劍橋大學的新研究表示，地表岩石的風化作用從大氣中移除的溫室氣體也許比之前預估的還少。
自然界可以利用岩石風化從大氣中移除二氧化碳，但發表在《美國國家科學院院刊》(PNAS)的成果提出這項機制事實上可能比科學家之前認為的還要微弱。這讓科學家得重新思考以數百萬年的時間尺度來看，岩石在減緩暖化的過程中究竟具有什麼樣的功能。
研究也顯示或許還有另一種前所未聞的作用可以從大氣中吸收二氧化碳，因此長時間尺度下能對氣候變遷造成影響。這是研究人員現在希望可以找到的目標。
大氣中的二氧化碳在風化過程中會分解岩石，然後被封存在沉積物裡面。這項過程是地球的碳循環中很重要的一道環節，它讓二氧化碳可以穿梭在陸地、大氣與海洋，進而影響全球溫度。
主要作者，劍橋大學地球科學系的Ed
Tipper表示：「風化作用就像是地球的溫控設備――也是地球為什麼適合居住的原因。科學家長久以來認為這讓地球的溫室效應不像金星一樣失去控制。」透過把二氧化碳封鎖在沉積物裡面，風化作用在長時間尺度下可以移除大氣中的二氧化碳，減輕溫室效應而讓全球溫度下降。
團隊最新的計算結果顯示世界各地的風化作用通量過往最多被高估了28%，其中最嚴重的是岩石分解特別快的山區河流。
他們也指出地球上最大的河川系統裡的其中三個――包括發源自西藏高原，彼此相鄰的黃河與怒江，以及北美的育空河――在長時間尺度下並不像之前認為的會吸收二氧化碳。
數十年來，西藏高原都被當成是長期下來會吸收碳的場所，因此可以調控氣候。全世界海洋裡的沉積物大約25%是從西藏高原產生。
Tipper說：「研究碳循環最好的地方之一便是河川，它們是陸地的血管，連結了固體地球與海洋。河川把風化產生的沉積物從陸地運送到海洋，然後把沉積物的碳封鎖到岩石裡面。」
「科學家測量河川的化學成分來推估風化速率已經有幾十年了，」共同作者Victoria
Alcock表示。「溶解鈉是風化產物中最常被測量的對象之一，但是我們證明並沒有這麼簡單，實際上鈉的來源經常是其他地方。」
地球岩石的基本組成最多的是矽酸鹽礦物。大氣中的二氧化碳和雨水混和之後會產生碳酸，當它們溶解矽酸鹽之後，就會把鈉釋放出來。
然而，團隊發現並非所有的鈉都是來自於風化作用。「我們發現世界各地河水中的鈉還有另一個來源，」共同作者Emily
Stevenson表示。「跟其他研究假設的不同，這些額外的鈉並非來自於風化的矽酸岩，而是集水區內部年代非常久遠的黏土受到侵蝕所產生的。」
Tipper和研究團隊為了尋找這些多出來的鈉是從何而來，他們探討了地球上八個最大的河川系統。這項任務總共歷時了16次長期的野外調查，並且在實驗室分析了數千具樣本。
他們發現答案是「陽離子交換池」(cation exchange pool)，這種水和黏土形成的稠狀膠體會跟著泥質沉積物一起被河水搬運。
像鈉這種帶有正電的離子稱為陽離子。交換池便是一群容易反應的陽離子微弱地鍵結在黏土顆粒上。它們很容易就會從膠體中跑出來跟河水裡的其他元素互相交換(比方說鈣)，過程只需要幾小時而已。
雖然1950年代就已經有人陳述了土壤中的陽離子交換池，但是對於河水中鈉的來源來說，科學家一直以來都沒有太關注它的地位。
「交換池裡的黏土的化學與同位素組成可以告訴我們它們的成分與來源，」共同作者Alasdair
Knight表示。「我們得出河川搬運的黏土有許多是來自古代的沉積物，而我們提出河川裡的鈉有一部分的來源必定就是這些黏土。」
黏土原先是在數百萬年前由陸地遭受的侵蝕作用所形成。在它們一路往下游的旅程當中會從周圍的水體獲取陽離子，到達海洋之後交換池便會從海水中汲取鈉。今日這些古代的黏土從海床抬升之後，便會連同其中的鈉一起被當今的河川侵蝕。
這些古老的鈉會從交換池的黏土交換到河水裡面，過去科學家誤以為它們是現今的風化作用留下來的溶解物質。
「光是要產生一個資料點就需要大量的實驗室工作，此外還得進行很多的數學運算，」Stevenson表示。「就像是要解構一個蛋糕，我們如同鑑識人員般分離出沉積物裡每一種重要的成分，然後把交換池與黏土留下來。科學家已經利用這種方法很長一段時間，而且也取得了成效，但我們卻找出了另外一種會提供鈉的原料，而且還要解釋它的來源。」
Tipper表示：「感謝多年來許多同事與學生辛苦地進行研究，我們樣品的涵蓋範圍才足以讓我們開始理解這種複雜的化學作用在全球尺度上是如何進行。」
科學家現在感到疑惑的是，從地質時間來看，還有什麼樣的作用可以吸收地球的二氧化碳？雖然他們還沒有確定的候選名單，但一個具有爭議的可能性是生物可以移除大氣裡的碳。另外一項理論則認為海床或火山島弧的矽酸鹽溶解可能具有一定的重要性。Tipper表示：「數十年來由於風化作用使得人們都著眼於陸地，或許現在我們應該要開始把視野拓展到其他地方了。」
Muddying the waters: rock breakdown
may play less of a role in regulating climate than previously thought
The weathering of rocks at the Earth’s surface
may remove less greenhouse gases from the atmosphere than previous estimates,
says new research from the University of Cambridge.
