世上最大滅絕事件的新線索
Rob Enslin
地球科學系的副研究員James Muirhead是這篇刊登於《自然通訊》(Nature Communications。麥克米倫出版公司，2017)論文的共同作者，其題名為「西伯利亞暗色岩最初產生的一波岩床引發了二疊紀末的大滅絕事件」(Initial
Pulse of Siberian Traps Sills as the Trigger of the End-Permian Mass
Extinction.)。
他們的發現提出稱為岩床(sill)的侵入火成岩體形成點燃了一連串事件，最後將二疊紀這段地質時期帶向終結。在過程當中超過95%的海洋物種和70%的陸地物種就此消失。
研究火山和構造作用交互關係的專家Burgess表示：「自地球上的生命從6億年以前出現開始，總共發生了五次主要的生物大滅絕事件。這些事件中的大部分多次被歸咎於火山爆發或小行星撞擊。透過重新檢視岩漿活動、氣候變遷和滅亡事件的時間點與關聯，我們可以建立出模型來解釋二疊紀末大滅絕事件的起因。」
他們研究的主要對象是位在俄羅斯的大型火成岩區域(LIP)，稱為西伯利亞暗色岩。這片遍布岩石且杳無人跡的區域占地超過130萬平方公里，形成此處的宏偉火山活動持續了將近50萬年。寬廣且平坦的火山可能噴出了極為大量的熔岩、灰燼和氣體，同時將環境中的二氧化硫、二氧化碳和甲烷濃度推升至相當危險的境界。
但這只是整段故事中的一部份。
Muirhead表示：「大滅絕事件和LIP岩漿活動發生的相對時間及長度，直到近期都還因為定年精準度而不甚明確。我們的模型基於高精準度的新定年資料，提出地表熔岩流的噴發時間太早因此不是大滅絕的成因。反之，岩漿活動中的一小段――持續時間較短且只形成LIP某部分的事件――才是引發一連串事件而造成大滅絕的元凶。」
所以元凶是什麼？答案是岩床形成期間散發出來的高熱。
Muirhead表示：「岩床散發出來的高熱使得封閉且富含氣體的沉積物受到接觸變質作用(岩石的礦物和組構因為受到溫度壓力影響而改變的作用)，因此釋放出引發生物滅絕所需的大量溫室氣體。我們的模型將滅絕事件的啟動跟最初一波岩床的產生畫上關聯。其代表了地球生物演化過程中一個重要的轉捩點。」
岩漿形成火成岩的方式有兩種。一種是噴出作用(extrusion)，其為岩漿透過火山口或地表的裂隙而噴出地表；另一種則是侵入作用(intrusion)，其為岩漿在已經形成的岩層之間移動或者穿過岩層，但沒有到達地表。常見的侵入岩體種類包括了岩床、岩脈和岩基。
Muirhead率領的團隊於西伯利亞通古斯盆地的岩床進行了這篇研究中的大部分內容，該岩床可能通過了石灰岩、煤層、碎屑岩和蒸發岩。他們認為是高溫熔岩加上含有碳氫化合物的煤炭，構成了大量溫室氣體釋放以及全球規模的氣候變遷得以發生的條件。
Burgess表示：「沉積物的成分和它們含有的碳氫化合物(石油和天然氣)多寡有助於我們瞭解LIP是否能夠引發大滅絕。」他補充說他們團隊的模型或許也能適用於其他跟LIP同時發生的生物滅絕事件。「大滅絕的發生過程可能只有一萬年或者更短――以地質事件的標準來說不過是眨眼之間――但它對生命演化的軌跡造成的影響至今仍然清晰可見。」
New Clues to Cause of World’s Greatest Extinction
A study by a
researcher in the Syracuse University College of Arts and Sciences offers new
clues to what may have triggered the world’s most catastrophic extinction,
nearly 252 million years ago.
James Muirhead, a research associate in the
Department of Earth Sciences, is the co-author of an article in Nature Communications
(Macmillan Publishers Limited, 2017) titled “Initial Pulse of Siberian Traps
Sills as the Trigger of the End-Permian Mass Extinction.”
His research involves Seth Burgess, the
article’s lead author and a geologist at the U.S. Geological Survey, and Samuel Bowring, the
Robert R. Shock Professor of Geology at the Massachusetts Institute of
Technology.
Their findings suggest that the formation of intrusive
igneous rock, known as sills, sparked a chain of events that brought the
Permian geological period to a close. In the process, more than 95 percent of
marine species and 70 percent of land species vanished.
“There have been five major mass extinctions since life
originated on Earth more than 600 million years ago,” says Burgess, who works
at the nexus of volcanic and tectonic processes. “Most of these events have
been blamed, at various times, on volcanic eruptions and asteroid impacts. By re-examining
the timing and connection between magmatism [the movement of magma], climate
change and extinction, we’ve created a model that explains what triggered the
end-Permian mass extinction.”
Central to their study is a large igneous province (LIP)
in Russia called the Siberian Traps. Spanning more than 500,000 square miles,
this rocky outpost was the site of nearly a million years of epic volcanic
activity. Broad, flat volcanoes likely dispelled significant volumes of lava,
ashes and gas, while pushing sulfur dioxide, carbon dioxide and methane to
dangerous levels in the environment.
But that’s only part of the story.
“Until recently, the relative timing and duration of mass
extinctions and LIP volcanism was obscured by age imprecision,” Muirhead says. “Our
model is based on new, high-resolution age data that suggests surface lava
flows erupted too early to drive mass extinction. Instead, there was a
subinterval of magmatism—a shorter, particular part of the LIP—that triggered a
cascade of events causing mass extinction.”
The trigger? Extreme heat given off during the formation
of sills.
“Heat from sills exposed untapped, gas-rich sediments to
contact metamorphism [the process in which rock minerals and texture are
changed by exposure to heat and pressure], thus liberating the massive
greenhouse gas volumes needed to drive extinction,” Muirhead says. “Our model
links the onset of extinction with the initial pulse of sill emplacement. It
represents a critical juncture in the evolution of life on Earth.”
There are two ways that magma forms igneous rock. One way
is extrusion, in which magma erupts through volcanic craters and cracks in the
Earth’s surface; the other is intrusion, whereby magma forces itself between or
through existing formations of rock without reaching the surface. Common types
of intrusion are sills, dykes and batholiths.
Sills in Siberia’s Tunguska Basin, where Muirhead’s team
carries out most of its research, likely pushed their way through limestone,
coal, clastic rocks and evaporates. The mixture of hot, molten rock and
hydrocarbon-bearing coals is thought to have set the stage for massive
greenhouse gas release and global-scale climate change.
"Sediment composition and the amount of hydrocarbons
[petroleum and natural gas] available within these sediments help us understand
whether or not an LIP can trigger a mass extinction," says Burgess, adding
that his team's model may apply to other extinction events coinciding with
LIPs. “Mass extinction can take 10,000 years or less—the blink of an eye, by
geological standards—but its effects on the evolutionary trajectory of life are
still observable today.”
原始論文：S. D.
Burgess, J. D. Muirhead, S. A. Bowring. Initial
pulse of Siberian Traps sills as the trigger of the end-Permian mass
extinction. Nature Communications,
2017; 8 (1) DOI: 10.1038/s41467-017-00083-9
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