I have gathered material to help you better understand how archaeologists estimate the approximate age of artifacts. The video clip below provides an overview of the main dating methods used. Please note that this topic is complex for those under 14 years of age.
Enjoy exploring!Clip produced by the Sherbrooke Museum of Nature and Science
Date: September 2018
Length: 9:17
An animated character representing Éric Graillon summarizes the main dating methods used: Stratigraphy, absolute dating methods (carbon-14 dating and optically stimulated luminescence), and typology. The topic is complex for those under 14 years of age.
On-screen text: Dating methods
On-screen clip, in colour, of the Éric Graillon character seen to the left of an image of planet Earth rotating in the background, with scrolling images of an exclamation mark, prehistoric humans, an arrowhead, and a question mark at the forefront.
[Animated character] One of the first questions that we are asked as archaeologists is: WHEN?
WHEN did the first humans arrive in Quebec?
WHEN was this artifact made?
WHEN? That is the BIG question in archaeology, especially prehistoric archaeology.
On-screen colour clip: Against a background of blue sky, mountains, and sand, the stages of human evolution are shown from monkey, to standing monkey, prehistoric human walking with an artifact, prehistoric human holding a lance, and to conclude, a human dressed in fur holding a stone slab with writings on it.
[A.C.] Prehistory covers the period between the beginning of humanity, about 2.8 million years ago, and the appearance of the first written documents. Therefore, prehistory does not end at the same time everywhere.
On-screen images in colour: Against a classroom background, an illustration of Jacques Cartier talking with Indigenous peoples along the shore. Jacques Cartier’s boat and crew are visible in the background.
[A.C.] In Quebec, prehistory ended when Jacques Cartier met Indigenous peoples in 1534. By writing accounts of his travels, Jacques Cartier ended the prehistoric period.
On-screen video: Jacques Cartier’s journal.
[A.C.] The question arises: How are sites and artifacts dated over such a vast period of time?
On-screen: Animated time line with an arrow moving -2.8M years to the left and toward the year 1534 to the right, against a background with a clock ticking counterclockwise.
On-screen text: 1st method: Stratigraphy
[A.C.] First method: Stratigraphy. Archeologists find their first clues in the field. They consult the dating work carried out by paleontologists and geologists.
On-screen colour images: Mixed forest in a mountain valley.
[A.C.] They analyzed the fossils found in different geological layers.
On-screen, colour illustration: A rocky surface on which images of artifacts and fossils are displayed: Shell, ammonite, trilobite, etc.
[A.C.] The appearance, evolution, and disappearance of species led archaeologists to create groups, first in stages, then periods, and lastly eras. They also produced the geologic time scale.
On-screen, colour animated table: The various eras are defined: Cenozoic, Mesozoic, Paleozoic, and Precambrian eras. Each era is divided into subcategories. The Cenozoic Era and its subcategories: The Quaternary, Neogene, and Paleogene periods. The Mesozoic Era and its subcategories: The Cretaceous, Jurassic, and Triassic periods. The Paleozoic Era and its subcategories: The Permian, Carboniferous, Devonian, Silurian, Ordovician, and Cambrian periods. The Precambrian Era is not subdivided.
[A.C.] The soils that contain Quebec’s artifacts are a part of the last period in the geologic time scale; the Quaternary Period, itself divided into the Pleistocene and the Holocene.
On screen : A second table shows the two Neogene, and Paleogene periods, each subdivided into subcategories : the Pliocene and Miocene for the Neogene are, and the Oligocene, Eocene and Paleocene for the Paleogene era.
On-screen: View of the Quaternary part of the Cenozoic Era, from the previous table. It is divided into two parts:Holocene, with the text: 11 500 years ago, and the Pleistocene with the text: 2.58M years ago.
[A.C.] Universally, the Pleistocene epoch corresponds with the arrival of the first representatives of humankind in Africa and their dispersion across Europe, Asia, Oceania, and, finally, America. In Canada, the Pleistocene epoch was defined by cycles of glaciation.
On-screen: Colour clip of the globe below the table and a prehistoric human travelling to different places around the globe.
On-screen: Colour interactive geographical map of Canada and the northern United States showing the melting of ice from around 9 000 to 7 000 years BP.
[A.C.] The Holocene epoch followed, with warmer temperatures. Warmer temperatures caused the vast ice sheets to melt, resulting in the end of the last glaciation period, that of the Wisconsin.
On-screen: Colour videos of scrolling landscapes.
[A.C.] With the retreat of the glaciers, soils built up over time, with layers being superimposed one over another. Generally, the thickness and buried remains of a soil are used as reference points to evaluate its age.
