Time to reset isotopic clocks? Free

Just as radiocarbon dating gives the ages of once-living materials up to tens of thousands of years old, longer-lived radioisotopes are used to date rocks that formed millions or billions of years ago. Uranium–lead dating is an especially useful method because two U isotopes decay with different half-lives into two isotopes of Pb. That redundancy offers a convenient shortcut: If the ²³⁸U/²³⁵U ratio is already known—and researchers have long supposed that it should be the same everywhere on Earth—then dates can be derived from the ²⁰⁶Pb/²⁰⁷Pb ratio alone. But recent work has cast doubt on that “lead–lead” dating method, as researchers have found that the ²³⁸U/²³⁵U ratio varies more than they thought it did, or could. Now, in a systematic, high-precision study, Joe Hiess and colleagues of the British Geological Survey have found not only the highest ²³⁸U/²³⁵U anomalies yet seen (more than 5 parts per thousand) but also a mean ²³⁸U/²³⁵U ratio almost 0.5 ppt less than the established value. As a result, lead–lead dates could be wrong by a million years or more. A lesser-used isotopic chronometer, based on samarium-146’s decay into neodymium-142, could be in for an even bigger revision: Michael Paul (Hebrew University, Jerusalem) and collaborators have measured the ¹⁴⁶Sm half-life to be 68 million years, 34% less than the currently used value of 103 million years. The discrepancy is not fully understood, but if the new value stands, it would mean that Earth’s mantle underwent differentiation much faster than previously thought. (J. Hiess et al., Science 335, 1610, 2012 ; N. Kinoshita et al., Science 335, 1614, 2012 .) —Johanna Miller