原文網址：https://press.uni-mainz.de/enhanced-chemical-weathering-a-solution-to-the-climate-crisis/
地球正在不斷變熱——今年全球的夏季也明顯看到這會有什麼樣的後果。但回顧地質歷史，全球暖化事件並不罕見：距今大約5600萬年前稱作古新世─始新世氣候最暖期(Paleocene–Eocene
Thermal Maximum ,PETM)的時代，地球的平均氣溫上升了5-8度。氣候會如此發展最有可能的原因是火山活動增加，連帶造成許多二氧化碳釋放到大氣當中。這股高溫持續了大約20萬年左右。回到2021年，約翰尼斯・古騰堡－美茵茲大學的教授Philip
Pogge von Strandmann探討了最後是什麼樣的效應結束了PETM暖化事件，使得全球氣溫降低而讓氣候回復到原狀。結果簡單來說，是雨水跟大氣中的二氧化碳結合成碳酸，使得岩石風化速度提高而把鈣鎂釋放出來。接著河川把這些鈣、鎂、碳酸帶到海洋，在此鈣、鎂以及碳酸(亦即二氧化碳)結合成不可溶的石灰岩。「換句話說，自然界有道回饋作用可以幫助調控氣候。高溫可以加速岩石的化學風化過程，使得大氣中的二氧化碳含量降低，讓氣候得以回復原狀，」Pogge
von Strandmann表示。
從d7Li的數據和模擬結果推測地球系統可能的變化過程。圖片來源：自然—地球科學
距今4000萬年前的氣候暖化用了兩倍的時間才回復
在PETM的1600萬年之後，於中新世中期氣候最暖期(Middle
Eocene Climatic Optimum, MECO)再度發生氣候暖化。雖然火山活動排放到大氣的二氧化碳量跟PETM時的差不多，但是氣候卻用了更久的時間才回穩。暖化的效應持續了漫長的40萬年，是PETM時的兩倍。為什麼這段時期的回復速度這麼慢？
Pogge von Strandmann與本論文的第一作者Alex
Krause和其他共同作者為了找尋答案，分析了4000萬年前的海洋碳酸岩以及黏土礦物，並將結果與5600萬年前的類似樣品對比。「就像在PETM期間一樣，風化侵蝕在MECO也變得更強了。然而在距今4000萬年前露出地表的岩石少了許多。當時地球許多地方都是廣大的雨林，其中的土壤大部分都是由黏土礦物組成，」研究人員解釋。跟岩石相反，黏土並不會風化；實際上，它們正是風化的產物。「因此就算當時的溫度很高，廣布的黏土土壤卻降低了岩石風化的效率，此作用稱為『土壤屏蔽』(soil
shielding)，」Pogge
von Strandmann指出。
加強風化來保護氣候
這項知識如何運用到現今的世界？「我們經由研究古氣候來找出能否運用某些方式對現在的氣候造成正面的影響，提升岩石的化學風化速率或許是其中一個選項。要達成此目的，我們可以把磨成細粉的岩石混進農田當中，」Pogge
von Strandmann表示。細顆粒的岩石會快速地受到侵蝕並結合大氣中的二氧化碳，進而調養氣候。這種過程中可以吸收二氧化碳的負排放技術(Negative
emissions technology, NET)在世界各國都是正在進行大量研究的主題。但於此同時，正如Pogge
von Strandmann所發現的，如果風化產生了黏土，移除二氧化碳過程的效率就會大幅降低。黏土會留住應該進到海裡的鈣和鎂，造成二氧化碳雖然持續流入海洋，卻不會困在那裡而是可以重新回到大氣。在這種情況，風化作用幾乎不會對氣候造成任何影響。
如果風化作用可以讓岩石顆粒完全溶解，那麼提升風化作用的想法就能百分之百發揮效用。但是，如果所有風化產物都變成黏土，此作用就完全失去了效力。實際結果很可能是在這兩種極端情形的中間地帶：就像PETM的岩石受到較強的侵蝕作用造成氣候更快回穩，而MECO則是以形成黏土為主。磨碎的岩石有多少會溶解，多少會成為黏土而保存下來跟許多區域性因素有關，像是世界各地先前存在的岩石與黏土量有多少。因此要確定提升風化作用是否為一個可行的方法，首先得找出每個潛在場址的風化作用會產生多少黏土。
與此研究相應的論文最近發表在《自然—地球科學》。參與此計畫的還有倫敦大學學院、英國艾塞克斯大學與荷蘭烏特勒支大學的研究人員。
Enhanced chemical weathering: A
solution to the climate crisis?
