Key Archaeological Dating Methods

Why This Matters

Dating methods form the backbone of archaeological interpretation—without them, we'd have artifacts but no story. You're being tested on your ability to distinguish between absolute dating (methods that give actual ages in years) and relative dating (methods that establish sequences without specific dates). Beyond this fundamental distinction, exams expect you to understand which method works for which materials, what time ranges each covers, and why certain contexts demand specific techniques.

These methods demonstrate core principles you'll encounter throughout the course: radioactive decay, stratigraphic superposition, typological change over time, and environmental recording in natural materials. When you see a question about dating, don't just recall the method's name—ask yourself what it measures, what materials it requires, and what its limitations are. That's what separates a 3 from a 5.

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Radiometric Methods: Measuring Atomic Decay

These techniques rely on the predictable decay of radioactive isotopes into stable daughter products. The ratio of parent to daughter isotopes acts as a natural clock, ticking away since the material formed or the organism died.

Radiocarbon Dating (C-14)

• Measures carbon-14 decay in organic materials—living things absorb $^{14}C$ from the atmosphere, and decay begins at death with a half-life of ~5,730 years
• Effective range: up to ~50,000 years—beyond this, too little $^{14}C$ remains for reliable measurement
• Requires organic samples like wood, bone, charcoal, or shell; calibration with dendrochronology corrects for atmospheric $^{14}C$ fluctuations

Potassium-Argon Dating

• Measures decay of $^{40}K$ to $^{40}Ar$ in volcanic rock—argon gas escapes during eruption, resetting the clock to zero
• Best for materials older than 100,000 years—the long half-life (1.25 billion years) makes it useless for recent deposits
• Critical for human evolution studies—dates volcanic layers bracketing hominin fossils at sites like Olduvai Gorge

Uranium-Series Dating

• Measures uranium isotope decay in calcium carbonate—useful for cave formations (speleothems), corals, and travertine
• Effective range: up to ~500,000 years—fills the gap between radiocarbon and potassium-argon
• Requires closed-system conditions—contamination or uranium leaching invalidates results

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Luminescence Methods: Trapped Energy Release

These techniques measure energy stored in mineral crystals since they were last exposed to heat or light. Radiation from surrounding sediments gradually fills electron traps in minerals; releasing this energy in the lab reveals how long ago the clock was reset.

Thermoluminescence (TL) Dating

• Measures trapped electrons released by heating—effective for ceramics, burnt flint, and heated sediments up to 500,000 years old
• Dates the last heating event—tells you when pottery was fired or when a hearth was last used
• Essential for sites lacking organic materials—provides absolute dates where radiocarbon cannot

Optically Stimulated Luminescence (OSL)

• Measures last light exposure in sediments—sunlight resets the luminescence signal, so burial starts the clock
• Effective for sediments up to ~100,000 years—dates when sand or silt was deposited and buried
• Requires total darkness during sampling—any light exposure after excavation contaminates the sample

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Relative Dating: Establishing Sequences

These methods don't provide calendar dates but establish which came first. They're foundational—you often need relative chronology before applying absolute methods.

Stratigraphy

• Based on the Law of Superposition—in undisturbed deposits, lower layers are older than upper layers
• Establishes relative chronology for entire sites—provides context for every artifact and feature found
• Disturbances complicate interpretation—bioturbation, pit-digging, and erosion can invert or mix layers

Seriation

• Tracks stylistic change in artifacts over time—pottery styles, projectile point shapes, and decorative motifs evolve predictably
• Creates relative sequences from artifact frequencies—popular styles rise and fall in popularity, creating diagnostic patterns
• Requires large sample sizes and defined contexts—works best when you can compare multiple assemblages from the same region

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Specialized Techniques: Unique Applications

These methods exploit specific properties of materials or environmental records, filling niches where other techniques fall short.

Dendrochronology (Tree-Ring Dating)

• Counts and matches annual growth rings—provides year-specific dates, sometimes to the exact season
• Extends back 10,000+ years in some regions—master chronologies built from overlapping sequences of living and dead wood
• Calibrates radiocarbon dates—tree rings of known age reveal past atmospheric $^{14}C$ variations

Archaeomagnetic Dating

• Records Earth's magnetic field in fired materials—magnetic minerals in clay align with the geomagnetic field when heated and lock in position upon cooling
• Requires regional master curves—the Earth's magnetic field wanders over time, creating datable patterns
• Best for in-situ fired features—hearths, kilns, and burned structures that haven't moved since firing

Amino Acid Racemization

• Measures conversion of L-amino acids to D-amino acids—living organisms maintain only L-forms; after death, slow conversion begins
• Effective for bones, shells, and teeth—useful when radiocarbon fails due to age or contamination
• Highly temperature-dependent—rates vary with burial environment, requiring site-specific calibration

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Quick Reference Table

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Self-Check Questions

1. Which two dating methods both rely on radioactive decay but target completely different time ranges and materials? What determines which one you'd use at a given site?

2. A site contains a burned clay hearth but no organic materials. Which absolute dating methods could you apply, and what would each one actually measure?

3. Compare and contrast stratigraphy and seriation: How does each establish chronological relationships, and what are the limitations of relying on either one alone?

4. Why is dendrochronology considered both a dating method and a calibration tool? What problem does it solve for radiocarbon dating?

5. An FRQ describes a cave site with flowstone layers capping archaeological deposits containing stone tools but no bone. Which dating method would be most appropriate for establishing the age of human occupation, and why?