新研究顯示距今5600萬年前的火山活動釋出的甲烷比以往認為的還多
由德國亥姆霍茲海洋研究中心的海洋物理學教授Christian
Berndt以及奧斯陸大學的海洋物理學教授Sverre
Planke主持，在挪威外海進行的國際鑽探活動證實了一項理論：距今大約5500萬年前發生的全球暖化事件，是由海底熱泉釋放出的甲烷導致。研究結果今日刊登在期刊《自然—地球科學》(Nature Geoscience)，其中指出這些活躍的熱泉出現在水深很淺甚至比海平面還高的地方，造成更加大量的甲烷可以進入大氣層當中。
每段岩芯看起來皆不盡相同，如同記下地球歷史的日記一般，專家可以閱讀這些岩芯而得出其中所含的資訊。圖片來源：Sandra Herrmann (IODP/JRSO)
距今5500萬年前左右，大西洋誕生了。在此之前，歐洲和美洲還連在一塊。隨著兩塊大陸開始分離，它們之間的地殼也被撕裂開來，冒出了大量的岩漿。這種張裂式的火山活動在世上幾個地點形成了大型火成岩區域(large
igneous provinces, LIP)，其中一個位於格陵蘭和歐洲之間，現在處在海平面數公里之下。德國亥姆霍茲海洋研究中心的海洋物理學教授Christian
Berndt以及挪威奧斯陸大學的海洋物理學教授Sverre
Planke主持的國際鑽探活動，從該LIP採集了許多樣品並在最近完成解析。
他們的研究結果今日刊登在期刊《自然—地球科學》。研究人員指出在水深很淺甚至比海平面還高的地方有許多活躍的熱泉，造成溫室氣體進到大氣的量比之前認為的還多出許多。
「在古新世和始新世交界的一百多萬年之間，發生了地球歷史上最強的幾次火山活動，」Christian
Berndt表示。根據現有的知識，這些火山活動造成全世界的氣候暖化了至少5℃並導致了大滅絕——這是人類時代之前最後一次全球大暖化，稱為古新世─始新世氣候最暖期(Paleocene-Eocene
Thermal Maximum ,PETM)。由於現今大部分的火山爆發會把氣膠釋放到平流層而造成氣溫降低，因此地質學家至今還無法解釋當時火山造成暖化的原因。
深入研究南非Karoo大火成岩區域的結果顯示，沉積盆地中有許多岩漿入侵造成的海底熱泉。這道觀察加上其他結果促成了一項假說：大量的溫室氣體二氧化碳與甲烷可能是經由海底熱泉進到大氣當中。「當2004年我們在挪威的同儕Henrik
Svensen和Sverre
Planke發表他們的結果，我們就想要立刻前往大西洋附近的古代熱泉系統進行鑽探以驗證這項假說，」Christian
Berndt表示。但實行起來並不簡單，「雖然整合海洋鑽探計畫(Integrated
Ocean Drilling Program ,IODP)同意我們的提案，卻從來沒有安排在行程表。因為我們需要用到立管鑽探(riser
drilling)，但這項技術當時我們還無法取得。」
隨著研究的進展，他們發現了不需要立管鑽探也能到達的熱泉系統。因此他們重新提出了鑽探計畫，終於2021年秋季——第一份提案呈交的17年後——展開了這次航程。
大約30名來自12國的科學家登上了科學鑽探船「聯合果敢號」，這班IODP(現更名為「國際海洋發現計畫」)的研究航次目的地為挪威外海的Vøring海底高原。「我們打了20口鑽井，其中5個直接打進了此地數千座海底熱泉的其中之一。鑽出來的岩芯就像記載地球歷史的日記一般，科學家閱讀它們之後得到了相當有趣的結果。」
作者表示該熱泉活動的時間就在古新世─始新世氣候最暖期不久之前，而且過沒多久，形成的坑洞在全球暖化開始的時候就被填平了。他們的數據相當出乎意料的地方是，該熱泉活動的深度非常淺，可能不到100公尺。就熱泉對氣候可能造成的衝擊來說，這點具有深遠的影響。Christian
Berndt解釋：「今日從深海熱泉進入水層中的甲烷，很快就會轉變成暖化能力弱上許多的二氧化碳。由於我們探討的熱泉位在裂谷中間，照理來說是當時最深的地方，因此我們推測其他熱泉也是位在淺海，甚至是海平面之上。這會造成更加大量的溫室氣體進到大氣當中。」
就今日的氣候暖化來說，從這些岩芯當中也可以得出某些有趣的結論。它們一方面證實了當時的全球暖化原因是天然氣水合物溶解，科學家近年來對於這項危機也有諸多討論。另一方面，它們也顯示出經過數千年後氣候會再次冷卻下來。因此地球的氣候系統確實能夠自行調控，但其時間尺度是當今的氣候危機進展速度無法比擬的。
Past climate warming driven
by hydrothermal vents
New study shows volcanism 56 million
years ago released more methane than thought
An international drilling expedition off
the Norwegian coast led by Christian Berndt, Professor of Marine Geophysics at
GEOMAR Helmholtz Centre for Ocean Research in Kiel, and Sverre Planke,
Professor of Marine Geophysics at the University of Oslo, confirms the theory
that methane emissions from hydrothermal vents were responsible for global
warming about 55 million years ago. The study, published today in the journal Nature Geoscience, shows that the vents
were active in very shallow water depth or even above sea level, which would
have allowed much larger amounts of methane to enter the atmosphere.
