Archaeological Dating Methods: Relative and Absolute Chronologies

Course Administration and Examination Returns

• Examination papers returned prior to $02:30$ in the classroom session.
• Students listed during paper distribution include:
  • Tyrese Chigwara
  • Holland
  • Harris Cuglitis
  • Tristan
  • Louise
  • Victoria (first student by this name)
  • Victoria (second student located further down)
  • Shoto
  • Robert
  • Lily
  • Lang
  • Vitrelia
  • Pulmer
  • Douglas
  • Elliot
  • Jensen
  • Alicia McKinnon
  • Katie McDermott
  • Hogan
  • Williams
  • Noah Fisher
  • Cade

Stylistic Variation and Frequency Seriation

• Foundations of stylistic change over time:

  • Material culture styles change continually throughout history; this is not solely a modern phenomenon.
  • Factors driving stylistic shifts include technological innovations, aesthetic choices, religious transformations, and political developments.
  • In archaeology, stylistic popularity within a single component or stratigraphic layer can be measured and compared against other stratigraphic layers.
  • The maximum frequency of a style within a site layer indicates its peak period of cultural popularity.
  • Principle of closed-category relative popularity: within a single functional class of artifacts (e.g., exclusively ceramic pots, arrowheads, automobiles, or hats), the increased popularity of one decorative style mathematically necessitates the decreased popularity of alternative styles.
  • Cross-category stylistic comparisons are invalid; shifts in automobile or hat designs do not correlate with or influence pottery decorative styles.

• Testing frequency seriation using colonial gravestones:

  • A classic demonstration study documented in introductory archaeology textbooks investigated changing gravestone decorative motifs in New England cemeteries.
  • Gravestones provided an ideal empirical control because dates of death are carved directly into the stones, allowing independent verification of relative seriation against absolute calendar dates.
  • Three sequential decorative styles on gravestone tympanums were analyzed:
  • Death's head motif
  • Cherub motif
  • Urn and Willow motif
  • Temporal distribution documented in the study:
  • $1750$ to $1759$: The Death's head motif constituted $100\%$ of the sample; Cherub and Urn and Willow designs exhibited $0\%$ representation.
  • $1760\text{s}$: The Cherub design emerged; as its popularity increased, the frequency of the Death's head motif experienced a corresponding decline.
  • Mid-to-late 18th century: The Cherub motif reached peak popularity without fully achieving $100\%$ market dominance because the Death's head motif lingered, and the Urn and Willow style began its initial appearance.
  • Replacement cycle: The Urn and Willow design steadily expanded in popularity, eventually superseding the Cherub motif.

• Mechanics and limitations of seriation graphs:

  • Graphic construction involves creating horizontal bar graphs representing percentage frequencies across stylistic classes.
  • Rearranging horizontal strips of paper representing artifact assemblages produces distinctive symmetric curves termed battleship-shaped curves or bipointed curves.
  • Bipointed curves reflect the universal lifecycle of artifact styles: introduction, progressive rise in popularity, peak frequency, gradual decline, and eventual obsolescence.
  • Example application: ceramic typologies from the American Southwest.
  • Fundamental limitation of seriation: bipointed curves demonstrate relative chronological sequence but cannot determine directional orientation (which end of the graph represents the oldest versus newest assemblage), as the graph can be inverted while retaining identical curve morphology.
  • Resolving directional polarity: determining "which end is up" requires external stratigraphic evidence (e.g., superposition of one style above another in an excavated site) to anchor the sequence temporally.
  • Seriation establishes relative chronological placement rather than absolute duration or calendar dates.

