Radiocarbon Dating

How It Works

Radiocarbon dating relies on the constant decay of carbon-14, a radioactive isotope of carbon that is continuously produced in Earth's upper atmosphere when cosmic rays interact with nitrogen atoms. Living organisms absorb carbon-14 through the carbon cycle (via photosynthesis for plants and consumption for animals). When an organism dies, it stops exchanging carbon with the environment, and the carbon-14 begins to decay with a half-life of 5,730 ± 40 years.

Calibration Curves

Scientists have developed precise calibration curves that convert raw radiocarbon ages into calendar dates. These curves are based on:

• Tree Rings (Dendrochronology): Annual growth rings from long-lived trees like bristlecone pines and European oaks provide a record going back more than 13,900 years.
• Marine Records: Coral records and foraminifera in varved sediments extend the record to approximately 50,000 years.
• Speleothems: Cave formations dated by uranium-thorium methods provide additional calibration points.
• Lake and Marine Sediments: Varves (annual layers in lake sediments) offer additional calibration data.

1. Dendrochronology (Tree-Ring Dating)

Tree rings provide an absolute chronology that matches radiocarbon dates with remarkable accuracy. Studies comparing radiocarbon dates with tree-ring sequences show agreement to within ±20 years for samples up to 11,000 years old. The International Tree-Ring Database contains over 4,000 chronologies from 6 continents that have been used to verify radiocarbon dates.

2. Uranium-Thorium Dating

Also known as U-series dating, this method has been used to date cave formations (speleothems) that also contain organic material suitable for radiocarbon dating. A 2018 study in Science examined 23 cave sites across Europe and Asia, finding agreement between uranium-thorium and calibrated radiocarbon dates within ±150 years over a 40,000-year range.

3. Varve Chronology

Annual sediment layers (varves) in lakes provide another independent chronology. The Cariaco Basin off Venezuela contains a 15,000-year varve sequence that correlates with radiocarbon dates to within ±50 years. Similarly, Lake Suigetsu in Japan has a continuous 52,800-year varve record that closely matches calibrated radiocarbon ages.

4. Ice Core Dating

Greenland and Antarctic ice cores contain annual layers that can be counted back tens of thousands of years. Trapped air bubbles in these ice cores contain organic materials that can be radiocarbon dated. The GRIP and GISP2 ice cores from Greenland show agreement with radiocarbon dates to within ±100 years over the past 40,000 years.

5. Archaeological Contexts

Radiocarbon dates from historical contexts with known ages provide strong verification. For example, Egyptian artifacts with known historical dates have been radiocarbon dated, showing agreement within the expected margin of error. The tomb of Tutankhamun (c. 1325 BCE) contained materials that were radiocarbon dated to 1320 ± 40 BCE, confirming the method's accuracy.

International Intercomparison Studies

Radiocarbon laboratories worldwide participate in regular intercomparison exercises to ensure consistency and accuracy. Recent examples include:

• SIRI (Sixth International Radiocarbon Intercomparison): Completed in 2013, involved 72 laboratories testing identical samples of varying ages. Results showed agreement within statistical margins for over 95% of measurements.
• TIRI/FIRI (Third/Fourth International Radiocarbon Intercomparison): These studies showed that AMS laboratories typically achieve an uncertainty of 2-3‰ (equivalent to ±16-24 years) for modern samples.

Laboratory Standards and Procedures

Modern radiocarbon dating employs rigorous quality control measures:

• Regular testing of international standards like Oxalic Acid II (NIST SRM 4990C)
• Analysis of background samples (materials with no measurable C-14)
• Process blanks to detect any contamination during sample preparation
• Analysis of samples of known age to confirm accuracy
• Duplicate measurements to assess precision

These procedures ensure that laboratories maintain a precision of ±20-30 years for samples less than 5,000 years old and ±100-400 years for samples approaching 40,000 years old.

Age Range Limitations

The practical limit for radiocarbon dating is approximately 50,000 years. After about 8-10 half-lives, the amount of remaining carbon-14 becomes too small to measure accurately. For older samples, other radiometric dating methods (such as potassium-argon or uranium-series) must be used.

Sample Contamination

Contamination with modern or ancient carbon can skew results. Modern laboratory protocols include rigorous pretreatment methods to remove contaminants:

• Acid-Base-Acid (ABA): Sequential treatment with acid, base, and acid again to remove carbonates and humic substances
• ABOX-SC: Advanced oxidation pretreatment for samples approaching the radiocarbon limit
• Ultrafiltration: Removal of degraded collagen fragments from bone samples

Reservoir Effects

Marine organisms and those in lakes can incorporate "old carbon" due to the marine reservoir effect, making them appear older than they actually are. Regional reservoir corrections have been developed through research:

• Global marine offset: approximately 400 years
• Local variations (ΔR values) range from -100 to +1500 years depending on ocean circulation
• Freshwater reservoir effects can range from 200-2000 years depending on the water body

Dietary Considerations

The radiocarbon date of human and animal remains can be affected by diet, particularly consumption of marine or freshwater resources. Stable isotope analysis (δ¹³C and δ¹⁵N) is routinely performed alongside radiocarbon dating to assess and correct for dietary effects.