Relative and Absolute Dating

Relative and Absolute Dating

Archaeologists use two kinds of dating methods: relative dating and absolute dating.

Author: Mary Beth Fitts, Assistant State Archaeologist, Raleigh

Imagine someone telling you a story where all the important events happened in the wrong order. It might be confusing, or even make no sense at all. Being able to tell how old things are and put them in the right order is one of the most important skills archaeologists have. We call this skill dating because it is how we organize our discoveries in time, like dates on a calendar.

Archaeologists use two kinds of dating methods: relative dating and absolute dating. In relative dating, we determine which things are older or younger based on their relationships. For example, we know from geology that soil layers near the surface of the ground are usually younger than those deeper down. This relationship helps archaeologists know that objects we find deep in the ground are older than things we find closer to the surface.

Using relative dating, archaeologists can tell that artifacts found in the layer of soil closest to the surface, like the penny, pipe, plate in this picture, are younger than those in the soil layers deeper down.
Using relative dating, archaeologists can tell that artifacts found in the layer of soil closest to the surface, like the penny, pipe, plate in this picture, are younger than those in the soil layers deeper down.

But how old is old? To find the specific age of an object, archaeologists use absolute dating. Absolute dating methods measure the physical properties of an object itself and use these measurements to calculate its age. One of the most useful absolute dating methods for archaeologists is called radiocarbon dating. It works by measuring carbon isotopes, which are versions of the element carbon. All isotopes of carbon have 6 protons but different numbers of neutrons. One of the carbon isotopes that occurs in nature is radioactive; it has 8 neutrons and is called carbon-14.

Carbon-14 molecule
Carbon-14 has 6 protons and 8 neutrons, two more neutrons than stable carbon.

All plants and animals take in radioactive and non-radioactive carbon when they eat and breathe. When they die, the carbon-14 in their wood and bone starts its radioactive decay process. This means the amount of carbon-14 goes down over time in a predictable way. However, the amount of stable (non-radioactive) carbon remains the same. In radiocarbon dating, the amounts of stable carbon and carbon-14 in a piece of bone or wood are counted. The ratio of stable carbon to carbon-14 is then used to calculate the date when the radioactive decay process started; in other words, the time when the plant or animal died. This means that by using radiocarbon dating, we can tell when someone in the past hunted an animal or cut down a tree at an archaeological site. We can then use these absolute dates to put those events in order and understand how people’s lives changed over time.

The Office of State Archaeology’s Underwater Archaeology Branch recently recovered two American Indian dugout log canoes. Since they are made of wood, we were able to use radiocarbon dating to learn how old they are. One is a 13-foot canoe found in the South River near Autryville, North Carolina. Radiocarbon dating of the hull found that it is between 610 and 670 years old. Another canoe, 24 feet in length was discovered in Lake Waccamaw. The radiocarbon date for this canoe tells us it is between 910 and 970 years old. The oldest canoe ever found in North Carolina is a canoe from Lake Phelps; radiocarbon dating of this canoe indicated it is about 4,300 years old!

Using radiocarbon dating, archaeologists discovered that this canoe found in Lake Waccamaw was made between 910 and 970 years ago.
Using radiocarbon dating, archaeologists discovered that this canoe found in Lake Waccamaw was made between 910 and 970 years ago.

Being able to date things allows archaeologists to understand how people’s lives changed over time and to tell histories that connect the past and present. So very tiny things like carbon isotopes can be used to answer really big questions!

 

Images:
-Stratigraphy. Graphic created by NC Office of State Archaeology.
-Carbon-14 atom. Graphic created by NC Office of State Archaeology
-Lake Waccamaw Canoe. Image by NC Office of State Archaeology