臨終之際：目睹6600萬年前消滅恐龍的隕石撞擊

臨終之際：目睹6600萬年前消滅恐龍的隕石撞擊

By Robert Sanders

整起滅亡事件的序幕是地面開始劇烈震動，當時包含北達科他州的北美大陸內海因而升起滔天巨浪。

6600萬年前的星體撞擊事件在一座內海產生了海嘯一般的巨浪，使得許多魚類、哺乳類、昆蟲和一隻恐龍因而喪生並被埋藏，成為地球最近一起大滅絕事件的首批罹難者。這處在撞擊一小時內發生的死亡現場在北達科塔州被挖掘出來，使其成為絕無僅有的化石場址。圖片來源：Robert DePalma

接著，微小的玻璃珠猶如彈雨般落下，力道之猛烈足以點燃陸上的植被。落到水中的玻璃珠則堵住了魚鰓，使魚群幾乎窒息。

升高的海面到達河口時逐漸隆起成10公尺高的水牆，使得河川一度逆流，數百、甚至數千條淡水魚(鱘魚、匙吻鱘等)跟著沖到沙洲上面。海水退去之後，擱淺的魚群被直徑達五釐米的玻璃珠雨轟擊，力道大到可以打穿數公分深的泥巴。細沙大小的土石以及微小的玻璃珠滂沱落下了十到二十分鐘之後，第二波巨浪再次淹沒沙洲，將魚群掩埋在礫石、沙子和泥巴混雜而成的沉積物中，直到6600萬年後才重見天日。

在北達科他州鮑曼市附近的地獄溪地層，古生物學家Robert DePalma六年來持續挖掘這座獨一無二的化石墳場，其中埋有層層相疊的魚骨，混雜著燃燒過後的樹幹、裸子植物的枝條、哺乳類的遺體、滄龍的骸骨、昆蟲碎片、一頭三角龍的殘骸、渦鞭澡(一種海洋微生物)、像蝸牛的海洋頭足類――菊石......等生物。這些化石證明了DePalma 2013年夏季第一次挖掘這裡之後，就一直盤旋在心頭的猜想：此處是小行星撞擊不久之後的災難現場。這場發生在白堊紀末，又稱白堊紀－第三紀界線的小行星撞擊事件，最終造成所有的陸生恐龍滅絕，也消滅了地球75%的物種。

「這是首次發現和白堊紀－第三紀界線有關，由大型生物大規模死亡而形成的化石組合。」堪薩斯大學的博士生DePalma表示。他也是佛羅里達州棕櫚灘自然歷史博物館的古生物研究員。「地球其他地方白堊紀－第三紀界線的地層中，從未發現有這麼多種生物，而且處於不同年齡層與生命史的不同階段，全部在同一天的同一時間死亡。」

層層交疊的魚類化石。顯示牠們是被沖上沙洲後，接著盪漾退去，造成牠們擱淺而同時死亡。

DePalma的研究同仁包括兩名加州大學柏克萊分校的地質學家以及其他歐美研究人員。在下周發表於期刊《美國國家科學院院刊》(Proceedings of the National Academy of Sciences)的論文中，他們敘述了這座命名為塔尼斯(Tanis)的化石場址，並且提出證據證明它和6600萬年前，撞擊墨西哥猶加敦半島的小行星(或彗星)之間有所關連。該撞擊事件在海床上撞出了巨大的希克蘇魯伯撞擊坑。之後小行星碎裂成數立方公里的粉塵，與大量蒸發的岩石一起進入大氣，形成的塵雲逐漸包覆整個地球，為地球史上最近一次大滅絕揭開了序幕。

Mark Richards是加州大學柏克萊分校地球與行星科學系的名譽教授，目前在華盛頓大學地球與太空科學系擔任系主任。他說：「這層一公尺半的岩石就像是保存了白堊紀結束之際的博物館。」

Richards 和加州大學研究所的教授Walter Alvarez，40年前首度提出白堊紀末的大滅絕是由彗星或小行星撞擊而造成。DePalma和荷蘭科學家Jan Smit向他們兩位討論掩埋並保存該處魚類的玻璃珠雨，以及海嘯般的巨浪。這些玻璃珠稱為似曜石(tektite)，是受到撞擊而融化的岩石在大氣中重新凝固所形成。

