Part 1: Relative and Radiocarbon Dating
A question I frequently hear about archaeology is, “How do archaeologists know how old something is?” Indeed, determining when an artifact or feature was made is a key part of learning about past civilizations.
There are several dating methods that help archaeologists figure out how old objects are. In fact, there are so many that it would be impossible to describe them all in one article. Hence, this post will discuss some of the most widely-used dating methods – stratigraphy, typology, seriation, and radiocarbon dating – and we will cover the rest in subsequent articles.
Relative Dating Methods
There are two overarching classes of dating methods: relative and absolute. Relative dating methods cannot determine the exact age of an object, but only which finds are older or younger than others. The most important relative dating method relies on a site’s stratigraphy.
When excavating an archaeological site, you can literally see the layers of dirt and debris that have accumulated over time. These layers are known as a site’s stratigraphy, and the law of superposition, first popularized by Sir William Matthew Flinders Petrie, states that the oldest stratigraphic layers are at the bottom. Thus, objects found near the top of a site are probably younger than the ones further down – unless something (like a burrowing animal) moved the items after burial.
Typology and Seriation
Other relative dating methods depend on examining the physical characteristics of archaeological finds. In a given culture – or amongst connected cultures – artifacts with similar styles (typologies) tend to be popular at specific times. This leads to the principle that “like goes with like,” or that objects that look the same were probably made during the same periods.
While studying the typologies of archaeological remains allows researchers to figure out which objects are close in age, seriation helps them track cultural trends.
Let me explain seriation through a hypothetical example. For instance, assume that archaeologists working in a given site have found the remains of many ceramic bowls that have distinctive, horizontal bands along their lips. By recording the stratigraphic layers in which they uncovered the bowls, the archaeologists noticed that they found a few bowls in the oldest layers of their site, lots of them in the middle layers, and then just a handful in the most recent layers.
Many assemblages, or groups of similar artifacts, go through this pattern: they emerge, rise to their peak in popularity, and then fade away. This pattern is known as the “battleship curve,” because it looks like a battleship when viewed from above (from the side???).
Here is a seriation diagram involving different types and time periods of tombstones at Old City Cemetery in Vancouver, Washington, where the data is laid out in the classic battleship curve pattern. Public domain photo, credits to the National Park Service.
By tracking the battleship curves of different assemblages of artifacts, archaeologists can observe the evolution of past technologies over time.
Absolute Dating Methods
Absolute dating methods can attach specific years to archaeological finds. More specifically, they provide ranges of possible years, because no absolute dating method is exact – with the possible exception of dendrochronology (tree ring dating). Each absolute dating method also requires a different kind of sample, which means that not every method can be used on each site. The most commonly-used absolute dating method needs carbon-based samples.
Radiocarbon dating was developed in the 1950s by chemist Willard F. Libby. It revolves around the element carbon-14 (14C): an isotope of carbon that is produced when the sun’s energy interacts with nitrogen atoms in the earth’s atmosphere.
Plants absorb 14C through photosynthesis, and animals consume it by eating plants or other animals. When plants and animals die they stop acquiring 14C. Since 14C is mildly radioactive and naturally decays into 12C, plants and animals slowly lose their 14C after they stop obtaining it.
Fortunately for archaeologists, 14C decays at a uniform rate. It takes 5730 years for half of the 14C in a given sample to decay, so scientists say that its half-life is 5730 years. When an organism is alive and acquiring carbon, its 14C content reaches an equilibrium with its environment. Thus, by measuring how much 14C is left in a dead plant or animal, and then checking this against the background 14C level in the atmosphere, scientists can discern how long ago that organism died.
Radiocarbon dating is an effective way to date carbon-based artifacts (e.g. charcoal, animal bones, seeds, etc.) that are up to 50,000 years old. Radiocarbon dates are usually expressed as a range of years BP, or before present (the year 1950), and might look something like 2000 ± 100 BP. Once archaeologists know the age of an organic sample, they can then apply that date to other archaeological remains found in the same stratigraphic layer.
Charcoal was a relatively common find on the excavations I worked on. Photo downloaded from Needpix.com.
Modern radiocarbon techniques can produce accurate results with small sample sizes, but radiocarbon dating still has drawbacks. The main issue is contamination.
If an archaeological sample comes into contact with another carbon-based object, it can alter the amount of 14C in that sample, making any subsequent radiocarbon dates inaccurate. Related to contamination is the reservoir effect.
14C mixes more slowly in water than it does air. Consequently, radiocarbon dates obtained from marine samples (any organism that lived in the sea) frequently appear older than the object actually is. This is a serious problem in regions like the Arctic, where indigenous peoples have long relied on marine mammals – including seals – to survive. Archaeological sites in the Arctic can be so inundated with seal oil that it becomes difficult to generate accurate radiocarbon dates, because many of the samples suffer from the marine reservoir effect.
Furthermore, radiocarbon dates must be calibrated. The amount of 14C in the earth’s atmosphere has fluctuated over time, which means that radiocarbon results do not translate directly into calendar years unless they are synced with reliable data about past 14C levels. This information, along with a powerful absolute dating method in its own right, comes from tree rings.
We will continue discussing archaeological dating methods here on StoneAgeMan with dendrochronology, or tree ring dating.
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