有機碳藏沉積物，大氣就能有氧氣
伍茲霍爾海洋研究所和哈佛大學最近進行的新研究或許有助於解決一項歷時已久的問題：岩石和沉積物如何保存一小部分的有機碳，避免它們遭到分解。主要作者，哈佛大學的博士後研究員Jordon
Hemingway之前就讀於伍茲霍爾海洋研究所，他說詳細了解這項作用的進行方式，可以解釋大氣中的氣體組成為何能長期維持穩定。這篇論文6月14日發表於期刊《自然》(Nature)。
富含有機物的內格羅河和富含沉積物的蘇里摩希河在亞馬遜盆地交匯時的景象。(圖片來源：Chris
Linder)
Hemingway指出大氣中的二氧化碳是一種無機碳。植物、藻類和某些細菌可以從空氣中抽出二氧化碳，當作原料組裝成體內的醣類、蛋白質和其他分子。此過程是光合作用的一部分，可以把無機碳轉換成「有機」碳，同時釋放氧氣到大氣當中。這些生物死後發生的作用則相反過來：微生物會逐漸分解生物遺體，消耗氧氣並把二氧化碳釋回大氣。
Hemingway表示地球適合居住的關鍵因素之一，便是上述的化學循環其實有點不平衡。某些原因造成一小部分的有機碳並未遭到微生物分解，而是持續保存在地下數百萬年。
「如果此循環的平衡相當完美，大氣中自由氧的生成速度有多快，消耗速度就有多快。」Hemingway說。「若要留下一些氧氣讓我們呼吸，就必須把某些有機碳藏在微生物無法分解的地方。」
研究人員根據現有的證據，發展出兩套理論解釋碳保存下來的原因。第一個理論稱為「選擇性保存」，他們認為某些有機碳分子可能很難被微生物破壞，因此沉積物裡的其他有機碳都分解之後，這些分子仍然可以完好無缺。另外一個理論稱為「礦物保護」，意思是有機碳能和周遭礦物形成強韌的化學鍵結，使得細菌無法把它們從礦物上面摘下來「吃掉」。
Hemingway說：「一直以來我們都很難分辨出哪個作用才是主要的。有機地球化學的分析工具還不夠靈敏。」Hemingway在這項研究中運用了「漸進式熱裂解氧化」法，分析從世界各地採集的沉積物樣品以驗證這兩個理論。他運用特別設計過的高溫爐把樣品逐步升到將近1000℃，同時測量加熱過程中有多少二氧化碳釋放出來。溫度較低時釋放出來的二氧化碳代表這種碳的化學鍵較弱；在高溫時釋放出來的則擁有較多能量才能打破的強力化學鍵。此外，他還運用碳定年法測量二氧化碳的年代。
「如果有機分子是因為細菌無法分解而選擇性地保存下來，我們預期樣品中的鍵結強度會侷限在一個很小的範圍。因為微生物應該只會留下幾種特別頑強的有機物，而把其他的全部分解掉。」他說。「但實際上我們沒有看到沉積物年代越久，鍵結的強度範圍也跟著縮小的現象；反而是鍵結的強度更加多變，代表許多不同類型的有機碳都保存了下來。我們認為這象徵了它們周圍有礦物保護著。」
Hemingway在樣品中另外看見的模式也支持了這項發現。像河口這類地點採到的細緻黏土，普遍來說碳的鍵結種類都比顆粒較粗的沉積物或砂礫來得更加多樣，顯示細粒沉積物的表面積更為廣大，可以黏附更多有機碳。
共同作者，伍茲霍爾海洋研究所的生物地球化學家Valier
Galy說：「這麼說好了，如果你把新罕布夏的花崗岩拿去磨碎，得到的會是沙子這類東西。它們相對來說比較大顆，所以可以跟有機物質接觸的表面積就比較小。你必須要經過地表的化學風化作用，才能得到細粒沉積物，像是屬於層狀矽酸鹽的黏土。」
雖然研究提供的證據強烈支持了兩個假說的其中一方，Hemingway和他的同事也立刻表明關於有機碳的謎題並未就此定調。Galy表示：「就算我們可以指出碳能保存下來是因為哪個機制，但我們並沒有提供任何訊息來闡明其他因素的影響，像是環境溫度。整體來說還需要考慮許多變因，而我們的論文可以當作一種參考，指引生物地球化學家未來的研究方向。」
Organic carbon hides in sediments,
keeping oxygen in atmosphere
A new study from researchers at the Woods
Hole Oceanographic Institution (WHOI) and Harvard University may help settle a
long-standing question—how small amounts of organic carbon become locked away
in rock and sediments, preventing it from decomposing. Knowing exactly how that
process occurs could help explain why the mixture of gases in the atmosphere
has remained stable for so long, says lead author Jordon Hemingway, a postdoctoral
researcher at Harvard and former student at WHOI. The paper publishes June 14
in the journal Nature.
