Why freshwater life survived the aste...

Why freshwater life survived the asteroid impact that wiped out most dinosaurs

There are 1 comment on the ITworld story from Jul 17, 2013, titled Why freshwater life survived the asteroid impact that wiped out most dinosaurs. In it, ITworld reports that:

Nearly 66 million years ago, an asteroid measuring more than 6 miles in diameter crashed into the Earth near the Yucatan Peninsula in Mexico.

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“Geologist [I'm Climate Change]”

Since: Mar 07

formerly Nuneaton

#1 Jul 17, 2013

While looking over data via search for Palaeo Tsunami in Central America I encountered a lot of papers on the K-T boundary impact & had to modify my ideas on it a bit.

It appears that Chixculub crater is smaller than I had previously believed & that the prior diameter is an inflow & uplift ring. The currently listed diameter is 180Km diameter and is surrounded by carbonate rich ejecta with lots of shocked quartz and glassy spherules in decreasing thickness with distance.@ gret distance the ejecta becomes a layer a few cm thick with again the shocked quartz @ the platinum group element anomaly. It is likely that the impactor was between 10 & 18Km diameter which is relatively small as asteroids go, although one listed article on Topix indicates that it may have been a comet.

Now the fun bit

The bulk of glassy spherules were trace element analysed & appear to correspond to mid to deep levels of continental crust (schist, gneiss & tonalite plutons of the craton fragment that was the Yucatan.
One location (Haiti) was analysed & found to be from a basaltic source not found in deep crustal rocks as the source would have been several orders of magnitude bigger than basaltic intrusions in typical continental crust (would be like the "great dike of SE Africa & easily visible in gravity & magnetics,

Now the real fun bit.

A region of glassy spherules exists in N. Mexico which appear to have been deposited 300,000 years earlier than the K-T boundary event with subsequent Cretaceous deposition & a real K-T boundary clay on top. The glassy spherules look almost identical to the Haitian examples & are also impact spherules.
A look over Gorgona island (Colombian forearc) features mid Cretaceous komatiites and picrites & also has been labelled as K-T age as a result of a tectonic bounding of all the rock formations and breccia + glass spherules & an early tertiary age carbonate rich sediment cap to the formation.

Another paper also lists a fallout of glassy spherules of continental provenance & a palaeo tsunami in the Danian of the earliest tertiary a few hundred Ka after the KT boundary impact. A hiatus in species diversification was also listed worldwide in the Danian after the K-T boundary.

It appears that there were 3 major impacts @ the time of the K-T boundary.

Gorgona (now a crater rim obducted onto the forearc of Colombia), which hit a basaltic oceanic large igneous province, producing a crater at least as big as Chixculub with a major basaltic tektite fall but no platinum group anomaly. The likely source would have been the outer chunk of a large differentiated asteriod such as 4 Vesta which looks a bit like a conker (USA.Buckeye) after being shock antipodes spalled by a big impact on one side.

Chixculub which hit a continent covered by very shallow water and produced a large crater & a mass extinction & platinum group element anomaly.

An unnamed mid size crater which produced a tsunami deposit and spherule fallout in Brazil in the early Danian (probably a crater of similar size to Popigai) either on the S. American or W. African margin covered by water deep enough to create a tsunami, and large enough to cause a mid size extinction.

The interesting bits were the fact that major extinction events appear to be the result of a big impact on a continent. The mass extinction being the result of a dust pall in the upper atmosphere.
Oceanic impacts (which produce a local steam bath) appear to be able to rain out the dust that would block photosynthesis very quickly. It seems that my math error involving asteroid size in ocean being too small to affect life has still got its size limit too small (possibly by a factor of 10 or more). The crater size able to produce a mass extinction in an ocean may need to be over 350Km diameter (even that size may be a mite too small).

Have a nice day: Ag

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