Iridium anomaly

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The iridium anomaly is the name given to the globally detectable increased concentration of the element iridium and other platinum metals in sedimentary rocks which were deposited 66 million years ago on the Cretaceous-Paleogene border .


The platinum metal iridium usually only occurs in very low concentrations of about 0.4  ppb in the earth's crust . In rocks of the Cretaceous-Paleogene border, most frequently in low-carbon claystones , iridium concentrations of 0.5 to 50 ppb have been observed worldwide. These sometimes enormously high concentrations, which clearly exceed the average value, were found not only for iridium but also for the other five platinum metals ruthenium , rhodium , palladium , platinum and osmium . Of these metals, the easiest way to detect iridium is through neutron activation . The increased concentrations of the other elements were only discovered when the term iridium anomaly was already coined.


The globally detectable anomaly was scientifically described for the first time in 1980 by the Nobel laureate in physics, Luis Walter Alvarez , his son, the geologist Walter Alvarez , and the chemists Frank Asaro and Helen Michel based on the analysis of rock layers near Gubbio in Italy and Stevns Klint in Denmark. The Italian samples contained about 30 times more iridium than normal, the proportion in the Danish samples even exceeded the normal concentration by 160 times. Since such high iridium concentrations of up to 550 ppb are only known from meteorites , the scientists concluded that the element was enriched by an impact event with worldwide effects, the impact of a massive meteorite. The diffusion took place through the atmospheric transport of the highly dispersed iridium-containing dust. Today, the Iridium anomaly was triggered by the Cretaceous-Paleogene Impact, the evidence of which is the Chicxulub Crater in the north of the Yucatán Peninsula in the Gulf of Mexico .

Earthly cause and counter arguments

Critics do not see impact theory as the only possible cause, because platinum metals can also be enriched through volcanic activity. However, the unusual iridium isotope ratios in the sedimentary rocks that can be detected all over the world speak against this . The impact theory is further supported by the chromium isotope ratios that occur in the same layers and largely correspond to those in chondritic meteorites ( stone meteorites ).

Today it is considered certain that the Iridium anomaly was caused by the impact of a celestial body 10 to 15 km in size .


  • G. Graup, B. Spettel, D. Herm, KF Weidlich: Mineralogy and phase-chemistry of an Ir-enriched pre-K / T layer from the Lattengebirge, Bavarian Alps, and significance for the KTB problem. In: Earth and Planetary Science Letters. Vol. 95, Amsterdam 1989, pp. 271-290. doi: 10.1016 / 0012-821X (89) 90102-7
  • W. Kiesling, P. Claeys: A Geographic Database Approach to the KT Boundary. In: E. Buffetaut, C. Koeberl (Ed.): Geological and Biological Effects of Impact Events. Springer, 2001, ISBN 3-540-42286-2 .
  • A. Shukolyukov, GW Lugmair: Isotopic Evidence for the Cretaceous-Tertiary Impactor and Its Type. In: Science. 282, 1998, pp. 927-929.

Web links

Individual evidence

  1. ^ LW Alvarez, W. Alvarez, F. Asaro, HW Michel: Extraterrestrial Cause for the Cretaceous-Tertiary Extinction . (PDF) In: Science . 208, No. 4448, June 1980, pp. 1095-1108.
  2. ^ Carl C. Swisher, José M. Grajales-Nishimura, Alessandro Montanari, Stanley V. Margolis, Philippe Claeys, Walter Alvarez, Paul Renne, Esteban Cedillo-Pardoa, Florentin JM. R. Maurrasse, Garniss H. Curtis , Jan Smit, Michael O. McWilliams: Coeval 40 Ar / 39 Ar Ages of 65.0 Million Years Ago from Chicxulub Crater Melt Rock and Cretaceous-Tertiary Boundary Tektites . (PDF) In: Science . 257, No. 5072, August 1992, pp. 954-958. doi : 10.1126 / science.257.5072.954 .
  3. ^ Paul A. Renne, Alan L. Deino, Frederik J. Hilgen, Klaudia F. Kuiper, Darren F. Mark, William S. Mitchell III, Leah E. Morgan, Roland Mundil, Jan Smit: Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary . (PDF) In: Science . 339, No. 6120, February 2013, pp. 684-687. doi : 10.1126 / science.1230492 .
  4. Johan Vellekoop, Appy Sluijs, Jan Smit, Stefan Schouten, Johan WH Weijers, Jaap S. Sinninghe Damsté, Henk Brinkhuis: Rapid short-term cooling Following the Chicxulub impact at the Cretaceous-Paleogene boundary . In: PNAS . 111, No. 21, May 2014, pp. 7537-7541. doi : 10.1073 / pnas.1319253111 .
  5. Stephen L. Brusatte, Richard J. Butler, Paul M. Barrett, Matthew T. Carrano, David C. Evans, Graeme T. Lloyd, Philip D. Mannion, Mark A. Norell, Daniel J. Peppe, Paul Upchurch, Thomas E. Williamson: The extinction of the dinosaurs . In: Biological Reviews, Cambridge Philosophical Society (Wiley Online Library) . 90, No. 2, May 2015, pp. 628–642. doi : 10.1111 / brv.12128 .