熱帶的板塊構造運動造成地球進入冰河期
過去5.4億年來，赤道附近的大型板塊碰撞事件造成了三次冰河期
Jennifer Chu
冰河期的全球氣溫會驟降，並在兩極形成面積遼闊的冰層與冰河。地球過去5.4億年來總共經歷了三次大型冰河期。
過往的構造運動把岩石暴露在印尼的熱帶環境之下，使得這些岩石可以吸收空氣中的二氧化碳。圖片來源：本研究人員
最近美國麻省理工學院、加州大學聖塔芭芭拉分校和加州大學柏克萊分校的科學家找出了這三次冰河期的可能成因。
在3月14日發表於《科學》(Science)的研究中，團隊表示距今最近的三個大型冰河期發生前，赤道附近都有構造運動把海洋板塊推擠到大陸板塊上方，造成山脈抬升。這種發生在熱帶地區的「弧陸碰撞」，使得數萬公里的海洋岩石暴露在熱帶環境。
科學家表示熱帶的高溫多雨會讓岩石和大氣發生化學反應。詳細來說，岩石裡的鈣和鎂會和大氣裡的二氧化碳產生化學反應，造成二氧化碳脫離大氣，形成石灰岩之類的碳酸鹽，而被長期封存起來。
研究人員表示數百萬平方公里的岩石經年累月進行上述的風化作用，可以吸收空氣裡的大量二氧化碳，使全球氣溫降低，最終造成地球進入冰河期。
「我們認為低緯度地區發生的弧陸碰撞造成了全球冷化。」麻省理工學院地球、大氣和行星科學系的副教授Oliver Jagoutz表示。「弧陸碰撞的範圍大概是1到5百萬平方公里，雖然聽起來相當廣大，實際上只是地表一條狹長的區塊。然而，如果位於正確的位置，就可以改變全球氣候。」
Jagoutz的共同作者包括加州大學聖塔芭芭拉分校的Francis
Macdonald 和Lorraine
Lisiecki，以及加州大學柏克萊分校的Nicholas Swanson-Hysell 和Yuem Park。
源於熱帶
海洋板塊被推擠到大陸板塊上方通常會產生新的山脈，並露出許多新鮮的岩石。這類海洋板塊跟大陸板塊之間的碰撞作用沿著稱為「縫合帶」(suture)的斷層帶進行。由於陸地經過數百萬年後會移動到別的地方，某些含有縫合帶的山脈(像是喜馬拉雅山)如今已經遠離一開始碰撞的位置。
2016年，Jagoutz 和他的同事回推了組成喜馬拉雅山的二條縫合帶的移動過程。他們發現這兩條縫合帶都是源自於同樣的板塊運動。八千萬年前，岡瓦那超大陸往北移動的途中一部分撞上了歐亞大陸，形成了第一條縫合帶，並讓許多海洋岩石露出地表；接著在五千萬年前，兩座大陸再度撞擊，產生了第二條縫合帶。
研究團隊發現兩次撞擊事件的地點都在熱帶，而且撞擊數百萬年後都發生了全球氣溫下降的事件――以地質時間尺度來說近乎是同時發生。研究團隊計算露出地表的海洋岩石(稱為蛇綠岩套)在熱帶地區跟二氧化碳反應的速率，結論顯示這兩條縫合帶的地點跟規模確實足以讓岩石封存夠多的二氧化碳，使全球氣溫降低並引發冰河期。
有趣的是，他們發現風化作用或許也是冰河期結束的原因：經過數百萬年之後，可以和大氣反應的海洋岩石最終會被侵蝕殆盡，取而代之露出地表的新岩石無法吸收那麼多二氧化碳，使得氣溫回升。
「我們證實風化作用可以啟動冰河期，也能讓它結束。」Jagoutz表示。「接著我們猜想：這有多常發生？如果我們的理論正確無誤，應該可以發現每次冷化事件發生，熱帶地區就有許多縫合帶。」
剖析世上的縫合帶
研究人員想要了解在更早以前的地球歷史中，冰河期和熱帶的弧陸碰撞是否也有類似關係。他們查閱了大量文獻，彙整所有大型縫合帶目前的位置。接著，他們運用模擬板塊運動的電腦模型，重建這些縫合帶的移動過程以及大陸板塊跟海洋板塊過去的位置。透過這種方式，研究人員可以指出這些縫合帶形成的大致位置和時間，以及它們的長度。
他們發現過去5.4億年，有三個時期熱帶地區形成了大型縫合帶(長度約10000公里)，而且也對應到三個著名的大型冰河期，分別是：奧陶紀晚期(4.55億至4.4億年前)、二疊紀―石炭紀(3.35億至2.28億年前)、新生代(3500萬年前到今日)。重點是他們發現如果大型縫合帶形成於熱帶之外的地區，就不會發生冰河期或是冰河擴張的事件。
過去5.4億年來地球的板塊不停移動。麻省理工學院的研究人員發現熱帶地區(綠色範圍)的大型構造運動(橘線)，可能是冰河期同時出現的原因。。圖片來源：本研究人員
「我們發現熱帶地區有大量縫合帶的時候，就會發生冰河擴張的事件。」Jagoutz說：「也就是說，只要把10000公里的縫合帶放到熱帶，就可以得到冰河期。」
他指出當代地球的冰河期，以及兩極擁有廣大冰層的原因，可能就是印尼長度超過10000公里、至今活動依然相當旺盛的大型縫合帶。
熱帶印尼擁有世上最大的幾座蛇綠岩體，使此區吸收、封存二氧化碳的效率在世上名列前茅。有鑑於人類排放的二氧化碳造成全球溫度不斷提高，有些科學家提出的解決方法，便是把大量蛇綠岩套磨碎之後，散佈到赤道各處以加速這種自然發生的冷卻作用。
但是Jagoutz表示把蛇綠岩套磨碎並運送到各處的過程中，可能會在無意之間產生更多二氧化碳。此外，也不確定在我們的有生之年，這種方法能否達到顯著的成效。
「要在人類的時間尺度下讓化學風化發揮成效是相當困難的。」Jagoutz表示。「化學風化是極為緩慢的地質作用，人類在地球上的活動完全無法影響到它。化學風化既不會傷害我們，也無法拯救我們。」
不過，賓州州立大學地球和礦物科學院的院長Lee Kump表示，至少我們能從這種緩慢進行的自然碳封存作用中，看到地球未來的一絲曙光。
「當今人類活動排放的大量二氧化碳，可以媲美地球歷史上火山最為活躍的時期。二氧化碳累積的速度遠遠超過岩石風化的抵銷能力。」未參與這項研究的Kump表示。「不過，如果有朝一日人類排放二氧化碳的速度減緩下來，岩石風化之類的自然作用就會開始進行耗時數千年的修復工作，慢慢把大氣中的二氧化碳濃度回復到人類世之前的水準。」
