Radioactivity revolutionized the 20th century, not only through weapons, electrical generation, and medical applications, but also by shining a light into Earth’s dark prehistory. Geologists in Darwin’s time could only indirectly guess at the deep time they were studying. The advent of radiometric dating allowed us to measure the age of the Earth and assign dates to the events laid out in the rock record.

But the science of geochronology hasn’t packed up and called it a day, of course. There’s always more to discover and improve upon. A pair of papers published recently in Science present changes to a couple of geologic clocks that will tweak previously calculated dates.

A bad samarium

We start with an element that doesn’t come up too often in casual conversation—samarium. It has a number of isotopic flavors, several of which are radioactive. Samarium-146, which ultimately decays to neodymium-142, is used to date some events in the formation of the solar system. It’s especially useful for dating the differentiation of planetary bodies and asteroids, when denser elements separated from lighter ones.

It’s often not the only decay series used to date these events, and samarium-146 has commonly been found to yield dates that don’t quite fall in line with other series. Previous measurements (performed between the 1950s and 1980s) of the half-life of samarium-146 have ranged from 50 million up to 103 million years; the latter became the accepted value. A group of researchers have now used greatly-improved instruments to revisit the particularly tricky measurement, and report a significant revision. They put the half-life at 68 million years.

This does make for some interesting shake-ups in the chronology of Earth’s formation, at least for work that relied solely on the samarium-146 clock. Those dates will shift closer to the initiation of planet formation, with some moving as much as 80 million years. For the most part, though, this seems to be more about tidying up the books than reworking timelines. The updated half-life brings samarium-146 dates in line with other decay series. So for events that were dated using multiple series, this merely erases the question marks surrounding that discrepancy.

Shaking up uranium

The other clock getting a tune-up is the familiar family of uranium-lead series, which are used on a wide variety of events, including the age of the Earth. Here, the question is not about the half-life, but about the abundance of uranium-235 with respect to the most common isotope, uranium-238.

Both isotopes are used for radiometric dating, and serve as convenient cross-checks on ages due to the difference between their half-lives. Uranium-238 decays to lead-206 with a half-life of 4.47 billion years, and uranium-235 decays to lead-207 with a half-life of about 700 million years. The ratio of these radiogenic (created by radioactive decay) isotopes of lead to non-radiogenic lead can also be used to calculate ages. In fact, lead-lead dating was the technique used by Clair Patterson in 1953 to obtain the first definitive age of the Earth. (His difficulties in the lab resulted in the discovery of lead air pollution and, ultimately, to the creation of unleaded gasoline.)

Since over 99 percent of uranium comes in the 238 flavor, getting enough uranium-235 to calculate a proper date can sometimes be a challenge. It has been assumed that the uranium in the Earth was perfectly mixed, so that the ratio of 238U/235U was invariably 137.88. Because of this, the process is commonly simplified by measuring the more abundant 238U and calculating the amount of 235U using that ratio. In lead-lead dating, the ratio is actually built into the age equation.

Recent high-precision work has turned up some rebel values that called the invariability of that ratio into question, though. To get a handle on this, researchers from the British Geological Survey and the Massachusetts Institute of Technology have carefully measured the 238U/235U ratio in a number of mineral specimens, including 44 zircon crystals, which are frequently used for this type of dating. They calculated an average ratio of 137.818 ±0.045. That may not sound like much of a difference, but it’s not to be ignored.

The impact of that change depends on the method used. For 235U-207Pb dates, the difference is greatest for older samples. Something previously dated as 4.5 billion years old would be off by about 400,000 years (a whopping 0.009% difference, to be fair). A similar 238U-206Pb date would have to move a little less than 30,000 years to fall in line.

The new uranium ratio has the biggest impact on lead-lead dating. In contrast to the other methods, the difference in dates actually decreases with the age of the sample, ranging from 700,000 to 1,000,000 years.

In the grand scheme, these updates obviously aren’t upsetting the apple cart. But they may upset an apple or two, and that’s enough to make scientists in a range of disciplines take notice.

Science, 2012. DOI: 10.1126/science.1215507 and 1010.1126/science.1215510 (About DOIs).