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The extiction of the dinosaurs from a chemist’s point ot view:
the abnormality of iridium at Gubbio, Italy and Woodside Creeck, New Zealand
Rossana Untaru
In the last 10 years, thanks to some books, films and cartoons, there has been a real explosion of ‘dinosaurmania’. People of all ages from all over the world have been affected. What exactly happened 135 million
years ago to make the dinosaurs vanish and devastate this planet?
I have tried, with my thorough research, to review the scientific literature regarding the event that has
triggered this mass extinction. The period is also know as the K-T boundary, and is located at the end of
Cretaceous period to Tertiary in the geological time scale as seen in table 1.
Table 1: Geological Time Scale
Eon
Era
Phanerozoic
Cenozoic
Mesozoic
Paleozoic
Period
Quaternary
Recent
Pleistocene
Abs.
Age
(Ma)
0.01
1.6
Tertiary
Pliocene
Miocene
Oligocene
Eocene
5.3
23.7
36.6
57.8
Paleocene
66
144
208
Cretaceous
Jurassic
Triassic
Permian
Carboniferou
s
Devonian
Silurian
Ordovician
Cambrian
Epoch
245
286
360
406
438
505
570
Age of
Events
Mammals
Ice age ends
Ice age begins
Humans
Scabland Floods
Columbia Basalts
Formation
Himalayas
of
Extinction of Dinosaurs
Reptiles
Flowering Plants
1st Birds and Mammals
1st Dinosaurs
1st land plants
1st fish
The K-T boundary is characterized by the second biggest worldwide extinction during which more than 80
% of terrestrial and marine species died. The 20% of survivors included salicaceous pelagic organisms
(diatoms, radiolarians), limnetic organisms, terrestrial higher plants, terrestrial mammals mainly small
carnivorous, insectivorous, and omnivorous vertebrates (Courtillot, 1999).
Many scientists around the world have spent several years of research trying to find answers to what
happened at the K-T boundary and their findings can be subdivided into two main theories that had split the
scientists involved. The two theories are the asteroid impact theory and the Deccan Traps worldwide
volcanism. The debate of K-T impact vs volcanism extinction began at the “Cretaceous-Tertiary
Environmental Change” meeting in Ottawa, Canada on 19 May 1981.The two theories debate the
extraterrestrial impact vs terrestrial volcanism with abrupt and short duration events vs gradual turnover with
iridium peak from extraterrestrial provenience vs volcanic origins, initial ‘nuclear winter’ type environment
followed by greenhouse warming vs greenhouse warming effects (leading to damage to reproductive system
of animals), and impact-induced acidification of marine waters vs carbon dioxide volcanic exhalation leading
to marine acidification. A summary of the effects of an asteroid impact can be seen in figure 1. On the other
hand the worldwide volcanism it would have had lasted for several hundreds of years with huge amounts of
carbon dioxide vaporised into the atmosphere leading to a severe, global greenhouse warming.
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Figure 1: Asteroid impact and its effect
Asteroid impact theory
The theory has been developed in late 1970s by the Alvarez team when Louis Alvarez suggested analysing
the amount of iridium content in a clay sample found by his sun Walter Alvarez. The samples have been
analysed at the Berkley Laboratories by using a novel technique – neutron activation analysis – and showed
iridium levels of 5 ppb. The results were published in 1980 in Science magazine "Extraterrestrial cause for
the Cretaceous-Tertiary extinction". Iridium is a metal that normally is found in very low concentrations in
Earth’s crust with the exception of few locations around the world such as New Zealand, Italy, and
Denmark. Iridium was found in a thin, clay layer deposited between Cretaceous and Triassic Period known
as the K-T boundary. The clay layer is quite unique as its texture is different from the strata below and above
it and contains the information of a mass extinction episode occurred about 67 million years (Ma) before
present (BP).
Iridium is a metal situated in group 9, period 6 of the Periodic Table of the elements. It can be found as a
free element mainly associated with platinum metals ore or with osmium. It is hard, brittle, and very resistant
to corrosion. Normally iridium is depleted in the Earth crust being stored in the Earth’s liquid core.
