Chicxulub asteroid impact: Stunning fossils record dinosaurs' demise

Image copyrightRobert DePalmaImage caption

The seismic shockwave would have triggered a water surge, known as a seiche

Scientists have found an extraordinary snapshot of the fallout from the asteroid impact that wiped out the dinosaurs 66 million years ago.

沉积物也显示出被水淹没的证据 - 撞击产生巨大海浪的后果。 / p>

细节报告在PNAS期刊上。

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图片copyrightRobert DePalmaImage标题Robert DePalma是堪萨斯大学博士生地质学学生

来自堪萨斯大学的Robert DePalma及其同事说，在一个名为Tanis的地方，挖掘工地给出了一个惊人的一瞥，可能只发生了几十个在巨大的小行星撞击地球后几分钟到达几分钟。

当这个12公里宽的物体猛烈撞击现在的墨西哥湾时，它将投掷数十亿大量的熔化和蒸发的岩石向各个方向飞向天空 - 并且跨越数千公里。

在Tanis，化石记录了这种珠粒大小的材料倒下的时刻在它的路径上扫射一切。

发现了鱼类中含有撞击诱发的碎片继承人鳃。他们会吸入充满周围水分的碎片。

还有琥珀中的颗粒，这是保存下来的树脂残留物。当他们进入树脂时，甚至可以辨别这些微小的玻璃状tektites留下的尾迹，使用技术术语。

Image coprightRobert DePalmaImage标题化石鱼被一块顶上堆积起来，因为它们被塞进了岸上

地球化学家已设法将沉降物质直接连接到海湾地区所谓的Chicxulub撞击点。他们还将这些碎片与6576万年前的日期进行了对比，这与世界其他地方的证据确定时间非常吻合。

来自通过安排Tanis矿床的方式，科学家们可以看到该地区被大量的水冲击。

虽然这种影响被认为产生了巨大的海啸，但它会尽管当时可能存在直接穿越美国陆地的海道，但是这波已经花了很多时间从海湾到达达科他州的3000公里。

相反，研究人员认为，当地的水可能会被地震冲击波更快地移动 - 相当于10级或11级地震 - 这种地震将在地球周围波动。这是一种被描述为一种生机的浪涌，它可以拾取其路径上的所有东西并将其倾倒到现在由团队报告的混乱的标本集中。

“ DePalma先生说：”这一层中充斥着大量的淡水鱼，陆地脊椎动物，树木，树枝，原木，海洋菊石和其他海洋生物。DePalma先生说。

{777} “海啸从火山口到达现场需要至少17个小时或更长时间，但地震波 - 以及随后的浪涌 - 将在几十分钟内到达，”他补充道。 { 777} 图片copyrightRobert DePalmaImage标题约会tektites给出影响的年龄 - 6576万年前

{25} PNAS论文将于周一上线，其作者包括加州地质学家沃尔特·阿尔瓦雷斯（Walter Alvarez），他与他的父亲路易斯·阿尔瓦雷斯（Luis Alvarez）一起，帮助制定了恐龙消亡的影响理论。{3}

Excavations in North Dakota reveal fossils of fish and trees that were sprayed with rocky, glassy fragments that fell from the sky.

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The deposits show evidence also of having been swamped with water - the consequence of the colossal sea surge that was generated by the impact.

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The detail is reported in PNAS journal.

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• Splosh! How to make a giant impact crater

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• Dinosaurs suffered catastrophic 'winter'

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• Asteroid strike made 'instant Himalayas'

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Image copyrightRobert DePalmaImage caption Robert DePalma is a University of Kansas doctoral student in geology

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Robert DePalma, from the University of Kansas, and colleagues say the dig site, at a place called Tanis, gives an amazing glimpse into events that probably occurred perhaps only tens of minutes to a couple of hours after the giant asteroid hit the Earth.

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When this 12km-wide object slammed into what is now the Gulf of Mexico, it would have hurled billions of tonnes of molten and vaporised rock into the sky in all directions - and across thousands of kilometres.

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And at Tanis, the fossils record the moment this bead-sized material fell back down and strafed everything in its path.

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Fish are found with the impact-induced debris embedded in their gills. They would have breathed in the fragments that filled the water around them.

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There are also particles caught in amber, which is the preserved remnant of tree resin. It is even possible to discern the wake left by these tiny, glassy tektites, to use the technical term, as they entered the resin.

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Image copyrightRobert DePalmaImage caption Fossilised fish piled one atop another as they were flung ashore by the seiche

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Geochemists have managed to link the fallout material directly to the so-called Chicxulub impact site in the Gulf. They have also dated the debris to 65.76 million years ago, which is in very good agreement with the timing for the event worked out from evidence at other sites around the world.

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From the way the Tanis deposits are arranged, the scientists can see that the area was hit by a massive surge of water.

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Although the impact is understood to have generated a huge tsunami, it would have taken many hours for this wave to travel the 3,000km from the Gulf to North Dakota, despite the likely presence back then of a seaway cutting directly across the American landmass.

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Instead, the researchers believe local water could have been displaced much more quickly by the seismic shockwave - equivalent to a Magnitude 10 or 11 earthquake - that would have rippled around the Earth. It is a type of surge described as a seiche, which would have picked up everything in its path and dumped it into the jumbled collection of specimens now being reported by the team.

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“A tangled mass of freshwater fish, terrestrial vertebrates, trees, branches, logs, marine ammonites and other marine creatures was all packed into this layer by the inland-directed surge,” said Mr DePalma.

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“A tsunami would have taken at least 17 or more hours to reach the site from the crater, but seismic waves - and a subsequent surge - would have reached it in tens of minutes,” he added.

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Image copyrightRobert DePalmaImage caption Dating the tektites gives an age for the impact - 65.76 million years ago

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The PNAS paper, which will go online on Monday, includes among its authors Walter Alvarez, the Californian geologist who, with his father Luis Alvarez, is credited with helping to develop the impact theory for the demise of the dinosaurs.

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The Alvarez pair identified a layer of sediment at the boundary of the Cretaceous and Palaeogene geological periods that was enriched with iridium, an element commonly found in asteroids and meteorites.

Iridium traces are also found to be capping the Tanis deposits.

Image copyrightRobert DePalmaImage caption Walter Alvarez pioneered the idea of an end-Cretaceous impact

Another member of the team, the University of Kansas' David Burnham, said the fossil hoard would yield critical new insights on the aftermath of the Chicxulub event.

“We've understood that bad things happened right after the impact, but nobody's found this kind of smoking-gun evidence,” he explained. “People have said, 'We get that this blast killed the dinosaurs, but why don't we have dead bodies everywhere？' Well, now we have bodies. They're not dinosaurs, but I think those will eventually be found, too.”

And Phil Manning, from the University of Manchester, the only British author on the paper, commented: “It's one of the most important sites in the globe now. You know, if you truly wanted to understand the last day of the dinosaurs - this is it,” he told BBC News.

The Chicxulub Crater - The event that changed life on Earth

Image copyrightNASAImage caption The outer rim (white arc) of the crater lies partly under Mexico's Yucatan Peninsula

• A 12km-wide object dug a hole in Earth's crust 100km across and 30km deep

• This bowl then collapsed, leaving a crater 200km across and a few km deep

• Today, much of the crater is buried offshore, under 600m of sediments

• On land, it is covered by limestone, but its rim is traced by an arc of sinkholes

• Scientists recently drilled into the crater structure to learn about its formation

Image copyrightMax Alexander/B612/Asteroid DayImage caption Mexico's famous sinkholes (cenotes) have formed in weakened limestone overlying the crater