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

Abstract

(1) The results of detailed biostratigraphic, lithological, isotopic-geochemical, and petro- magnetic analysis of the sedimentary sequence at the Cretaceous-Paleogene boundary in the Gams area, Eastern Alps, Austria, point to two stages in the evolution of the transi- tional layer at the Cretaceous-Paleogene boundary. During the earlier one of these stages (which lasted for approximately 1500 years, as follows from conservative estimates of the sedimentation rate), the transitional layer was formed under the eect of volcanic aerosol. The later one caused the presence of titanomagnetite, gold, and copper, as well as high concentrations of Ir, As, Pb, Cr, and other elements in the rocks. The occurrence of an Ir anomaly in the lower part of the transitional layer and the presence of titanomagnetite, Cu, and Au in it were related to volcanic activity. During the later stage, the character of sedimentation was aected by the fall of an asteroid (meteorite), and traces of its material are discernible as beads of metallic Ni, awaruite, and diamond crystals. The conclusions drawn from the results of our analysis principally dier from all preexisting data on the transitional layer between the Cretaceous and Paleogene and provide another look at the reasons for the mass extinction of living organisms at 65 Ma. These data eliminate the need for opposing volcanism to an impact event: both took place, but the changes in the biota were induced by volcanism, as also was the appearance of the Ir anomaly itself, whereas the fall of a cosmic body occurred approximately 500-800 years later. INDEX TERMS: 1051 Geochemistry: Sedimentary geochemistry; 1519 Geomagnetism and Paleomagnetism: Magnetic mineralogy and petrology; 1630 Global Change: Impacts of global change; KEYWORDS: K/T boundary, volcanism, impact event, Eastern Alps.