2018年4月18日 星期三

山脈受到侵蝕或許會增加大氣中的二氧化碳


山脈受到侵蝕或許會增加大氣中的二氧化碳
科學家長久以來認為,陡峭的山脈可以從大氣中吸收二氧化碳:當新鮮的岩石因為侵蝕作用而露出地表,山坡上的這些礦物就會開始跟空氣中的二氧化碳進行化學反應,造成岩石「風化」的同時消耗二氧化碳而形成方解石之類的碳酸鹽礦物。

然而,一項由伍茲霍爾海洋研究所(WHOI)的人員進行的新研究卻澈底顛覆了此想法。在412日發表於期刊《科學》(Science)的論文中,科學家聲稱侵蝕作用也會產生新的二氧化碳氣體,而且釋回大氣的速度可以比新露出的岩石吸收二氧化碳的速度還快上許多。
「這和長久以來的理論相反,之前認為山地越多代表侵蝕作用和風化作用會更加劇烈,進而減少更多的二氧化碳。結果顯示實際情形還要複雜許多。」論文主要作者,哈佛大學的博士後研究員Jordon Hemingway表示。
這些多的二氧化碳並非全都來自於地質作用。反之,山地土壤中的微生物吃掉岩石中年代古老的有機碳時便會釋出二氧化碳為副產物。當微生物代謝這些礦物時,它們會吐出二氧化碳。
研究人員在探討世界上侵蝕最為劇烈的山脈之一――台灣的中央山脈之後領悟到了這項結論。這座陡峭的山脈每年會遭到三次以上的大型颱風重創,每次事件帶來的狂風豪雨會以物理作用侵蝕岩石和土壤。
Hemingway和他的同事檢驗了中央山脈的土壤、底岩和河川沉積物的樣品,從中尋找岩石裡的有機碳的代表性訊號。結果他們的發現令他們感到十分驚訝。
Hemingway指出:在土壤剖面最底部的地方基本上是未風化的岩石。然而一到土壤下層,我們所見的岩石就變得十分易碎,但還未完全分解。不過在底岩中可以看見的有機碳在此處卻似乎已經完全消失。」他進一步指出團隊注意到在土壤的這個位置脂質有變多的現象,其為細菌所產生。
「我們還不知道是什麼種類的細菌造就此事,這需要我們在研究中尚未用到的基因體學、總體基因體學和其他微生物學工具才能解答。而這便是研究的下一步。」本研究的資深作者,WHOI的海洋地球化學家Valier Galy表示。他也是Hemingway在麻省理工學院和WHOI聯合計畫中的指導教授。
研究團隊也迅速指出微生物釋放出的二氧化碳總量不足以對氣候變遷造成任何立即影響――因為這種過程是在地質時間尺度下發生。WHOI團隊的研究成果或許可以讓我們更加瞭解跟山脈(或者是岩石圈)有關的碳循環實際上是如何運作,而讓我們得到更多線索去解開地球自形成之後是如何調控自身的二氧化碳濃度。
Hemingway表示:「當我們看向從前,最令人感興趣的是這些作用如何使地球數百萬年來大氣中的二氧化碳濃度保持在差不多的水準。地球因此擁有適當的氣候和環境促使複雜的生命型態出現。」他接著說明:「地球從古至今雖然二氧化碳的多寡隨著時間而有所變動,但都保持在一個穩定範圍。這項研究讓我們對地質作用透過何種機制造成此種現象有了最新瞭解。」

Mountain erosion may add carbon dioxide to atmosphere
Scientists have long known that steep mountain ranges can draw carbon dioxide (CO2) out of the atmosphere—as erosion exposes new rock, it also starts a chemical reaction between minerals on hill slopes and CO2 in the air, “weathering” the rock and using CO2 to produce carbonate minerals like calcite.
A new study led by researchers from the Woods Hole Oceanographic Institution (WHOI), however, has turned this idea on its head. In paper released on April 12th in the journal Science, the scientists announced that the erosion process can also be a source of new CO2 gas and can release it back into the atmosphere far faster than it’s being absorbed into newly exposed rock.
“This goes against a long-standing hypothesis that more mountains mean more erosion and weathering, which means an added reduction of CO2. It turns out it’s much more complicated than that,” says Jordon Hemingway, a postdoctoral fellow at Harvard University and lead author on the paper.
The source of this extra CO2 isn’t entirely geological. Instead, it’s the byproduct of tiny microbes in mountain soils that “eat” ancient sources of organic carbon that are trapped in the rock. As the microbes metabolize these minerals, they spew out carbon dioxide.
The researchers came to this realization after studying one of the most erosion-prone mountain chains in the world—the central range of Taiwan. This steep-sided range is pummeled by more than three major typhoons each year, each of which mechanically erode the soil and rock through heavy rains and winds.
Hemingway and his colleagues examined samples of soil, bedrock, and river sediments from the central range, looking for telltale signs of organic carbon in the rock. What they found there surprised them.
“At the very bottom of the soil profile, you have basically unweathered rock. As soon as you hit the base of the soil layer, though, you see rock that’s loose but not yet fully broken down, and at this point the organic carbon present in the bedrock seems to disappear entirely,” notes Hemingway. At that point in the soil, the team also noticed an increase in lipids that are known to come from bacteria, he adds.
“We don’t yet know exactly which bacteria are doing this—that would require genomics, metagenomics, and other microbiological tools that we didn’t use in this study. But that’s the next step for this research,” says WHOI marine geochemist Valier Galy, senior author and Hemingway's advisor in the MIT/WHOI Joint Program.
The group is quick to note that the total level of CO2 released by these microbes isn’t severe enough to have any immediate impact on climate change—instead, these processes take place on geologic timescales. The WHOI team’s research may lead to a better understanding of how mountain-based (or “lithospheric”) carbon cycles actually work, which could help generate clues to how CO2 has been regulated since the Earth itself formed.
“Looking backwards, we’re most interested in how these processes managed to keep the levels of CO2 in the atmosphere more or less stable over millions of years. It allowed Earth to have the climate and conditions it’s had—one that has promoted the development of complex life forms,” says Hemingway. “Throughout our Earth’s history, CO2 has wobbled over time, but has remained in that stable zone. This is just an update of the mechanism of geological processes that allows that to happen,” he adds.
原始論文:Jordon D. Hemingway, Robert G. Hilton, Niels Hovius, Timothy I. Eglinton, Negar Haghipour, Lukas Wacker, Meng-Chiang Chen, Valier V. Galy. Microbial oxidation of lithospheric organic carbon in rapidly eroding tropical mountain soils. Science, 2018; 360 (6385): 209 DOI: 10.1126/science.aao6463
引用自:Woods Hole Oceanographic Institution. "Mountain erosion may add CO2 to the atmosphere." ScienceDaily. ScienceDaily, 12 April 2018.


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