[X] My Assistant

[Guest_BruceMoomaw_*]

...in Antarctica. This was the biggest mass extinction event in the history of multicellular life -- it wiped out something like 95% of all existing marine species as opposed to only about 50% for the impact that ended the dinosaurs -- and it has been one of the biggest remaining geological mystery stories. An even bigger giant impact has always been a serious suspect -- but because no crater could be found, other suspects have also been high on the list, ranging from the titanic flood volcanic outburst of the "Siberian Traps" region to a huge buildup of CO2 in the deep sea that was then suddenly released due to geological events (a planet-wide version of the Lake Nyos CO2 eruption that smothered hundreds of people in a matter of seconds in Cameroon). Indeed, there has been a recent comprehensive book on the mystery which ended by concluding that the mystery is still utterly unsolved.

Well, we seem finally to have found a crater that is both gigantic enough, and of just the right age, to be the culprit -- in the only place on Earth it could have hidden for so long: beneath the Antarctic ice layer.

http://www.space.com/scienceastronomy/060601_big_crater.html

It isn't quite settled yet, but this looks like an impact crater fully 500 km wide -- more than twice as wide as Chiczulub -- and thus quite big enough to have done the job, if it really IS an impact crater.

[NMRguy]

What evidence has been provided that would suggest that this large round-shaped structure should lie at the Permian-Triassic (P-T) boundary? Granted, the P-T extinction was a momentous event in the history of life on this planet. But has any evidence been cited other than a giant ringed structure that resembles a crater has been found under half a mile of Antarctic ice?

More specifically, has any sediment been found anywhere in the strata of the Permian-Triassic boundary (about 251 million years ago) that would suggest extraterrestrial activity? My understanding is no. Antarctica shared borders with Australia, India, and southern Africa in the Permian time, and these lands are not buried under thick layers of polar ice. For a collision of this magnitude, one might expect shocked quartz or perhaps rare earth metals (eg. iridium) to be at least as common as that of the Cretaceous-Tertiary boundary. I just haven’t seen that evidence show up in the rocks.

[Guest_BruceMoomaw_*]

The lack of iridium might be explained if the impactor was a comet nucleus rather than an asteroid, but I'll admit that the lack of shocked quartz is suspicious. Obviously we'll have to wait and see more.

[tty]

There is some limited evidence of an impact layer at the P/Tr boundary. The best evidence is actually from Antarctica (Graphite Peak), see e. g.

http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2321.pdf

http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1875.pdf

tty

[Guest_Richard Trigaux_*]

There was already the Bedout crater associated with the permian extinction. It is as big as Chixculub, and, depending on geochemical/biological/ecological conditions, it could have caused much more extinctions than the Chixculub, for instance if, in the epoch, there was less biodiversity.

The newly discovered circular structure in the Antarctic is a good candidate too, provided there will be drills into it to really assess its meteoritic origin.

By the way, 500kms large craters are rare into the solar system, and they all are ancient (3.3 -3.7 billions years for the Moon. I strongly suspect that the bodies which created them belong to a special class of large meteorites, which resulted from some event in the solar sytem into this time. Most of large craters are caused by comets, which seem to have a sharp size limit.

We also are lucky to still see it. since 250 millions years, all the oceanic crust was absorbed into Earth, that makes about only 30% of the surface of that epoch which is still observable.

[NMRguy]

My position was not to be overly pessimistic, but I find it misleading when the headline of a news story is "Antarctic crater linked to big die-off". Blame it on media sensationalism, I guess. But these scientists now have a great hypothesis, and they must now go dig up evidence (literally) to prove their position. Greenland and Antarctica are areas of the Earth that still have secrets locked away under all of that ice.

[ljk4-1]

By the way, 500kms large craters are rare into the solar system, and they all are ancient (3.3 -3.7 billions years for the Moon. I strongly suspect that the bodies which created them belong to a special class of large meteorites, which resulted from some event in the solar sytem into this time. Most of large craters are caused by comets, which seem to have a sharp size limit.