Table 2: Comparison between the concentration of iridium and other elements
Element
Universe
Sun
(ppb)
(ppb)
Meteorites
(ppb)
Crustal rock
(ppb)
Iridium
2
2
5.5x102
0.4
Platinum
5
9
103
37
Palladium
2
3
6.7x102
6.3
Iron
1.1x105
1.0x106
2.2x108
6.3x107
From the values in table 2 it can be seen that iridium is found in low concentration 0.4 ppb (parts per billion)
compared with another trace metal such as palladium in Earth’s crust but the concentration raises sharply in
extraterrestrial objects (meteorites, asteroids, comets). The sties around the world with a positive anomaly
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iridium concentration have been found at several marine and continental K-T boundary section ssuch as
Gubbio, Italy, Steves Klint - Denmark, at seven different sites in New Zealand (Hollis, 2003), Caravaca Spain, El Kef - Tunisia, Brazos River – US, and near Gubbio – Italy (Courtillot, 1999). According to
Heymann et al 1996, at Woodside Creek, New Zealand there is a sharp increase by a factor of 1500 in
iridium concentration just below the boundary layer whereas the decrease in the above layer is less sharp.
The highest peak found at Woodside Creek was of 197 ng/cm2.
The sample analysed by Alvarez team was from the pelagic sedimentary deposits near Gubbio, Italy and was
containing a band of brown clay between the Cretaceous limestone and the Tertiary red limestone.
Furthermore the Cretaceous limestone was found to contain large amounts of Forminifera globutrucana
crustacean fossils (carbonate-producing single-cell animals) but those fossils were absent from the clay layer,
and in the red limestone layer few crustacean fossils reappeared but are different from those of the upper
Cretaceous.
In 1981 Glen Penfield and Antonio Camargo, two petroleum geophysicists, published in “Sky & Teleskope”
magazine the finding of a large crater located at Chicxulub in the northwest Yucatan peninsula in Mexico, as
seen in figure 2.
Figure 2: Chicxulub Location – Yucatan Peninsula at the time of impact. In red other sites where there is an iridium anomaly
(Hollis, 2003)
From the geophysical data found at the impact basin Sharpton et al, 1996, suggested that the crater has a
diameter of less than 200 km and found evidence of impact breccia layer of 300 meters thickness of shocked
minerals shock metamorphism with planar deformations in quartz, feldspar, and zircons; fused mineral and
whole-mineral melts. In addition they have found as well high concentrations of iridium, rhenium, and
osmium suggesting extraterrestrial origins. Bellow this layer was found a unit of melt rock dated by 40Ar/39Ar
techniques as 65.07 ± 0.1 Ma old (Swisher et al 1992).
Bohor et al 1984 found the presence of extensive grain fracturing, shock mosaic, shifted crystal lattice, glass
lamellae and high pressure SiO polymorph as seen in figure 8. Shocked quartz forms as a result of the
application of a very high pressure of ~ 10 – 50 kPa. The grains have been found in Italy, Denmark, Spain,
north-central Pacific, and New Zealand.
Tektites and microtektites are other structure forming during an impact; they can be ejected over large
distances. Generally are low water and volatile contents, are black, silicate glass bodies similar to obsidian but
with a different chemical composition suggesting meteoritic contamination (Glass, 1990). They have been
found to be part of K-T boundary sediments.
The impact being aquatic had lead to the formation of tsunami up to 150 meters high disturbing the pelagic
sediments. At K-T boundary at Brazos River, Texas (Bourgeois et al, 1988), Beloc, Haiti (Carey et all, 1993) and
northeast of Mexico (Smit et all, 1992) have found coarse clastic deposits in contrast with the deep-water fine
deposits found bellow and above the coarse layer. Located in Baja California, Mexico it was recognized a
Pacific margin stratigraphic sequence. The dating with laser-heating 40Ar/39Ar biotite, hornblende, and
plagioclase, pumice lapilli tuff in the middle of the valley fill gave an age of 65.5 +/- 0.6. Also the findings of
shocked and liquefied sediments at the same site supported for catastrophic land sliding secondary to bolide
impact. (Busby, 2002 and Yip et al, 2002).
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Analysis by means of High Pressure Liquid Chromatography (HPLC) for fullerenes C60 and C70 that forms
during wildfires in K-T clays are consistent with the theory of wildfires developed post-impact (Heymann et
al 1996).
Deccan Trap Theory
Deccan Traps, located in west central India, are considered to be the largest volcanic provinces with an area
of almost 500,000 km2 covered by basaltic lava. In 1977 McLean started to look at the perturbation of
carbon cycle at the K-T boundary but only in 1979 that he began to associate those perturbation with the
Deccan traps. McLean proposed, in 1981, that the Deccan Traps mantle plume volcanism perturbed the
carbon cycle at K-T boundary that had as result the mass extinction and iridium release from volcanic fumes
(Courtillot, 1999).
