Above: The warship Mary Rose as depicted in the Anthony Roll, a record of ships in the English Tudor navy produced during the 1540s (Gerry Bye, Wikimedia Commons)
It takes effort to learn a new language. But once you’ve mastered it, it can open up a world of information. Isotopes—atoms of the same element that have different numbers of neutrons—are a lot like a foreign language. Once you know how to interpret them, you can learn a lot more about the world around you.
At a very basic level, atoms tell you what things are made of. Water, for example, is made of two hydrogen atoms and one oxygen atom. But atoms have other stories to tell, too. Their isotopes can shed light on the history of Earth’s climate, the lives of your ancestors, and even who is killing endangered animals.
Oxygen isotopes reveal the history of Earth’s climate
Atoms of the same element always have the same number of protons. But the number of neutrons can vary. For example, all stable oxygen atoms have 8 protons. However, while some have 8 neutrons, others have 9 or 10.
There are three naturally-occurring isotopes of hydrogen. Protium (hydrogen-1), has one proton and no neutrons). Deuterium (hydrogen-2), has one proton and one neutron. And tritium (hydrogen-3), has one proton and two neutrons (Dirk Hünniger, Wikimedia Commons)
These different oxygen isotopes all behave the same way in a chemical reaction. However, they don’t always behave the same way physically. For instance, consider the isotopes oxygen-16 (8 protons plus 8 neutrons gives an atomic weight of 16) and oxygen-18 (8 protons plus 10 neutrons). While the ratio between oxygen-16 and oxygen-18 is fairly stable in ocean water, it can vary greatly in ice.
Water molecules that contain oxygen-18 are heavier than water molecules that contain oxygen-16. As a result, when water evaporates, molecules that contain oxygen-18 tend to evaporate more slowly. And when evaporated water condenses again, water molecules containing oxygen-18 tend to condense more quickly—much more quickly if temperatures are very cold.
As a result, the concentration of oxygen-18 in very old ice can provide information on prehistoric patterns of heating and cooling. Ice that has evaporated and condensed many times contains a higher concentration of oxygen-18 than ice that has not evaporated and condensed as often.
For example, during periods when temperatures frequently rise above 0° C, sea ice evaporates and recondenses more often. As a result, the ice will have a higher concentration of oxygen-18. So when they wanted to confirm the dates of the last ice age, climate scientists measured the concentration of oxygen-18 in the ice sheet that covers Greenland. They were able to determine that the Earth’s temperature cooled down significantly about 110,000 years ago and warmed up again about 20,000 years ago.
Did you know? In the 1500s, a return fishing trip from England to Iceland took three to six months. A return trip to Newfoundland took even longer.
Carbon isotopes uncover Renaissance history
In 1545, while attempting to repel a French invasion, the English warship Mary Rose sank. When it was brought to the surface over 400 years later, the ship still contained thousands of dried cod bones. English sailors ate a lot of cod in the 1500s! But where did those fish come from? Were they caught nearby or were they transported long distances?
Carbon isotopes helped solve that mystery! Like oxygen-16 and oxygen-18, the ratio of carbon-12 to carbon-13 in Earth’s atmosphere is fairly constant. But depending on where they grow and live, plants and the animals that eat them accumulate different ratios of these two isotopes.
Researchers determine the concentration of different isotopes in a sample by using a process called mass spectrometry. First, molecules are heated and vapourized. Next, electrons are removed to turn the molecules into electrically charged ions. The resulting ions are accelerated through a magnetic field. As they pass through the magnetic field, lighter ions—isotopes with fewer neutrons—will change direction more more than heavier ions. By comparing the current produced by ions that follow different paths, a mass spectrometer can be used to determine the relative concentrations of two different isotopes in a sample.
All plants absorb carbon dioxide from the air. Through photosynthesis, they use the carbon to create the carbohydrates they need for energy. Grasses that are native to temperate climates build carbohydrates mostly out of carbon-12 and release most of the carbon-13 back into the atmosphere. Grasses that grow closer to the equator incorporate both isotopes of carbon into their carbohydrates.
So by analyzing the ratio of carbon-12 (with 6 neutrons) to carbon-13 (with 7 neutrons) in a piece of bone, scientists can learn a lot about what an animal ate and where it came from. This is especially true for animals that live on land, but isotopes can still say a lot about where fish came from.
Among other tests, archaeologists measured the ratio of carbon-12 to carbon-13 in the cod bones they found on the Mary Rose and compared them to other cod caught around the same time. Many of the cod bones had isotope ratios unlike those found in cod caught near England. But they had very similar isotope ratios to cod caught as far away as Iceland. One sample even showed similarities to cod caught near Newfoundland. That’s how far 16th-century England was prepared to send ships to find food for its growing navy!
Isotopes can even be used to catch ivory poachers. As conservationists work to protect the endangered African elephant, poachers continue to kill elephants and sell the ivory in their tusks on the black market. Gathering evidence to convict a poacher can be difficult, especially since it is legal to sell small amounts of ivory from certain countries.
Elephants that live in different parts of Africa have slightly different diets. As a result, they have different ratios of carbon, oxygen, and hydrogen isotopes in their bodies.
Researchers have analyzed hundreds of pieces of ivory and created a map linking specific ratios to different locations. So it is now possible to tell where an ivory sample came from by analyzing the isotopes of carbon, nitrogen, and hydrogen it contains. In fact, scientists can trace ivory back to its source with 95% confidence. Identifying where a piece of ivory came from can help prosecute poachers, and help elephant populations recover.
As you learn to interpret the stories that isotopes have to tell, you begin to understand how scientists can use them to better understand the past, and even help build a better future.
Website with a general introduction to stable isotopes, with links to additional resources:
Website and scientific article about how isotope analysis was used to discover the origins of cod bones found on the warship Mary Rose: