Mega-tsunamis more common than we think
Enormous comets may have often bombarded our oceans causing tsunamis that dwarf ones seen today, says a small group of scientists.
But most critics are yet to be convinced there's evidence to back claims about such recent, frequent mega-impacts.
Conventional wisdom has it that the Earth suffers 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.
And 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.
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," says Bryant, who is associate dean of science at the university.
Evidence from Google Earth
Bryant used satellite images from Google Earth to identify inland dunes in the shape of arrowheads that he says are signs of mega-tsunamis.
The tsunamis would have displaced marine deposits containing marine fossils, he says, dumping them inland as 'chevron' dunes.
"We've found that chevrons are everywhere, everywhere around the world's coasts," he says.
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.
Bryant says 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 6 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, says Bryant.
Dating of sediments to the north of the craters suggests the impact happened 1500 years ago, he says, and the well-preserved chevrons also date to around the same time.
Indian Ocean crater
Bryant says chevrons about 4800 years old around the Indian Ocean are associated with a 29 kilometre wide impact crater located thousands of kilometres to the southeast of Madagascar.
"There are chevrons around the Indian Ocean that all point back to this one crater site," he says.
And 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 4000 years referring to an event that could have been the impact.
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 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
Earth scientist Professor Richard Arculus of the Australian National University says he accepts 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 not yet convinced.
"They're heading in the right direction," he says, but 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 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," says McSaveney.
But as Arculus says, 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 says. "We're inclined to be conservative."