地震研究顯示有大量的水被拖往地球內部
Talia
Ogliore
科學家最近對整個馬里亞納海溝進行極為詳細的地震學研究，結果指出板塊在海底緩慢碰撞時帶到地球深處的水量是之前預估的三倍左右。
美國聖路易斯華盛頓大學文理學院的研究人員表示，觀測世上最深的海溝得到的結果對於全球水循環來說具有相當重要的啟發。
剛在華盛頓大學完成博士學位研究的Chen
Cai 表示：「研究人員知道隱沒帶可以把水帶進地球內部，但不知道到底有多少。」以他為第一作者的這篇研究發表於11月15日當期的《自然》(Nature)期刊。
此研究的經費來自美國國家科學基金會海洋科學部門，計畫主任Candace
Major 評論：「研究顯示隱沒帶遠比之前認為的還能攜帶更多水到地球深處，直到地表之下數十公里的地方。這項結果強調出隱沒帶在地球的水循環中具有的重要地位。」
Cai在這項研究中的研究顧問，華盛頓大學聖路易斯分校文理學院的地球與行星科學特聘教授Doug
Wiens表示：「過往的研究在預估有多少水隱沒至超過60英里(約100公里)深的地方時有很大的分歧。計算過程中的不確定性主要來自隱沒下去的上部地函一開始含有的水分多寡。」
太平洋板塊的西側和馬里亞納板塊在馬里亞納海溝緩緩聚合，在此太平洋板塊滑行至馬里亞納板塊下方而沒入地函深處。為了進行這項研究，研究人員部署了涵蓋馬里亞納海溝，由19個被動式海底地震儀組成的地震網，再加上七個架設在島上的地震儀。他們運用這些儀器在一年多的時間中傾聽地球發出來的各式聲響――從周遭噪訊(ambient
noise)至實際發生的地震。
新的地震觀測依據地震波在跟水結合能力不同的岩石中運動時的相對速度，以三維方式呈現出更加細緻的圖像顯示太平洋板塊如何彎折到地球內部。
送水下去
岩石可以運用幾種不同的方式跟水結合並留住它們。
板塊碰撞並彎曲的地方會密布許多斷層線，上方的海水便能沿著這些裂隙往下流入地殼與上部地函，接著被留存下來。在特定的溫度壓力條件之下化學反應會形成含水礦物，這些「濕潤」的礦物使得水能以非液態的形式保存在板塊的岩石當中。然後板塊會繼續下潛至地函更深處，並把水一直往下帶。
之前在馬里亞納海溝之類的隱沒帶進行的研究發現隱沒板塊可以攜帶水分，但他們無法確定到底有多少，以及可以到達多深的地方。
Cai表示：「之前研究的慣用做法是利用人工震源，但這樣只能顯示震波進入板塊後最上方3至4英里(約5-7公里)的情況。」
他指的這類地震學研究是利用海洋研究船上的空氣槍轟出的聲波，來做出海底岩石內部構造的影像。
「這無法非常精確得知岩石的厚度或是含水程度。」Cai說。「我們的研究嘗試定出這些數字。如果水可以滲透到板塊更深的地方，就能夠待在那裏並被運往地球內部更深的地方。」
地球水循環的重要環節
Cai和Wiens得到的地震影像顯示出在馬里亞納海溝，含水岩石的分布區域可以到達海床下方將近20英里(約32公里)――遠比之前認為的更加深入。
這一大塊含水岩石所能擁有的水量相當驚人。
單就馬里亞納海溝來說，此區隱沒進地函的水量是之前預估的四倍。將此外推可以預估世上其他海溝內部的情況。
Wiens表示：「如果其他古老、低溫的隱沒板塊也擁有差不多厚的含水地函，對於全球來說進入60英里(約100公里)以下地函的水量預估值，勢必要增加三倍左右。」
在地球上，進入到地下的水總有一天會回到地表。海平面從地質時間來看一直處於相對穩定的狀態，變化幅度不超過1000英尺(約300公尺)。代表在隱沒帶進入地球內部的海水必定會以某種方式回到海中，而不會一直累積在地球內部。
科學家認為大部分從海溝進入地底的水分，會在數百公里外的火山噴發時以水蒸氣的形式返回地球大氣。但回顧這篇新研究的預估結果，進入到地球內部的水量似乎遠遠超過回來的。
Wiens說：「從火山島弧噴回地表的水量預估值可能十分不精確。」他希望這篇研究可以激發其他研究人員重新審視他們用來解釋水如何回到地表的模型。「這項研究或許能促使其他研究人員重新計算他們的成果。」
Wiens進一步拓及馬里亞納海溝之外的地區，他和一組科學團隊最近在阿拉斯加外海架設了類似的地震觀測網，以觀察此處的海水如何進入地球內部。
Wiens問道：「隱沒帶之間會因為板塊彎折時產生的斷層類型，造成進入地球內部的水量有很大的差異嗎？」「之前有人提出阿拉斯加和中美洲會有這樣的差異，不過目前還沒有人可以跟我們在馬里亞納海溝做的研究一樣，觀察到更加深部的構造。」
Seismic study reveals
huge amount of water dragged into Earth’s interior
Slow-motion collisions
of tectonic plates under the ocean drag about three times more water down into
the deep Earth than previously estimated, according to a first-of-its-kind
seismic study that spans the Mariana Trench.
