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Geoarchaeology
Closing the dating gap

Dating archaeological artifacts is rarely an easy task, especially those from 50,000 to 100,000 years ago — a time period for which many otherwise reliable dating methods are not very accurate. Assigning dates to artifacts from that period, called the “chronological gap,” may now be somewhat easier, however, thanks to a new method involving quartz crystals.

A new method called quartz hydration dating can date this ancient quartz pendant from Mexico. The technique could make it easier to date artifacts 50,000 to 100,000 years old. Image courtesy of Johann Wolfgang Goethe University, Frankfurt, Germany.

For the past two decades, Jonathon Ericson of the University of California, Irvine, and colleagues have been developing a technique called quartz hydration dating. It relies on a natural process that happens when fractured quartz reacts with water: Whenever a rock containing quartz fractures naturally or is cut or polished by human activities (such as tool or statue building), the quartz at the surface is exposed. Over time, water diffuses into the quartz, forming a virtually invisible layer that grows on the fresh surface of the quartz grain. “The thickness of the hydration layer can be used as a measure of the time elapsed since the grain was fractured,” Ericson says, as the team will report in next month’s Journal of Archaeological Science.

Quartz hydration dating is applicable to any time period, Ericson says, “but it is especially significant because of its effectiveness in that chronological gap.” Radiocarbon dating is accurate up to 50,000 years ago and potassium-argon dating is accurate in rocks older than 100,000 years. Other techniques, including argon-argon isotope dating and luminescence, are available for that time period but have limiting factors.

Indeed, “lots of important developments took place in this time range, but the array of chronometric tools available to monitor them is limited,” says James O’Connell, an archaeologist at the University of Utah in Salt Lake City. Ericson’s approach could be valuable, he says, especially in places where other techniques are not available, such as archaeological sites in Australia where O’Connell has been working. There, organic preservation is often limited but quartz is common. Reliable quartz dating paired with dates derived through luminescence or other techniques, he says, could yield “very useful results.”

However, O’Connell cautions, more work is necessary to solve the dating dilemma. “No single dating technique solves all chronometric problems,” he says. “But anytime one can draw on more than one dating technique, it’s a good thing.”

Ericson, whose background is in geophysics and archaeology, developed quartz hydration dating with nuclear physicists from China and Germany, based on previous research he had done on obsidian hydration dating, which dates the hydration layer on obsidian. But, Ericson says, obsidian has a number of different physical and chemical properties that make the dating technique a little bit problematic. As quartz is ubiquitous, usually pure and fairly large-grained, the researchers decided it might make a good mineral to date. Over the years, they tested quartz grains from natural formations and from artifacts around the world using a nuclear particle accelerator. They eventually developed an equation of the process of hydration that “anyone with a particle accelerator or a secondary ion mass spectrometer” can use, Ericson says.

Dating of quartz grains “opens up so many doors,” Ericson says, and not just for dating artifacts, but also for figuring out when geologic events occurred, such as earthquakes, meteorite impacts or volcanic activity — anything that breaks a quartz crystal — as well as authenticating statues and determining past climates. In the future, he hopes that scientists can expand the hydration technique to other minerals to give archaeologists and geologists yet another tool for understanding the past.

Megan Sever

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