Bone Age Dating (Nitrogen, Fluorine, and Uranium Dating)

• Principles of relative bone dating:

  • Bone age dating (nitrogen-fluorine-uranium dating) establishes relative age sequences within osteological collections (such as cemetery populations).
  • It provides an economical relative dating method when financial resources are insufficient for extensive radiocarbon testing.
  • Nitrogen chemical mechanics:
  • Living bone contains approximately $4\%$ nitrogen within bone collagen.
  • Following death and nutrient cessation, nitrogen progressively leaches out of the bone matrix into the surrounding depositional environment.
  • In comparing two bones from identical burial environments, a bone possessing $2\%$ nitrogen is older than a bone possessing $3\%$ nitrogen.
  • Fluorine and uranium chemical mechanics:
  • Soluble fluorine and uranium present in groundwater and surrounding sediments are progressively absorbed into bone mineral over post-depositional time.
  • Higher concentrations of absorbed fluorine and uranium correlate with greater chronological age, behaving inversely to nitrogen depletion.
  • Matrix dependency and environmental caveats:
  • Rates of chemical leaching and absorption depend heavily on local ambient temperature and soil moisture/humidity within the burial matrix.
  • Elevated temperature and elevated moisture significantly accelerate both nitrogen loss and fluorine/uranium absorption.
  • Reliable relative dating requires that all compared osteological samples share identical depositional environments, microclimates, and matrix conditions.

• The Piltdown Man hoax investigation:

  • In $1911$, amateur archaeologist Charles Dawson recovered a hominin cranium and a primate mandible within a gravel quarry in Sussex, England, claiming to have discovered the evolutionary missing link between apes and humans.
  • Prevailing early 20th-century evolutionary assumptions held that human ancestors developed large brains prior to developing bipedal locomotion and tool use.
  • The specimens appeared to validate this model by featuring a human-like cranial vault coupled with an ape-like, primitive jaw whose teeth supposedly matched the cranial articulation.
  • In the $1950\text{s}$, chemical bone age testing (nitrogen, fluorine, and uranium analysis) proved the cranium and mandible were chronological mismatches:
  • Cranium: Retained virtually no nitrogen, demonstrating genuine antiquity (identified as late Homo erectus or Homo heidelbergensis, dating to several hundred thousand years old).
  • Mandible: Contained $4\%$ nitrogen, proving it was modern skeletal material obtained from a recently deceased zoo baboon.
  • Physical tampering: The baboon molars had been artificially filed down to simulate human flat-wear mastication patterns, and the bone was chemically stained by boiling in acid to produce an artificial fossilized patina.
  • Paleoanthropological perspective: The evolutionary concept of a single "missing link" is scientifically obsolete, as evolutionary lineages branch rather than develop through singular transitional stages.

Absolute Dating Principles and Epigraphic Evidence

• Concept of absolute (chronometric) dating:

  • Absolute dating yields ages calibrated in solar/calendar years.
  • "Absolute" does not denote complete absence of error; dates include statistical margins of error and measurement uncertainties.

• Epigraphic, numismatic, and historical dating:

  • The most direct form of absolute dating occurs when calendar dates are recorded directly on material culture (e.g., dedicatory architectural inscriptions, coins, tombstones).
  • Terminus post quem principle: Discovery of a coin minted in AD $1800$ within a stratigraphic layer establishes that the layer dates to the year AD $1800$ or later; the layer cannot date to $1799$ because the artifact had not yet been manufactured.
  • Circulation considerations: Coins frequently remained in circulation for extended periods in antiquity compared to modern currency.
  • Historical calendar systems include Maya, Egyptian, and Mesopotamian records dating back thousands of years.

