Two spinel populations from the Cretaceous-Paleogene (K/T)...

Introduction

[2] The nature of the transitional layer at the boundary of the Cretaceous and Paleogene (K/T boundary layer), which is known to occur at all continents, except only Antarctica, and at the oceanic floor (deep sea drilling data) remains a matter of vivid discussions. The anomalies of Ir and other PGE elements detected in clays from the K/T boundary layer [Alvarez et al., 1980 and others] gave rise to the paradigm that the massive extinction of the biota was related to an impact even and triggered the study of the boundary at several regions worldwide. The hypothesis was underlain by the reasonable idea that the high Ir concentrations (which are higher than those known in terrestrial rocks) were caused by the fall of a meteorite (or asteroid) [Alvarez et al., 1980 and others].

[3] At the same time, data were published that an Ir anomaly may occur both below and above the K/T boundary [Ellwood et al., 2003; Graup and Spettel, 1989; and others]. Moreover, Ir anomalies were also found in rocks showing no relations at all to the Cretaceous-Paleogene boundary [Dolenec et al., 2000; Keller and Stinnesbeck, 2000; and others].

[4] Our detailed biostratigraphic, lithological, isotopic-geochemical, and magnitological examination of the stratigraphic section of the sedimentary deposits at the Cretaceous-Paleogene boundary near Gams, Eastern Alps, Austria, led us to conclude that the boundary layer was formed in two stages [Grachev et al., 2005, 2006, 2007]. During the first of these stages (which lasted for approximately 1500 years at conservative assays of the sedimentation rate), the boundary layer was produced under the effect of volcanic aerosol, which predetermined the occurrence of titanomagnetite, Au, and Cu, along with high concentrations of Ir, As, Zn, Ni, Cr, and other element. The presence of an Ir anomaly in the lower portion of the boundary layer, as well as the occurrence of titanomagnetite, Au, and Cu in it, was related to volcanic activity. During the second stage, the character of sedimentation was affected by the fall of an asteroid (meteorite), whose traces are identified in the form of spherules of pure Ni, awaruite, and diamond crystals. Our conclusions differ from the results obtained by all other researchers dealing with the K/T boundary layer.

Figure 1

[5] This paper reports our newly obtained data on the composition of spinel from the Gams stratigraphic sequence not far from the village of Gams in Styria, Eastern Alps, where the Gams River exposes a number of sites with the K/T boundary layer (Figure 1).