小行星撞擊地點的岩石記錄了恐龍滅絕的第一天
消滅恐龍的小行星狠狠撞上地球之後，不但引發了大火與海嘯，也把巨量的硫燒成氣體，遮蔽陽光使得全球冷化，最終造成恐龍殞落。
這幅藝術家的示意圖表現了消滅所有非鳥類恐龍的小行星撞擊。圖片來源：NASA/Don
Davis.
上述是科學家長久以來的假想情境。不過最近德州大學奧斯汀分校領導的新研究證明了確實如此。他們發現的強力證據來自撞擊坑裡厚達數百公尺的岩石――其形成於撞擊之後的最初24小時。
證據包含了海嘯回流之後帶來的一些木炭與混雜的岩石，而且明顯缺乏了硫。德州大學奧斯汀分校地球科學院地球物理研究所的教授Sean
Gulick表示，帶有這些證據的岩石提供了迄今最為詳盡的紀錄，讓我們看見終結恐龍時代的浩劫發生時的景象。
領導這項研究的Gulick表示：「我們從事發地點取回的紀錄含有這次事件的大量訊息。」由他共同主持的這項2016國際海洋發現計畫的科學鑽探任務，從位於猶加敦半島外海的撞擊地點取出了岩石。「它們(岩石)告訴我們撞擊過程親眼看來會是什麼樣子。」
這項研究9月9日發表在《美國國家科學院院刊》(Proceedings of the National Academy of
Sciences)，參與的國際團隊包含了二十多名科學家。之前德州大學奧斯汀分校地球科學院主持與共同主持的研究，描述了撞擊坑的形成過程，以及撞擊地點的生物復原速度有多快，成果構成了本研究的基礎。
撞擊過後數小時內填入撞擊坑的物質大部分是產生自撞擊地點，或者是周遭墨西哥灣的海水灌回撞擊坑時將其席捲的物質攜帶進來。短短一天之內撞擊坑就堆積了將近425英尺厚(約130公尺)的沉積物――這是科學家在地質紀錄中看過最快的沉積速率。如此驚人的堆積速率代表這些岩石可以記錄撞擊坑內部與附近的環境，在撞擊過後數分鐘至數小時間發生了什麼事情。此外也能讓我們推測這場撞擊事件帶來什麼樣的長期效應，使得地球75%的物種因而滅絕。
這是從填滿撞擊坑的岩石中取出的一部分岩芯。科學家發現了熔化或破碎的砂岩、石灰岩、花崗岩等岩石，卻沒有找到含硫礦物――儘管此地區很大一部分的岩石都有含硫。這項發現代表撞擊把含硫岩石蒸發成硫酸鹽氣膠釋放到大氣當中，造成了全球尺度的氣溫降低。圖片來源：國際海洋發現計畫(International
Ocean Discovery Program)
Gulick形容說這以區域層級來看是一時的煉獄，但接著影響全球的則是長期氣溫降低。
「就像是被烤熟之後又放入冰箱，」Gulick表示，「雖然不是所有的恐龍，但許多恐龍都在那天死亡了。」
研究人員估計小行星撞擊的能量相當於100億顆二次大戰時用的原子彈。撞擊釋出的爆風點燃了數千公里外的花草樹木；引發的巨型海嘯則深入遠達伊利諾州的內陸。研究人員在撞擊坑內部找到具有跡象顯示為海水回流時沉積的沙層，他們發現緊鄰其上方或內部含有木炭以及和土壤真菌有關，可作為生物標記的化學物質。代表海嘯過後回流的海水將燒焦的地表物質拖入隕石坑裡面。
普渡大學的教授Jay
Melosh是研究撞擊坑的專家，他說發現野火的發生證據讓科學家知道他們對小行星撞擊的理解方向是對的。
未參與這項研究的Melosh表示：「當天是生命史上極為重要的日子。而這份紀錄清楚記載了事發地點的經歷。」
不過，研究最重要的發現之一則是岩芯樣品裡看不到的東西。撞擊地點周圍具有大量富含硫的岩石，但在岩芯裡卻找不到硫。
這項發現支持了一項理論：小行星撞擊使得位於撞擊地點的含硫礦物蒸發，並將硫釋放到大氣當中。這對地球氣候來說是重大災難，因為硫可以反射陽光而造成全球氣溫降低。研究人員估計撞擊至少使3250億噸的硫釋放到大氣當中。為了具體呈現出這有多少，可以跟1883年的喀拉喀托火山噴發比較：該次噴發使地球氣候在後續五年的溫度平均下降了華氏2.2度(約攝氏1.22度)――而小行星撞擊釋出的硫是它的數萬倍。
雖然小行星撞擊在地區層級來說造成了重大毀滅，但是全球氣候變遷才是造成生物大滅絕的原因，這讓恐龍以及當時生存在地球上的大多數生物徹底消失。
「真正的死因必定跟大氣有關。」Gulick表示，「唯有影響到大氣才能發生像這樣的全球大滅絕。」
研究經費來自於數個國際和國家基金組織，包括美國國家科學基金會。
Rocks at asteroid impact site record
first day of dinosaur extinction
When the asteroid that wiped out the
dinosaurs slammed into the planet, the impact set wildfires, triggered tsunamis
and blasted so much sulfur into the atmosphere that it blocked the sun, which
caused the global cooling that ultimately doomed the dinos.
