Fossils record dinosaur-killing impact

[beyond]

Artwork: The impact hit with the energy equivalent to 10 billion Hiroshima bombs

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

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

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

The detail is reported in PNAS journal.

Robert DePalma 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.

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 in all directions - and across thousands of kilometres.

And at Tanis, the fossils record the moment this material strafed everything in its path.

Fish are found with the impact-induced debris embedded in their gills.

There are also fragments 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.

Geochemists have managed to link the fall-out 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 direct agreement with the ages for the event worked out from evidence at other sites around the world.

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

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.

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.

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.

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.

The Tanis deposits are also capped with iridium traces.

Prof Phil Manning, from the University of Manchester, is the only British author on the paper.

He described the Tanis site as astonishing.

"If you truly wanted to understand the last days of the dinosaurs, this is it," he told BBC News.

Chicxulub Crater - The impact that changed life on Earth

The outer rim (white arc) of the crater lies under the Yucatan Peninsula itself, but the inner peak ring is best accessed offshore

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

The crater's centre rebounded and collapsed again, producing an inner ring

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

Mexico's famous sinkholes (cenotes) have formed in weakened limestone overlying the crater