Tectonics in the tropics
trigger Earth’s ice ages
Major tectonic collisions near the equator have
caused three ice ages in the last 540 million years.
Over the last 540 million years, the Earth has weathered three
major ice ages — periods during which global temperatures plummeted, producing
extensive ice sheets and glaciers that have stretched beyond the polar caps.
Now scientists at MIT, the
University of California at Santa Barbara, and the University of California at
Berkeley have identified the likely trigger for these ice ages.
In a study published today
in Science,
the team reports that each of the last three major ice ages were preceded by
tropical “arc-continent collisions” — tectonic pileups that occurred near the
Earth’s equator, in which oceanic plates rode up over continental plates,
exposing tens of thousands of kilometers of oceanic rock to a tropical
environment.
The scientists say that the
heat and humidity of the tropics likely triggered a chemical reaction between
the rocks and the atmosphere. Specifically, the rocks’ calcium and magnesium
reacted with atmospheric carbon dioxide, pulling the gas out of the atmosphere
and permanently sequestering it in the form of carbonates such as limestone.
Over time, the researchers
say, this weathering process, occurring over millions of square kilometers,
could pull enough carbon dioxide out of the atmosphere to cool temperatures
globally and ultimately set off an ice age.
“We think that
arc-continent collisions at low latitudes are the trigger for global cooling,”
says Oliver Jagoutz, an associate professor in MIT’s Department of Earth,
Atmospheric, and Planetary Sciences. “This could occur over 1-5 million square
kilometers, which sounds like a lot. But in reality, it’s a very thin strip of
Earth, sitting in the right location, that can change the global climate.”
Jagoutz’ co-authors are
Francis Macdonald and Lorraine Lisiecki of UC Santa Barbara, and Nicholas
Swanson-Hysell and Yuem Park of UC Berkeley.
A tropical trigger
When an oceanic plate
pushes up against a continental plate, the collision typically creates a
mountain range of newly exposed rock. The fault zone along which the oceanic
and continental plates collide is called a “suture.” Today, certain mountain
ranges such as the Himalayas contain sutures that have migrated from their
original collision points, as continents have shifted over millenia.