海嘯或是盪漾(seiche)？

這處新發現的場址是西部內陸海道的一個海灣。Richards和Alvarez判斷此處魚群擱淺，接著被掩埋的原因並非一般的海嘯。此處距離撞擊地點約有三千公里，因此撞擊產生的海嘯波在未消散的前提下，要到達這裡至少需要10到12小時；然而，在撞擊後的45分鐘到一小時之內，似曜石就會落下來，如果此時海床還沒裸露，就不會形成似曜石打在海床上的泥坑。

反之，他們主張撞擊產生的震波(相當於規模10到11的地震)在10分鐘內於封閉的海灣產生了「盪漾」，這種駐波的效應就像地震襲擊時水缸裡的水會晃動噴濺一樣。Richards說雖然大地震經常會在封閉水體內產生盪漾，卻很少受人矚目。2011年規模9.0的東日本大地震發生的30分鐘後，就在8000公里外的一處挪威峽灣形成了約2公尺高的盪漾。

「地震波在撞擊後的9到10分鐘到達這裡，使水面可以在似曜石球粒全數落下之前發生擾動。」Richards表示。「這些球粒會撞擊地面形成凹洞，你可以看到過往是軟泥的岩層因此變形，接著礫石又把球粒給覆蓋住。過往從來沒有人有觀察到類似的結構。」

Alvarez在數十年前估算了似曜石的飛行時間。結果指出似曜石會以拋物線的軌跡從高空降下，終端速度可以到達每小時160公里至320公里。

Richards說：「想像當時你站在那裡被這些玻璃球粒轟擊，大概會因此送命。」許多科學家認為這些急速落下的岩屑具有的能量足以讓整個北美洲陷入火海，甚至擴及全世界。

從坦尼斯化石層取出的似曜石，這些玻璃球粒的直徑只有1毫米左右。

「確實有明確記錄顯示希克蘇魯伯撞擊事件產生了海嘯，不過沒有人知道海嘯可以進到內海多裡面的地方。」DePalma表示。「Mark加入研究團隊之後，發現一個重要的關聯：從撞擊地點傳來的地震波到達此處的時間，跟撞擊拋射出的物質飛過來的時間差不多。這是一個相當重大的突破。」

至少有兩次相隔大約20分鐘的強烈盪漾淹沒了陸地，帶來厚約2公尺的沉積物把化石給覆蓋住。再上面是一層黏土，其中富有的金屬元素銥在地球上相當罕見，但在小行星和彗星中卻很常見。這層黏土稱為白堊紀－第三紀界線或是白堊紀－古近紀界線，它標記了白堊紀的結束，也代表了第三紀，或稱古近紀的開始。

銥元素

全世界6600萬年前的岩層中都可以找到銥元素。1979年，Alvarez和他的父親，加州大學柏克萊分校的諾貝爾得主Luis Alvarez首度指出其重要性。他們提出白堊紀－第三紀界線的銥元素和同時發生的大滅絕，都是由彗星或小行星撞擊造成的。

撞擊使得海床下的岩石融化，也粉碎了小行星本身。這些塵埃和融化的岩石噴濺到平流層後，被氣流輸送到全世界，遮蔽陽光數月，甚至數年之久。最終碎屑會從天空落下，不只是似曜石，還有大陸地殼的岩屑，其中含有一種因為撞擊導致結晶構造變形的礦物――衝擊石英。

撞擊過後，最慢從大氣中落下的是小行星粉碎形成富含銥元素的塵埃，也標記了白堊紀的句點。

「銥是小行星或彗星留下的指紋。當時我們提出撞擊假說來解釋大滅絕的唯一根據，便是在岩層當中發現異常高濃度的銥。」Alvarez表示。「之後有越來越多證據支持我們的假說，但我從來沒有想過可以找到像這樣的案發現場。」

天體撞擊理論的關鍵證據是在加勒比海，鄰近墨西哥猶加敦半島的海底，發現埋起來的希克蘇魯伯撞擊坑。定年結果顯示它的形成年代正好就是大滅絕發生的時候。此外，世界各地的白堊紀－古近紀界線中也發現了衝擊石英和玻璃球粒。坦尼斯則是首次在同個地方發現衝擊產生的岩屑，以及撞擊過後立即死亡的動物。

