September Editorial      

New evidence for a catastrophic meteorite strike during the Younger Dryas.

The weather cooled dramatically and stayed cold for more than a millennium. Glaciers spread across most of the higher latitudes of the Northern Hemisphere. For example, results from the Greenland ice core show that the highest points of Greenland were approximately 13^oC(27^oF) colder than today. In the UK the mean temperature dropped by approximately 5oC. Although the effects of Younger Dryas were probably most drastic in the Northern Hemisphere, climatic changes were seen across the Earth. There has been no other period since with cooling of the amount, extent and rapidity of that seen during Younger Dryas.

One of the prevailing theories is that the cooling effect resulted from the sudden shutdown of the Northern Atlantic Ocean currents which circulate warm tropical waters north-westward. By this theory the shutdown was caused by a massive glacial lake which burst and flooded a huge torrent of icy water into the ocean from the St Lawrence seaway or Mackenzie River. The sudden influx of icy cold fresh water would lower the temperature of the seas and oceans and disrupt ocean currents. This in turn would cause a substantial drop in atmospheric temperature. Although this theory has been gaining momentum in recent years, the truth is that no-one has yet found a geological event which would explain what happened.

In 2007 some scientists suggested that a comet or a very large meteorite could have caused the sudden atmospheric changes of the Younger Dryas. Now new research published in the PNAS by Mukul Sharma (of Dartmouth College in Hanover, N.H.) and his colleagues provides support for the impact theory. The researchers examined tiny spherical (marble-like) rocks from Pennsylvania and New Jersey which were formed right at the beginning of the Dryas period. These so-called ‘spherules’ contain minerals that could only have been produced through extreme heat, that is in temperatures upwards of 1700^oC.

"The only place where you can make these [spherules] on the surface of the Earth is in a blast furnace," Sharma said. "And not everywhere in a blast furnace — in the hottest part. Clearly, these objects were produced in an impact fireball."

Although the spherules were found in Pennsylvania and New Jersey, they actually weren’t local. Close chemical analysis showed the presence of rare earth element patterns and the strontium (Sr) and neodumium (Nd) isotopes of the spherules closely match those from areas in southern Quebec along the Gulf of St Lawrence. So how did the spherules get as far as Pennsylvania and New Jersey? Sharma believes that what his research has found is evidence of a meteor which struck southern Quebec. A meteor so large and powerful that it could punch through more than 1 km of ice (at the time of impact, the region was covered by a continental ice sheet, rather as Antarctica and Greenland are covered today). Even after punching through the ice, the meteor still had enough energy to generate high enough temperatures to create the spherules. The huge mushroom cloud of debris thrown up by the impact covered a large chunk of the north American continent with rubble - rubble which included the Pennsylvanian and New Jersey spherules..

There is no actual proof that the meteor caused sudden cooling across the globe. It does, however, suggest there was at least one major event affecting the atmosphere that occurred right around the same time.

But not everyone is equally convinced. For a start the spherules found in Pennsylvania have never been accurately dated. As Sharma himself admitted: ‘We are assuming they are Younger Dryas, but [dating] is one of the things that should be done better’.

Secondly, the analysis of osmium (Os) and iridium (Ir) in the spherules suggest that the stones are definitely terrestrial, so there is no hard data connecting them to a meteorite. Finally, no-one has yet found the crater in Quebec where the theoretical meteorite is supposed to have hit. However, Sharma suggests that the crater is there, buried deep under the bedrock waiting to be discovered. Quebec is home to a number of craters already, including the Manicouagan impact crater which is one of the largest and oldest craters found on Earth. New craters in this area are still being discovered, including one discovered in 2001 which is dated to the start of the Younger Dryas (although this crater is definitely not the crater of Sharma’s meteor). More data is still needed to convince the sceptics.

Reference

1. Yingzhe Wua, Mukul Sharmaa, Malcolm A. LeCompteb, Mark N. Demitroffc, and Joshua D. Landisa. Origin and provenance of spherules and magnetic grains at the Younger Dryas boundary. PNAS (2013) 110(38), E3557–E3566.