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Re: Giant Impact Near India -- Not Mexico -- May Have Doomed Dinosaurs



On Fri, Oct 16, 2009 at 01:06:31AM +0200, David Marjanovic scripsit:
>>  There's also the point that asteroid impacts that happen to occur
>>  around the time there's already major flood vulcanism going on seem
>>  to do more ecological damage and produce extensive mass extinctions.
>
> 1) The biggest Phanerozoic crater that is securely dated is the  
> Chicxulub crater... that's unfortunately a sample size of 1.

Granted.  Though there's the small point that there isn't anything
bigger that's really firmly attested as a crater; Bedout is about the
same size, but the crater-ness is still pretty much inferred if getting
more solidly inferred.

http://www.sciencemag.org/cgi/content/abstract/1093925

Next down is Manicouagan, at about 100 km diameter; since it's a cube
law for excavated volume, that's way smaller than half the impact size.

> 2) The main episode of flood basalt eruptions ended 100,000 years
> before  the K-Pg boundary; on a global scale, it had no impact except
> a transient rise in (CO2 and therefore) temperature.

100 kyears doesn't seem too long for there to have been a lingering
diversity loss from a CO2 and temperature spike, though.  No direct
extinction other than in regions over which the lava flowed, but it
doesn't seem implausible that the terrestrial ecosystems had not reached
peak diversity again before Chicxulub.

> BTW, while the marine P-Tr mass extinction was indeed a sight to behold,  
> the terrestrial one could have been less severe than the K-Pg one. It's  
> all a bit strange.

That one looks very much like something got to ocean chemistry directly,
doesn't it?

> Are any flood basalts dated to the D-C boundary (Hangenberg Event)?
> Because that's a mass extinction that looks much more severe now than
> it  did 10 years ago, and it's the first one for which an impact was
> seriously proposed as the cause (I think in the 1960s).

Don't know of any flood basalts dated to the D-C boundary.

There's the Siljansringen, Charlevoix, and Woodleigh craters all from
about that time, so maybe the correct principle is "multiple
catastrophic events in close succession", rather than needing both a
volcano and a rock.

-- Graydon