[Aqualist] Quaternary in the news

[Tim Barrows]

SCIENCE media office

5 October 2007

Nearly 12,900 years ago as the world was warming after the last ice 
age it was suddenly plunged back into the cooler for another 1300 
years. Temperatures dropped by as much as 12C and ice sheets 
advanced. The sudden changes were thought to be global in scale, but 
now research is suggesting that the Southern Hemisphere was spared.

The cooling episode, known as the Younger Dryas event, was likely 
linked to a disruption of ocean circulation in the Atlantic. If this 
cycle were disrupted, as researchers think it might be if the 
Greenland Ice Sheet were to rapidly melt, it would have big 
implications for the climate. The important question is "how big."

The Younger Dryas was definitely felt in the Northern Hemisphere, but 
whether its effects were global is a matter of debate. At the Waiho 
Loop site in New Zealand, wood has been found in glacial debris 
dating to the time when the cooling took place, suggesting that 
glaciers responded to the Younger Dryas event.

But palaeoclimatologist Tim Barrows at The Australian National 
University has his doubts. He has previously dated the remains of 
glaciers in Australia and found no evidence for cooling after the 
last Ice Age. Using a new method for dating glacial sediments similar 
to radiocarbon dating, he measures radioactive elements in rock 
surfaces that are produced by nuclear reactions between minerals and 
cosmic rays. Barrows and colleagues now report in Science that 
boulders from the Waiho Loop, date to 10,500 years, more than 1000 
years after the Younger Dryas event had ended.

They support their finding with a new sea surface temperature record 
from a core off the coast of New Zealand. The record from the core 
shows no sign of the Younger Dryas cooling. In fact, the core data 
suggests that temperatures actually warmed during the time period. 
"Heat accumulating in the Southern Hemisphere as the North cooled is 
precisely what we expected to see if the ocean conveyor belt were 
shut down in the North Atlantic", comments Barrows.

"This study that shows no obvious cooling in the Southern Hemisphere, 
suggesting that previous theories of the hemispheres being 
climatically synchronous is questionable" comments geomorphologist 
Derek Fabel at the University of Glasgow. "It is good to see 
re-examination of critical Earth history events using new techniques, 
and this particular study shows that there is still a lot about the 
time period of the Younger Dryas that we still do not understand" 
adds geologist Ken Verosub at UC Davis.

On the bright side, if global warming triggers a collapse of an ice 
sheet which triggers another ice age, Australia and New Zealand will 
be toasty warm.