Methods of Scientific Dating
An image of the Grand Canyon, with layers of the Earth exposed. (Source: National Park Service)
Our past is unwieldy, a maze that stretches across billions of years, but a maze that must be solved. This most basic step in scientific pursuit is naturally the most fundamental; nothing can be known of an object or event if we don’t know when it happened. Luckily scientists have a long list of tools at hand to reconstruct the labyrinthine past.
Scientific dating falls into one of two main categories: absolute dating and relative dating. As their names suggest, absolute dating seeks to find a definitive numerical age of a given specimen (or as close as possible), while relative dating arranges past events in rough sequence with each other. While absolute dating is definitionally more precise, relative dating is still a valuable tool, with scientists using both methods to assemble a timeline of the ancient past.
Willard Libby, who won the Nobel Prize in Chemistry for discovering radiocarbon dating. (Source: UCLA)
In the realm of absolute dating, radiocarbon dating makes use of unstable carbon isotopes to pinpoint the age of an organic specimen. Carbon-14 is formed through the interaction of cosmic rays with nitrogen in the upper atmosphere. From there, Carbon-14 enters the carbon cycle, where it embeds itself in plant life and animals.
Because Carbon-14 is unstable, it begins to break down after the organism dies. By measuring the amount of Carbon-14 and comparing it to the rate at which it decays, scientists are able to pin down the age of a given specimen, provided the specimen is younger than 50,000 years. A lower carbon amount means an older specimen, as it has more time to decay. Other elements such as potassium can also be used to date items in different chronological groups.
Another absolute dating method is archaeomagnetic dating, which makes use of the Earth’s magnetic field and its effect on a given object. As an object (say a piece of pottery) is heated to a certain point, it passes over the Curie point where it loses its magnetic properties. As it cools again it regains this property, preserving within it a snapshot of the Earth’s magnetic field at that time. Based on the estimated flow of the field, one can match the object to when the field’s direction last corresponded to the object’s snapshot, allowing an estimate of its age.
A trilobite, often used as an index fossil. (Source: Smithsonian Institution)
Relative dating can’t hope to be as precise as carbon dating or archaeomagnetism, but it’s still a useful tool, especially in the dating of the Earth’s strata. One means of accomplishing this is by using index fossils. These fossils allow paleontologists to estimate the age of surrounding specimens. They require a specimen that has a wide range geographically but a fixed range temporally, allowing some certainty in estimating the age of the area wherever the index fossils are found.
Index fossils are useful for understanding the ancient past, but it’s obviously unsuited for objects from human history. Seriation on the other hand applies a similar principle but with man-made objects. On a basic level, seriation is simply the ordering of data into a chronological order. In an archaeological context, seriation relies on identifying stylistic attributes of a given culture to see how often they appear in their objects. For example, if object A and object B share three attributes, they’re closer in time to each other than object C that only shares one attribute.
When these techniques are combined together, scientists can quickly date large areas of geologic samples. For instance, when an index fossil such as an ammonite is absolute dated, you can look for fossils of the same species in other geologic formations to get a good approximation of the age of other fossils around the ammonite.
There are dozens of other methods of scientific dating, from the ultra precise to the merely chronological, but both types are invaluable tools in reconstructing the past. From simple human artifacts, to fossils frozen in time millions of years ago, scientists are able to pinpoint the age of all manner of artifacts, providing the foundation for scientific pursuit and discovery.
Bowman S. Radiocarbon Dating. University of California Press; 1990.
O’Brien MJ (Michael J, Lyman RL. Seriation, Stratigraphy, and Index Fossils: the Backbone of Archaeological Dating. Kluwer Academic Publishers; 1999.
Rink, W. Jack., and Jeroen. Thompson. Encyclopedia of Scientific Dating Methods. Ed. W. Jack. Rink and Jeroen. Thompson. Dordrecht: Springer Netherlands, 2020. Web.
Vaknin, Yoav et al. “The Earth’s Magnetic Field in Jerusalem During the Babylonian Destruction: A Unique Reference for Field Behavior and an Anchor for Archaeomagnetic Dating.” PloS one 15.8 (2020): e0237029–e0237029. Web.