Scientists explain: how is ancient material dated

“As an archaeologist, especially if you dig a lot, you always have to consider time. This involves both absolute time and so-called relative time, or what happens before or after an event. Therefore, the whole digging process, as you get down through the layers of soil, is about when things have happened in relation to each other,” explains Charlotte Hedenstierna-Jonson, researcher at the Department of Archaeology and Ancient History.

In general, different soil layers represent different events and time periods. The deeper below the ground’s surface something is found, the older it is. Stratigraphies, i.e. cross-sections of the various soil layers, can also show exact times in the form of a layer of soot from a documented fire or ash from a known volcanic eruption, for example.

“The techniques I use the most myself are these stratigraphies, but also object-based dating, typologies and building time scales based on objects,” says Hedenstierna-Jonson.

Archaeologists have been cataloguing objects for a long time and thus have a good understanding of how things have looked during different historical eras. Coins are especially useful because they often show when they were minted.

If a larger wooden object is found, such as a ship, the annual growth rings in the wood can be used to produce very precise dates. The method is called dendrochronology and there are extensive catalogues of variations in the patterns of annual rings over at least the last thousand years. However, a well-preserved and relatively large piece of wood with many annual rings is required to produce reliable answers.

“If you have that, as well as access to the bark, then you can really tell exactly when it fell during that year,” notes Hedenstierna-Jonson.

The carbon-14 method, which is perhaps the most well-known dating technique, is not quite as accurate. It only works on organic material such as bones, food waste or excrement. With an accuracy of about 30 years, it can pinpoint when the person, animal or plant died.

Carbon-14 in organisms

The carbon-14 method is what is known as a radiometric method based on measuring the ratio between different variants, or isotopes, of the element carbon. Carbon-14 is a radioactive isotope absorbed by plants during photosynthesis and then by animals via food. When an organism dies, uptake ceases and the radioactive carbon-14 atoms in it decay into nitrogen. The proportion of carbon-14 in relation to other carbon variants will then decrease according to a time-determined pattern that can be measured.

“I usually say that we have a trillion carbon atoms per carbon-14 atom in us. Therefore, we need a huge machine to detect them. Carbon-14 has a half-life of 5,730 years and we can measure a sample with an age down to eight or ten half-lives, so it's 50,000 or 60,000 years max,” explains Bryan Lougheed.

Lougheed is a climate scientist at the Department of Earth Sciences and uses the carbon-14 method himself to date the shells of small marine organisms called foraminifera. They can be used as a kind of time capsule of ancient climates.

Other methods for older fossils

The carbon-14 method does not work on fossils or rocks older than 50,000 years. In such cases, there are other radiometric methods to investigate the radioactive decay in other elements.

“If you want to know how old the earth is, for example, you can use something called the uranium-lead method. There are different types of uranium that decay into different types of lead. The two that are being examined have half-lives of about 4.5 billion years and 700 million years. With those methods, they were able to determine the age of the earth at 4.5 billion years in the 1950s,” says Lougheed.

The radioactive decay in these substances occurs more slowly than in carbon-14 and the precision will therefore not be as high either. If uranium-lead is used, the margin of error is a million years here or there compared to carbon-14, where the margin of error can be as low as 30 years.

In addition to analysing radioactive decay, geologists also use so-called magnetic dating when they want to find out how long ago a certain volcano erupted, for example. Then they study how magnetic minerals in the lava are oriented. The Earth’s magnetic field varies, namely, and the magnetic poles even switch places with each other from time to time, meaning the magnetic north pole becomes the south pole and vice versa. This results in magnetic minerals being fixed in what was a north-south direction when the lava solidified. How and when the Earth's magnetic field varied is fairly well researched.

Just as archaeologists use stratigraphies, geologists use stratigraphic sequences for relative dating that can sometimes stretch back billions of years. Rocks formed from sediments, for example limestone or clay, accumulate in different layers that can be dated partly by magnetic dating but also by radiometric dating of embedded fossils. In this way, chronological sequences have been pieced together concerning very long periods of time.

Of all the life that has existed on earth, only an extremely small proportion has left traces. Therefore, fossils do not provide a comprehensive picture of when today’s species arose. However, those answers can be found in our genetic material.

“When we analyse DNA, we can see mutations, i.e. the changes in the DNA. They largely happen in a random way and can almost be considered in the same way as the decay of an atom. They accumulate so that the more mutations distinguish one species from another, the further back in time these two species have a common ancestor. This means that we can use the genetic differences between a human and, for example, a mouse or a chimpanzee,” explains Mattias Jakobsson, Professor of Genetics at the Department of Organismal Biology, who researches human evolution.

Although mutations occur randomly, there is a mutation rate.

“If you take human DNA from one generation to the next, from an individual who has a child, you expect maybe five mutations on average. Then you can calculate how many changes you expect per generation or year and use the mutation rate to date a common predecessor that existed several million years back in time,” adds Jakobsson.

This method can be used if you want to find out when humans and chimpanzees diverged, for example. The answer will then end up somewhere around 7­–8 million years ago.

­It was through mutation studies that Jakobsson and his research team came to the conclusion in 2017 that modern humans, Homo sapiens, appeared around 300,000 years ago. This made headlines as it was 100,000 years earlier than previous estimates.

Using DNA from 2,000-year-old bone material from South Africa, they investigated how common certain specific mutations were in different population groups. In this way, they could track when a mutation occurred.

“We used these tools and looked at how common these changes were in the genome of Stone Age individuals. Based on that, we could calculate how far back in time all humans have a common ancestor. In all comparisons, we landed in the order of between 250,000 and 350,000 years back in time,” says Jakobsson.

Åsa Malmberg