The Earth is getting hotter and
consequences have been made manifest this summer around the world. Looking back
in geological history, global warming events are not uncommon: Around 56
million years ago, during the period known as the Paleocene–Eocene Thermal
Maximum (PETM), the temperatures rose by an average of 5 to 8 degrees Celsius.
This development was most likely linked to increased volcanism and the
associated release of masses of carbon dioxide into the atmosphere. The higher
temperatures persisted for about 200,000 years. Back in 2021, Professor Philip
Pogge von Strandmann of Johannes Gutenberg University Mainz (JGU) had already
investigated the effect that eventually led to global cooling and climatic
recovery after the PETM warming. In short: Rainwater combined with the
atmospheric carbon dioxide, resulting in carbonic acid that caused enhanced
weathering of rock, thus releasing calcium and magnesium. Rivers then
transported the calcium, magnesium, and carbonic acid into the oceans where the
calcium, magnesium – and also the carbon dioxide – came together to form
insoluble limestone. "In other words, there is a feedback effect that
helps control the climate. High temperatures accelerate the chemical rock
weathering process, reducing the levels of carbon dioxide in the atmosphere,
allowing the climate to recover," said Pogge von Strandmann.
Climate required
twice as long to regenerate 40 million years ago
Climate warming occurred again 16 million years after
the PETM during the Middle Eocene Climatic Optimum or MECO. Although volcanic
activity resulted in the discharge of roughly the same amounts of carbon
dioxide into the atmosphere as during the PETM, it took far longer for the
climate to restabilize. The warming effect lasted for an immense 400,000 years,
twice as long as in the PETM. Why was recovery so slow during that period?
In searching for an answer, Pogge von Strandmann and
co-authors, including first author Alex Krause, began analyzing
40-million-year-old oceanic carbonates and clay minerals to compare the results
with those for similar 56-million-year-old examples. "Just as during the
PETM, there was also intensified weathering and erosion in the MECO. However,
there was far less exposed rock on the Earth's surface 40 million years ago.
Instead, the Earth was extensively covered by a global rainforest the soil of
which largely consisted of clay minerals," explained the researcher. In
contrast with rock, clay does not weather; in fact, it is actually the product
of weathering. "So despite the high temperatures, the widespread clay soil
prevented rocks from being effectively weathered, a process known as soil
shielding," the geoscientist pointed out.
Enhanced
weathering for climate protection
How can we use this knowledge in today's world?
"We study paleoclimates to determine whether and how we can positively
influence our present climate. One option might be to boost the chemical
weathering of rock. To help achieve this, we could plough finely crushed rock
into our fields," said Pogge von Strandmann. The fine-grained particles of
rock would erode rapidly, resulting in the binding of atmospheric carbon
dioxide, thus enabling the climate to recuperate. Negative emissions
technologies (NETs) such as this involving the absorption of carbon dioxide are
the subjects of intense research across the globe. At the same time, however,
if the weathering results in the formation of clay, the effects of the process
would be significantly less efficient, as Pogge von Strandmann has discovered.
Clay retains the calcium and magnesium that would otherwise be delivered to the
ocean. The carbon dioxide would continue to flow into the oceans, but it would
not be bound there and would be able to escape back into the atmosphere. In
this case, the weathering effect would have next to no influence on the
climate.
If the rock particles fully dissolve as a result of
weathering, the enhanced weathering concept would turn out to be 100 percent
efficient. However, if all the weathered materials were turned into clay, this
would in its turn completely nullify the effect. In reality, the actual outcome
would probably be somewhere between the two extremes: While there was enhanced
erosion of rock in the PETM so that the climate normalized more rapidly, clay
formation was predominant during the MECO. The extent to which the crushed rock
dissolves and how much of it is preserved as clay depends on a range of local
factors, such as the globally pre-existing levels of clay and rock. So in order
to establish whether the process of enhanced weathering is a viable approach,
it would first be necessary to find out how much clay is formed during the
weathering process at each potential location.
The corresponding research paper has recently been
published in Nature Geoscience. Also
involved in the project were researchers at University College London and the
University of Essex in the UK as well as Utrecht University in the Netherlands.
原始論文：Alexander J.
Krause, Appy Sluijs, Robin van der Ploeg, Timothy M. Lenton, Philip A. E. Pogge
von Strandmann. Enhanced clay formation key in sustaining the Middle
Eocene Climatic Optimum. Nature Geoscience, 2023; 16 (8): 730
DOI: 10.1038/s41561-023-01234-y
引用自：Johannes Gutenberg Universitaet Mainz.
"Enhanced chemical weathering: A solution to the climate
crisis?."
原文網址：https://press.uni-mainz.de/enhanced-chemical-weathering-a-solution-to-the-climate-crisis/
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