On the Atlantic coast of the U.S., archaeologists found oyster shells left by Native Americans more than 4,000 years ago. In Morocco, paleontologists excavated the fossils of a dinosaur that roamed Earth 168 million years ago. How did the researchers determine these ages? When examining remnants from the past, experts use radiometric dating, a versatile technique that involves counting radioactive atoms of certain elements that are still present in a sample. The particular elements studied, as well as the details of the process, depend on the approximate age of the object that scientists hope to date.
For human or animal remains and artifacts from the past 50,000 years or so, researchers look at levels of carbon 14 in the sample. Also called “radiocarbon,” this isotope is generated by cosmic rays colliding with nitrogen in Earth’s atmosphere, says José Capriles, an archaeologist at Pennsylvania State University. Chemically, carbon 14 behaves exactly like its stable siblings (carbon 12 and carbon 13), allowing plants to absorb it during photosynthesis and then pass it up the food chain. While alive, animals and plants tend to contain the same levels of carbon 14 as their environment. But “as living organisms die, they stop consuming or incorporating radiocarbon,” Capriles says, and “the process of radioactivity kicks in,” with the isotope decaying back into nitrogen. So researchers compare the amount of carbon 14 with the levels of carbon 12 and carbon 13 to determine how much time has passed since an organism perished.
The amount of carbon 14 in a dead organism decays exponentially, falling to one half of its initial value after about 5,730 years. Using an accelerator mass spectrometer, researchers can readily measure the radiocarbon in a sample. The trickier task is estimating how much of it should have been present in the environment when the organism was alive, which can then serve as a baseline for comparison.
“Solar flaring and other events can influence how much radiocarbon is in the upper atmosphere,” Capriles says. “And there’s also [a] somewhat different distribution of radiocarbon over the entire world.” Based on measurements from tree rings, ice cores and other sources, researchers have devised calibration curves that show how the concentration of carbon 14 in the environment has changed over time. The Northern Hemisphere, Southern Hemisphere and marine environments all have separate calibration curves, Capriles says. To achieve the most precise dating, he and other archaeologists also consider factors that cause local variations in atmospheric radiocarbon.
Capriles studies the earliest occupants of South America, who arrived from the north and began dispersing throughout the continent around 15,000 years ago. At each archaeological site, he and his colleagues “just want to know when people were there,” he says. “How long did they stay there? What was their intensity of occupation?” To reconstruct a site’s time line, Capriles adds, “there's no better method out there than to use radiocarbon dating” on bones, cloth, seeds and any other organic material he finds. But at sites older than about 50,000 years, almost all of the carbon 14 in a dead organism has already decayed, so researchers must turn to longer-lived elements.
Originating in Earth’s mantle, some radioactive elements reach the surface through volcanic processes and become trapped inside mineral crystals in soil and rock. Over the course of millions of years, uranium 235 and uranium 238, for example, undergo multistep decays to isotopes of lead, making them ideal for paleontology: researchers can determine the age of a sample by measuring the ratio of lead to uranium isotopes. But using this technique to date fossils from creatures that lived millions of years ago, such as dinosaurs, is far from straightforward. “Fossils themselves usually can’t be dated directly,” says Sarah Gibson, a paleontologist at St. Cloud State University, who studies fish evolution about 230 million to 150 million years ago, during the early Mesozoic era.
Fossils form through various processes, the most common of which is called permineralization. When a deceased organism is buried, permineralization can preserve its hard parts, such as bones. As water seeps into the remains, the minerals in the water fill the gaps in the bones, solidifying into a crystalline structure that eventually replaces the organic material. By the time minerals form a fossil, they are no longer “fresh”—the uranium inside has already been decaying for millions of years. Attempting to date one directly would yield a false result—much older than the organism itself. As a result, scientists must “rely on the geologic formations that are around or adjacent to the fossils” to calculate their age, Gibson explains. Because fossils are usually found in sedimentary rock layers, paleontologists can date them by examining the minerals above or below the sedimentary rock.
Zircon, a mineral commonly found in igneous rocks, proves particularly useful. As zircon forms in cooling magma, its crystal structure contains uranium but no lead. Thus, any lead present in a sample of zircon must have formed via radioactive decay of uranium. This feature allows geologists to date volcanic ash flows that are interspersed with layers of sedimentary rock like a prehistoric layer cake. Any fossils found in the sedimentary rock must be younger than the ash below and older than the ash above.
So far, so good. But what if there is no ash layer close to the fossils? “It’s not always cut-and-dried,” Gibson says. “At the fossil sites that I’ve worked on in Utah, we have to trace the [rock] beds from Arizona [that have already been dated] northward and try to correlate [them] to different geologic formations [in Utah]. And then we can get an estimate for how old or young something is, based on the relative position.” The approach is much like tracing one layer of a birthday cake around to the opposite side of the cake. In other cases, researchers can date fossilized remains using nearby “index fossils” of species known to have existed during a specific narrow time frame. Yet another technique, magnetostratigraphy, studies the magnetic signatures left in rocks by Earth’s magnetic field as its orientation slowly shifts.
To understand how fish anatomy changed over time, Gibson depends on the results of uranium-lead dating, magnetostratigraphy and index fossils. Because of the difficulties in determining the age of rock layers, however, she does not perform radiometric dating herself. “You can’t specialize in everything. That's why you get the colleagues that can do that kind of work,” she says. “I really do rely on the work of other geologists and chemists to figure out these specific dates.”
The accuracy and precision of both radiocarbon dating and uranium-lead dating have improved in recent decades as scientists have learned more about Earth’s past. In fact, researchers released a major update of the radiocarbon calibration curves this year. From the first animals to the rise of human civilization, “we’re all working together, building upon each other’s studies to piece [together] this snapshot of time,” Gibson says.