Maybe the body that made this crater - assuming all this is true - was one of
the last of the great planetesimals that first formed Earth? It just got delayed
for whatever reason. But ouch for the creatures living then!

Or maybe it was a protoplanet offset by the Jovian worlds migrating?

[tty]

Please note that there is absolutely no evidence linking this (probable) crater to the P/Tr boundary. In my opinion it may well be too large to explain the P/Tr extinction. An impact of that size would probably kill almost everything except some taxa living at great depth in the oceans or deep cave systems.

tty

[Guest_Richard Trigaux_*]

My position was not to be overly pessimistic, but I find it misleading when the headline of a news story is "Antarctic crater linked to big die-off". Blame it on media sensationalism, I guess. But these scientists now have a great hypothesis, and they must now go dig up evidence (literally) to prove their position. Greenland and Antarctica are areas of the Earth that still have secrets locked away under all of that ice.

I somewhat agree. If we look at the images in the article, we' see that the structure is VERY ROUGHLY circular. Many other circles of similar diametre also appear on the gravimetric map, as the poor resolution create a blur of this size around any punctual object. So it is a bit fast to claim that it is the big new hypothesis. With such a aproximate identification, the Hudson bay in Canada, or the northern plain of Italy, would be candidates too.

[dvandorn]

...With such a aproximate identification, the Hudson bay in Canada, or the northern plain of Italy, would be candidates too.

I thought shocked rock beds had been found radial to Hudson Bay, indicating that it is, indeed, an ancient impact crater.

Is my memory failing me again?

-the other Doug

[edstrick]

Hudson's bay is an "epicontinental sea", a cold version of the many shallow seas that have flooded over parts of continents during much of earth's history. Not an impact.

[blobrana]

Hum,
i too seem to recall that it was once speculated that the Bay was formed by an impact.
However it may be that they hadn't found the Chesapeake bay impact structure.

(35 million years ago, a meteorite smashed a 83 kilometre hole in Earth's surface.)

But i agree that it is just speculation as to what the gravity anomaly is. If it is an impact crater then it could date from 545 million years ago, (an upper limit of 100 million years is dictated by a tectonic fault line that cuts through the feature). Personally i think it`s an old caldera.

BTW Another intriguing `crater` worth looking at in Canada (with say google earth) is at
Position: Latitude 76.628177° Longitude -109.037557°

[Guest_BruceMoomaw_*]

I believe the references are not to Hudson Bay as a whole, but to that uncannily neat semicircular bite out of its southwestern shore -- which really does look, at least to the untrained eye, like a possible impact crater.

[Bob Shaw]

I believe the references are not to Hudson Bay as a whole, but to that uncannily neat semicircular bite out of its southwestern shore -- which really does look, at least to the untrained eye, like a possible impact crater.

Bruce:

Yup. Canada is well-served by the Crater Gods! The Laurentian Shield has some very ancient craters on it, too - and Sudbury's metal ores are assumed to be associated with an impact. I suspect, though, that gravimetric surveys are of less value there than in many other parts of the world, due to the large and homogenous lump of continent which forms the landscape.

Bob Shaw

[tty]

Bruce:

Yup. Canada is well-served by the Crater Gods! The Laurentian Shield has some very ancient craters on it, too - and Sudbury's metal ores are assumed to be associated with an impact. I suspect, though, that gravimetric surveys are of less value there than in many other parts of the world, due to the large and homogenous lump of continent which forms the landscape.

Bob Shaw

I once happened to fly across Ungava in spring when the snow had melted but the lakes were still ice-covered and noted at least four perfectly circular lakes that looked mighty suspicious crater-wise.

Incidentally the Sudbury crater is particularly interesting since it is a counter-example to the often repeated "fact" that a meteor impact cannot cause volcanism.


tty