COMBINED OSMIUM AND STRONTIUM ISOTOPIC STUDY OF THE CRETACEOUS-TERTIARY BOUNDARY AT SUMBAR, TURKMENISTAN - A TEST FOR AN IMPACT VS A VOLCANIC HYPOTHESIS

Osmium and strontium isotopic ratios at the Cretaceous-Tertiary bounda ry at Sum-bar, Turkmenistan, display a negative hyperbolic covariation superimposed on the longterm trend, which displays a positive covaria tion. The minimum ratio for Os and the maximum ratio for Sr occur at t he boundary clay. Volcanism with a mantle or crustal source cannot acc ount for the isotopic data. The low Os-187/Os-188 and the high Sr-87/S r-86 ratios can be explained by an impact, whereby Os was derived from the bolide and the Sr-87/Sr-86 ratio was enhanced by acid rain and/or a tsunami following the event.