• Maya vigesimal numerical notation:

  • The Maya numerical system operates on a base-$20$ (vigesimal) positional system rather than a base-$10$ (decimal) system.
  • Fundamental glyphic units:
  • Dot represents $1$
  • Bar represents $5$
  • Stylized clamshell glyph represents $0$
  • Historical significance of the zero: The mathematical concept of zero was independently invented only three times in human history, with the Maya being one of those independent origins.
  • Positional calculations:
  • Lowest positional tier: units (ones), representing values multiplied by $20^0 = 1$.
  • Second positional tier: twenties, representing values multiplied by $20^1 = 20$.
  • Third positional tier: four-hundreds, representing values multiplied by $20^2 = 400$.
  • Example calculation: A representation featuring one dot in the upper position ($1 \times 20$) and two bars with two dots in the lower position ($12 \times 1$) computes as:     $1 \times 20 + 12 = 32$

Dendrochronology (Tree-Ring Dating)

• Biological mechanism of annual ring formation:

  • Dendrochronology provides the most precise absolute dating technique available, exceeding the accuracy of radiocarbon dating, and is used to calibrate the radiocarbon timescale.
  • Applicable exclusively in temperate geographic zones exhibiting pronounced seasonal cycles; tropical trees do not develop distinct, regular annual growth rings due to the lack of strong seasonal dormancy.
  • Seasonal cell formation cycle:
  • Early spring: Melting snow and rising temperatures provide abundant moisture, stimulating rapid, expansive cellular division (early wood/large cells).
  • Summer to autumn: Increasing thermal stress and decreasing moisture slow cellular division, yielding progressively smaller, densely packed cells (late wood/small cells).
  • Winter: Growth arrests completely; freezing ground restricts water uptake.
  • Spring renewal: The sudden resumption of large cellular growth adjacent to the microscopic, dense cells of the preceding autumn creates the distinct macroscopic annual ring line.
  • Ring width variability: Favorable meteorological conditions produce broad annual rings, whereas droughts, high heat, and unfavorable conditions yield narrow, compressed rings.

• Master chronologies and archaeological sampling:

  • Master chronologies are composite sequences assembled by overlapping ring sequences from living trees, historic timbers, and prehistoric architectural wood.
  • Cross-dating matches distinctive climatic growth patterns across specimens, extending chronological sequences back more than $10{,}000\text{ years}$ in arid environments like the American Southwest.
  • Sampling methodology: Core specimens are collected using hollow drill bits inserted into structural timbers (e.g., prehistoric Pueblo door lintels).
  • Taxonomic and microclimatic controls:
  • Dendrochronological comparisons require identical tree species, as moisture and temperature responses differ across taxa.
  • Local topographic microclimates dictate growth rates; for instance, ponderosa pines (Pinus ponderosa) on a dry, south-facing valley slope display growth profiles distinct from the same species on an adjacent, moist north-facing slope (e.g., Kamloops valley).
  • Analytical precision:
  • Presence of intact outer bark or the outermost terminal growth ring permits dating the tree's felling to the precise calendar year.
  • Samples missing outer rings provide only terminus post quem estimates based on average species lifespans.
  • Case study: L'Anse aux Meadows (Newfoundland Norse site):
  • Excavated during the $1960\text{s}$ and $1970\text{s}$; early radiocarbon estimates broadly dated Norse occupation from the late $980\text{s}$ to approximately AD $1010$–$1020$.
  • Dendrochronological analysis conducted on structural wood established the exact calendar year of occupation as AD $1021$.

• The marine reservoir effect on dendrochronological materials:

  • Marine organisms and submerged terrestrial materials absorb ancient dissolved inorganic carbon from seawater, inducing an apparent aging effect.
  • Wood recovered from marine inundation contexts (such as samples analyzed from the Oak Island excavations in Nova Scotia) can yield radiocarbon ages artificially centuries older than their true calendar age due to marine carbon contamination.

Radiocarbon Dating Principles and Mechanisms

• Historical development:

  • Developed between $1949$ and $1950$ by chemist Willard Libby.
  • Libby established research proof-of-concept in $1949$, and operational laboratory dating systems became accessible to external researchers around $1950$.