Atmospheric carbon dioxide (CO2),
Hemingway notes, is an inorganic form of carbon. Plants, algae, and certain
types of bacteria can pull that CO2 out of the air, and use it as a
building block for sugars, proteins, and other molecules in their body. The
process, which occurs during photosynthesis, transforms inorganic carbon into
an “organic” form, while releasing oxygen into the atmosphere. The reverse
occurs when those organisms die: microbes start to decompose their bodies,
consuming oxygen and releasing CO2 back into the air.
One of the key reasons Earth has remained habitable
is that this chemical cycle is slightly imbalanced, Hemingway says. For some
reason, a small percentage of organic carbon is not broken down by microbes,
but instead stays preserved underground for millions of years.
“If it were perfectly balanced, all the free oxygen
in the atmosphere would be used up as quickly as it was created,” says Hemingway. “In order to have oxygen left
for us to breathe, some of the organic carbon has to be hidden away where it
can’t decompose.”
Based on existing evidence, researchers have
developed two possible reasons why carbon is left behind. The first, called
“selective preservation,” suggests that some molecules of organic carbon may be
difficult for microorganisms to break down, so they remain untouched in
sediments once all others have decomposed. The second, called the “mineral
protection” hypothesis, states that molecules of organic carbon may instead be
forming strong chemical bonds with the minerals around them—so strong that
bacteria aren’t able to pluck them away and “eat” them.
“Historically, it’s been hard to tease out which
process is dominant. The tools we have for organic geochemistry haven’t been
sensitive enough,” says Hemingway. For this study, he turned to a method called
“ramped pyrolysis oxidation”, or RPO, to test the hypotheses in sediment
samples from around the globe. With a specialized oven, he steadily raised the
temperature of each sample to nearly 1000 degrees Celsius, and measured the
amount of carbon dioxide it released as it warmed. CO2 released at
lower temperatures represented carbon with relatively weak chemical bonds,
whereas carbon released at high temperatures denoted strong bonds that took
more energy to break. He also measured the age of the CO2 using
carbon dating methods.
“If organic molecules are being preserved because of
selectivity—because microbes aren’t able to break them down— we would expect to
see a pretty narrow range of bond strength in the samples. Microbes would have
decomposed the rest, leaving only a few stubborn types of organic carbon
behind,” he says. “But we actually saw that the diversity of bond strengths
grows rather than shrinks with time, indicating that a wide range of organic
carbon types are being preserved. We think that means they’re getting
protection from minerals around them.”
Hemingway also saw a pattern in the samples
themselves that supported his findings. Fine clays like those found at river
outlets had a consistently higher diversity of carbon bonds than coarse or
sandy sediments, suggesting that fine sediments provide more surface area on
which organic carbon could attach itself.
“If you take, say, granite from New Hampshire and
break it down, you’ll get a sort of sand. Those grains are relatively large, so
there’s not that much surface available to interact with organic matter. You
really need fine sediments created via chemical weathering at the
surface—things like phyllosilicate clays,” says Valier Galy, a biogeochemist at
WHOI and co-author on the paper.
Although this work provides strong evidence for one
hypothesis over another, Hemingway and his colleagues are quick to note that it
doesn’t provide a definitive answer to the organic carbon puzzle. “We were able
to put our finger on the mechanism by which carbon is being preserved, but we
don’t provide information about other factors, like sensitivity to temperature
in the environment, for instance. There are a lot of other factors to consider.
This paper is intended as a sort of waypoint to direct biogeochemists in their
research,” says Galy.
原始論文：Jordon D.
Hemingway, Daniel H. Rothman, Katherine E. Grant, Sarah Z. Rosengard, Timothy
I. Eglinton, Louis A. Derry & Valier V. Galy. Mineral protection
regulates long-term global preservation of natural organic carbon. Nature,
2019 DOI: 10.1038/s41586-019-1280-6
引用自：Woods Hole Oceanographic Institution.
"Organic carbon hides in sediments, keeping oxygen in atmosphere."
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