甲烷氣爆在北極海海床造成許多巨大坑洞

原文網址：www.sciencedaily.com/releases/2017/06/170601151803.htm

甲烷氣爆在北極海海床造成許多巨大坑洞

即使這些坑洞是在大約12000年前左右形成，時至今日此處仍源源不絕地漏出甲烷。甲烷是種強力的溫室氣體，因而在日趨溫暖的氣候下受到大量關注。

研究第一作者，北極天然氣水合物、氣候與環境中心的教授Karin Andreassen表示：「在上個冰河期期間，這些坑洞的所在地區被相當厚的冰層覆蓋，跟現今的南極西部十分相似。隨著氣候暖化及冰層逐漸崩塌，大量甲烷突然釋放而造成了許多巨型坑洞。時至今日仍有甲烷從此處釋出。」

現今在這些坑洞的附近及內部，可以觀察到有超過600道氣焰(gas flare)冉冉上升，持續釋放溫室氣體到水層當中。

「但和冰層消融後這些溫室氣體發生的氣爆來說，現在的情況根本不算什麼。當時釋放的甲烷量必定令人嘆為觀止。」

西伯利亞的坑洞相形失色

這些坑洞最初有部分是在1990年代觀測到。但新式科技顯現出坑洞的分布範圍比過往認為的還要廣，也呈現出更精細的圖像可供解讀。

Andreassen表示：「我們聚焦在300公尺到1公里寬的坑洞，在此地區我們定位了將近100個這樣大小的坑洞。但是寬度比300公尺小的坑洞還有數百個這麼多。」

相較之下，陸地上於西伯利亞吉丹半島和亞馬爾半島出現的大型氣爆坑洞只有50到90公尺寬，但它們的形成過程也許有相似之處。

北極海有大量甲烷以水合物(hydrate)的形式封存於海床上。水合物是一種類似冰塊，由水和天然氣混和而成的固態物質，在高溫低壓下可以維持穩定狀態。不論是在過去或現在，冰層都提供了絕佳的環境，使得下方可以形成天然氣水合物。

壓力累積至難以承受的地步

現今的海床曾負有厚達2000公尺左右的冰塊，其產生的重量十分龐大。在冰層之下，甲烷氣體會從深處的油氣層往上升，但卻無法突破冰層，使得甲烷會在沉積物中以天然氣水合物的形式存在。由於氣體持續從下方供應的緣故，水合物會因此處於過壓狀態。

Andreassen表示：「隨著冰層快速後退，水合物會集中成土丘，最終開始融化並且膨脹，而產生過壓狀態。原理就跟壓力鍋一樣：如果你不去控制釋放鍋中的壓力，壓力就會不斷累積，最終你的廚房就會發生一場重大災難。這些土丘在數千年來一直處於過壓狀態，一旦蓋子突然掀開，它們就會瞬間崩毀並往水中釋放出甲烷。」

今日冰層下方也有同樣的作用正在進行

如此大型的甲烷噴發事件似乎相當稀有，因此可能很容易就被忽略。

Andreassen表示：「儘管它們的發生頻率很低，像這樣的氣爆造成的影響仍可能比緩緩進行的甲烷滲漏還要大。不過一口氣突然釋出的大量甲烷是否可以到達大氣層仍有待觀察。我們評估在上個冰河期，直接被冰層影響的油氣層面積有兩個俄羅斯這麼大。意謂同時期曾發生類似甲烷劇烈釋放事件的地區，可能還要大上許多。」

另一項要關注的事實是，今日南極西部和格陵蘭的冰層之下，也有油氣層的分布。

Andreassen總結：「我們的研究呈現給學界一個很好的過去類比，顯示現今正在後退的冰層前緣，未來可能會如何釋出甲烷。」

Massive craters formed by methane blow-outs from the Arctic sea floor

Even though the craters were formed some 12,000 years ago, methane is still leaking profusely from the craters. Methane is a potent greenhouse gas, and of major concern in our warming climate.

"The crater area was covered by a thick ice sheet during the last ice age, much as West Antarctica is today. As climate warmed, and the ice sheet collapsed, enormous amounts of methane were abruptly released. This created massive craters that are still actively seeping methane " says Karin Andreassen, first author of the study and professor at CAGE Centre for Arctic Gas Hydrate, Environment and Climate.

Today more than 600 gas flares are identified in and around these craters, releasing the greenhouse gas steadily into the water column.

"But that is nothing compared to the blow-outs of the greenhouse gas that followed the deglaciation. The amounts of methane that were released must have been quite impressive."

Siberian craters small in comparison

A few of these craters were first observed in the 90-ties. But new technology shows that the craters cover a much larger area than previously thought and provides more detailed imaging for interpretation.

"We have focused on craters that are 300 meters to 1 kilometre wide, and have mapped approximately 100 craters of this size in the area. But there are also many hundred smaller ones, less than 300 meters wide that is" says Andreassen.

In comparison, the huge blow-out craters on land on the Siberian peninsulas Yamal and Gydan are 50-90 meters wide, but similar processes may have been involved in their formation.

The Arctic ocean floor hosts vast amounts of methane trapped as hydrates, which are ice-like, solid mixtures of gas and water.These hydrates are stable under high pressure and cold temperatures. The ice sheet provides perfect conditions for subglacial gas hydrate formation, in the past as well as today.

Unbearable pressure builds up

Some 2000 metres of ice loaded what now is ocean floor with heavy weight. Under the ice, methane gas from deeper hydrocarbon reservoirs moved upward, but could not escape. It was stored as gas hydrate in the sediment, constantly fed by gas from below, creating over-pressured conditions.

"As the ice sheet rapidly retreated, the hydrates concentrated in mounds, and eventually started to melt, expand and cause over-pressure. The principle is the same as in a pressure cooker: if you do not control the release of the pressure, it will continue to build up until there is a disaster in your kitchen. These mounds were over-pressured for thousands of years, and then the lid came off. They just collapsed releasing methane into the water column" says Andreassen.

Similar processes are ongoing under ice sheets today

Major methane venting events such as this appear to be rare, and may therefore easily be overlooked.

"Despite their infrequency, the impact of such blow-outs may still be greater than impact from slow and gradual seepage. It remains to be seen whether such abrupt and massive methane release could have reached the atmosphere. We do estimate that an area of hydrocarbon reserves twice the size of Russia was directly influenced by ice sheets during past glaciation. This means that a much larger area may have had similar abrupt gas releases in the overlapping time period " says Andreassen.

Another fact to consider is that there are reserves of hydrocarbons beneath the load of West Antarctica and Greenland ice sheets today.

"Our study provides the scientific community with a good past analogue for what may happen to future methane releases in front of contemporary, retreating ice sheets" concludes Andreassen.

原始論文：K. Andreassen et al. Massive blow-out craters formed by hydrate-controlled methane expulsion from the Arctic seafloor. Science, June 2017 DOI: 10.1126/science.aal4500

引用自：CAGE - Center for Arctic Gas Hydrate, Climate and Environment. "Massive craters formed by methane blow-outs from the Arctic sea floor." ScienceDaily. ScienceDaily, 1 June 2017.