That’s the scenario scientists have hypothesized.
Now, a new study led by The University of Texas at Austin has confirmed it by
finding hard evidence in the hundreds of feet of rocks that filled the impact
crater within the first 24 hours after impact.
The evidence includes bits of charcoal, jumbles of
rock brought in by the tsunami’s backflow and conspicuously absent sulfur. They
are all part of a rock record that offers the most detailed look yet into the
aftermath of the catastrophe that ended the Age of Dinosaurs, said Sean Gulick,
a research professor at the University of Texas Institute for Geophysics (UTIG)
at the Jackson School of Geosciences.
“It’s an expanded record of events that we were able
to recover from within ground zero,” said Gulick, who led the study and co-led
the 2016 International Ocean Discovery Program scientific drilling mission that
retrieved the rocks from the impact site offshore of the Yucatan Peninsula. “It
tells us about impact processes from an eyewitness location.”
The research was published in the Proceedings of the National Academy of
Sciences on Sept. 9 and builds on earlier work co-led and led by the
Jackson School that described how the crater formed and how life quickly
recovered at the impact site. An international team of more than two dozen
scientists contributed to this study.
Most of the material that filled the crater within
hours of impact was produced at the impact site or was swept in by seawater
pouring back into the crater from the surrounding Gulf of Mexico. Just one day
deposited about 425 feet of material — a rate that’s among the highest ever
encountered in the geologic record. This breakneck rate of accumulation means
that the rocks record what was happening in the environment within and around
the crater in the minutes and hours after impact and give clues about the
longer-lasting effects of the impact that wiped out 75% of life on the planet.
Gulick described it as a short-lived inferno at the
regional level, followed by a long period of global cooling.
“We fried them and then we froze them,” Gulick said.
“Not all the dinosaurs died that day, but many dinosaurs did.”
Researchers estimate the asteroid hit with the
equivalent power of 10 billion atomic bombs of the size used in World War II.
The blast ignited trees and plants that were thousands of miles away and
triggered a massive tsunami that reached as far inland as Illinois. Inside the crater,
researchers found charcoal and a chemical biomarker associated with soil fungi
within or just above layers of sand that shows signs of being deposited by
resurging waters. This suggests that the charred landscape was pulled into the
crater with the receding waters of the tsunami.
Jay Melosh, a Purdue University professor and expert
on impact cratering, said that finding evidence for wildfire helps scientists
know that their understanding of the asteroid impact is on the right track.
“It was a momentous day in the history of life, and
this is a very clear documentation of what happened at ground zero,” said
Melosh, who was not involved with this study.
However, one of the most important takeaways from the
research is what was missing from the core samples. The area surrounding the
impact crater is full of sulfur-rich rocks. But there was no sulfur in the
core.
That finding supports a theory that the asteroid
impact vaporized the sulfur-bearing minerals present at the impact site and
released it into the atmosphere, where it wreaked havoc on the Earth’s climate,
reflecting sunlight away from the planet and causing global cooling.
Researchers estimate that at least 325 billion metric tons would have been
released by the impact. To put that in perspective, that’s about four orders of
magnitude greater than the sulfur that was spewed during the 1883 eruption of
Krakatoa — which cooled the Earth’s climate by an average of 2.2 degrees
Fahrenheit for five years.
Although the asteroid impact created mass destruction
at the regional level, it was this global climate change that caused a mass
extinction, killing off the dinosaurs along with most other life on the planet
at the time.
“The real killer has got to be atmospheric,” Gulick
said. “The only way you get a global mass extinction like this is an
atmospheric effect.”
The research was funded by a number of international
and national support organizations, including the National Science Foundation.
原始論文：Sean P. S. Gulick, Timothy J. Bralower, Jens Ormö, Brendon
Hall, Kliti Grice, Bettina Schaefer, Shelby Lyons, Katherine H. Freeman, Joanna
V. Morgan, Natalia Artemieva, Pim Kaskes, Sietze J. de Graaff, Michael T.
Whalen, Gareth S. Collins, Sonia M. Tikoo, Christina Verhagen, Gail L.
Christeson, Philippe Claeys, Marco J. L. Coolen, Steven Goderis, Kazuhisa Goto,
Richard A. F. Grieve, Naoma McCall, Gordon R. Osinski, Auriol S. P. Rae, Ulrich
Riller, Jan Smit, Vivi Vajda, Axel Wittmann, and the Expedition 364
Scientists. The
first day of the Cenozoic. PNAS, 2019 DOI: 10.1073/pnas.1909479116
引用自：University of Texas at Austin. "Rocks at asteroid impact
site record first day of dinosaur extinction."
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