🦴Intro to Archaeology Unit 6 – Dating Methods in Archaeology

Key Concepts and Terminology

• Chronology involves arranging events or artifacts in the order they occurred or were created
• Stratigraphy is the study of rock layers (strata) and the layering process (stratification)
  • Law of Superposition states that in a sequence of undisturbed sedimentary rocks, the oldest layers are at the bottom and the youngest layers are at the top
• Absolute dating determines the specific age of an object or event, often expressed in years before present (BP) or years ago (ya)
• Relative dating establishes the order of events or artifacts without specifying exact ages
• Half-life is the time required for half of the original amount of a radioactive isotope to decay
• Calibration curves are used to convert radiocarbon ages to calendar dates, accounting for variations in atmospheric carbon over time
• Terminus post quem (TPQ) is the earliest possible date for an event or artifact based on its stratigraphic position or associated datable materials
• Terminus ante quem (TAQ) is the latest possible date for an event or artifact based on its stratigraphic position or associated datable materials

Types of Dating Methods

• Relative dating methods determine the order of events without providing specific dates (stratigraphy, seriation)
• Absolute dating methods assign specific ages or date ranges to events or artifacts (radiocarbon dating, dendrochronology)
• Chronometric dating techniques measure time-dependent processes, such as radioactive decay or chemical changes (potassium-argon dating, amino acid racemization)
• Biochronology uses the presence of fossil organisms to establish relative ages of sedimentary rocks (index fossils)
• Archaeomagnetic dating measures the orientation of magnetic minerals in fired clay artifacts, comparing them to known changes in the Earth's magnetic field over time
• Tephrochronology uses volcanic ash layers (tephra) as time-specific markers to correlate and date archaeological sites and geological deposits
• Luminescence dating techniques, such as optically stimulated luminescence (OSL) and thermoluminescence (TL), measure the accumulated radiation dose in mineral grains to determine the time since they were last exposed to sunlight or heat
• Incremental dating methods count annual growth layers in organic materials (tree rings, coral, ice cores) or chemical precipitates (speleothems, varves) to establish high-resolution chronologies

Relative Dating Techniques

• Stratigraphy analyzes the vertical and horizontal relationships between layers of sediment or rock
  • Steno's Laws of Stratigraphy include the Law of Superposition, Law of Original Horizontality, Law of Lateral Continuity, and Law of Cross-Cutting Relationships
• Seriation arranges artifacts in chronological order based on changes in their style, form, or frequency over time
  • Frequency seriation assumes that the popularity of an artifact type rises gradually, peaks, and then declines
• Fluorine dating compares the relative amounts of fluorine absorbed by bones from the same site, with older bones containing more fluorine
• Paleomagnetism measures the orientation of magnetic minerals in rocks, which can be compared to known reversals in the Earth's magnetic field to establish relative ages
• Linguistic dating estimates the age of languages or language families based on the rates of vocabulary change or the accumulation of phonological and grammatical differences

Absolute Dating Techniques

• Radiocarbon dating measures the decay of carbon-14 in organic materials to determine the time since the organism died
  • The half-life of carbon-14 is approximately 5,730 years
  • Calibration is necessary to convert radiocarbon ages to calendar dates due to variations in atmospheric carbon-14 levels over time
• Potassium-argon (K-Ar) and argon-argon (Ar-Ar) dating measure the decay of potassium-40 to argon-40 in volcanic rocks and minerals
  • The half-life of potassium-40 is approximately 1.3 billion years
• Uranium-series dating measures the decay of uranium isotopes to thorium and lead in calcium carbonate materials (speleothems, corals, shells)
  • The half-lives of uranium-234 and uranium-235 are 245,000 years and 704 million years, respectively
• Fission track dating counts the tracks left by the spontaneous fission of uranium-238 in minerals such as zircon and apatite
• Dendrochronology counts and matches tree ring patterns to create a master chronology that can be used to date wood samples
• Varve chronology counts annual layers of sediment deposited in lakes or marine environments
• Amino acid racemization measures the ratio of D-form to L-form amino acids in organic materials, which changes at a predictable rate over time

