Re: extinction (long)

[Tommy Tyrberg <tommy.tyrberg@norrkoping.mail.telia.com>]

First a few thoughts about ?world-wide wildfires?. World-wide does not mean 
?the whole world?. All studies of ejecta distribution indicate that the 
high-energy debris would be very unevenly distributed. There would under 
almost any circumstances be a heavy concentration antipodal to Chicxulub 
(in India or thereabouts) but otherwise the distribution would depend 
strongly on the direction of the impact and the rotation of the Earth. It 
has even been suggested that the distribution of wildfires might be useful 
for determining the direction of the impactor.

Also studies on the effects of hydrogen bombs (fortunately mostly 
theoretical) show that the presence or absence of cloud-cover, the wetness 
of the vegetation and the presence of snow-cover greatly influences the 
energy required for actual ignition.

It should also be noted that sites relatively near the impact site (such as 
Colorado) could only have been hit by relatively low-energy debris (an 
object moving at 3 kms-1 has less than 15% of the energy of one moving at 8 
kms-1). This is because only objects moving in strongly curved paths can 
hit close to the primary impact site.
Admittedly high-energy objects that had completed one or more orbits might 
impact practically anywhere, but by this time (at least a couple of hours 
after the impact), the flux would have been to low for ignition. Any pieces 
moving at more than 11 kms-1 would of course be ejected into orbits around 
the sun (actually closer to 12 kms-1 in some cases due to the rotation of 
the Earth). Many of these, plus those ejected into more-or-less stable 
orbits around the Earth will ultimately fall back. There must have been 
lots of shooting-stars for a LONG time after Chicxulub.

It is not at all clear (and indeed unlikely) that soot from even very large 
wildfires would be distributed world-wide, in contrast to particulate 
matter from the impact. The latter was carried to the top of the atmosphere 
entrained in the rising fireball (which was large enough to ?punch through? 
the atmosphere) or ballistically distributed into the top of the 
atmosphere. The larger pieces would come down fast while the fine material 
would remain in the stratosphere for a long time and be fairly evenly 
distributed across the world by high-altitude winds.
The soot on the other hand is produced at ground level and carried up into 
the atmosphere by the convection produced by the heat of the fire. This 
means that it will under normal circumstances stay within the troposphere 
and be washed out by precipitation within a fairly short period. It will 
also mostly remain in the hemisphere where it was formed, since there is 
relatively little tropospheric exchange across the tropical zone. This is 
supported by experience. Volcanic debris from explosive eruptions, which is 
lofted to the stratosphere, spreads world-wide. Examples: Pinatubo, 
Krakatoa, Tamboro. Smoke, even from very large fires, does not. Examples: 
very large forest-fires in Indonesia a few years ago, oil-fires in Kuwait 
1991, large-scale fire-raids in 1943-45. It has been suggested that 
fire-storms might cause strong enough convection to carry smoke into the 
stratosphere (?nuclear winter?), but I have never heard of any case where 
this has actually happened.
Gaseous products of the fires (e. g. CO2, CO, NOx and a large range of 
hydrocarbons many of which would be highly toxic and/or mutagenic) would 
remain in the atmosphere for a longer time, and these might well have a 
world-wide effect.

It is true that flood basalt eruptions are ?quiet?, but that does not mean 
that they do not have large-scale environmental effects. The Lakagigar 
eruption of 1783 (the only historical (small) flood basalt eruption) had 
strong effects on weather in the northern hemisphere, presumably through 
sulphuric acid emissions. The strange haziness and cold weather was noted 
by Benjamin Franklin who was then living in France and even correctly 
attributed by him to the eruptions on Iceland, though he thought that it 
was Mount Hekla that was erupting. The local effects on Iceland were 
devastating, mostly because of a bluish sulfurous haze that covered most or 
all of Iceland during the summer of 1783 stunting vegetation and poisoning 
cattle. The resulting famine is remembered as móduhardhindin ?the haze 
famine? and killed an estimated third of the population. It has even been 
suggested that the bad harvests caused by the Lakagigar eruption in Europe 
may have been a contributing factor in causing the French revolution. 
Incidentally it is thought that the cattle were mostly killed by fluorine 
poisoning, so it would be interesting to know whether there is a world-wide 
?fluorine peak? in 1783. Unfortunately it is difficult to think of two 
elements with more different chemical characteristics than Ir and F.
In this context it is interesting to note that since dinosaurs (and many 
other organisms) occur in the intertrappan deposits in India at least the 
early stages of the Deccan volcanism were survivable, even on the Greater 
India plate itself. Since this was situated on top of the Reunion hotspot 
at the time, extinction and re-colonization (all within the late 
Maastrichtian) is unlikely. It would take a pretty remarkable ramp-up of 
the volcanism after the deposition of the inter-trappan beds to cause mass 
extinction even in the antipodes. Is there any evidence of this?

