II
THE VARIOUS THEORIES
The mass cause
extinction at K/T limit gave rise to many hypotheses. Certains are rather
whimsical, the other serious ones, like the marine regressions and climatic
deteriorations which resulted from this (Ginsburg 1964). Many (except the volcanic
assumption, Courtillot 1987) are nevertheless very speculative: no convincing
fact accredits the supernova, star or unknown planet thesis, the earth meeting
with galaxy plan clouds, a viral epidemic, the groups senescence, etc... The in
vogue impact theory of a meteorite with the earth (Alvarez 1980) received the
support of many arguments (iridium traces, shocked quartz spherules and grains,
spinels, Chicxulub crater).
A certain number
of arguments which it proposes can be interpreted, with much relevance, by the
volcanic origin concurrent assumption. Other arguments go against this
assumption:
1) recent data
suggests that the the crater impact, close to Chixculub, would be former to
border K/T
2) consecutive
siliceous deposits to a " tsunami " generated by the impact are
long-term and noninstantaneous deposits
3) the impact
event, marked by the Iridium anomaly, is above the siliceous deposits
4) all collected
data converge towards a causes plurality (Gerta Keller 1996; MacLeod 1999).
In any event,
all these arguments can establish only the meteoric impact potential
possibility. The causality report with the mass extinction at the K/T limit
remains purely hypothetical. Moreover, to allot to a possible local meteoric
impact and its catastrophic consequences on the whole planet, the mass
extinction at the K/T limit, seems, in the knowledge current state, a very
hazardous generalization. Causal plurality, dates and durations problems and,
finally the extinctions selectivity, are major difficulties that the
impact model does not solve.
A contrario, the probabilistic model proposes a synthetic explanatory model which
answers all these impact model difficulties.
Any mass
extinctions explanatory model at the K/T limit must be, in addition, able to
answer the various arguments which we will expose hereafter.
The mass
extinction selectivity at the K/T limit is, we must repeat it, fundamental. The animal
species majority for which the skeleton requires calcium significant
requirements disappear. Almost no higher weight terrestrial vertebrate than
20 kg crosses the final Cretaceous (Reichholf 1993). Let us point out the
skeleton importance in the large animals (20 % of the weight) (Dejours 1994).
The Birds with reptilian characteristics, appeared to the superior Jurassic,
die out with the final Cretaceous whereas take to their rise the modern birds,
without teeth, in the Eocene..
Paleobiogeographic
arguments: the fossiliferous sites examination of a terrestrial reptiles group
which dominated Mesozoic, the Dinosaurs, brought a probabilistic model
unambiguous confirmation. It was noted :
1) The Dinosaurs
fossils emergence always occurs after a transgression i.e. a marine
sedimentation (Furon 1959) which founds the probabilistic calcium trophic
chain.
2) A contrario,
without preliminary transgression, one does not find Dinosaurs fossils. The
direct link between the Dinosaurs appearance and radiation probability and a
calcic medium presence thus firmly appears established.
3) The mass
extinctions at the K/T limit intervene, generally, following the groups
explosive radiation which disappear. Only the evolution probabilistic model
interprets these two facts orders by a same factor: a trophic chain
introduction and favorable calcium biominéralisation probabilistic factors then
their disturbance, whereas the models majority take into account only the mass
extinctions to the finish-Cretaceous.
4) The trophic
chain and the probabilistic calcium biomineralization factors establish a link
between the acme then the "simultaneous" disappearance of very
distant animal groups, Vertebrates or Invertebrates, by a common denominator
possession: a significant calcic metabolism.
5) Stratigraphy
attests sedimentation change at the K/T limit. An argillaceous layer "
separates, in the majority of the cuts, the Cretaceous of the Tertiary sector
and, in a general way... all the areas where the passage terms between the two
systems could be highlighted " (De Bonis 1991). In Gubbio, this layer contains
50 % of clay, whereas the layers majority lower than K/T limit contains
approximately 95 % calcium carbonate and 5 % of clay (Alvarez, Asaro 1993). In
addition, at the ocean floor, the organic origin calcareous sedimentation
coming from micro-organisms skeleton accumulation stops at the Maestrichtian
end. The carbonates concentration, at the K/T limit , to the sedimentary sites
of El Kef (Tunisia), Caravaca (Spain) and Hole 761 C (off Australia), testifies
clearly to the reduction, on this level, of the calcium carbonate
concentration: El Kef (from approximately 50 to 5 %), Caravaca (from
approximately 70 to 20 %), Hole 761 C (from approximately 90 ŕ.70.%) (Robin,
Rocchia 1993).
Arguments a
contrario:
1) The
Foraminifera genera majority disappear with the finish-Cretaceous (Orbitoids,
Orbitolinids). They have a significant calcic metabolism; their thick and heavy
tests can reach 6 cm in diameter (Orbitolina concava). On the other hand,
Silicoflagellates appear with the final Cretaceous. The Diatoms and the
Radiolaria ones (93 % of survivors), with the siliceous skeleton, which thrive
in the abundant marine siliceous material facies, pullulate with the Cenozoic
one, where they constitute sediments considerable masses: diatomites and "
muds with radiolaria ". Dinoflagellates, with cellulose theca, also thrive
(78 % of survivors) and one often perceives K/T clays limit like a "
Dinoflagellates soup " (Smit Ian 1996). Among the Vertebrates ones,
Elasmobranchii, fishes with cartilageous internal skeleton, survive 67 %,
whereas Osteichthya, with osseous skeleton, disappear almost completely (4 % of
survivors).
2) The mass
extinction duration is incompatible with a short catastrophe (celestial body
impact, star or unknown planet, etc...). It corresponds to the various mass
extinctions, like the Dekkan volcanicity estimate duration, about 500.000
years, based mainly on the magnetic field opposite polarity (Courtillot 1987).
The trophic chain disturbance and the calcium biomineralization factors is a
progressive phenomenon which can occur only for one " geological "
length of time and which can engender as well many gradual extinctions
(Inoceramids and Rudists) as well as sudden ones (Coccolithophoridae and
Globigerina). This extinctions temporal diversity is well in phase with the
disturbing probabilistic biomineralization factors multiplicity.
3) Mesozoic,
often considered as the Reptiles era, can be considered, more precisely, like
the calcium era. The orogenetic activity and volcanicity apparition to
finished Cretaceous and the environment chemical and climatic variations which
it involves modify the sedimentary composition and break the trophic calcium
chain (phosphates, carbonates, etc...). Thus will disappear the Dinosaurs, as
well herbivorous as carnivorous. At the same time as the Dinosaurs, Plesiosaurs
and Pterosaurs disappear, just as many marine and terrestrial animals, many of
big sizes or adapted to a significant calcic metabolism. The biomineralization
factors disturbance also will reach the marine organisms majority with
calcareous skeleton. This specificity of the attack from organisms with calcium
skeleton and metabolism cannot be understood but within the framework of the
probabilistic model.
Next : III The probabilistic model