In 2016, Jagoutz and his
colleagues retraced the movements of two sutures that today make up the
Himalayas. They found that both sutures stemmed from the same tectonic
migration. Eighty million years ago, as the supercontinent known as Gondwana
moved north, part of the landmass was crushed against Eurasia, exposing a long
line of oceanic rock and creating the first suture; 50 million years ago,
another collision between the supercontinents created a second suture.
The team found that both
collisions occurred in tropical zones near the equator, and both preceded
global atmospheric cooling events by several million years — which is nearly
instantaneous on a geologic timescale. After looking into the rates at which
exposed oceanic rock, also known as ophiolites, could react with carbon dioxide
in the tropics, the researchers concluded that, given their location and
magnitude, both sutures could have indeed sequestered enough carbon dioxide to
cool the atmosphere and trigger both ice ages.
Interestingly, they found
that this process was likely responsible for ending both ice ages as well. Over
millions of years, the oceanic rock that was available to react with the
atmosphere eventually eroded away, replaced with new rock that took up far less
carbon dioxide.
“We showed that this
process can start and end glaciation,” Jagoutz says. “Then we wondered, how
often does that work? If our hypothesis is correct, we should find that for
every time there’s a cooling event, there are a lot of sutures in the tropics.”
Exposing Earth’s
sutures
The researchers looked to
see whether ice ages even further back in Earth’s history were associated with
similar arc-continent collisions in the tropics. They performed an extensive
literature search to compile the locations of all the major suture zones on
Earth today, and then used a computer simulation of plate tectonics to
reconstruct the movement of these suture zones, and the Earth’s continental and
oceanic plates, back through time. In this way, they were able to pinpoint
approximately where and when each suture originally formed, and how long each
suture stretched.
They identified three
periods over the last 540 million years in which major sutures, of about 10,000
kilometers in length, were formed in the tropics. Each of these periods
coincided with each of three major, well-known ice ages, in the Late Ordovician
(455 to 440 million years ago), the Permo-Carboniferous (335 to 280 million years
ago), and the Cenozoic (35 million years ago to present day). Importantly, they
found there were no ice ages or glaciation events during periods when major
suture zones formed outside of the tropics.
“We found that every time
there was a peak in the suture zone in the tropics, there was a glaciation
event,” Jagoutz says. “So every time you get, say, 10,000 kilometers of sutures
in the tropics, you get an ice age.”
He notes that a major
suture zone, spanning about 10,000 kilometers, is still active today in
Indonesia, and is possibly responsible for the Earth’s current glacial period
and the appearance of extensive ice sheets at the poles.
This tropical zone includes
some of the largest ophiolite bodies in the world and is currently one of the
most efficient regions on Earth for absorbing and sequestering carbon dioxide.
As global temperatures are climbing as a result of human-derived carbon
dioxide, some scientists have proposed grinding up vast quantities of ophiolites
and spreading the minerals throughout the equatorial belt, in an effort to
speed up this natural cooling process.
But Jagoutz says the act of
grinding up and transporting these materials could produce additional,
unintended carbon emissions. And it’s unclear whether such measures could make
any significant impact within our lifetimes.
“It’s a challenge to make
this process work on human timescales,” Jagoutz says. “The Earth does this in a
slow, geological process that has nothing to do with what we do to the Earth
today. And it will neither harm us, nor save us.”
However, Lee Kump, dean of
the College of Earth and Mineral Sciences at Penn State University, sees at
least one silver lining for this slow, natural sequestration process in the
Earth’s future.
“Emissions of carbon
dioxide from human activity today rival the most massive volcanic episodes in
Earth history, far exceeding the capacity of rock weathering feedbacks to
counter the buildup,” says Kump, who was not involved in the research.
“However, as anthropogenic carbon emissions wane, natural restoration processes
like these will begin the multimillennial repair job of restoring atmospheric
carbon dioxide to pre-Anthropocene levels.”
原始論文：Francis A.
Macdonald, Nicholas L. Swanson-Hysell, Yuem Park, Lorraine Lisiecki, Oliver
Jagoutz. Arc-continent collisions in the tropics set Earth’s climate
state. Science, 2019; eaav5300 DOI: 10.1126/science.aav5300
引用自：Massachusetts Institute of Technology.
"Tectonics in the tropics trigger Earth's ice ages.”
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