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21-Oct-2017 08:48

"You see a bunch of repetitions of features in sediments, but you don't know where to start counting." Argon-argon dating of volcanic ash, or tephra, in these sediments provided that anchor, he said, synchronizing the methods and making each one more precise.

The argon-argon analyses were conducted both in Berkeley and Amsterdam to eliminate interlaboratory bias.

That boundary had previously been dated at 65.5 million years ago, give or take 300,000 years.

According to a paper by Renne's team in the April 25 issue of Science, the best date for the Cretaceous-Tertiary, or K/T, boundary is now 65.95 million years, give or take 40,000 years.

"Where it really adds up is in dating events in the early solar system," Renne said.

"A 1 percent difference at 4.5 billion years is almost 50 million years." One major implication of the revision involves the formation of meteorites, planetessimals and planets in the early solar system, he said.

"The importance of the argon-argon technique is that it is the only technique that has the dynamic range to cover nearly all of Earth's history," Renne said.

"What this refinement means is that you can use different chronometers now and get the same answer, whereas, that wasn't true before." Renne noted that the greater precision matters little for recent events, such as the emergence of human ancestors in Africa 6 million years ago, because the uncertainty is only a few tens of thousands of years.

Renne's group had proposed using the astronomical tuning approach to calibrate the argon-argon method as early as 1994, but lacked ideal sedimentary sequences to realize the full power of this approach.

At Zumaia in the Basque country of northern Spain, sediments laid down around the end of the Cretaceous period show layers of light limestone and dark marl reflecting warm and cool periods, respectively, in Earth's climate.

These alternating climatic periods are caused by 100,000-year and 405,000-year cycles in Earth's orbital eccentricity.

"The evolution of the early solar system - the accretion of planetessimals, the differentiation of bodies by gravity while still hot - happened very fast.

Argon-argon dating is now no longer at odds with that evidence, but is very consistent with it." Renne has warned geologists for a decade of uncertainty in the argon-argon method and has been correcting his own data since 2000, but it took a collaboration that he initiated in 1998 with Jan R.Argon-argon dating, developed at UC Berkeley in the 1960s, is based on the fact that the naturally-occurring isotope potassium-40 decays to argon-40 with a 1.25-billion-year half-life.



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