About 55 million years ago, the Atlantic Ocean was
born. Until then, Europe and America were connected. As the continents began to
move apart, the Earth’s crust between them ruptured, releasing large volumes of
magma. This rift volcanism has led to the formation of large igneous provinces
(LIPs) in several places around the world. One such LIP was formed between
Greenland and Europe and now lies several kilometres below the ocean surface.
An international drilling campaign led by Christian Berndt, Professor of Marine
Geophysics at GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany, and
Sverre Planke, Professor of Marine Geophysics at the University of Oslo,
Norway, has collected extensive sample material from the LIP, which has now
been evaluated.
In their study, published today in the journal Nature Geoscience, the researchers can
show that hydrothermal vents were active at very shallow depths or even above
sea level, which would have allowed much larger quantities of greenhouse gases
to enter the atmosphere than previously thought.
"At the Paleocene-Eocene boundary, some of the
most powerful volcanic eruptions in Earth’s history took place over a period of
more than a million years," says Christian Berndt. According to current
knowledge, this volcanism warmed the world's climate by at least five degrees
Celsius and caused a mass extinction - the last dramatic global warming before
our time, known as the Paleocene-Eocene Thermal Maximum (PETM). Geologists have
not yet been able to explain why, as most modern volcanic eruptions cause
cooling by releasing aerosols into the stratosphere.
Further studies of the Karoo large igneous province
in South Africa revealed an abundance of hydrothermal vents associated with
magmatic intrusions into the sedimentary basin. This observation among others
led to the hypothesis that large amounts of the greenhouse gases carbon dioxide
and methane could have entered the atmosphere through hydrothermal venting.
"When our Norwegian colleagues Henrik Svensen and Sverre Planke published
their results in 2004, we would have loved to set off immediately to test the
hypothesis by drilling the ancient vent systems around the North
Atlantic," says Christian Berndt. But it wasn't that easy: "Our
proposal was well received by the Integrated Ocean Drilling Program (IODP), but
it was never scheduled because it required riser drilling, a technology that
was not available to us at the time."
As the research progressed hydrothermal vent systems
were discovered that were within reach of riserless drilling. Thus, the
drilling proposal was resubmitted, and the expedition could finally begin in
autumn 2021 – 17 years after the first proposal was submitted.
Around 30 scientists from 12 nations took part in the
IODP (now the International Ocean Discovery Program) research cruise to the
Vøring Plateau off the Norwegian coast on board the scientific drilling ship
"JOIDES Resolution". Five of the 20 boreholes were drilled directly
into one of the thousands of hydrothermal vents. The cores obtained can be read
by scientists like a diary of the Earth's history. The results were compelling.
The authors show that the vent was active just before
the Paleocene Eocene Thermal Maximum and that the resulting crater was filled
in a very short time, just as the global warming began. Quite unexpectedly,
their data also show that the vent was active in a very shallow water depth of
probably less than 100 metres. This has far-reaching consequences for the
potential impact on the climate. Christian Berndt: "Most of the methane
that enters the water column from active deep-sea hydrothermal vents today is
quickly converted into carbon dioxide, a much less potent greenhouse gas. Since
the vent we studied is located in the middle of the rift valley, where the
water depth should be greatest, we assume that other vents were also in shallow
water or even above sea level, which would have allowed much larger amounts of
greenhouse gases to enter the atmosphere".
As far as today's climate warming is concerned, there
are some interesting conclusions to be drawn from the cores. On the one hand,
they do not confirm that the global warming at that time was caused by the
dissolution of gas hyrates – a danger that has been much discussed in recent
years. On the other hand, they show that it took many millennia for the climate
to cool down again. So the Earth system was thus able to regulate itself, but
not on time scales relevant to today's climate crisis.
原始論文：Christian
Berndt, Sverre Planke, Carlos A. Alvarez Zarikian, Joost Frieling, Morgan T.
Jones, John M. Millett, Henk Brinkhuis, Stefan Bünz, Henrik H. Svensen, Jack
Longman, Reed P. Scherer, Jens Karstens, Ben Manton, Mei Nelissen, Brandon
Reed, Jan Inge Faleide, Ritske S. Huismans, Amar Agarwal, Graham D. M. Andrews,
Peter Betlem, Joyeeta Bhattacharya, Sayantani Chatterjee, Marialena
Christopoulou, Vincent J. Clementi, Eric C. Ferré, Irina Y. Filina, Pengyuan
Guo, Dustin T. Harper, Sarah Lambart, Geoffroy Mohn, Reina Nakaoka, Christian
Tegner, Natalia Varela, Mengyuan Wang, Weimu Xu, Stacy L. Yager. Shallow-water
hydrothermal venting linked to the Palaeocene–Eocene Thermal Maximum. Nature
Geoscience, 2023; DOI: 10.1038/s41561-023-01246-8
引用自：Helmholtz Centre for Ocean Research Kiel
(GEOMAR). "Past climate warming driven by hydrothermal vents: New study
shows volcanism 56 million years ago released more methane than thought."
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