Fossil Dating Techniques

Why This Matters

Understanding how we know when things happened in human evolution is just as important as knowing what happened. Every claim about hominin origins, migration patterns, and evolutionary relationships rests on dating techniques—without them, we'd have fossils but no timeline. You're being tested on your ability to distinguish between absolute dating (which gives numerical ages) and relative dating (which establishes sequences), and to know which method works for which materials and time ranges.

These techniques demonstrate core principles of radioactive decay, chemical accumulation, geological layering, and biological succession. Exam questions often ask you to select the appropriate dating method for a given scenario or explain why certain techniques can't be used together. Don't just memorize method names—know what each technique actually measures, what materials it works on, and its effective time range. That's what separates a 5 from a 3.

---

Radioactive Decay Methods

These techniques measure the predictable breakdown of unstable isotopes into stable daughter products. The key principle is that radioactive decay occurs at a constant rate (half-life), allowing scientists to calculate elapsed time from the ratio of parent to daughter isotopes.

Carbon-14 Dating

• Measures decay of $^{14}C$ to $^{14}N$—works only on organic materials that were once living
• Half-life of ~5,730 years limits effectiveness to materials younger than ~50,000 years old
• Primary method for archaeological remains—ideal for bone, wood, charcoal, and shell within its range

Potassium-Argon Dating

• Measures decay of $^{40}K$ to $^{40}Ar$—used exclusively on volcanic rocks, not fossils directly
• Effective for materials millions to billions of years old—essential for dating early hominin sites in East Africa
• Dates the volcanic layers bracketing fossils—provides minimum and maximum ages for specimens found between flows

Uranium Series Dating

• Measures decay chain of uranium isotopes ($^{234}U$ to $^{230}Th$)—works on calcium carbonate materials
• Effective range of ~500,000 years—bridges the gap between Carbon-14 and Potassium-Argon methods
• Ideal for cave deposits, coral, and flowstones—crucial for dating cave sites with hominin remains

---

Trapped Charge Methods

These techniques measure electrons that accumulate in crystal lattices when exposed to natural radiation. The principle: radiation from surrounding sediments knocks electrons into "traps" within mineral structures, and the number of trapped electrons indicates time since the material was last heated or exposed to sunlight.

Electron Spin Resonance (ESR) Dating

• Measures trapped electrons in tooth enamel and cave deposits—non-destructive method preserves specimens
• Effective range from thousands to several million years—fills critical gaps in the hominin fossil record
• Particularly valuable for dating teeth directly—unlike K-Ar, doesn't require volcanic context

Thermoluminescence (TL) Dating

• Measures radiation dose accumulated since last heating—"clock" resets when material is burned or fired
• Effective for ~500,000 years—works on pottery, burned flint, and heated sediments
• Key method for dating archaeological hearths and artifacts—reveals when humans used fire or created tools

---

Stratigraphic Methods

These techniques establish relative ages by examining the position and characteristics of geological layers. The foundational principle is superposition: in undisturbed sequences, older materials lie beneath younger ones.

Stratigraphy

• Uses the Law of Superposition—older layers deposited first lie below younger layers
• Provides relative dating and contextual information—tells you what came before and after, not exact years
• Essential for understanding depositional environment—reveals whether fossils were in situ or redeposited

Biostratigraphy

• Uses index fossils to correlate rock layers across regions—certain species existed only during specific time periods
• Based on faunal succession principle—fossil assemblages change predictably through time
• Allows dating by association—if you find a hominin with known pig species, you can estimate the hominin's age

Magnetostratigraphy

• Records Earth's magnetic field reversals preserved in rocks—polarity "flips" create a global timeline
• Provides precise correlation across distant sites—same reversal pattern appears worldwide
• Integrated with radiometric dates for calibration—magnetic reversals are independently dated using K-Ar

---

Chemical Accumulation Methods

These techniques measure substances that accumulate in or alter materials over time. The principle: certain chemical changes occur at predictable rates, allowing age estimation based on the degree of change.

Fluorine Dating

• Measures fluorine absorbed from groundwater into buried bones—more fluorine means longer burial time
• Provides relative dating only—absorption rates vary by local conditions
• Famous for exposing the Piltdown hoax—showed the skull and jaw had different burial histories

Amino Acid Dating (Racemization)

• Measures conversion of L-amino acids to D-amino acids—living organisms have only L-forms; after death, conversion begins
• Effective for tens of thousands to millions of years—useful for materials beyond C-14 range
• Temperature-sensitive—requires understanding of site's thermal history for accurate results

---

Annual Growth Methods

These techniques count or measure regular growth patterns that occur on known time cycles. The principle: some natural processes create distinct, countable markers at predictable intervals.

Dendrochronology

• Counts annual tree rings—each ring represents one year of growth
• Provides exact calendar dates—precision unmatched by any other method
• Master chronologies extend back ~12,000 years—overlapping patterns from living and dead trees create continuous records

---

Quick Reference Table

---

Self-Check Questions

1. A researcher discovers hominin fossils sandwiched between two volcanic ash layers at a 3.2-million-year-old East African site. Which dating method would provide absolute dates, and why can't Carbon-14 be used here?

2. Compare and contrast ESR and Thermoluminescence dating: what do they both measure, and what determines which one you'd use for a given sample?

3. Which two methods were instrumental in exposing the Piltdown forgery, and what principle did each one demonstrate?

4. You're dating a cave site with no volcanic material but abundant flowstone deposits and hominin teeth. Which combination of methods would give you the most reliable chronology?

5. Explain why biostratigraphy and magnetostratigraphy are considered relative dating methods even though they can be correlated with absolute dates. How do they complement radiometric techniques?