On-screen: Colour illustration of a cross-section of soil, with several layers in which artifacts are seen. An arrow moves up and down.
[A.C.] The deeper we go in the soil, the further we go back in time. On the condition, of course, that the soil has not been disturbed by the action of freezing and thawing, the falling of trees, and holes dug by burrowing animals, or landscaping work.
On-screen: The same illustration of a rocky surface over which are shown a table of the geologic time scale along with images of fossils associated with each era.
On-screen: The same illustration of a table of a section of the Cenozoic Era and the planet Earth indicating the arrival of humans in North America.
[A.C.] Thus, thanks to stratigraphy and the discovery of ancient artifacts, we can associate the arrival of the first humans in the Eastern Townships to the last two millennia of the Pleistocene epoch, about 12 000 years ago. That is the first method. You are no doubt wondering if more accuracy is possible...
On-screen text: Second method: Absolute dating techniques
[A.C.] Second method: Absolute dating techniques
On-screen: Image of a time line with the decades 1950 to 1990. An arrow points to 1947 on the time line.
[A.C.] Before 1947, dating artifacts could only be done from a relative perspective, meaning there was no quantification of the number of years.
Two illustrations of projectile points are shown on screen.
[A.C.] For example: One artifact would be deemed older than another... Dating was based on observations in the field.
On the time line, at year 1947, is a black-and-white photo of a man’s face.
[A.C.] In 1947, Willard Frank Libby conducted the first carbon-14 dating, a dating method carried out independently in a laboratory by chemists and physicians.
On-screen: Clip of a chemical element composed of red and purple balls with white rings swirling around them.
[A.C.] Want to know how it works?
On-screen text: Carbon-14 dating
[A.C.] Carbon-14 dating
On-screen: Video of dead leaves on the ground, followed by a representation of carbon 12, carbon 13, and carbon 14 isotopes with captions indicating how many protons and neutrons they have.
[A.C.] In nature, carbon is represented by three isotopes. i.e. carbon 12, carbon 13, and carbon 14, which contain 6, 7, and 8 neutrons respectively.
On-screen: Video of a bunch of raspberries.
[A.C.] In a living being, 99% of carbon is in the form of carbon 12; 1% in the form of carbon 13; and carbon 14 is only present in trace amounts. Luckily so, as it is radioactive.
On-screen: Three graphic representations indicating the amounts of each isotope: A large carbon 12 bar, a small carbon 13 bar, and a dot to represent carbon 14.
On-screen: Graph with two axes, the vertical representing the amount of carbon, and the horizontal representing time. The curve indicates that the amount of carbon diminishes over time.
[A.C.] More specifically, scientists have observed that living matter contains one carbon-14 atom for every 750 billion carbon-12 atoms.
On-screen: Photo of bones.
[A.C.] When an organism dies, its carbon-14 mass begins disintegrating. Its amount lessens by a half every 5 730 years. This is what is called radioactive half-life.
On-screen: Clip of a mammoth on the move.
[A.C.] By measuring the amounts of carbon 12 and carbon 14, the date of the death of a plant or an animal can be determined. Results are expressed in years.
On-screen: Video of a forest at sunrise. A close-up of a tree trunk appears.
[A.C.] The more measurements scientists have been taking, the more they have realized that the concentrations of carbon 12 and carbon 14 must have varied over time. They had to review their results. The challenge was to determine the concentrations of carbon 12 and carbon 14 at different times.
On-screen: 9 illustrations of cross-sections of tree trunks in which growth rings can be seen.
[A.C.] The solution came from ancient tree trunks. Scientists analyzed their growth rings and measured concentrations of carbon 12 and carbon 14. This is the field of “dendrochronology.”
On-screen: Illustration of a close-up of a cross-section of a tree trunk on which different sections of rings are indicated by coloured circles.
[A.C.] Using these discoveries, in 1993, they created a calibration curve to fine-tune their measurements. This is how carbon-14 dating and its calibration are carried out.
On-screen: Clip of a carbon-14 isotope whose electrons are orbiting a cluster of protons and neutrons.
[A.C.] Results could only be inferred, as the method was only applicable to organic matter and not to stone artifacts.
On-screen: Image of a bone and a stick to the left and an arrowhead marked with a red X.
[A.C.] Of the three Palaeoindian sites in the Eastern Townships, none could be dated using carbon 14. Fortunately, there are other means…
On-screen text: Optically stimulated luminescence.
[A.C.] Optically stimulated luminescence
On-screen: Image of an animated character in front of a chemistry lab containing an oven.
[A.C.] This alternative method of absolute dating was developed in recent decades, in parallel with the evolution of carbon-14 dating. Optically stimulated luminescence can only be used to date inorganic materials.
