Temporal link between impact events and biotic crises &#8211; The case of the Puchezh-Katunki impact structure (Russia)
Authors: Alwmark Sanna , Jourdan Fred , Ferrière Ludovic , Alwmark Carl , Koeberl Christian 
Published: October 2020

Geochemical evidence for a collision between a large (~10 km) extraterrestrial body and Earth at the end of the Cretaceous, ca. 66 million years ago, and the conclusion that this event was the cause for the global Cretaceous-Paleogene boundary extinction event [1], triggered an intense worldwide search for evidence of other impact events that could be temporally correlated to other extinction events [2, 3]. To date, no convincing links to any other extinction events have been presented [2].
&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; Connecting extinction events with a specific impact event/crater is complicated by the fact that the inventory of terrestrial impact craters remains largely incomplete, due to, e.g., erosion and sedimentary cover. In addition, our understanding of impact diagnostic markers in sedimentary strata is far from complete, especially in the case of target rocks that lack quartz or when dealing with extraterrestrial projectiles with &#8220;not so distinctive&#8221; geochemical signature, such as achondrites. On top of these considerations, the ages of a large number of the known terrestrial impact structures are poorly constrained. A precise, and most importantly accurate, age for an impact event is crucial when attempting to tie a specific impact event to a biosphere episode recorded in the stratigraphic record.
The 40-km-diameter Puchezh-Katunki impact structure [4], located ~400 km northeast of Moscow (Russia), with an estimated age ranging from ~164 to 203 Ma [5, 6], has been discussed as a possible trigger for four different biotic crises (Fig. 1; [7-12]).
Here we present new 40Ar/39Ar data on impact melt rock samples from the Vorotilovo Deep Well drill core, retrieved from the center of the Puchezh-Katunki impact structure [4,6]. The new age obtained for the structure provides valuable data regarding relationships between impact events and extinction events in general and specifically the proposed relationship with some Mesozoic extinction/biotic events, and also contributes to the improvement of the age database of terrestrial impact structures.
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Laser step-heating of four whole-rock samples of impact melt rock from the Puchezh-Katunki impact structure were run in duplicate (Fig. 1). The samples (V1018, V1144, and V1251) come from the upper parts of the drill core. The new 40Ar/39Ar data allow us to precisely and accurately date the formation of the Puchezh-Katunki impact structure to 195.8 &#177; 1.0 Ma (2&#963;; mean square of weighted deviates (MSWD) = 2.0; P = 0.11). This new age makes Puchezh-Katunki one of the most precisely dated impact structures on Earth (2&#963; relative error: 0.51%).
As previously noted, only one global mass extinction has been confirmed to be the direct result of an impact event. The possible relation between impact events and global mass extinctions has been extensively discussed in the literature [2, 3], and lower limits on crater sizes that cause global effects have been debated as well (op.cit.). For example, there is little evidence that the late Eocene impact events (e.g., the Popigai and Chesapeake Bay impact events) have caused any global biotic effects. It is of course possible that less severe, perhaps regional, biotic events have also played an important role in the evolution of life (e.g., [13]). Understanding these events can yield important insights into e.g., the relationship between environmental changes and extinctions (i.e., causality), and how life recovers after such sudden events. Despite not causing global mass extinction, impact events of significant magnitude to cause regional havoc (e.g., resulting in impact craters equivalent to the size of Puchezh-Katunki) will have an impact on life (e.g., [14]) at least on a regional to continent-wide scale, and could possibly have caused both yet undetected biotic events as well as some of the (minor) extinction events that are recognized in the stratigraphic succession. However, since the geological timescale is continuously refined and ages updated, connecting even a well-dated impact structure with an event in the stratigraphic record is by no means straight-forward.
Our new age of 195.8 &#177; 1.0 Ma for the formation of Puchezh-Katunki excludes any relationship with the previously mentioned biotic/extinction events, and clearly illustrates that connecting poorly constrained crater ages with extinction events is troublesome. Therefore, we stress that using poorly constrained or outdated ages leads to erroneous or problematic conclusions (see also e.g., [15, 16]). We also want to emphasize that proving synchronicity between an impact event and a biotic event is not the same thing as proving causality.

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Fig. 1. Comparison of previous age data with our results from 40Ar/39Ar analyses of Puchezh-Katunki impact melt rocks, with plateau ages and mini-plateau &#8220;ages&#8221; (2&#963; errors, all ages in million years). For references to previous age estimates see [5, 6]. For references to environmental/biotic events that have been suggested to possibly be synchronous with the Puchezh-Katunki impact event, see text. Abbreviations: Baj/Bat = Bajocian/Bathonian, Pl/To = Pliensbachian/Toarcian, Si/Pl = Sinemurian/Pliensbachian, Tr = Triassic.
Acknowledgements
We are very grateful to our late friend and colleague Dr. Svetlana Bogdanova for providing the samples investigated here. SHA is supported by VR (2017-06388).
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References
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