The eruption started just before the K-T boundary and is thought to have lasted for about 200.000 to
500.000 years. The release of large amounts of volcanic ash, carbon dioxide and other volatiles during an
eruption forms acid rain, greenhouse effect, and ozone depletion. The volcanic theory included an initial
drop in the temperatures of 3-5° C due to the dust released during the eruption, followed by a long-term
drastic climate changes due to an increase in global temperature of about 5° C. Furthermore the oceans were
not able to dissolve such a great amount of carbon dioxide and as a result the photosynthesis has been
dramatically reduced killing the least adaptable marine organisms.
For the presence of iridium pikes the scientists sustaining the volcanic scenario had agreed that it was
accumulated from the material erupted. But recent measurements of volcanic emissions for Hawaiian
eruptions have shown too low iridium levels to can account for the high concentrations found at K-T
boundary samples (Finnegan, 1990).
On the other hand Abbott and Isley they had found that the Deccan volcanism started after the Chixculub
meteorite impact as a consequence (Abbot & Isley, 2002).
Recent mathematical models showed that the greenhouse warming effect from Deccan Traps volcanism was
of about 2°C and that is too weak to can be associated with the mass extinction occurred at K-T boundary
(Caldeira and Rampino 1990) Furthermore that increase can be regarded as the cause of extinction of
temperature-dependent sex-determination species (Rage, 1998).
Conclusion
Up to today it can’t be said that the scientists around the world have reached a final conclusion, hence the
debate continues more animated than ever. Every year there are several papers published around the world
with new evidence to back one or the other theory but still has to be found a conclusive proof that can be
considered acceptable by both parts.
References
Abbot, D.H. and Isley, A.E., Extraterrestrial influences on mantle plume activity, EARTH AND PLANETARY
SCIENCE LETTERS, 205 (1-2): 53-62 DEC 30 2002
Alvarez L. W., Alvarez W., Asaro F. and Michel H. V., Extraterrestrial causes for the Cretaceous-Tertiary extinction,
Science, 208, 1095-1108, 1980
Alvarez W., Asaro F., Michel H. V. and Alvarez L. W., Iridium anomaly approximately synchhronous with terminal
Eocene extinctions, Science, 216, 886-888, 1982.
Bohor, B.F., E.E. Foord, P.J. Modreski, and D.M. Triplehorn. 1984. Mineralogic Evidence for an impact event at the
Cretaceous-Tertiary boundary. Science 224: 867-869.
Bourgeois J. T., Hensen T. A., Wilberg. P. L. and Kauffman E. G., A tsunami deposit at the Cretaceous-Tertiary
boundary in Texas, Science, 241, 567-570, 1988
Busby, CJ; Yip, G; Blikra, L; Renne, PCoastal landsliding and catastrophic sedimentation triggered by CretaceousTertiary bolide impact: A Pacific margin example? GEOLOGY, 30 (8): 687-690 AUG 2002
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Lunar Planet. Sci. Conf. 251-252, 1993.
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38-40, 52
Finnegan et all, 1990, Iridium and other trace-metal enrichments from Hawaiian volcanoes; Sharpton and Ward eds,
Global Catastrophes in Earth History, Geological Society of America, Special Paper 247
Glass B. P., Tektites and microtektites: key facts and interferences, Tectonophysics, 171, 393-404, 1990.
Glass, B.P., and J. Wu. 1993. Coesite and shocked quartz discovered in the Australasian and North American
microtektite layers. Geology 21: 435-438.
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Bollettino della Comunità Scientifica in Australasia
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Heymann, D. 1996, Fullerenes of Possible Wildfire Origin in Creatceous-Tertiary Boundary Sediments; Sharpton and
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Creek, eastern Marlborough, New Zealand
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20, 99-103, 1992
Smit J., Roep T. B., Alvarez W., Claeys P. and Montanari A., Deposition of channel deposits near the CretaceousTertiary boundary in northeastern Mexico: Catastrophic or normal sedimentary deposits? and Is there evidence for
Cretaceous-Tertiary boundary-age deep-water deposits in the Caribbean and Gulf of Mexico? Comment, Geology , 22,
953-954, 1994
Swisher C. C. III, Grajales N.J. M. Montanari A., Margolis S.V., Claeys P., Alvarez W., Renne P, Cedillo P. E.,
Maurrasse F. J-M., R., Curtis G., Smit J. and McWilliams M., Coeval Ar-Ar ages of 65 millions years ago from
Chixculub crater melt-rock and Cretaceous-Tertiary boundary tektites, Science, 257, 954-958, 1992.
Yip, G; Blikra, L; Renne, Pacific coastal landsliding and catastrophic sedimentation triggered by Cretaceous-Tertiary
bolide impact: A Pacific margin example? GEOLOGY, 30 (8): 687-690 AUG 2002.
Rossana Untaru, BSc (Waikato) MSc (Waikato)
Email: [email protected]
Original manuscript in English
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