RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES6001, doi:10.2205/2005ES000189, 2005

Introduction

[2]  The discovery of anomalies of Ir and other platinum-group elements in clays at the boundary between the Cretaceous and Paleogene (the so-called Cretaceous-Tertiary, or K/T, boundary) [Alvarez et al., 1980, 1984; Ganapathy et al., 1981; Preisinger et al., 1986; Smit and Hertogen, 1980] gave rise to the paradigm that the mass extinction of the biota was induced by an impact event and had been an impetus for studying this boundary throughout the world. This hypothesis was supported by the reasonable idea that high Ir concentrations, much higher than those know in terrestrial rocks, were related to the fall of a meteorite (or an asteroid) [Alvarez et al., 1980].

[3]  The establishment of the impact paradigm of the mass extinction of the biota was facilitated by the discovery of the world's largest Chicxulub meteoritic crater in Yucatan, Mexico [Hildebrand et al., 1991; Smith et al., 1992]. Moreover, some rock units at the K/T boundary were found out to bear shocked quartz and coesite [Bohor et al., 1984; Koebler, 1997; Preisinger et al., 1986; and several others].

[4]  Later papers by Alvarez et al. [1980] demonstrated that the Ir anomaly in the transitional layer at the K/T boundary was present in virtually all of the inspected rock sequences, both in continents and in deep-sea drilling holes in oceans [Alvarez et al., 1992; Hsu et al., 1982; Kyte and Bostwick, 1995; and several others]. The problem of the mass extinction of the biota at the Cretaceous-Paleogene boundary seemed to be resolved, although an almost unanimous consensus was distorted by several researchers who doubted the validity of the impact hypothesis and put forth arguments in support of magmatic (related to a mantle plume volcanism) reasons for the development of the transitional layer [Officer and Drake, 1985; Officer et al., 1987; Zoller et al., 1983]. In particular, several scientists pointed to data on the multiplicity of Ir anomalies and the possibility of explaining the unusual geochemistry of the transitional layer at the K/T boundary by the effect of volcanic activity [Officer et al., 1987]. Research in early 1990s provided new, more detailed information on transitional layers at the K/T boundary. Along with new finds of shocked quartz in the transitional layers of different regions of the world, such high-pressure minerals as coesite and stishovite were found, as well as spinel with high ( > 5%) Ni concentrations and diamond [Carlisle and Braman, 1991; Hough et al., 1997; Leroux et al., 1995; Preisinger et al., 2002]. Although all of these materials considered together provided irrefutable evidence of an impact event, the mechanisms relating it to the mass extinction of the biota remained uncertain.

[5]  Meanwhile newly obtained data were amassing that indicated that Ir anomalies could occur both below and above the K/T boundary [Ellwood et al., 2003; Graup and Spettel, 1989; Tandon, 2002; Zhao et al., 2002; and others]. Furthermore, Ir anomalies were found in rocks with no relation at all to the Cretaceous-Paleogene boundary [Dolenec et al., 2000; Keller and Stinnesbeck, 2000; and others]. Hence, the Ir anomaly itself, which was originally considered one of the milestone of the impact hypothesis for the mass extinction of living organisms at the K/T boundary [Alvarez et al., 1980], could not be anymore (in light of newly obtained data) regarded as a geochemical indicator of such phenomena. It is also pertinent to recall that data on the Permian-Triassic boundary also did not confirm that the reasons for the extinction of that biota were of an impact nature [Zhou and Kyte, 1988].

[6]  The idea that the fundamental changes in the biota at the K/T boundary were related to volcanic processes became topical again [Grachev, 2000a, 2000b], particularly after the detailed studying of anomalies of Ir and other PGE in plume-related basalts in Greenland, at the British Islands, and Deccan [Crocket and Paul, 2004; Phillip et al., 2001; Power et al., 2003], which made it possible to explain the high Ir concentrations in sediments by the transportation of this element by aerosols during volcanic eruptions, as was earlier hypothesized in [Zoller et al., 1983].

[7]  All of these discrepancies became so obvious that W. Alvarez, one of the main proponents of the impact hypothesis, admitted that "...although I have long been a proponent of impact at the K/T boundary, I hold no grief for all extinctions being caused by impact. If the evidence for a flood-extinction link is compelling, we should accept that conclusion" [Alvarez, 2002, p. 3]. He also wrote: "It would be useful to the community of researchers to have a compilation of evidence for impact and for volcanism at prominent extinction levels. This is probably something that should be prepared by a group of workers experienced in the field" [Alvarez, 2002, p. 4].

