Puchezh-Katunki impact structure: No link with any extinction event revealed by 40Ar/39Ar

by Ralph MT 
Geological map of Puchezh-Katunki impact structure from Holm-Alwmark (2021).

Isotope geochemistry provide a key in unraveling the absolute ages of rocks and even impact events in the past. The quest for worldwide search for evidence of impact events that can be temporally correlated to extinction events and major biotic crisis were prompted by the established notion supported by geochemical evidence of a large asteroid impact that ended the Cretaceous period around 66 million years ago and is associated with one of the massive extinction events that has occurred on Earth, also known as the K-T boundary. One example for this quest is the Puchezh-Katunki impact structure which is located in the central part of the East European platform, in the Privolzhsky Federal District of Russia. Its impact structure is well-preserved with an 8–10 km-in diameter central uplift surrounded by an annular depression that is about 40 km in diameter and is presently buried under a maximum of 120 m of Upper Jurassic, Cretaceous, and Cenozoic sediments. 

Currently, the Puchezh-Katunki impact structure, with an age range from ~164 to 203 Ma, were previously considered to be contemporaneous with the following extinction events: (1) the end-Triassic extinction 201.3 Ma; (2) the Pliensbachian-Toarcian extinction at 183.8 Ma; (3) the Toarcian ocean anoxic event which is estimated to be younger than 183.22 Ma; (4) the Bajocian-Bathonian at 168.2 Ma; and (5) the minor extinction event at the Sinemurian-Pliensbachian Stage. While the possibility is intriguing, the following claims must be supported by reliable and reproducible data, not only impact diagnostic criteria, but also accurate and precise radioisotopic ages of the impact structures/events, and as of now, only the K-T extinction boundary is confirmed. 

When the age of an impact event is accurately and precisely established, a fundamental question arises – what is the possible relationship between the impact event and a recorded event in the biosphere at the same period of time, such as a mass extinction?. In this study by Holm-Alwmark (2021), new 40Ar/39Ar data were obtained on impact melt rock samples from the Vorotilovo Deep Well drill core, retrieved from the center of the Puchezh-Katunki impact structure. Small fragments (~200 mg or less) of each of the impact melt rock samples were crushed, washed, dried, handpicked and then irradiated in preparation for the 40Ar/39Ar analysis. 40Ar/39Ar analyses were carried out at the Western Australian Argon Isotope Facility, at Curtin University. Ar isotopes were measured in static mode using a low-volume in ARGUS VI mass spectrometer. The Ar isotopic data were then processed using the ArArCALC software and the ages were calculated using the recommended decay constants. This method obtained the new age for the structure and provides valuable data regarding the proposed relationships between this impact event and several proposed Mesozoic extinction events, contributing to the improvement of the age database of terrestrial impact structures. 

Results showed that the Puchezh-Katunki impact structure in Russia has been accurately 40Ar/39Ar dated at 195.9 ± 1.0 Ma, making it one of the most precisely dated impact structures on Earth. The new robust age of 195.9 ± 1.0 Ma presented, based on three plateau and a miniplateau ages, safely excludes all of the possible connections with currently known extinctions/biotic events, based on the currently accepted ages of the stratigraphic boundaries. Furthermore, it is highly recommended that statistical analysis of ‘‘comet or asteroid showers”, impact events cyclicity, and/or suggestions of connections between impact structures on poorly constrained ages of the impact crater should be considered with extreme caution.


Holm-Alwmark, S., Jourdan S., Ferriere, L., Alwmark, C., Koeberl, C. (2021) Resolving the age of the Puchezh-Katunki impact structure (Russia) against alteration and inherited 40Ar* - No link with extinctions. Geochimica et Cosmochimica Acta 301, 116-140. https://doi.org/10.1016/j.gca.2021.03.001 

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