The Berkeley Science Review: Articles

Rock of Ages
Synchronizing geological clocks (view PDF)
by Katie Peek

Once upon a time, the Earth was an inhospitable place for giant cold-blooded creatures—a meteorite had just blanketed the atmosphere with dust, blocking out the sun and sending temperatures plummeting. Until recently, the date of that extinction event had been placed at sixty-five million years ago, give or take a few million. A team of geologists at Berkeley and the Universities of Utrecht and Vrije in the Netherlands has recalibrated the method used to calculate the date of the dinosaur extinction event. That technique, argonargon radiometric dating, allows geologists to determine the ages of rocks over most of the geologic past. "We're trying to decipher Earth's history," says Paul Renne, Director of the Berkeley Geochronology Center and a geologist in UC Berkeley's Department of Earth and Planetary Science.

Just how do geologists go about deciphering Earth's history? The sequence of sedimentary rock layers reveals their relative ages. But in order to compare the geologic record to, say, the archaeological record, scientists need to know absolute ages. Says Renne, "Precise and accurate age determinations are critical for causality arguments in geology, paleontology, archeology, and cosmology." Enter radiometric dating: a radioactive element can tell a geologist how old a rock is by how much of the so-called "parent" isotope has decayed into the "daughter" isotope. In the case of argon-argon dating, the parent is potassium-40, one of three naturally occurring potassium isotopes (each differs in its number of neutrons). Every 1.25 billion years, half the potassium-40 in a sample decays into argon-40. This long halflife means the technique can be used to date rocks over most of Earth's 4.5-billion-year history. "The further you go in the past, the more of the daughter product accumulates in your rock," says Alan Deino of the Berkeley Geochronology Center, a member of the Berkeley team. Measuring the accumulation of argon-40 provides a clue, but to solve the mystery of the rock's age, the original amount of potassium-40 also needs to be known.

Geologists can use the stable potassium- 39 isotope to figure out how much potassium- 40 was present when the rock formed, because rocks crystallize with a constant amount of potassium-40 relative to potassium- 39. But measuring potassium and argon separately requires splitting a sample and using two different measurement techniques, increasing the chance for error. Geologists solve this problem by bombarding their rock samples with neutrons, which converts a fraction of the potassium-39 into argon-39. "The miracle of the 40-39 dating process," says Deino, "is that you can measure potassium in your rock via the proxy of argon-39 at the same time as you measure the radiogenic component, which is the argon-40."

Geologists translate the argon measurements into an absolute age by simultaneously measuring the 40-39 ratio in their mystery rock and in a sample from a standard rock of known age. Comparing the two 40-39 ratios yields an age for the unknown sample. The most widely used standard for the argon-argon method comes from a site called the Fish Canyon Tuff, a large volcanic deposit in the San Juan Mountains in Colorado. Fish Canyon has become the standard because it is a uniform formation that contains many different minerals and is in a location accessible to geologists.

The accuracy of the Fish Canyon age dictates the accuracy of argon-argon dating. Previously, that age came from a multistage laboratory process that was subject to relatively large errors. A more accurate Fish Canyon age applied to past and future experiments could improve all argon-argon ages. The Netherlands and Berkeley teams have calculated a better Fish Canyon age by linking it to another, independent method known as astronomical tuning.

Over tens of thousands of years, the Earth's orbit around the Sun changes, affecting the intensity of the sunlight at its surface, or insolation. "Astronomers have very carefully measured all the orbital parameters of the solar system and combined them all in models so that you can take a present-day observation of the state of the solar system and say what it was like a hundred thousand years ago," says Deino. An increase in insolation causes heavier rains, increasing freshwater runoff in the Mediterranean basin, and the resulting circulation patterns in the sea cause more organic matter to be present in the sediments that form during that time, darkening them. Identifying those darker sedimentary layers and linking them to astronomical models yields precise ages for the relatively young rocks that formed within the past few million years. To calculate a more accurate Fish Canyon age, the Netherlands team first found sedimentary layers on the northern coast of Morocco that could be dated astronomically and that also contained the potassium-rich minerals required for precise argon-argon dates. Since the Moroccan rocks could be age-dated by both methods, they would allow the geologists to directly compare the two scales. "If you assume your astronomical clock is the correct one," says Deino, "then you can back-calculate a better age for your standard."

The Netherlands team analyzed the Moroccan rocks and asked the Berkeley group to do the same. "The point of having two laboratories," says Deino, "is a cross-validation of techniques." The two teams independently concluded that the canonical Fish Canyon age was a little too young for the argon-argon clock to agree with the astronomical one. They advocate adjusting the age of the standard in order to synchronize the two. Their new and improved Fish Canyon age aligns argon-argon dating with the astronomical record and increases its precision and accuracy.

There is, of course, more work to be done. To be sure the calibrations are correct, Deino says it is important to go elsewhere in the world to repeat the test. But the higher precision already allows for better absolute ages of much of geologic history. The Netherlands- Berkeley team fi rst applied the new calibration to the layer of rock that formed during the dinosaur extinction: that devastating meteorite impact happened 65.96 million years ago, give or take a mere 40,000 years.

Katie Peek is a graduate student in astronomy.

Want to know more? Check out:
A virtual tour of the Argon lab at the Berkeley
Geochronology Center
bgc.org/facilities/argon_lab.html

A current outline of rock ages over Earth's
history
geosociety.org/science/timescale/
timescl.htm
and
earth-time.org

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