Dating Techniques in Archaeology

Dating methods determine the age of archaeological finds. They are categorized into relative dating, which establishes the sequence of events, and absolute dating, which provides specific calendar dates or age ranges.

Relative Dating Methods

Relative dating establishes that an object or layer is older or younger than another without providing an exact date.

Stratigraphy

Stratigraphy is the study of the layers of soil and rock deposited over time. The law of superposition states that in undisturbed deposits, deeper layers are older than layers above them. Excavators use this to build a chronological sequence of human occupation at a site.

Typology

Typology involves classifying artifacts based on shared characteristics like shape, material, or decoration. Over time, styles change in a predictable manner. By identifying these stylistic evolution sequences, researchers place artifacts in a relative order.

Seriation

Seriation orders artifacts into a series based on changes in their frequency. It assumes that a specific style becomes popular, peaks in use, and then declines. Archaeologists map these frequency distributions to determine the relative age of different sites or assemblages.

Fluorine Dating

This method measures the absorption of fluorine from groundwater by buried bones. Older bones generally contain more fluorine. It is a relative method because the rate of fluorine absorption depends on local soil conditions, making it unreliable for comparing sites in different geographical areas.

Absolute Dating Methods

Absolute dating assigns a specific numerical age or a calibrated age range to an artifact or feature.

Radiocarbon (Carbon-14) Dating

Radiocarbon dating is the most common method for organic materials like wood, charcoal, bone, and shell. Living organisms absorb Carbon-14 throughout their lives. Upon death, this absorption stops, and the isotope begins to decay at a constant rate. The half-life of Carbon-14 is 5,730 years. It is effective for samples up to 50,000 years old.

Potassium-Argon Dating

Potassium-Argon dating measures the decay of radioactive potassium into argon gas within volcanic rock. It is used to date geological strata associated with early hominid fossils. Because of its long half-life, it is useful for dating sites ranging from 100,000 to billions of years old.

Dendrochronology

Dendrochronology, or tree-ring dating, relies on the annual growth rings of trees. Each year, a tree adds a new ring. The width of these rings varies based on climate conditions, creating unique patterns. By matching patterns from archaeological wood to master sequences of known ages, researchers can date wooden structures precisely to the year.

Thermoluminescence (TL)

Thermoluminescence measures the accumulated radiation in minerals like quartz or feldspar found in pottery or burnt flint. When these materials are heated to high temperatures, such as during firing in a kiln, trapped electrons are released. After firing, electrons begin to accumulate again. Re-heating the sample in a laboratory releases these electrons as light, which is measured to determine the time elapsed since the last heating.

Electron Spin Resonance (ESR)

ESR is used for dating materials that cannot be dated by thermoluminescence, such as tooth enamel or certain carbonates. It measures the buildup of trapped electrons due to natural background radiation. It is particularly valuable for sites dating from 50,000 to two million years ago.

Optically Stimulated Luminescence (OSL)

OSL dates the last time mineral grains like sand were exposed to sunlight. When sediment is buried, it stops being exposed to light, and radiation from the surrounding soil builds up within the minerals. Researchers use light in the laboratory to release this signal. This method is effective for dating the deposition of sediment layers where no organic material exists.

Comparison of Dating Techniques

Important Facts and Concepts

• The Three-Age System is the foundational classification of prehistory into Stone, Bronze, and Iron ages. It was proposed by Christian Jürgensen Thomsen in 1836 to organize Danish museum collections.
• Willard Libby received the Nobel Prize in Chemistry in 1960 for his development of the radiocarbon dating method.
• Calibration is necessary for radiocarbon dates because the concentration of Carbon-14 in the atmosphere has fluctuated over time. Researchers use tree-ring data to correct these variations and obtain accurate calendar dates.
• Cross-dating occurs when artifacts with known dates are found in layers with artifacts of unknown dates. This allows the unknown objects to be assigned the same approximate age as the known ones.
• Palynology, or pollen analysis, provides relative dating and environmental context. Different plant species produce distinct pollen grains that preserve well in stagnant water or peat bogs. Changes in pollen frequency in a stratigraphic column reflect changes in climate and human vegetation management.

Archaeomagnetism dates fired clay features like hearths and kilns. The Earth’s magnetic field changes direction and intensity over time. Baked clay retains the magnetic signature of the Earth’s field at the time it was cooled. By comparing this signature to known historical records of magnetic field shifts, the feature can be dated.