Osmium and strontium isotopic ratios at the Cretaceous-Tertiary boundary at Sumbar, Turkmenistan, display a negative hyperbolic covariation superimposed on the long-term trend, which displays a positive covariation. The minimum ratio for Os and the maximum ratio for Sr occur at the boundary clay. Volcanism with a mantle or crustal source cannot account for the isotopic data. The low 187-Os/188-Os and the high 87-Sr/86-Sr ratios can be explained by an impact, whereby Os was derived from the bolide and the 87Sr/86Sr ratio was enhanced by acid rain and/or a tsunami following the event.

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