「Robert DePalma在北達科他州挖到的場址是個完全出人意料的重大發現。此處具有極其詳細的資訊，顯示撞擊造成的結果。」Alvarez說：「這真的令我喜出望外！」

似曜石

Jan Smit是荷蘭阿姆斯特丹自由大學退休的沉積地質學教授，在衝擊產生的似曜石這方面，是舉世公認的專家。Smit加入DePalma的團隊分析並定年坦尼斯出土的似曜石。在坦尼斯，由軟糊糊的松脂硬化形成的琥珀，把許多似曜石完美地保存下來。

「我在2015年前往該場址的時候，他(DePalma)就在我的眼前挖出一段長約四公尺、燒得焦黑的樹幹，而且上面還覆有琥珀。琥珀就像氣凝膠一樣，可以困住從天而降的似曜石。」Smit說。「這是一項重大發現。因為樹脂，也就是琥珀可以把似曜石給完全包覆起來，使它們成為我見過的似曜石中置換程度最低的，連1%都不到。我們對它們進行定年，結果顯示它們就是在白堊紀－第三紀界線形成的，分毫不差。」

他們也首次發現卡在魚鰓中的似曜石。

「匙吻鱘游泳時會目瞪口呆似地把嘴巴張開，把水中微小的顆粒、食物碎屑給網到鰓耙裡面，再把它們吞下去，就跟鯨鯊和鬚鯨的覓食方法一樣。」Smit表示。「有趣的是，牠們也會吸入似曜石，代表撞擊事件直接造成的首批受害者之一便是這些魚群。」

Smit也強調到此處埋有一頭三角龍和一頭鴨嘴龍的遺骸，正是毋庸置疑的證據顯示撞擊當時恐龍仍然活著。

「我們還有許多驚人的發現，之後或許可以證明出它們具有更加珍貴的資訊。」Smit說，「這座完美的沉積岩層需要從各方面來徹底研究。我認為我們可以找出從希克蘇魯伯撞擊坑拋射過來的物質的詳細抵達順序。這是墨西哥灣附近的沉積岩中從未辦到的研究。」

「我們花了40年才找到這座很有可能是絕無僅有的化石場址。」Smit說，「因此，我們必須要謹慎挖掘這座場址，並用心學習其中蘊含的知識。對於已經處在學術生涯末期的我來說，這是一個再好不過的禮物。Walter也有同感。」

跟DePalma、Smit、Richards、Alvarez共同進行這項研究的作者包括美國堪薩斯大學的David Burnham；荷蘭阿姆斯特丹自由大學Klaudia Kuiper；英國曼徹斯特大學的Phillip Manning；美國佛羅里達大西洋大學的Anton Oleinik；美國南達科塔黑山地質研究院的Peter Larson；美國佛羅里達國際大學的Florentin Maurrasse；比利時天主教魯汶大學的Johan Vellekoop；美國棕櫚灘自然歷史博物館的Loren Gurche。

66 million-year-old deathbed linked to dinosaur-killing meteor

The beginning of the end started with violent shaking that raised giant waves in the waters of an inland sea in what is now North Dakota.

Then, tiny glass beads began to fall like birdshot from the heavens. The rain of glass was so heavy it may have set fire to much of the vegetation on land. In the water, fish struggled to breathe as the beads clogged their gills.

The heaving sea turned into a 30-foot wall of water when it reached the mouth of a river, tossing hundreds, if not thousands, of fresh-water fish — sturgeon and paddlefish — onto a sand bar and temporarily reversing the flow of the river. Stranded by the receding water, the fish were pelted by glass beads up to 5 millimeters in diameter, some burying themselves inches deep in the mud. The torrent of rocks, like fine sand, and small glass beads continued for another 10 to 20 minutes before a second large wave inundated the shore and covered the fish with gravel, sand and fine sediment, sealing them from the world for 66 million years.

This unique, fossilized graveyard — fish stacked one atop another and mixed in with burned tree trunks, conifer branches, dead mammals, mosasaur bones, insects, the partial carcass of a Triceratops, marine microorganisms called dinoflagellates and snail-like marine cephalopods called ammonites — was unearthed by paleontologist Robert DePalma over the past six years in the Hell Creek Formation, not far from Bowman, North Dakota. The evidence confirms a suspicion that nagged at DePalma in his first digging season during the summer of 2013 — that this was a killing field laid down soon after the asteroid impact that eventually led to the extinction of all ground-dwelling dinosaurs. The impact at the end of the Cretaceous Period, the so-called K-T boundary, exterminated 75 percent of life on Earth.

