新的證據顯示史上最嚴重的大滅絕是由火山造成
在全球古代岩石中找到的汞，支持了2億5200萬年前的「大死亡」是由火山爆發造成的理論
研究人員表示埋藏在古代岩石中的汞是迄今最為強力的證據，顯示地球歷史上最嚴重的大滅絕是由火山造成。
2億5200萬年前的滅絕事件在全球各地造成了極為慘重的災情，使得科學家稱其為「大死亡」。在這場持續了數十萬年的浩劫中，超過百分之95的生物就此消失在地球上。
包括美國辛辛那提大學和中國地質大學在內的古生物學家團隊，表示他們在分布於世界各地的十幾個地質紀錄中，發現此時的汞濃度都出現了高峰，強烈支持了火山爆發是這場全球浩劫的元兇。
這項研究本月發表在期刊《自然通訊》(Nature
Communications)。
火山爆發燃燒了大量的碳質沉積物，而把汞蒸氣釋放到大氣當中。這些汞最終落到全球各地的海洋沉積物裡面，形成的化學訊號標記了這場浩劫，也揭示了之後的恐龍時代。
研究主要作者，中國地質大學的副教授沈俊說：「火山爆發會噴出氣體並燃燒有機物，諸如此類的火山活動使得大量的汞釋放到地表。」
這場大滅絕發生的時間科學家稱為二疊紀―三疊紀界線(Permian-Triassic Boundary)。恐龍崛起之前的陸地與海洋生物絕大部分都在這場滅絕事件中消失，其中包含了某些遠古猛獸，像是外貌十分兇惡的麗齒獸，牠們就像是劍齒虎和科摩多龍的混合體。
火山爆發發生在現今的俄羅斯中部，稱為「西伯利亞暗色岩」的火山系統。其中大多數並非從錐狀的火山噴出，而是沿著地表張開的裂隙噴發。這些裂隙噴發不但十分頻繁而且歷時許久，延續了數十萬年的時間。
辛辛那提大學文理學院的地質教授Thomas Algeo表示：「大型而猛烈的火山爆發通常會釋放很多汞到大氣當中。」
他接著說：「汞對研究人員來說是個較為新穎的指標。利用汞來探討火山如何影響地球歷史上的重要事件，是近期十分熱門的研究題目。」
研究人員利用牙形石(conodont)來定年含有汞的沉積岩。牙形石是類似七鰓鰻的生物留下來的尖銳牙齒化石，就跟當時地球上的多數生物一樣，牙形石也在這場浩劫中滅亡了。
極為長期的火山爆發噴出了多達300萬立方公里的火山灰。要顯現出這有多麼龐大，可以跟1980年發生在美國華盛頓州的聖海倫火山爆發比較。當時噴發出的火山灰只有1立方公里，即便如此，遠至奧克拉荷馬州的汽車擋風玻璃上依然有火山灰飄落(相距約3000公里)。
Algeo說事實上，西伯利亞暗色岩噴發至大氣中的物質，多到其中的溫室氣體足以讓地球的平均溫度增加攝氏10度。
他說氣候暖化可能是這場大滅絕中造成最多生物死亡的原因之一。不過酸雨也會破壞許多水體，並讓全球海洋酸化。此外，水溫升高也會讓溶氧量降低，造成更多的死區。
「我們一直想找出是什麼造成了最嚴重的破壞。而有跡象顯示適應低溫環境的生物是傷亡最重的。」Algeo表示，「因此我猜測溫度變化可能是首要死因，而酸化和其他排放到環境中的毒素，可能擴大了暖化的影響。」
火山在這段極為漫長的時間中接連不斷地噴發，造成地球的食物鏈無法回復。
「強度不是最重要的，持續時間才是。」Algeo說。「火山持續爆發的時間越長，對環境造成的壓力就越大。」
他說生物多樣性不停地遭到破壞，造成地球從這場浩劫中回復的速度極為緩慢。
地球45億年的歷史中總共經歷了五次生物大滅絕。
其中一場發生在6500萬年前的大滅絕造成恐龍覆亡。科學家利用另外一種元素――銥而找出了嫌疑犯。他們認為一顆巨大的流星撞擊墨西哥，造成該次大滅絕。
撞擊使得極為熾熱的大量塵土衝進大氣層。這些含有銥的物質之後落回地面，使得全世界的地質紀錄中都可以發現銥。
沈俊表示汞的化學訊號提供了強而有力的證據，顯示形成西伯利亞暗色岩的火山爆發就是二疊紀大滅絕的罪魁禍首。研究人員現在的目標是要精確定出噴發的規模有多大，以及哪些環境變化是造成生物大量死亡的最大原因，尤其是陸上的動植物。
沈俊表示二疊紀大滅絕也可以闡明今日發生的全球暖化，可能會造成什麼影響而引發下次大滅絕。如果二疊紀生物大量死亡的原因確實是全球暖化，那現今的暖化會把人類和野生生物引領到什麼樣的未來？
沈俊說：「人類把碳釋放到大氣中的行為，就好比是二疊紀末的西伯利亞火山爆發，釋放出大量二氧化碳一樣。」
Algeo說這讓他們十分擔憂。
「大多數生物學家認為我們正邁入另一場大滅絕――也就是第六次大滅絕。我也抱持著同樣看法。」Algeo表示，「我們應該要體認到這是一件非同小可的事情，因為它會損害人類全體的利益，因此我們必許採取行動來把傷害降到最低。」
生活在沙漠這類邊陲地帶的人們會是首批受害者。全世界將出現更多氣候難民。
「我們可能會在受損最嚴重的地區看到更多饑荒與大量遷徙的人民。這是一個涵蓋全球的議題，我們每一個人都應該體認到此點並主動採取作為。在問題真正釀成危害之前，先解決掉總是容易得多。」
New evidence suggests volcanoes caused
biggest mass extinction ever
Mercury found in ancient rock around the world supports theory that
eruptions caused 'Great Dying' 252 million years ago
Researchers say
mercury buried in ancient rock provides the strongest evidence yet that
volcanoes caused the biggest mass extinction in the history of the Earth.
