Isotope analysis in archaeology facts for kids
Imagine detectives using science to solve ancient mysteries! That's a bit like what Isotope analysis does in archaeology. It helps scientists learn about the past. They can figure out how old sites and objects are. They can also discover what ancient people ate and where they traveled. Even past environments can be understood.
This information comes from looking at the amounts of different isotopes in a sample. Isotopes are like different versions of the same atom. The most common isotopes studied in archaeology are carbon, oxygen, nitrogen, strontium, and calcium.
An isotope is an atom of an element with a different number of neutrons. This changes its atomic weight. Isotopes can be stable or unstable (also called radioactive). Unstable isotopes break down at a steady rate over time. Scientists first found stable isotopes in 1913. Most were identified by the 1930s.
Archaeology was slow to use isotope studies at first. Other sciences like chemistry and biology quickly used isotope analysis in the 1950s and 1960s. This was after machines called mass spectrometers became common. It wasn't until the 1970s that isotope analysis became a key part of archaeological study.
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What are Isotopes?
Carbon Isotopes
Carbon is found in all living things. This includes bones, charcoal, and old food bits. It's super important for dating things using radiocarbon dating. The amounts of different carbon isotopes naturally change over time.
Scientists can study ancient air bubbles trapped in ice. By looking at the carbon dioxide (CO2) in these bubbles, they build a timeline of these changes. Plants like grasses take in CO2 during photosynthesis. Animals then eat these plants. So, the animals get the same carbon "signature" as the plants.
Later, humans might eat those animals. By matching the carbon isotopes from an old sample to the ice record, scientists can guess its age. After an organism dies, it stops taking in CO2. The unstable 14C carbon then slowly decreases. This steady decrease is called its decay rate.
Oxygen and Nitrogen Isotopes
Oxygen and nitrogen also have different isotopes. Their amounts vary depending on location and climate. Our bodies absorb oxygen from water (H2O). This oxygen is used to grow tissues.
Like carbon, oxygen isotope differences can point to specific places. The amounts of oxygen isotopes can help scientists figure out past climates. They can also learn about ancient diets and water use. It even helps understand how people moved around and lived their lives.
Strontium Isotopes
Strontium is found in our bones and teeth. We get it from the food and water we consume. Each place on Earth has its own unique strontium isotope signature. So, if scientists find a certain strontium ratio in a bone or tooth, they can match it to a region.
Tooth enamel forms when a person is a child. So, strontium from tooth enamel shows where someone lived when they were young. Bones, however, are always changing. This means strontium in bones tells us about an adult's diet and location.
If the strontium ratios in bones and teeth are the same, the person likely stayed in one area their whole life. If they are different, scientists can map where the person was born and where they died. This helps understand how people moved. For example, studies at Stonehenge showed that both people and cattle traveled long distances. Some strontium ratios matched areas in Scotland and Wales.
Calcium Isotopes
Calcium, like strontium, is found in bones and teeth from our diet. Calcium is more easily stored in humans and animals who eat mostly plants. So, if a sample has a lot of calcium compared to strontium, it means the animal likely ate more plants.
How Isotope Analysis Works
Getting the Sample Ready
Before isotopes can be separated, the right part of the bone or tooth must be taken out. These parts include collagen, carbonate, and apatite. Each part needs different ways to be removed. Scientists also have to be careful about how decayed or dirty the sample might be.
For collagen (a protein in bones), here are some ways to get it out:
- Small bone pieces are soaked in a weak acid. This removes the hard mineral part. If there's still old organic stuff, it might need another soak. Then the clean collagen is dried.
- Another way is to use a salt solution to separate the collagen from the bone. Then it's dried.
- Powdered bone can be soaked in a stronger acid. This is good for very old or damaged bones. But it also has a higher risk of getting dirty from other things.
Scientists always check the cleaned sample. They make sure it's pure enough before testing it. Many modern machines can even check this automatically.
Using a Mass Spectrometer
A mass spectrometer is a special machine. It separates and measures the different isotopes in a sample. Archaeologists often use a type called an Isotope Ratio Mass Spectrometer (IRMS). It has an inlet, an ion source, a mass analyzer, and detectors.
The sample usually goes into the machine as a gas. For example, oxygen and carbon are turned into carbon dioxide gas.
Strontium is harder to handle as a gas. Instead, it's heated in a vacuum until it turns into ions (charged atoms). This method is called Thermal Ionization Mass Spectrometry (TIMS). Recently, scientists are also looking at a less invasive method for strontium called laser ablation.
Inside the machine, electrons hit the gas. This turns the molecules into a beam of ions. This beam then splits into smaller beams based on their mass. This creates a "mass spectrum." The machine then measures how strong each beam is. This tells scientists the ratio of the different isotopes.
Amazing Discoveries
Paranthropus Diet
Plants can be identified by their carbon isotope ratios. This is because of how they do photosynthesis. Some plants (C3 plants) have more 12C. Others (C4 plants) have more 13C. This carbon signature passes up the food chain. So, it can tell us what ancient people and animals ate.
Isotope analysis helped scientists understand the diets of different Paranthropus species. These were ancient human relatives. They found that P. boisei likely ate a lot of grasses and sedges. P. aethiopicus showed a similar diet. But P. robustus ate a wider variety of foods. Also, carbon isotope analysis showed that around 2.37 million years ago, early humans started eating more C4 plants.
"Ötzi the Iceman" and Ancient Life
Ötzi is a Neolithic man found in 1991. He was discovered in an Alpine glacier between Austria and Italy. Ötzi was incredibly well-preserved because he was dried out and frozen in ice. Radiocarbon dating showed he lived about 5,200 years ago.
Scientists used different isotope methods on Ötzi's bones and teeth. His teeth suggested he was born and spent his early childhood near where the Eisack and Rienz rivers meet. But his bones showed that as an adult, he moved to a different valley. More recent studies of his gut contents suggest his movements might have been seasonal.
White Sands Trackway and Early Americans
The earliest strong proof of humans in the Americas was thought to be from the Clovis culture. This was between 11,050 and 10,800 years ago. However, human footprints were found at White Sands National Park in New Mexico. These tracks were controversially dated to between 23,000 and 21,000 years ago. This was during the Last Glacial Maximum (a very cold period).
Besides human prints, the trackway also shows prints from a Columbian mammoth and a giant ground sloth. Scientists could guess the latest time the prints were made by knowing when these animals died out. More exact dates came from radiocarbon dating of ditch grass seeds found in the prints. These seeds gave the 23,000-21,000 year old date.
However, radiocarbon dates are not always perfect. This dating is still debated. Some scientists think the tracks were made by the Clovis culture. They believe the older dates might be wrong. Older seeds could have washed onto the wet clay and been pressed into the prints. Also, aquatic plants like ditch grass get their carbon from their environment. If that environment had less 14C, it could make the dates seem older than they are.
Images for kids
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The decay rate of the radioactive 14C isotope, used in radiocarbon dating artefacts (public domain image).
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Mass spectrometer, used to separate and measure elemental ions (public domain image).