“This is the first mass death assemblage of large organisms anyone has found associated with the K-T boundary,” said DePalma, curator of paleontology at the Palm Beach Museum of Natural History in Florida and a doctoral student at the University of Kansas. “At no other K-T boundary section on Earth can you find such a collection consisting of a large number of species representing different ages of organisms and different stages of life, all of which died at the same time, on the same day.”

In a paper to be published next week in the journal Proceedings of the National Academy of Sciences, he and his American and European colleagues, including two University of California, Berkeley, geologists, describe the site, dubbed Tanis, and the evidence connecting it with the asteroid or comet strike off Mexico’s Yucatan Peninsula 66 million years ago. That impact created a huge crater, called Chicxulub, in the ocean floor and sent vaporized rock and cubic miles of asteroid dust into the atmosphere. The cloud eventually enveloped Earth, setting the stage for Earth’s last mass extinction.

“It’s like a museum of the end of the Cretaceous in a layer a meter-and-a-half thick,” said Mark Richards, a UC Berkeley professor emeritus of earth and planetary science who is now provost and professor of earth and space sciences at the University of Washington.

Richards and Walter Alvarez, a UC Berkeley Professor of the Graduate School who 40 years ago first hypothesized that a comet or asteroid impact caused the mass extinction, were called in by DePalma and Dutch scientist Jan Smit to consult on the rain of glass beads and the tsunami-like waves that buried and preserved the fish. The beads, called tektites, formed in the atmosphere from rock melted by the impact.

Tsunami vs. seiche

Richards and Alvarez determined that the fish could not have been stranded and then buried by a typical tsunami, a single wave that would have reached this previously unknown arm of the Western Interior Seaway no less than 10 to 12 hours after the impact 3,000 kilometers away, if it didn’t peter out before then. Their reasoning: The tektites would have rained down within 45 minutes to an hour of the impact, unable to create mudholes if the seabed had not already been exposed.

Instead, they argue, seismic waves likely arrived within 10 minutes of the impact from what would have been the equivalent of a magnitude 10 or 11 earthquake, creating a seiche (pronounced saysh), a standing wave, in the inland sea that is similar to water sloshing in a bathtub during an earthquake. Though large earthquakes often generate seiches in enclosed bodies of water, they’re seldom noticed, Richards said. The 2011 Tohoku quake in Japan, a magnitude 9.0, created six-foot-high seiches 30 minutes later in a Norwegian fjord 8,000 kilometers away.

“The seismic waves start arising within nine to 10 minutes of the impact, so they had a chance to get the water sloshing before all the spherules (small spheres) had fallen out of the sky,” Richards said. “These spherules coming in cratered the surface, making funnels — you can see the deformed layers in what used to be soft mud — and then rubble covered the spherules. No one has seen these funnels before.”

The tektites would have come in on a ballistic trajectory from space, reaching terminal velocities of between 100 and 200 miles per hour, according to Alvarez, who estimated their travel time decades ago.

“You can imagine standing there being pelted by these glass spherules. They could have killed you,” Richards said. Many believe that the rain of debris was so intense that the energy ignited wildfires over the entire American continent, if not around the world.

“Tsunamis from the Chicxulub impact are certainly well-documented, but no one knew how far something like that would go into an inland sea,” DePalma said. “When Mark came aboard, he discovered a remarkable artifact — that the incoming seismic waves from the impact site would have arrived at just about the same time as the atmospheric travel time of the ejecta. That was our big breakthrough.”

At least two huge seiches inundated the land, perhaps 20 minutes apart, leaving six feet of deposits covering the fossils. Overlaying this is a layer of clay rich in iridium, a metal rare on Earth, but common in asteroids and comets. This layer is known as the K-T, or K-Pg boundary, marking the end of the Cretaceous Period and the beginning of the Tertiary Period, or Paleogene.

Iridium

In 1979, Alvarez and his father, Nobelist Luis Alvarez of UC Berkeley, were the first to recognize the significance of iridium that is found in 66 million-year-old rock layers around the world. They proposed that a comet or asteroid impact was responsible for both the iridium at the K-T boundary and the mass extinction.

