[Aqualist] Quaternary science in the news

[Tim Barrows]

http://www.abc.net.au/news/newsitems/200611/s1790224.htm

Mega-tsunamis more common than we think: scientists

By Anna Salleh for ABC Science Online

Enormous comets may have often bombarded our oceans in the past, 
causing tsunamis that dwarf ones seen today, a small group of scientists says.

But most critics are yet to be convinced there is evidence to back 
claims about such recent, frequent mega-impacts.

Conventional wisdom has it that the Earth suffered such violent hits 
from space only twice every million years.

But scientists including Australian geomorphologist Associate 
Professor Ted Bryant of the University of Wollongong have been 
studying what they say is evidence of massive objects slamming into 
the Earth's oceans as recently as 500 years ago.

They say these kilometre-wide objects are likely to have been comets.

Prof Bryant says there have been up to 10 such impacts in the past 
10,000 years, based on research with others, including Assistant 
Professor Dallas Abbott from the Lamont-Doherty Earth Observatory at 
Columbia University.

Prof Bryant says these would have caused mega-tsunamis 10 times 
bigger than the 2004 Asian tsunami, one of the largest 
earthquake-generated tsunamis the world has ever seen.

"Aceh was a dimple compared to what we're looking at," said Professor 
Bryant, who is associate dean of science at the university.
Evidence from Google Earth

Prof Bryant used satellite images from Google Earth to identify 
inland dunes in the shape of arrowheads that he says are signs of 
mega-tsunamis.

He says the tsunamis would have displaced marine deposits containing 
marine fossils, dumping them inland as 'chevron' dunes.

"We've found that chevrons are everywhere, everywhere around the 
world's coasts," he said.

Prof Abbot used sea surface altimetry, which measures the height of 
the sea surface to get an image of the seabed, to identify possible 
underwater craters, which could be evidence of the impact that caused 
the tsunamis.

Prof Bryant says Prof Abbot also looked for melted material in cores 
from the seabed around the craters to confirm impacts caused them.

The chevrons and craters were linked by the direction the chevrons 
were pointing.

For example, two chevrons identified six kilometres inland from the 
Gulf of Carpentaria in Australia both pointed north in the direction 
of two craters found in the Gulf of Carpentaria itself, Prof Bryant says.

He says dating of sediments to the north of the craters suggests the 
impact happened 1,500 years ago, and the well-preserved chevrons also 
date to around the same time.
Indian Ocean crater

Prof Bryant says chevrons about 4,800 years old around the Indian 
Ocean are associated with a 29-kilometre wide impact crater located 
thousands of kilometres to the south-east of Madagascar.

"There are chevrons around the Indian Ocean that all point back to 
this one crater site," he said.

He says this is supported by evidence from an anthropologist on the 
team who found 170 myths and legends from the area dating back about 
4,000 years referring to an event that could have been the impact.

Prof Bryant says other evidence of a mega-tsunami as recently as 500 
years ago has been found on the eastern coast of Australia.

He and Prof Abbott have linked this one to an impact crater south of 
Stewart Island in New Zealand.

None of the research has been published but some of it will be 
presented at an American Geophysical Union meeting in San Francisco next month.
Mixed reception

Earth scientist Professor Richard Arculus of the Australian National 
University says he accepts Prof Bryant's evidence of mega-tsunamis.

But he says working out what caused them and when will require more evidence.

His colleague, marine sediment specialist Dr Bradley Opdyke, is also 
yet to be convinced.

"They're heading in the right direction," he said.

But he believes more evidence is required to prove the existence of 
the Indian Ocean crater.

New Zealand-based tsunami expert Dr Mauri McSaveney of GNS Science 
says there is pretty good evidence that there are more large craters 
on our planet than mainstream scientists think there should be.

While he says Prof Bryant's claims are "perfectly plausible" and the 
best available evidence suggests the New Zealand crater is one from 
the Holocene period, this could still be wrong.

"He has yet to convince me and a lot of others," Dr McSaveney said.

But as Prof Arculus says, Prof Bryant is fighting against a tradition 
in earth sciences that suggests everything we see around us is the 
product of slow processes rather than sudden catastrophic events.

"Geologists are naturally anti-catastrophe," he said.

"We're inclined to be conservative."