10 Methods Scientists Use to Date Things

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Left and right, archaeologists are radiocarbon dating objects: fossils, documents, shrouds of Turin. They do it by comparing the ratio of an unstable isotope, carbon-14, to the normal, stable carbon-12. All living things have about the same level of carbon-14, but when they die it begins to decay at uniform rate—the half-life is about 5,700 years, and you can use this knowledge to date objects back about 60,000 years.

However, radiocarbon dating is hardly the only method that creative archaeologists and paleontologists have at their disposal for estimating ages and sorting out the past. Some are plainly obvious, like the clockwork rings of many old trees. But there are plenty of strange and expected ways to learn about the past form the clues it left behind.

1. Camel on Your Knife

It's wasn't so long ago that megafauna ruled the American continent. Sloths and wooly mammoths pushed their weight around; horses and camels had their day. But after the end of the last Ice Age those animals disappeared, so when scientists turn up traces of those animals on archaeological remains, those remains go way back.

Last year, the University of Colorado's Doug Bamforth analyzed a cache of 80-plus tools that a Boulder, Colorado, man accidentally unearthed in his yard. Those tools showed protein residue from camels and horses, so Bamforth dated them to the Clovis people who lived around about 13,000 years ago. (Not all scientists accept the accuracy of these tests, but that's nothing new in archaeology).

2. Locked Away DNA

Medieval manuscripts have a lot more to say than simply the words on their pages; often they're written on parchment made from animal skins, and organic material keeps its secrets for a long time. Literary historian Timothy Stinson developed a way to extract the DNA from parchment itself, and if you can tell what animal a parchment was derived from, you might be able to tell more about what time and place the document originated.

3. The Secret Life of Dung

Moa, the giant flightless birds of New Zealand, may have been extinct for at least 500 years, but their dung is surprisingly resilient. On cave floors and buried in shelters, researchers found dung from the moa, with some of the samples being 15 cm (nearly six inches) in length. The contents of the droppings give more than a window into the giant bird's eating habits—they preserve a record of what the long-gone moa's ecosystem was like.

The arid conditions of New Zealand caves provide the perfect place for poo preservation. Australia should, too, the researchers say, but the droppings of ancient marsupials just haven't turned up. As professor Alan Cooper says, "A key question for us is 'where has all the Australian poo gone?'"

4. Nuclear Forensics

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If you think your metal detector has uncovered some treasures, try finding vintage plutonium in the backyard. Jon Schwantes of the Pacific Northwest National Laboratory was called in to analyze a sample of plutonium-239 accidentally discovered in a safe during the cleanup of the Hanford nuclear site in Washington. One clue was the "signature" left by the reactor—every reactor's is different. The fingerprint of this discarded material led him to a reactor not in Hanford, but in Oak Ridge, TN. It also led him to the conclusion that it was created in 1944, meaning it was created during the Manhattan Project, making it one of the world's oldest-known samples of enriched plutonium. [Image courtesy of Popular Mechanics.]

5. Chemical Warfare

A pile of skeletons probably wouldn't tell us much more than the obvious. But University of Leicester archaeologist Simon James sees evidence that, to him, dates the first known chemical warfare attack back to 256 A.D.

In that year, Persians attacked a Roman garrison at Dura-Europos in Syria; when they tried to mine under the walls, Romans tried to counter by mining under the Persian tunnels. Archaeologists found the pile of Roman bodies in one of the tunnels, but no cause of death. James thinks it was asphyxiation. In the tunnels, he says, there was bitumen and sulfur—materials that, when burned, give off toxic gas. So, he says, the Persians probably used chemical warfare to do in their rivals.

6. The Magnetic Fields

One classical way to date objects is to take note of what strata of rock they occupy—rocks come in layers, with the oldest at the bottom. But those rocks also carry less obvious information—their magnetic signatures. The Earth's magnetic field varies all the time, by both strength and orientation. At the time rocks form, however, their magnetic materials acquire the particular orientation of the planet's magnetism at the time, giving geologists a window into the Earth's magnetic past.

7. Ice Cores

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You've probably heard about ice cores, but what are they exactly? Ice sheets are laid down in layers, and the layer corresponding to each year is a little different. The important thing for climate researchers is that the oxygen isotopes present in a layer can help show what the temperature was that year. So by extracting a cylindrical core sample containing layers that go way back, they can build a model of the climate of the past. [Image courtesy of AccuWeather.com.]

8. Pollen

Finally, pollen is good for something besides making you sneeze. Deposits of pollen deep in the ground can reveal what the vegetation was like at that time, and ergo, what the area's climate might have been like. Radiocarbon dating has become the standard method to date organic material, making pollen deposits sort of useless in that regard. But pollen can still help scientists interpret the environment of the past.

9. Volcanic Ash

Everything, it seems, has a fingerprint, and volcanoes are no exception—each eruption contains a chemical mix that is all its own. So if you knew the specific signature of say, the 79 A.D. eruption of Mt Vesuvius that buried Pompeii, you could look for that signature elsewhere in Italy and know that it came from the same eruption. Thus, any objects in that "tephra," the name for solids ejected during a single eruption, date to that era of Roman history, and anything below it would be older. This dating system is called tephrochronology.

10. Thermoluminescence

You probably know that radiation you can't see is flying all around you, but you might not know that not only do objects absorb that radiation, they also let their trapped radiation go when heated up. Knowing this, an archaeologist could heat up an object, watch how much radiation is released and determine how old the thing might be.

It's particularly useful for ceramics. When a potter in Ancient Greece fired his kiln and baked a pot, that released the clay's stored electrons and reset the clock to zero. During all those centuries it sat in the ground, it began storing radiation again at a steady rate. So when a curious 21st century scientist unearths the pot and heats it again, she can measure the radiation released, crunch some numbers and figure out how long ago the pot was first fired.

Andrew Moseman writes about science for publications like Popular Mechanics, Discover, Scientific American and Big Think from his Brooklyn apartment beneath the elevated tracks. He's from Nebraska, and he claims that ex-Huskers are starting a hive in New York City. Just so you know.

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May 4, 2009 - 9:29am
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