Starting from the Cambrian (540 mln years ago), our planet survived five large-scale catastrophes that led to mass extinction of living organisms. These catastrophes took place in the late Ordovician (438 mln years), Devonian (370 mln years), Permian (251 mln years), Triassic (200 mln years) and Cretaceous (65 mln years). In the last cataclysm almost 75 percent of all terrestrial and marine organisms died, including dinosaurs* that had appeared on the Earth 230 mln years ago and lived during subsequent 170 mln years, i.e. in the Mesozoic era. Why did they die out? One more attempt to answer this question was made in the article by Andrei Grachev, Dr. Sc. (Geol. & Min.), chief research assistant of the RAS Institute of Physics of the Earth named after O. Schmidt, published in Zemlya i Vselennaya (The Earth and Universe) magazine.
In 1980, American physicist Luis Alvarez (Nobel Prize Winner, 1968) recommended his son Walter, a qualified geologist, to apply a newly developed neutron activation method to study elements of the platinum group discovered in the clays of Gubbio Slit in Italy. It has turned out that in the 2 cm thick layer on the edge of Cretaceous and Palaeogene deposits, the concentration of iridium is much higher than it was recorded for the terrestrial rocks before. Scientists have established that this phenomenon is typical of meteorites; on the basis of this discovery the Alvarez family concluded that the mass extinction of biota and the fall of a big cosmic body (asteroid) were interrelated. This hypothesis was also supported by discovery of the meteorite crater Chikhulub, the largest on the Earth, on Yucatan Peninsula (Mexico) that had appeared 65 mln years ago, i.e., coincides with the time
* See: Yu. Avsyuk et. al., "Did Dinosaurs Die Out Suddenly?", Science in Russia, No. 3, 2002; V. Alifanov, A. Averyanov, "Time of Dinosaurs", Science in Russia, No. 5, 2003.–Ed.
Dinosaur eggs found (a) in the deposits of the boundary period between the Cretaceous and Palaeogene, ragments of egg shells (b). Guangdong, China, 2002.
boundary between the Cretaceous and Paleogenic systems. Moreover, in a number of sections of deposits there was found quartz with signs of impact action.
Studies carried out after the publication of these works showed: abnormal concentration of iridium in the transition layer was fixed in almost all sections under consideration, located both on the continents and in deep-water wells. The problem of mass extinction of biota at the turn of the Cretaceous and Palaeogene seemed to be resolved, but some scientists doubted the correctness of the proposed paradigm and were committed to the idea of the magmatic origin of the transition layer. In other words, they connected its geochemical peculiarities with volcanic activities.
Meanwhile, it was established that iridium concentration abnormalities were frequent in the layers above and below the transition layer; moreover, they were discovered in isolated rocks. Thus, abnormality of this element could not be classified as a geochemical indicator of mass extinction of biota. In addition, in 1983, in the course of research works on degassing of volcanoes on the Hawaiian Islands, American geologist U. Zoller and his colleagues established: iridium gets into the atmosphere together with fluorine in the form of hexaftoride and then is spread by a volcanic aerosol.
In the late 1980s, scientists made two important discoveries. First, they established that dinosaurs lived in polar areas. In 2004, the geologist Eric Buffeto from the National Center of Scientific Research of France, on the basis of traces of animals discovered in the Cretaceous deposits at Alaska, made the following conclusion: climatic changes, namely cooling, could not be the reason of their dying, as most scientists believed (according to paleographic reconstructions, at that time Alaska was located to the north of its present position). Secondly, in the same year Chinese scientists from the Institute of Paleontology under the Academy of Sciences in Beijing headed by Zhao Dji-Ku made another important discovery: in the south of China, in the province of Guangdong, researchers found some layings of eggs of dinosaurs in river deposits. The age of the rocks corresponded to the time boundary between the Cretaceous and Palaeogene.
The thorough examination of the shell of these eggs upwards along the section revealed abnormal changes of its thickness and pathological degeneration of micro-structure. It was accompanied by sharp changes in the concentration of such chemical elements as arsenic, lead, zinc, nickel and iridium. All these elements were consumed by dinosaurs with food, water and air, which prevented normal embryonal development of giant reptiles.
It should be pointed out that the said changes were gradual and took about 250 thous. years. Thus, extinction of dinosaurs was not instant, which contradicts the aforesaid asteroid hypothesis.
