than 20° Celsius, facilitating life on our planet. To maintain such stable temperatures, Earth must have a ‘thermostat’ that regulates the concentration of atmospheric carbon dioxide over geological timescales, influencing global temperatures. The erosion and weathering of rocks are important parts of this ‘thermostat.’ A team led by LMU geologist Aaron Bufe and Niels Hovius from the German Research Centre for Geosciences has now modeled the influence of these processes on carbon in the atmosphere. Their surprising result: CO₂ capture through weathering reactions is highest in low-relief mountain ranges with moderate erosion rates and not where erosion rates are fastest.

Weathering occurs where rock is exposed to water and wind. "When silicates weather, carbon is removed from the atmosphere and later precipitated as calcium carbonate. By contrast, weathering of other phases – such as carbonates and sulfides or organic carbon contained in rocks – releases CO₂. These reactions are typically much faster than silicate weathering", says Hovius. “As a consequence, the impact of mountain building on the carbon cycle is complex.”

Weathering model shows common mechanisms

To address this complexity, the researchers used a weathering model to analyze fluxes of sulfide, carbonate, and silicate weathering in a number of targeted study regions – such as Taiwan and New Zealand – with large ranges in erosion rates. “We discovered similar behaviors in all locations, pointing to common mechanisms,” says Bufe.

Further modelling showed that the relationship between erosion and CO₂-fluxes is not linear, but that CO₂ capture from weathering peaks at an erosion rate of approximately 0.1 millimeters per year. When rates are lower or higher, less CO₂ is sequestered and CO₂ may even be released into the atmosphere. “High erosion rates like in Taiwan or the Himalayas push weathering into being a CO₂ source, because silicate weathering stops increasing with erosion rates at some point, whereas the weathering of carbonates and sulfides increases further,” explains Bufe.

In landscapes with moderate erosion rates of around 0.1 millimeters per year, the rapidly weathering carbonates and sulfides are largely depleted, whereas silicate minerals are abundant and weather efficiently. Where erosion is even slower than 0.1 millimeters per year, only few minerals are left to weather. The biggest CO₂ sinks are therefore low-relief mountain ranges such as the Black Forest or the Oregon Coast Range, where erosion rates approach the optimum. “Over geological timescales, the temperature to which Earth’s ‘thermostat’ is set therefore depends strongly on the global distribution of erosion rates,” says Bufe. To understand the effects of erosion on Earth’s climate system in greater detail, Bufe thinks that future studies should additionally consider organic carbon sinks and weathering in floodplains.

 

原始論文：Aaron Bufe, Jeremy K. C. Rugenstein, Niels Hovius. CO 2 drawdown from weathering is maximized at moderate erosion rates. Science, 2024; 383 (6687): 1075 DOI: 10.1126/science.adk0957

引用自：Ludwig-Maximilians-Universität München. "Rock weathering and climate: Low-relief mountain ranges are largest carbon sinks."