Dating Techniques in Archaeology

Why This Matters

Every archaeological discovery is only as meaningful as its place in time. When you're studying great discoveries in archaeology, you're not just learning what was found—you're learning when it existed and how that timing reshapes our understanding of human history. Dating techniques are the backbone of archaeological interpretation, transforming scattered artifacts into coherent narratives of human evolution, migration, and cultural development. The difference between a 40,000-year-old cave painting and a 4,000-year-old one isn't just a number—it completely changes what that painting tells us about human cognitive development.

You're being tested on more than just matching techniques to their definitions. Exams want you to understand why archaeologists choose specific methods for specific contexts, what limitations affect each technique, and how multiple methods work together to build reliable chronologies. Think about the underlying principles: radioactive decay, biological processes, geological layering, material properties. Don't just memorize that radiocarbon dating works on organic materials—know that it measures isotope decay, understand its 50,000-year limit, and recognize when an archaeologist would need to reach for a different tool entirely.

---

Radiometric Methods: Measuring Atomic Decay

These techniques rely on a fundamental principle of physics: radioactive isotopes decay at predictable rates, allowing scientists to calculate elapsed time from measurable isotope ratios. Different isotopes have different half-lives, making each method suitable for different time ranges.

Radiocarbon Dating (C-14)

• Measures carbon-14 decay in organic materials—living organisms absorb $^{14}C$ from the atmosphere, and decay begins at death with a half-life of approximately 5,730 years
• Effective range up to ~50,000 years—beyond this, too little $^{14}C$ remains for accurate measurement, making it ideal for recent prehistory but useless for early hominid sites
• Calibration with dendrochronology corrects for atmospheric variation—raw radiocarbon dates must be converted to calendar years using calibration curves built from tree-ring data

Potassium-Argon Dating

• Measures the ratio of $^{40}K$ to $^{40}Ar$ in volcanic materials—potassium-40 decays to argon-40 with a half-life of 1.25 billion years, trapping argon gas in rock crystals
• Effective for materials over 100,000 years old—this makes it essential for dating early hominid sites in East Africa's volcanic Rift Valley
• Dates the rock, not the artifact—archaeologists date volcanic ash layers above and below fossils to bracket their age, a technique called bracketing

Uranium-Series Dating

• Measures uranium isotope decay in calcium carbonate formations—uranium is water-soluble while its decay products are not, so carbonate deposits trap a "clock" that starts at formation
• Effective range from thousands to over 500,000 years—bridges the gap between radiocarbon's limit and K-Ar's minimum, crucial for Middle Pleistocene sites
• Essential for dating cave sites—speleothems (stalactites, stalagmites) and flowstone layers that seal archaeological deposits can be precisely dated

---

Luminescence Methods: Measuring Trapped Energy

These techniques measure accumulated radiation damage in mineral crystals since their last "resetting" event—either heating or light exposure. Electrons become trapped in crystal defects over time, and laboratory stimulation releases this energy as measurable light.

Thermoluminescence Dating (TL)

• Measures radiation dose accumulated since last heating—when ceramics are fired or flint is burned, the "clock" resets to zero, and electrons begin accumulating again
• Effective for dating ceramics and burnt materials up to 500,000 years—fills a critical gap where organic materials for radiocarbon dating are absent
• Provides age ranges rather than precise dates—typical uncertainty of 5-10% means a 100,000-year-old sample might date to 90,000-110,000 years

Optically Stimulated Luminescence (OSL)

• Measures the last time sediment grains saw sunlight—exposure to light resets the electron trap, so burial begins a new accumulation period
• Effective for dating sediments up to ~100,000 years—ideal for sites in sandy or silty environments where datable organics are scarce
• Dates the depositional event itself—unlike dating an artifact, OSL tells you when that layer of soil was buried, establishing site chronology directly

---

Relative Dating Methods: Establishing Sequence Without Numbers

These techniques don't provide calendar dates but establish which things are older or younger than others. They're foundational to archaeological interpretation and often guide decisions about where to apply more expensive absolute dating methods.

Stratigraphy

• Based on the law of superposition—in undisturbed deposits, older materials lie beneath younger materials, creating a vertical timeline through excavation
• Provides contextual relationships between finds—an artifact's stratigraphic position tells you what came before and after it, even without absolute dates
• Essential for interpreting site formation processes—disturbances like pit-digging, erosion, or animal burrowing complicate the record and must be identified

Seriation

• Arranges artifacts by stylistic or functional change over time—pottery styles, tool types, or burial practices evolve in patterned ways that create relative sequences
• Reveals cultural trends and technological shifts—the gradual replacement of one artifact style by another reflects changing preferences, trade networks, or innovations
• Requires regional artifact sequences as reference—seriation works by comparing unknown assemblages to established typological sequences for that area

---

Specialized Methods: Unique Materials and Contexts

Some dating techniques exploit specific properties of particular materials, offering precision or applicability that general methods can't match.

Dendrochronology (Tree-Ring Dating)

• Analyzes annual growth rings to determine exact calendar years—trees add one ring per year, with ring width varying by climate conditions, creating unique regional patterns
• Provides year-specific precision unmatched by any other method—a wooden beam can be dated to the exact year (sometimes season) it was cut
• Calibrates radiocarbon dates and reconstructs past climates—long tree-ring sequences extending thousands of years provide both calibration data and environmental records

Archaeomagnetic Dating

• Analyzes magnetic minerals in fired materials—when clay is heated above ~600°C, iron minerals align with Earth's magnetic field and lock in place upon cooling
• Requires regional calibration curves—Earth's magnetic field direction and intensity change over time, and these changes must be mapped locally
• Ideal for dating hearths, kilns, and ovens—any fired clay feature that hasn't moved since cooling can preserve a magnetic record

Amino Acid Dating (Racemization)

• Measures the conversion of L-amino acids to D-amino acids after death—living organisms maintain only L-forms, but after death, amino acids slowly convert to a 50/50 mixture
• Effective for bones and shells up to several million years old—extends far beyond radiocarbon's range for organic materials
• Highly sensitive to temperature history—warmer conditions accelerate racemization, so results require careful environmental calibration

---

Quick Reference Table

---

Self-Check Questions

1. Which two dating methods both rely on radioactive decay but operate on completely different timescales? What determines which one an archaeologist would choose?

2. A site contains a hearth with burnt flint, pottery fragments, and no preserved organic material. Which dating techniques could potentially be applied, and why?

3. Compare and contrast stratigraphy and seriation as relative dating methods. How might an archaeologist use them together to build a regional chronology?

4. Why is dendrochronology considered essential for radiocarbon dating, even though they're independent techniques? What problem does their combination solve?

5. An FRQ asks you to explain how archaeologists date early hominid fossils in East Africa's Rift Valley. Which method would you discuss, and what specific feature of that region makes it possible?