[8]  It is also worth mentioning two other approaches to the problem of relations between impact events and plume magmatism.

[9]  In one of them, an attempt was undertaken to relate the onset of plume magmatism to the decompression and melting of deep lithospheric layers under the effect of the development of craters of about 100 km diameter. Here the impact itself is considered to be a triggering mechanism for the origin of a plume [Jones et al., 2003; and others]. Aside from the implausibility of this process from the physical standpoint [Molodenskii, 2005, in press], there is direct and only one evidence of the impossibility of this process: the He isotopic signature of plume basalts. As it is well known, these basalts have a 3He/ 4He ratio more than 20 times10-6, which could be caused by the uprise of the melts from depths of more than 670 km, i.e., from the lower mantle or, more probably, from the core/mantle boundary (D primeprime layer) [Grachev, 2000b and references therein].

[10]  In the latter instance, perhaps because no scientifically plausible resolution of the mass extinction could be found, it was proposed to regard mass extinction in the Phanerozoic as an accidental coincidence with coeval plume magmatism and impact events [White and Saunders, 2005].

[11]  Nevertheless, the problem remains unsettled as of yet. How can one explain the fact that the study of the transitional layers at the K/T boundary over the past 25 years, with the use of state-of-the-art analytical equipment and techniques, did not result in the solution of this problem?

[12]  In our opinion, the answer to this question stems from the methods employed in these studies: the layer was inspected as a single whole, and its characteristics obtained with different techniques were ascribed to the whole thickness of this rock unit. At the thickness of the transitional layer at the K/T boundary varying from 1 cm [Preisinger et al., 1986] to 20 cm [Luciani, 2002], sampling sites were commonly spaced 5-10 cm apart, or 1-2 cm apart near the boundary [Gardin, 2002; Keller et al., 2002 and others]. With regard to the known sedimentation rates of about 2 cm per 1000 years [Stuben et al., 2002 and references therein], the transitional layers should have been produced over time spans from 500 to 10,000 years.

[13]  The time during which an impact event could affect the character of sedimentation can be estimated from the numerical simulations of the nuclear winter scenario, according to which the duration of this event at the Earth's surface should range from 10 to 30 days [Turko et al., 1984]. Because of this, even if such events took place in the geologic past, and even if some records of them could be discerned in sediments, evidence of these events cannot be identified visually but require a detailed and scrupulous investigation.

2005ES000189-fig01
Figure 1
[14]  The most interesting known stratigraphic successions in the Eastern Alps the boundary for which between the Cretaceous and Paleogene is considered proved faunistically are three localities in which transitional layers were determined to have Ir anomalies. One of them is situated not far from Gosau, southeast of Salzburg [Preisinger et al., 1986], in the Ellengraben, near the village of Rusbach. Another one is located in the Bavarian Alps, southwest of Salzburg [Graup and Spettel, 1989], and the third one lies east of the village of Gams in Styria, where the Gams River and other streams in some places exposes the K/T boundary [Lahodynsky, 1988] (Figure 1).

[15]  In the first sections, the transitional layer is 1 cm thick [Preisinger et al., 1986], but we failed to identify and sample it in the field. The second locality is characterized by stratigraphic gaps, and it is unclear whether the stratigraphic succession is continuous [Graup and Spettel, 1989]. An exposure most suitable for studying the transitional layer at the K/T boundary is the third one, in the vicinity of Gams, which is now under the protection of UNESCO. A monolithic rock block from this stratigraphic sequence was made available for us for studying under the supervision of the administration of the Museum of Natural History in Vienna.

[16]  This publication is presents the results of the detailed and complex examination of boundary layer in the Gams area.


RJES

Citation: Grachev, A. F., O. A. Korchagin, H. A. Kollmann, D. M. Pechersky, and V. A. Tsel'movich (2005), A new look at the nature of the transitional layer at the K/T boundary near Gams, Eastern Alps, Austria, and the problem of the mass extinction of the biota, Russ. J. Earth Sci., 7, ES6001, doi:10.2205/2005ES000189.

Copyright 2005 by the Russian Journal of Earth Sciences

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