Slashdot Mirror
101 Ways To Kill The Dinosaurs
blank writes "Everyone knows there are many impact craters on Earth; around 170 in fact. This article from the Seattle PI points out that more than one of those impacts could have caused the extinction of the Dinosaurs. In Ukraine, scientists found that a well-known crater had been inaccurately dated - the correct date puts the impact sometime around when the Dinosaurs disappeared..."
The impact itself would not adversely affect radiometric dating techniques in terms of screwing up decay rates, or isotopic ratios, etc. That's all misinformed FUD that you might hear people spew.
While I'm a geologist who has done a lot of radiometric dating (U-Pb and 40Ar/39Ar), I don't know much about how this particular crater is being dated. However, I can imagine that the heating and melting caused by an impact event actually HELPS you date the event.
Consider a rock. It may have some amount of 40K and 235U and 238U and 232Th that decay over time. The daughter products of those decays (39Ar from K and 206Pb and 207Pb from the U and Th) accumulate within the rock (actually within specific crystals, but I won't get into details) over time. The minerals, however, are only closed systems below a certain temperature (it varies depending on the specific mineral) called the closure temperature. Above the closure temperature, solid state diffusion creates an open system and the daughter products can escape from the crystal structure.
Now if the impact hits and melts the target rocks (heating them way above the closure temperatures of any minerals they may contain), the radiometric clocks in the target materials will be reset! Once the melt cools and solidifies, though, the "rock clocks" will become closed systems again, and radiogenic daughter products will accumulate again.
The best material to go after from the melted target would be stuff that quickly cooled to form glass. Things like tektites. Since they are glassy, we know they cooled very quickly (too fast to form mineral grains). So dating glassy tektites (probably with 40Ar/39Ar, a fancy variant of K-Ar dating) is one way to pinpoint the timing of the impact.