Sampling and Preparation Processes

• Sampling strategies must consider the research questions, available materials, and potential sources of contamination
  • Judgmental sampling selects specimens based on their perceived relevance or representativeness
  • Random sampling gives each potential specimen an equal chance of being selected
  • Systematic sampling selects specimens at regular intervals (spatial or temporal) within a defined population
• Sample preparation varies depending on the dating method and material type
  • Radiocarbon dating requires the removal of contaminants (rootlets, carbonates) and the isolation of the carbon-bearing fraction (collagen, cellulose, charcoal)
  • Potassium-argon dating involves the separation of potassium-bearing minerals (feldspars, micas) from the rock matrix
  • Uranium-series dating requires the chemical extraction and purification of uranium and thorium from the sample matrix
• Pretreatment methods aim to remove contaminants and isolate the target material for dating
  • Acid-base-acid (ABA) pretreatment is commonly used for charcoal and wood samples in radiocarbon dating
  • Ultrafiltration is used to isolate collagen from bone samples for radiocarbon dating
• Quality control measures, such as the use of blanks, standards, and replicates, ensure the accuracy and precision of the dating results

Interpreting Dating Results

• Dating results are typically reported as a central value (mean, median, or mode) and an associated uncertainty (standard deviation, confidence interval)
  • Radiocarbon ages are reported as years before present (BP) with a one or two-sigma uncertainty
  • Calibrated dates are reported as calendar years (BCE/CE or BP) with a probability distribution
• Interpreting dating results requires consideration of the sample context, potential sources of error, and the research questions being addressed
  • Stratigraphic relationships can help constrain the interpretation of dating results
  • The association between the dated material and the archaeological event of interest must be evaluated
• Outliers and inconsistencies in dating results may indicate contamination, disturbance, or the presence of multiple components
• Bayesian statistical methods can be used to combine multiple lines of chronological evidence and refine the probability distributions of dates
• The interpretation of dating results should be integrated with other archaeological and environmental data to construct robust chronologies and models of past human behavior

Limitations and Challenges

• The accuracy and precision of dating methods are influenced by factors such as sample size, contamination, and calibration uncertainties
  • Small sample sizes can lead to large uncertainties in the dating results
  • Contamination by younger or older carbon can skew radiocarbon dates
  • Variations in atmospheric carbon-14 levels over time require calibration to convert radiocarbon ages to calendar dates
• The range and applicability of dating methods are limited by the availability and preservation of suitable materials
  • Organic materials are susceptible to decay and may not survive in certain environments (acidic soils, tropical climates)
  • Some dating methods are only applicable to specific material types (potassium-argon for volcanic rocks, uranium-series for carbonates)
• The cost and time required for dating analyses can be a limiting factor in archaeological research
  • Radiocarbon dating is relatively expensive compared to other dating methods
  • Some dating methods require specialized equipment and expertise that may not be widely available
• The interpretation of dating results can be complicated by issues of sample context, association, and mixing
  • The relationship between the dated material and the archaeological event of interest may be unclear or indirect
  • Post-depositional disturbances (bioturbation, erosion) can mix materials of different ages within a single context

Applications in Archaeological Research

• Dating methods provide a temporal framework for understanding past human societies and their interactions with the environment
  • Chronologies can be used to trace the spread of cultures, technologies, and ideas across space and time
  • Dated archaeological sequences can be correlated with paleoenvironmental records to investigate human-environment interactions
• High-resolution dating techniques (dendrochronology, varve chronology) can reveal short-term events and processes in the archaeological record
  • Tree ring dates can be used to study the construction and repair of wooden structures (buildings, ships)
  • Varve sequences can provide annual-scale records of past climate, vegetation, and human activities
• Dating methods are crucial for understanding the tempo and mode of cultural change and the emergence of social complexity
  • The timing and duration of key transitions (Neolithic, Bronze Age, Iron Age) can be established through the dating of archaeological sites and artifacts
  • Dated settlement patterns and material culture assemblages can be used to infer changes in population size, social organization, and economic systems over time
• The integration of multiple dating methods and lines of evidence can provide more robust and reliable chronologies
  • The use of Bayesian statistics to combine stratigraphic information with radiocarbon dates can refine site chronologies and phase durations
  • The cross-dating of archaeological materials (pottery, coins) with historical events or inscriptions can anchor floating chronologies to absolute time scales
• Advances in dating methods and technologies continue to expand the range and resolution of archaeological chronologies
  • The development of accelerator mass spectrometry (AMS) has enabled the radiocarbon dating of small and poorly preserved samples
  • The refinement of calibration curves and statistical methods has improved the precision and accuracy of radiocarbon dates
  • The application of new dating methods (optically stimulated luminescence, electron spin resonance) has extended the temporal range of archaeological investigations beyond the limits of radiocarbon dating