Scientific methods of dating fossils

Scientists use two kinds of dating techniques to work out the age of rocks and fossils. The first method is called relative dating. This considers the positions of the.
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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 A.

Accuracy of Fossils and Dating Methods

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. 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.

Relative Dating

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. 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.

How Do Scientists Date Ancient Things?

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. Finally, pollen is good for something besides making you sneeze.


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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. 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.

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. Within hours or days of a volcanic eruption, tephra — fragments of rock and other material hurled into the atmosphere by the event — is deposited in a single layer with a unique geochemical fingerprint. Researchers can first apply an absolute dating method to the layer.

Relative and Absolute Dating

They then use that absolute date to establish a relative age for fossils and artifacts in relation to that layer. Anything below the Taupo tephra is earlier than ; anything above it is later.


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Generally speaking, the more complex a poem or piece of pottery is, the more advanced it is and the later it falls in the chronology. Egyptologists, for example, created a relative chronology of pre-pharaonic Egypt based on increasing complexity in ceramics found at burial sites. Sometimes called carbon dating, this method works on organic material.


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Both plants and animals exchange carbon with their environment until they die. Afterward, the amount of the radioactive isotope carbon in their remains decreases. Measuring carbon in bones or a piece of wood provides an accurate date, but only within a limited range. It would be like having a watch that told you day and night.

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Also called single crystal argon or argon-argon Ar-Ar dating, this method is a refinement of an older approach known as potassium-argon K-Ar dating, which is still sometimes used. Both methods date rock instead of organic material. As potassium decays, it turns into argon. But unlike radiocarbon dating, the older the sample, the more accurate the dating — researchers typically use these methods on finds at least , years old.

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While K-Ar dating requires destroying large samples to measure potassium and argon levels separately, Ar-Ar dating can analyze both at once with a single, smaller sample. The uranium-thorium method is often helpful for dating finds in the 40, to ,year-old range, too old for radiocarbon but too young for K-Ar or Ar-Ar. Silicate rocks, like quartz, are particularly good at trapping electrons.

Researchers who work with prehistoric tools made from flint — a hardened form of quartz — often use thermoluminescence TL to tell them not the age of the rock, but of the tool. Green plants absorb the carbon dioxide, so the population of carbon molecules is continually replenished until the plant dies. Carbon is also passed onto the animals that eat those plants. After death the amount of carbon in the organic specimen decreases very regularly as the molecules decay. Samples from the past 70, years made of wood, charcoal, peat, bone, antler or one of many other carbonates may be dated using this technique.