THE mystery of exactly when dinosaurs were wiped out is a step closer to being solved after Scots researchers pinpointed the date of a catastrophic comet impact believed to have dealt the death blow to the species.
Scientists at the Scottish Universities Environmental Research Centre (SUERC) in East Kilbride, South Lanarkshire, used advanced radioactive dating techniques to create the most accurate ever estimate of when the impact occurred.
They have narrowed it down to 66,038,000 years ago – coinciding to "within a gnat's eyebrow" of the date of the dinosaurs' extinction, which the study also reveals occurred around 180,000 years earlier than previously thought.
Both events are precise to within the same 11,000-year margin of error, making them effectively simultaneous in geological time.
The revised dates clear up confusion over whether the impact actually occurred before or after the extinction, which was characterised by the almost overnight disappearance from the fossil record of land-based dinosaurs and many ocean creatures.
Although the extinction of the dinosaurs was first linked to a comet or asteroid collision by scientists at the University of California Berkeley in 1980, the latest findings – the result of an international collaboration between researchers in Scotland, Holland and the US – provides the first quantitative proof, rather than speculation, an impact event was to blame.
A 110-mile-wide crater in the Yucatan peninsula off Mexico is believed to mark the point where the six-mile diameter comet struck, plunging the Earth into a deadly nuclear winter and spelling doomsday for dinosaurs.
However, the researchers are keen to stress the impact was probably not the sole cause of extinction. Dramatic climate variation over the previous million years, including prolonged cold spells and sustained volcanic eruptions, is believed to have brought many creatures to the brink of extinction, with the impact kicking them over the edge.
Paul Renne, director of the Berkeley Geochronology Center and professor in residence of earth and planetary science at the University of California, Berkeley, said: "The impact was clearly the final straw that provided the tipping point. We have shown these events are synchronous to within a gnat's eyebrow and therefore the impact clearly played a major role in extinctions, but it probably wasn't just the impact."
In Scotland, the research was led by Glasgow University geochronologist Dr Darren Mark, who headed up the SUERC team as it conducted independent analyses on rock samples from Haiti and Montana. The Haiti sample would reveal how long ago the comet impact occurred, while volcanic ash collected from the Hell Creek Formation in Montana – the source of many dinosaur fossils – would pinpoint the extinction date.
The team used a technique known as argon-argon dating, which tests the ratio of radioactive potassium in a sample of rock to its decay product, argon. Because the process of conversion from radioactive potassium to argon is so slow, the scientists can use the rocks like extremely slow stopwatches – counting down from the date they landed on the Earth's surface.
Analysis of the volcanic samples pushed the extinction date back by 180,000 years compared to a previous estimate calculated in 1993.
Mr Mark said: "This study shows the power of high precision geochronology. Many people think precision is just about adding another decimal place to a number, but it's far more exciting than that. It's more like getting a sharper lens on a camera. It allows us to dissect the geological record at greater resolution and piece together the sequence of Earth history."