“Argon-argon dating” is a common technique scientists use to reveal when a geological event, such as a volcanic eruption, took place. The problem is that it is difficult to apply it to recent events. To address this problem, a team of researchers sought help from an ancient Roman writer. In a study published Friday in Science Advances, scientists used argon-argon dating to date pumice samples from the Mount Vesuvius eruption. They compared this age to the historical date (August 24, 79 CE, according to Pliny the Younger, who witnessed the tragedy from afar), allowing them to fine-tune the calibration of the argon-argon technique. Their success in applying the argon-argon dating method to a relatively recent geological event opens the door to specifying other recent volcanic eruptions, with implications for estimating future problems. “We wanted to see how precisely and precisely we could date something that is very recent,” Paul Renne, lead author of the study and a researcher at the Berkeley Geochronology Center, told Gizmodo. “So part of this study was simply to see if we could double the historical age of the eruption of Vesuvius that buried Pompeii.” In geochronology, AD 79 is “like yesterday,” Renne explains. However, not everyone agrees with the date of the eruption of Vesuvius indicated by Pliny. For example, some historians theorize that a coin found in Pompeii could only have been produced in September of that year, meaning the tragedy must have occurred later in the fall. However, when comparing it to other contemporary Roman coins, Caroline Hasler, another researcher at the Berkeley Geochronology Center and co-author of the study, determined that the coin was probably made before September. That, along with other research, assured the team that August 24, 79 CE was a reliable reference point. Then came argon-argon dating. Researchers know that the potassium-40 isotope (or version of an element) in minerals naturally transforms, through radioactivity, into the argon-40 isotope at a particular rate. In extremely hot magma chambers, argon escapes from these mineral crystals as a gas. But once the volcano erupts and the minerals are expelled and begin to cool, the argon becomes trapped within the mineral and begins to build up. As such, researchers can use the accumulation of argon-40 to determine when the eruption took place. That’s why it’s easier to date older eruptions: there’s more argon buildup to measure. Paul Renne and colleagues at the Berkeley Geochronology Center. © Paul Renne/Berkeley Geochronology Center and UC Berkeley Using Pliny the Younger’s date as a reference point and eight samples of volcanic material from Vesuvius, the team was able to fine-tune the argon-argon dating technique to estimate that the volcano had erupted in 87 CE, give or take 13 years. In other words, the newly calibrated dating method can be used to date something as “recent” as the destruction of Pompeii “within a decade, which is pretty good,” Renne noted. Their achievement also allowed the team to more accurately calculate the rate at which potassium-40 decays into argon-40. Why dates are important Reliable dating techniques are particularly important for scientists investigating the consequences of very ancient geological events. When looking for a link between two events, such as a volcanic eruption and a mass animal extinction, an exact chronological alignment is often the most direct evidence they can hope for. Additionally, by shedding light on a volcano’s eruptive history, scientists can better predict future ones and thus mitigate risks to surrounding populations. It may also help resolve another geochronological question. The two standard dating techniques in the field are argon-argon dating and another similar method that analyzes the transformation of uranium to lead. However, these two “rock clocks” do not indicate the same age for the same event. “I mean, it’s close, but we can do better,” Renne said. “We’ve intercalibrated those two systems pretty well, but there’s always room for improvement, and this particular evidence could now be incorporated into that technique.” In the future, the study could even help align other dating methods, such as carbon-14 (used to date organic material). “It’s nice that all the different watches give you the same answer,” he concluded.
