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Comment on "Impact Ejecta Layer from the Mid-Devonian: Possible Connection to Global Mass Extinctions"

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Science  23 Jan 2004:
Vol. 303, Issue 5657, pp. 471
DOI: 10.1126/science.1090332

The interesting report by Ellwood et al. (1) demonstrated the applicability of magnetic susceptibility field methods to the identification of distal impact deposits. However, the evidence that they have presented for a bolide impact during the Middle Devonian, immediately preceding the Eifelian-Givetian (E-G) stage boundary at ∼380 million years ago (Ma), requires further confirmation and, in particular, better evidence for shock metamorphism. The images identified by Ellwood et al. (1) as shocked quartz grains are not convincing, and the orientation measurements suffer from an insufficient number of observations. The possible presence of an extraterrestrial component also needs confirmation: As and V anomalies are certainly not indicative of a cosmic component, and Ir data, which would be more characteristic, are absent.

Even if there were strong evidence for a mid-Devonian impact, the interpretation presented by Ellwood et al. (1) would be too far-reaching in one key respect. In the title of the report, Ellwood et al. announce a possible cause-and-effect relation between the putative bolide impact and a global mass extinction. This extinction in question is the late Eifelian Kacák/otomari event (24). Ellwood et al. claim that this biospheric perturbation “may represent the extinction of as many as 40% of all marine animal genera” (1). They cite the work of Sepkoski (5) as their only reference for this claim, but the 40% figure is not derived from this work. The crucial figure 6 from Sepkoski (5) illustrates the extinction pattern of only “well-preserved” marine genera during the Devonian substages, and the E-G passage is marked by demise level of approximately 15% (Fig. 1). More significant, however, is that this so-called mass extinction of the late Eifelian is at the sixth position among the Devonian intervals in a relative ranking, and has only half the magnitude of the peak that marks the late Frasnian event, a true mass extinction. Indeed, Walliser (2) characterized the impact-related Kacák/otomari biotic crisis as a minor two-step event that constituted at most a third-order extinction limited primarily to pelagic biota. Its main cause is seen to be the anoxic conditions recorded in widespread, long-lasting (perhaps 1 million years in duration) black-shale deposition during a pulse of marine transgression (24). The sulfide-forming and redox-sensitive trace metals, reported by Ellwood et al. (1) as evidence for an impact, may be alternatively thought as a geochemical signature of oxygen deficiency only.

Fig. 1.

Two selected impact events plotted against percent extinctions of “well-preserved” marine genera through 16 substages of the Devonian [adapted with permission from figure 6 of Sepkoski (5); copyright © 1996 by Springer-Verlag]. As in the original diagram, end-stage extinction data are not plotted at the graphed stage boundary lines, but slightly before them. Loch, Lochkovian; Prag, Pragian; Emsi, Emsian.

Altogether, the term “global Kacák/otomari mass extinction” is an original proposal of Ellwood et al. (1), and an overestimate of the actual biotic changes at this stratigraphic horizon. Similarly negligible ecosystem collapse, at least in an immediate feedback, is established for the well-dated mid-Frasnian Alamo impact event in Nevada (6). The insistent search for causal link between an extraterrestrial impact of unknown magnitude and an obligatorily major extinction represents a circular argument [see, for example, (7, 8)], and such a link has not been demonstrated in this case.

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