In 2004-2008, Russian scientists Andrei Grachev, Dr. Sc. (Geol. & Min.), Vladimir Tselmovich, Cand. Sc. (Phys. & Math.) (RAS Institute of Physics of the Earth named after O. Schmidt) and Oleg Korchagin, Cand. Sc. (Geol. & Min.) (RAS Institute of Geology), in cooperation with the Austrian paleontologist Heintz Hollman from the Museum of Natural History in Vienna studied a geological section located in the National Park Eizen Eisenurerzen near the village of Gams in Eastern Alps, where the local river opened in several places the transitional layer between the Cretaceous and Palaeogene. The results of a complex analysis showed a distinct vertical zonality of the transitional layer (it is about 2 cm thick), which made it possible to divide it into 2 parts. The lower part is characterized by high the concentrations of arsenic, zinc, copper, lead, chrome, iridium, cobalt and nickel, while in the upper part concentrations of these elements are much lower. It is very important to point out the high concentrations of arsenic, reaching 60g/t, which is 12 times higher than the maximum permissible norm for living organisms (they simply die). This fact cannot be explained by the asteroid theory as the concentration of arsenic in meteorites does not exceed 2g/t.
Andrei Grachev and his team-workers identified four abnormalities of iridium: the first one was identified on the border line in the transitional layer; another two
Native metals from Hams Section: a-silver (upper left-silver crystal, below-massive formations of silver), gold and copper (to the right) found in volcanic aerosols (Hawaii) and in the basalts of mantle plumes (Greenland, Eastern Siberia); b-platinum (at the top-dendritic grain, below-rounded grains of zircon covered with a thin platinum film); c-rhenium-a result of degassing of alloys getting to the surface from the lower mantle (pure rhenium is absent in the earth crust).
abnormalities–below, and one–above. According to the results of mineralogical analysis the lower part of Hams section is characterized by presence of native metals (gold, silver, copper, platinum, rhenium), sulfides (pyrite, arsenic pyrite, molybdenite, etc.) and sulphates (baryte). It is the first place on the Earth where pure platinum and native rhenium were discovered, while such paragenesis of minerals is typical of modern hydrothermal systems. Among oxides there was also found titano-magnetite, similar in its composition with its analogs found in basalts. In the middle part of the transition layer these minerals are not represented.
In the course of a thorough examination of the upper part of the transition layer scientists found balls of pure nickel and diamonds, including minerals of hexagonal modification–lonsdaleite, typical mineral of iron meteorites. In addition, there were also found such minerals as awaruite and muassonite.
Studies of benthic organisms living in the ground and at the bottom of reservoirs and planktonic foraminifera (i.e. unicellular protozoa, "wearing" lime shells) carried out by Oleg Korchagin showed: they are not represented in the middle part of the transitional layer but reappear in the upper part. Such "disappearance" of foraminifera is explained by an abrupt change of the habitat caused by toxic metals absorbed by sea water. By the way, according to the results of analysis of marine deposits on the edge of the Cretaceous and Palaeogene carried out in other regions of the world, abnormal concentration of arsenic is typical of the first millimeters of the transition layer. For example, concentration of arsenic in the Caravaca Section (Spain) is 400 times higher than the maximum permissible norm.
Finally, the author of the article noted that abnormal content of toxic metals in marine deposits in the transition layer between the Cretaceous and Palaeogene is a global phenomenon. The same fact was established in the
Nickel and diamonds found in Hams Section: a, b-spherules of pure nickel absent in the crust; c-hexagonal modification diamond typical of iron meteorites.
course of studies of egg shells and bones of dinosaurs. Consequently, ingress of these metals to marine and fresh water basins could be explained by an effect of volcanic aerosol (its effect on the environment is well studied on the example of historic volcanic explosions).
Another important fact established during research works in the Hams Section was that scientists identified two phases in the development of the transition period between the Cretaceous and Palaeogene. During the first phase (that lasted about 1,500 years, according to the conservative estimation of deposition speed), the layer was formed under an effect of volcanic aerosol which is considered to be the source of native metals, sulfides and high concentrations of toxic metals. It also explains abnormalities of iridium and other elements in the bottom part of the transition layer.
The second deposition phase was affected by a fall of an asteroid (meteorite) that left its traces in the form of balls of pure nickel, awaruite and diamond crystals.
Conclusions made by Russian scientists stand in a marked contrast to the data of other researchers, who are studying the transition layer between the Cretaceous and Palaeogene and enable us to find out what was the reason of extinction of living organisms about 65 mln years ago. No doubt, we should not oppose: the volcan-ism to the fall of an asteroid: both events are a reality, but changes in the biota as well as abnormalities of iridium are referred only to volcanism as the fall of a cosmic body took place later.
In conclusion we would like to add: the results of these research works would discussed at the International Simposium on the transition period between the Cretaceous and Palaeogene held in Austria in 2009 and published in the monograph printed by the Geological Service of Austria.
A. Grachev, "The Mystery of Extinction of Dinosaurs", "Zemlya i Vselennaya", No. 3, 2010
Prepared by Sergei MAKAROV
Новые публикации: |
Популярные у читателей: |
Новинки из других стран: |
Контакты редакции | |
О проекте · Новости · Реклама |
Цифровая библиотека Украины © Все права защищены
2009-2024, ELIBRARY.COM.UA - составная часть международной библиотечной сети Либмонстр (открыть карту) Сохраняя наследие Украины |