The
observations from the deepest ocean trench in the world have important
implications for the global water cycle, according to researchers in Arts &
Sciences at Washington University in St. Louis.
“People
knew that subduction zones could bring down water, but they didn’t know how
much water,” said Chen Cai, who recently completed his doctoral studies at
Washington University. Cai is the first author of the study published in
the Nov. 15 issue of the journal Nature.
“This research shows that subduction zones
move far more water into Earth’s deep interior — many miles below the surface —
than previously thought,” said Candace Major, a program director in the
National Science Foundation’s Division of Ocean Sciences, which funded the
study. “The results highlight the important role of subduction zones in Earth’s
water cycle.”
“Previous
estimates vary widely in the amount of water that is subducted deeper than 60
miles,” said Douglas A. Wiens, the Robert S. Brookings Distinguished
Professor in Earth and Planetary Sciences in Arts & Sciences and Cai’s
research adviser for the study. “The main source of uncertainty in these
calculations was the initial water content of the subducting uppermost mantle.”
To
conduct this study, researchers listened to more than one year’s worth of
Earth’s rumblings — from ambient noise to actual earthquakes — using a network
of 19 passive, ocean-bottom seismographs deployed across the Mariana Trench,
along with seven island-based seismographs. The trench is where the western
Pacific Ocean plate slides beneath the Mariana plate and sinks deep into the
Earth’s mantle as the plates slowly converge.
The
new seismic observations paint a more nuanced picture of the Pacific plate
bending into the trench — resolving its three-dimensional structure and
tracking the relative speeds of types of rock that have different capabilities
for holding water.
Bringing water down
Rock
can grab and hold onto water in a variety of ways.
Ocean
water atop the plate runs down into the Earth’s crust and upper mantle along
the fault lines that lace the area where plates collide and bend. Then it gets
trapped. Under certain temperature and pressure conditions, chemical reactions
force the water into a non-liquid form as hydrous minerals — wet rocks —
locking the water into the rock in the geologic plate. All the while, the plate
continues to crawl ever deeper into the Earth’s mantle, bringing the water
along with it.
Previous
studies at subduction zones like the Mariana Trench have noted that the
subducting plate could hold water. But they could not determine how much water
it held and how deep it went.
“Previous
conventions were based on active source studies, which can only show the top
3-4 miles into the incoming plate,” Cai said.
He
was referring to a type of seismic study that uses sound waves created with the
blast of an air gun from aboard an ocean research vessel to create an image of
the subsurface rock structure.
“They
could not be very precise about how thick it is, or how hydrated it is,” Cai
said. “Our study tried to constrain that. If water can penetrate deeper into
the plate, it can stay there and be brought down to deeper depths.”
An important part of
Earth’s water cycle
The
seismic images that Cai and Wiens obtained show that the area of hydrated rock
at the Mariana Trench extends almost 20 miles beneath the seafloor — much
deeper than previously thought.
The
amount of water that can be held in this block of hydrated rock is
considerable.
For
the Mariana Trench region alone, four times more water subducts than previously
calculated. These features can be extrapolated to predict the conditions under
other ocean trenches worldwide.
“If
other old, cold subducting slabs contain similarly thick layers of hydrous
mantle, then estimates of the global water flux into the mantle at depths
greater than 60 miles must be increased by a factor of about three,” Wiens
said.
And
for water in the Earth, what goes down must come up. Sea levels have remained
relatively stable over geologic time, varying by less than 1,000 feet. This
means that all of the water that is going down into the Earth at subduction
zones must be coming back up somehow, and not continuously piling up inside the
Earth.
Scientists
believe that most of the water that goes down at the trench comes back from the
Earth into the atmosphere as water vapor when volcanoes erupt hundreds of miles
away. But with the revised estimates of water from the new study, the amount of
water going into the earth seems to greatly exceed the amount of water coming
out.
“The
estimates of water coming back out through the volcanic arc are probably very
uncertain,” said Wiens, who hopes that this study will encourage other
researchers to reconsider their models for how water moves back out of the
Earth. “This study will probably cause some re-evaluation.”
Moving
beyond the Mariana Trench, Wiens along with a team of other scientists has
recently deployed a similar seismic network offshore in Alaska to consider how
water is moved down into the Earth there.
“Does
the amount of water vary substantially from one subduction zone to another,
based on the kind of faulting that you have when the plate bends?” Wiens asked.
“There’s been suggestions of that in Alaska and in Central America. But nobody
has looked at the deeper structure yet like we were able to do in the Mariana
Trench.”
原始論文：Chen Cai, Douglas A. Wiens, Weisen Shen, Melody
Eimer. Water input into the Mariana subduction zone estimated from
ocean-bottom seismic data. Nature, 2018; 563 (7731): 389
DOI: 10.1038/s41586-018-0655-4
引用自：Washington
University in St. Louis. "Seismic study reveals huge amount of water
dragged into Earth's interior."
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