The impact would have melted the bedrock under the seafloor and pulverized the asteroid, sending dust and melted rock into the stratosphere, where winds would have carried them around the planet and blotted out the sun for months, if not years. Debris would have rained down from the sky: not only tektites, but also rock debris from the continental crust, including shocked quartz, whose crystal structure was deformed by the impact.

The iridium-rich dust from the pulverized meteor would have been the last to fall out of the atmosphere after the impact, capping off the Cretaceous.

“When we proposed the impact hypothesis to explain the great extinction, it was based just on finding an anomalous concentration of iridium — the fingerprint of an asteroid or comet,” said Alvarez. “Since then, the evidence has gradually built up. But it never crossed my mind that we would find a deathbed like this.”

Key confirmation of the meteor hypothesis was the discovery of a buried impact crater, Chicxulub, in the Caribbean and off the coast of the Yucatan in Mexico, that was dated to exactly the age of the extinction. Shocked quartz and glass spherules were also found in K-Pg layers worldwide. The new discovery at Tanis is the first time the debris produced in the impact was found along with animals killed in the immediate aftermath of the impact.

“And now we have this magnificent and completely unexpected site that Robert DePalma is excavating in North Dakota, which is so rich in detailed information about what happened as a result of the impact,” Alvarez said. “For me, it is very exciting and gratifying!”

Tektites

Jan Smit, a retired professor of sedimentary geology from Vrije Universiteit in Amsterdam in The Netherlands who is considered the world expert on tektites from the impact, joined DePalma to analyze and date the tektites from the Tanis site. Many were found in near perfect condition embedded in amber, which at the time was pliable pine pitch.

“I went to the site in 2015 and, in front of my eyes, he (DePalma) uncovered a charred log or tree trunk about four meters long which was covered in amber, which acted as sort of an aerogel and caught the tektites when they were coming down,” Smit said. “It was a major discovery, because the resin, the amber, covered the tektites completely, and they are the most unaltered tektites I have seen so far, not 1 percent of alteration. We dated them, and they came out to be exactly from the K-T boundary.”

The tektites in the fishes’ gills are also a first.

“Paddlefish swim through the water with their mouths open, gaping, and in this net, they catch tiny particles, food particles, in their gill rakers, and then they swallow, like a whale shark or a baleen whale,” Smit said. “They also caught tektites. That by itself is an amazing fact. That means that the first direct victims of the impact are these accumulations of fishes.”

Smit also noted that the buried body of a Triceratops and a duck-billed hadrosaur proves beyond a doubt that dinosaurs were still alive at the time of the impact.

“We have an amazing array of discoveries which will prove in the future to be even more valuable,” Smit said. “We have fantastic deposits that need to be studied from all different viewpoints. And I think we can unravel the sequence of incoming ejecta from the Chicxulub impact in great detail, which we would never have been able to do with all the other deposits around the Gulf of Mexico.”

“So far, we have gone 40 years before something like this turned up that may very well be unique,” Smit said. “So, we have to be very careful with that place, how we dig it up and learn from it. This is a great gift at the end of my career. Walter sees it as the same.”

Co-authors with DePalma, Smit, Richards and Alvarez are David Burnham of the University of Kansas, Klaudia Kuiper of Vrije Universiteit, Phillip Manning of Manchester University in the United Kingdom, Anton Oleinik of Florida Atlantic University, Peter Larson of the Black Hills Institute of Geological Research in South Dakota, Florentin Maurrasse of Florida International University, Johan Vellekoop of Katholieke Universiteit Leuven in Belgium and Loren Gurche of the Palm Beach Museum of Natural History.

原始論文：DePalma, Robert A.; Smit, Jan; Burnham, David; Kuiper, Klaudia; Manning, Phillip; Oleinik, Anton; Larson, Peter; Maurrasse, Florentin; Vellekoop, Johan; Richards, Mark A.; Gurche, Loren; Alvarez, Walter. Prelude to Extinction: a seismically induced onshore surge deposit at the KPg boundary, North Dakota. PNAS, 2019

引用自：University of California - Berkeley. "66-million-year-old deathbed linked to dinosaur-killing meteor: Fossil site preserves animals killed within minutes of meteor impact."

原文網址：https://news.berkeley.edu/2019/03/29/66-million-year-old-deathbed-linked-to-dinosaur-killing-meteor/