The day the dinosaurs' world fell apart

Wednesday, September 11, 2019

Scientists have a recording of the worst day on Earth; certainly the worst day in the last 66 million years.

It takes the form of a 130m section of rock drilled from under the Gulf of Mexico.

These are sediments that were laid down in the seconds to hours after a huge asteroid had slammed into the planet.

You'll know the event we're talking about - the one researchers now think was responsible for the demise of the dinosaurs and the rise of mammals.

The high-resolution account of this catastrophe was recovered by a UK/US-led team, who spent several weeks in 2016 drilling into what remains of the crater produced by the impact.

Today, this 200km-wide structure is positioned under Mexico's Yucatan Peninsula, with its best preserved central portions sitting just offshore of the port of Chicxulub.

The team pulled up a great long core of rock but it's a particular 130m-long section that essentially documents the first day of what geologists call the Cenozoic Era, or as some others like to refer to it: the Age of Mammals.

  • Stunning fossils record dinosaurs' demise
  • Splosh! How to make a giant impact crater
  • Asteroid strike made 'instant Himalayas'
  • 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

The section is a nightmarish jumble of shattered material, but its contents are arranged in such a way that scientists say they can discern a clear narrative.

The bottom 20m or so, is dominated by glassy debris. This is the rock that was melted by the heat and pressure of the impact. It lapped across the base of the crater in the subsequent seconds and minutes.

This then transitions to a lot of fragmented melt rock - the result of explosions as water rushed over the hot material.

The water came from the shallow sea covering the area at the time. It would have been pushed temporarily out of the way by the impact but when it came back in and made contact with the broiling rock, it would have set off violent reactions. Something similar occurs at volcanoes where magma interacts with seawater.

This phase covers the first minutes to an hour. But the water keeps coming, filling up the crater, and the top 80-90m of the core section is built from all the debris that was in this water and ultimately rained out. Larger fragments initially followed by finer and finer material.

The timescale for this is the first hours after the impact.

And then, right at the top of the 130m core section is evidence of a tsunami. The sediments all dip in one direction and their organisation suggests they were deposited in a high-energy event.

Scientists say the impact would have generated a giant wave pulse that would have crashed on to shorelines hundreds of kilometres from the crater. But this outward train would also have had a return pulse and it's the debris carried in this tsunami that caps the top of the rock sequence.

"This is all still Day One," says Prof Sean Gulick from the University of Texas at Austin. "Tsunamis move at the speed of a jet plane. Twenty-four hours is a generous amount of time for the waves to move out and come back in again," he told BBC News.

Prof Gulick's team is confident in the tsunami interpretation because mixed in with the deposits are soil markers, and charcoal - evidence of the great fires that would have been triggered on nearby landmasses by the heat of the impact - all brought back to the crater by the returning wave pulse.

Strikingly, what the team doesn't see anywhere in the 130m core is the presence of sulphur. That's surprising because the asteroid would have hit a seafloor made up by perhaps a third to half with sulphur-containing minerals, such as gypsum.

For some reason, the sulphur must have been preferentially ejected or vaporised. But this only goes to support the popular theory now for how the dinosaurs met their demise.

So much sulphur mixed with water and injected into the sky would have dramatically cooled the climate, making life a struggle for all sorts of plant and animal life.

"There was a global climate model run with just a hundred gigatonnes (billion tonnes) of sulphur injected into the atmosphere and that came up with 25C cooling for at least 15 years, which puts most of the planet below freezing," said Prof Gulick.

"And the conservative estimate for the amount of sulphur released is 325 gigatonnes. That's orders of magnitude more than you would get from any volcano like a Krakatau which can also cool the climate for a short period."

The mammals pulled through this calamity, the dinosaurs did not.

Source: BBC News