As for the discussion of the Chicxulub drilling that started this thread I 
fail to see how a single borehole would make it possible to decide how 
large the crater is. Actually the YAX-1 hole is 60-70 km from the center of 
the crater and so would be well OUTSIDE the crater rim if Chicxulub was 
only 100 km in diameter.
The melt sheet may be thinner than expected (IF the boring really went 
completely through the melt sheet and the (mostly) unmelted rock 
encountered in the lower part is not just megablocks from the collapsing 
crater wall), but then how many melt sheets have been drilled through 
before? Zero. So the expected thickness is entirely based on theoretical 
models. How many times have these been validated previously by comparison 
with an actual large crater? Also zero. Further the cenote ring is 
definitely there and it has a diameter of some 200 km (have a look at 
http://www.geotimes.org/apr03/NN_chicx.html). Is there any other case known 
where an impact crater manifests itself in a precisely circular structure 
at twice the diameter of the crater itself?

Actually the figure depends on how one defines the crater diameter. If one 
uses the outer limit of the deep crater structure (ring faults) then 
Chicxulub is probably 160-170 km across. This is probably the best measure 
when comparing it with deeply eroded terran craters such as Sudbury or 
Vredevort. If it is measured across the crater rim the diameter is probably 
200-220 km. This is more relevant when comparing it with lunar and Martian 
craters, which we see from above and which are usually fairly well 
preserved. In either case it?s a goddam big crater. There are very few 
larger ones of post-Late Heavy Bombardment age on either the Moon or Mars.

A few thoughts on environmental effects of a large impact that have not 
been much noted:

If the total amount of vaporized seawater was 10^12 tons (the  figure of 
3-7 x 10^11 tons seems definitely on the low side) this would be equivalent 
to about 4 mm of precipitation if spread evenly over the northern 
hemisphere. Certainly a very significant quantity but not catastrophic by 
itself. However much of the seawater vaporized by the impact would have 
been carried into the stratosphere together with massive amounts (megatons) 
of chlorine (also iodine, bromine &c). This must have had drastic effects 
on the ozone layer. Incidentally, since most of the chlorine in seawater is 
chloride of sodium, the skies at dawn and dusk would probably have been 
yellow for quite a while after the impact. Note that vaporization would 
also have gone on for a while after the impact. Once the fireball had 
dispersed seawater would pour into the crater to be boiled away until the 
temperature of the interior had been quenched to below 100 centigrade. This 
water vapour would stay in the troposphere though, and rain out fairly 
quickly.

The tsunamis from the impact would probably have been high enough to break 
at the edge of the continental shelf, at least in the Caribbean and parts 
of the Atlantic. This would have greatly reduced the effects onshore, 
except where the shelves were narrow, but on the other hand it would have 
violently disturbed vast amounts of sediment that would normally never be 
affected by wave action. Large-scale shelf collapse and turbidity-currents 
are likely and a major ?belch? of methane from methane hydrates is quite 
possible. The magnitude 10-13 earthquake would have had similar effects, 
particularly in the Caribbean area.

Finally I recommend visiting:

http://www.lpi.usra.edu/meetings/lpsc2003/lpsc2003.download.html

which has a lot of interesting information on the results of the YAX-1 
borehole, written by impact researchers with no paleontological axes to grind.


Tommy Tyrberg