On-screen: Video of a log fire in front of which an illustration of a projectile point appears.
[A.C.] Not all inorganic materials: Only those which have already been exposed to significant temperatures, such as firestone, pottery cookware, or chipped stone objects thrown into a fire.
On-screen: Visual of a continuum on a graph indicating the accumulated energy, which increases over time in the environment.
[A.C.] The principle behind optically stimulated luminescence is the property of certain crystals, such as quartz and feldspar, which accumulate natural irradiation energy from the soil and release it in the form of light when heated up.
On-screen: Photo of quartz crystal with a luminous point.
[A.C.] Once all of a crystal’s energy has been released, the counters return to zero and energy accumulation can start again.
On-screen: Illustration of the same laboratory with an oven containing a stone that emits light.
[A.C.] Thus, to measure the age of a heated stone, scientists take the object from the site, heat it up in a laboratory, and measure the quantity of light it emits. They get a first measurement, which reveals the amount of accumulated radiation.
On-screen: Video of the banks of a river with stones along the shore.
[A.C.] To determine how many years this represents, they measure the natural radiation of the land where it was found.
On-screen: Illustration of the laboratory in which an oven contains a stone that is being irradiated.
[A.C.] After, in the laboratory they re-expose the piece of stone to radiation equivalent to a thousand years in the field. After irradiating the stone, they heat it up and measure the quantity of light emitted. Thus, through a simple rule of three and some correctional factoring, they calculate the time that has passed since the last time the object was heated in its natural environment.
On-screen: Image of a paperweight on which the rule of three is represented in an animated clip. Age = Quantity of light emitted 1 x 1 000 years ÷ Quantity of light emitted 2
[A.C.] Optically stimulated luminescence was used to date sediments on the Cliche-Rancourt site and the firestone on the Kruger 2 site.
On-screen: Transition clip of a calm river and an image of a stone emitting light and waves.
[A.C.] This concludes the section on optically stimulated luminescence and absolute dating methods. It is important to note that neither of these two methods can really be used to measure the age of a projectile point. The age of a projectile point is determined based on extrapolations. Can artifacts tell us anything else?
On-screen text: 3rd method - typology
[A.C.] 3rd method – typology
On-screen: Visual of an animated character in front of a blackboard on which both sides and a cross-section of a single projectile point are shown.
[A.C.] Typology is a method based on classifying artifacts based on their appearance.
On-screen: Illustration of a collection of arrowhead projectiles of various shapes and sizes.
[A.C.] We will now look at stone-chipping techniques, as well as the shapes created and the materials used. In prehistoric times, this was the main method used for ceramics and projectile points. Designs and shapes are associated with cultures and traditions. One must be very methodical and use feedback loops.
On-screen: Illustration of radar rays demonstrating a feedback loop.
[A.C.] When a discovery is made, it is described in detail and its location and position in the soil are recorded.
On-screen: Illustration of the laboratory with an oven containing a stone that emits waves.
[A.C.] If possible, objects that can be dated using absolute dating methods are sent to a laboratory. Results are validated with other discoveries from nearby sites.
On-screen: Close-up clip of a hand moving a computer mouse.
[A.C.] If the results converge, the artifact's information is integrated into a database to validate other discoveries.
On-screen text: Conclusion.
[A.C.] Conclusion
On-screen: Clip of flashing red and blue police car lights, followed by a close-up clip of a 360-degree view on microscopes.
[A.C.] Site dating can be compared to crime scene investigation. There are no witnesses, and all evidence must be gathered to try and understand what happened.
On-screen: Clip of a forest in a valley.
[A.C.] The depth at which an artifact is found and the thickness of the soil layer provide the first clues.
On-screen: Image of an isotope with spinning electrons, followed by a clear quartz crystal.
[A.C.] Organic matter and heated stones are precious clues once they have been dated in a laboratory.
On-screen: Visual of an animated character in front of a blackboard on which both sides and a cross-section of a single projectile point are shown.
[A.C.] Finally, typology is used to identify cultural associations. It helps to reconstitute history.
On-screen: Illustration of the lab.
[A.C.] Pooling all the evidence, including that from all measuring techniques, and the results of digs and analyses, makes is possible to develop a deeper understanding. Just as when identifying a perpetrator, the convergence and consistency of clues lead to reliable results. The more archaeological discoveries are made, the more clues are accumulated, and the more we can develop a time scale of events to which we can link artifacts.
On-screen: Clip of a no-digit dial whose needle rotates counterclockwise, followed by an abstract animation of triangles that are being deconstructed.
[A.C.] New artifacts are discovered every year. Who knows, maybe the dating technologies of tomorrow will provide an even more accurate picture of Quebec’s prehistoric settlement.