The findings, published in PNAS, suggest Earth’s natural mechanism for removing carbon dioxide
(CO2) from the atmosphere via the weathering of rocks may in fact be weaker
than scientists had thought – calling into question the exact role of rocks in
alleviating warming over millions of years.
The research also suggests there may be a previously
unknown sink drawing CO2 from the atmosphere and impacting climate changes over
long timescales, which researchers now hope to find.
Weathering is the process by which atmospheric carbon
dioxide breaks down rocks and then gets trapped in sediment. It is a major part
of our planet’s carbon cycle, shuttling carbon dioxide between the land, sea
and air, and influencing global temperatures.
“Weathering is like a planetary thermostat - it’s the
reason why Earth is habitable. Scientists have long suggested this is why we
don’t have a runaway greenhouse effect like on Venus,” said lead author Ed
Tipper from Cambridge’s Department of Earth Sciences. By locking carbon dioxide
away in sediments, weathering removes it from the atmosphere over long
timescales, reducing the greenhouse effect and lowering global temperatures.
The team’s new calculations show that, across the
globe, weathering fluxes have been overestimated by up to 28%, with the
greatest impact on rivers in mountainous regions where rocks are broken down
faster.
They also report that three of the largest river
systems on Earth, including the neighbouring Yellow and Salween Rivers with
their origins on the Tibetan Plateau and the Yukon River of North America, do
not absorb carbon dioxide over long timescales - as had been thought.
For decades, the Tibetan plateau has been invoked as
a long-term sink for carbon and mediator of climate. Some 25% of the sediment
in the world’s oceans originate from the plateau.
“One of the best places to study the carbon cycle are
rivers, they are the arteries of the continents. Rivers are the link between
the solid Earth and oceans – hauling sediments weathered from the land down to
the oceans where their carbon is locked up in rocks,” said Tipper.
“Scientists have been measuring the chemistry of
river waters to estimate weathering rates for decades,” said co-author Victoria
Alcock “Dissolved sodium is one of the most commonly measured products of
weathering – but we’ve shown that it’s not that simple, and in fact sodium
often comes from elsewhere.”
Sodium is released when silicate minerals, the basic
building blocks of most of Earth’s rocks, dissolve in carbonic acid - a mix of
carbon dioxide in the atmosphere and rainwater.
However, the team found not all sodium comes from
this weathering process. “We’ve found an additional source of sodium in river
waters across the globe,” said co-author Emily Stevenson. “That extra sodium is
not from weathered silicate rocks as other studies assume, but in fact from
very old clays which are being eroded in river catchments.”
Tipper and his research group studied eight of the
largest river systems on Earth, a mission involving 16 field seasons and
thousands of lab analyses in search of where that extra sodium was coming
from.
They found the answer in a soupy ‘gel’ of clay and
water – known as the cation exchange pool – which is carried along by muddy
river sediment.
The exchange pool is a reactive hive of cations –
positively charged ions like sodium - which are weakly bonded to clay
particles. The cations can easily swap out of the gel for other elements like
calcium in river water, a process that can take just a few hours.
Although it has been described in soils since the
1950s, the role the exchange pool plays in supplying sodium to rivers has been
largely neglected.
“The chemical and isotopic makeup of the clays in the
exchange pool tell us what they are made of and where they’ve come from,” said
co-author Alasdair Knight. “We know that many of the clays carried by these
rivers come from ancient sediments, and we suggest that some of the sodium in
the river must come from these clays.”
The clays were originally formed from continental
erosion millions of years ago. On their journey downstream they harvested
cations from the surrounding water –
their exchange pool picking up sodium on reaching the sea. Today, after
being uplifted from the seafloor, these ancient clays – together with their
sodium - are now being eroded by modern rivers.
This old sodium, which can switch out of the clays in
the exchange pool and into river water, has previously been mistaken as the
dissolved remnants of modern weathering.
“Generating just one data point took a huge amount of
work in the lab and we also had to do a lot of maths,” said Stevenson. “It’s
like unmixing a cake, using a forensic approach to isolate key ingredients in
the sediments, leaving behind the exchange pool and the clays. People have used
the same methods for a really long time - and they work - but we’ve been able
to find an extra ingredient that provides the sodium and we need to account for
this.”
“It's thanks to the hard work of many collaborators
and students over many years that our samples had the scope to get to grips
with this complex chemical process at a global scale,” said Tipper.
Scientists are now left to puzzle over what else
could be absorbing Earth’s carbon dioxide over geological time. There are no
certain candidates – but one controversial possibility is that life is removing
carbon from the atmosphere. Another theory is that silicate dissolution on the
ocean floor or volcanic arcs may be important. “People have spent decades
looking on the continents for weathering - so maybe we now need to start
expanding where we look,” said Tipper.
原始論文：Edward T.
Tipper, Emily I. Stevenson, Victoria Alcock, Alasdair C. G. Knight, J. Jotautas
Baronas, Robert G. Hilton, Mike J. Bickle, Christina S. Larkin, Linshu Feng,
Katy E. Relph, and Genevieve Hughes. Global silicate weathering flux
overestimated because of sediment–water cation exchange. PNAS,
2020 DOI: 10.1073/pnas.2016430118
引用自：University of Cambridge. "Muddying the
waters: Weathering might remove less atmospheric carbon dioxide than
thought."
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