The extinction 252 million
years ago was so dramatic and widespread that scientists call it "the
Great Dying." The catastrophe killed off more than 95 percent of life on
Earth over the course of hundreds of thousands of years.
Paleontologists with the
University of Cincinnati and the China University of Geosciences said they
found a spike in mercury in the geologic record at nearly a dozen sites around
the world, which provides persuasive evidence that volcanic eruptions were to
blame for this global cataclysm.
The study was published
this month in the journal Nature
Communications.
The eruptions ignited vast
deposits of coal, releasing mercury vapor high into the atmosphere. Eventually,
it rained down into the marine sediment around the planet, creating an
elemental signature of a catastrophe that would herald the age of dinosaurs.
"Volcanic activities,
including emissions of volcanic gases and combustion of organic matter,
released abundant mercury to the surface of the Earth," said lead author
Jun Shen, an associate professor at the China University of Geosciences.
The mass extinction
occurred at what scientists call the Permian-Triassic Boundary. The mass
extinction killed off much of the terrestrial and marine life before the rise
of dinosaurs. Some were prehistoric monsters in their own right, such as the
ferocious gorgonopsids that looked like a cross between a sabre-toothed tiger
and a Komodo dragon.
The eruptions occurred in a
volcanic system called the Siberian Traps in what is now central Russia. Many
of the eruptions occurred not in cone-shaped volcanoes but through gaping
fissures in the ground. The eruptions were frequent and long-lasting and their
fury spanned a period of hundreds of thousands of years.
"Typically, when you
have large, explosive volcanic eruptions, a lot of mercury is released into the
atmosphere," said Thomas Algeo, a professor of geology in UC's McMicken
College of Arts and Sciences.
"Mercury is a
relatively new indicator for researchers. It has become a hot topic for
investigating volcanic influences on major events in Earth's history,"
Algeo said.
Researchers use the sharp
fossilized teeth of lamprey-like creatures called conodonts to date the rock in
which the mercury was deposited. Like most other creatures on the planet,
conodonts were decimated by the catastrophe.
The eruptions propelled as
much as 3 million cubic kilometers of ash high into the air over this extended
period. To put that in perspective, the 1980 eruption of Mount St. Helens in
Washington sent just 1 cubic kilometer of ash into the atmosphere, even though
ash fell on car windshields as far away as Oklahoma.
In fact, Algeo said, the
Siberian Traps eruptions spewed so much material in the air, particularly
greenhouse gases, that it warmed the planet by an average of about 10 degrees
centigrade.
The warming climate likely
would have been one of the biggest culprits in the mass extinction, he said.
But acid rain would have spoiled many bodies of water and raised the acidity of
the global oceans. And the warmer water would have had more dead zones from a
lack of dissolved oxygen.
"We're often left
scratching our heads about what exactly was most harmful. Creatures adapted to
colder environments would have been out of luck," Algeo said. "So my
guess is temperature change would be the No. 1 killer. Effects would
exacerbated by acidification and other toxins in the environment."
Stretching over an extended
period, eruption after eruption prevented the Earth's food chain from
recovering.
"It's not necessarily
the intensity but the duration that matters," Algeo said. "The longer
this went on, the more pressure was placed on the environment."
Likewise, the Earth was
slow to recover from the disaster because the ongoing disturbances continued to
wipe out biodiversity, he said.
Earth has witnessed five
known mass extinctions over its 4.5 billion years.
Scientists used another
elemental signature -- iridium -- to pin down the likely cause of the global
mass extinction that wiped out the dinosaurs 65 million years ago. They believe
an enormous meteor struck what is now Mexico.
The resulting plume of
superheated earth blown into the atmosphere rained down material containing
iridium that is found in the geologic record around the world.
Shen said the mercury
signature provides convincing evidence that the Siberian Traps eruptions were
responsible for the catastrophe. Now researchers are trying to pin down the
extent of the eruptions and which environmental effects in particular were most
responsible for the mass die-off, particularly for land animals and plants.
Shen said the Permian
extinction could shed light on how global warming today might lead to the next
mass extinction. If global warming, indeed, was responsible for the Permian
die-off, what does warming portend for humans and wildlife today?
"The release of carbon
into the atmosphere by human beings is similar to the situation in the Late
Permian, where abundant carbon was released by the Siberian eruptions,"
Shen said.
Algeo said it is cause for
concern.
"A majority of
biologists believe we're at the cusp of another mass extinction -- the sixth
big one. I share that view, too," Algeo said. "What we should learn
is this will be serious business that will harm human interests so we should
work to minimize the damage."
People living in marginal
environments such as arid deserts will suffer first. This will lead to more
climate refugees around the world.
"We're likely to see
more famine and mass migration in the hardest hit places. It's a global issue
and one we should recognize and proactively deal with. It's much easier to
address these problems before they reach a crisis."
原始論文：Jun Shen,
Jiubin Chen, Thomas J. Algeo, Shengliu Yuan, Qinglai Feng, Jianxin Yu, Lian
Zhou, Brennan O’Connell, Noah J. Planavsky. Evidence for a prolonged
Permian–Triassic extinction interval from global marine mercury records. Nature
Communications, 2019; 10 (1) DOI: 10.1038/s41467-019-09620-0
引用自：University of Cincinnati. "New evidence
suggests volcanoes caused biggest mass extinction ever.”
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