• Atmospheric and isotopic physics:

  • Naturally occurring isotopes of carbon:
  • Carbon-12 ($^{12}\text{C}$): Stable isotope, constitutes the vast majority of carbon in living organisms.
  • Carbon-13 ($^{13}\text{C}$): Stable minor isotope.
  • Carbon-14 ($^{14}\text{C}$): Unstable radioactive isotope.
  • Trace, short-lived unstable isotopes include $^{8}\text{C}$, $^{9}\text{C}$, and $^{10}\text{C}$.
  • Production reaction:
  • Extraterrestrial cosmic radiation generates high-energy thermal neutrons in the upper atmosphere.
  • Neutrons collide with abundant atmospheric nitrogen gas ($^{14}\text{N}$), transmuting it into radioactive carbon-14 ($^{14}\text{C}$).
  • Biological uptake and equilibrium:
  • Atmospheric $^{14}\text{C}$ oxidizes into carbon dioxide and enters the global biosphere through floral photosynthesis.
  • Primary producers absorb $^{14}\text{C}$ directly; herbivores and carnivores ingest it through trophic transfer.
  • While an organism is alive, continuous metabolic exchange maintains a steady equilibrium ratio between $^{14}\text{C}$ and stable $^{12}\text{C}$.

• Radioactive decay and half-life dynamics:

  • Upon biological death, metabolic intake of carbon ceases.
  • Existing $^{14}\text{C}$ undergoes continuous radioactive beta decay, reverting into nitrogen-14 ($^{14}\text{N}$) with the emission of beta particles and energy.
  • Half-life ($t_{1/2}$) of $^{14}\text{C}$ is $5{,}730 \pm 40\text{ years}$.
  • Decay rate progression:
  • Beginning with an initial mass of $20\text{ g}$ of $^{14}\text{C}$:
  • After $1\text{ half-life}$ ($5{,}730\text{ years}$): $10\text{ g}$ remains.
  • After $2\text{ half-lives}$ ($11{,}460\text{ years}$): $5\text{ g}$ remains.
  • After $3\text{ half-lives}$ ($17{,}190\text{ years}$): $2.5\text{ g}$ remains.
  • Effective dating boundaries:
  • Detectable $^{14}\text{C}$ diminishes exponentially until reaching background detection thresholds between $40{,}000$ and $50{,}000\text{ years}$.
  • Dates approaching $50{,}000\text{ years}$ exhibit severe statistical uncertainties (e.g., $50{,}000 \pm 30{,}000\text{ years}$), whereas dates around $40{,}000\text{ years}$ retain acceptable standard errors (e.g., $40{,}000 \pm 500\text{ years}$).

Radiocarbon Dating Methodologies and Analytical Challenges

• Analytical techniques:

  • Conventional radiocarbon dating (Liquid Scintillation Counting):
  • Measures radioactive decay emissions (photons released during beta particle decay events).
  • Requires large, subterranean liquid scintillation counting chambers shielded with multiple feet of concrete and lead to block interference from cosmic rays.
  • Major operational limitation: requires large sample masses (approximately "two fist-sized" volumes of wood charcoal) and prolonged monitoring periods; extremely aged samples with few remaining atoms require excessively long, expensive counting intervals.
  • Accelerator Mass Spectrometry (AMS):
  • Utilizes a particle accelerator to physically separate and count individual $^{14}\text{C}$ atoms directly against stable isotopes based on atomic mass.
  • Requires minuscule sample quantities (e.g., a few milligrams or specks of ash).
  • Eliminates the need to wait for decay events, bypassing the primary temporal bottleneck of scintillation counters.

• Methodological constraints and anomalies:

  • Destructive testing requirement: Samples must be chemically cleaned and combusted to pure graphite carbon, destroying the dated material; consequently, precious artifacts are rarely dated directly.
  • Dating by stratigraphic association: Non-artifactual organic materials (e.g., animal bones, ecofactual charcoal) located within the same stratigraphic layer as an artifact are dated, transferring the temporal determination to the associated artifact.
  • Exceptional direct dating of inorganic material: A Near Eastern steel sword was dated directly because iron was carburized into steel using organic carbon sources; the resulting date reflected the death of the organic fuel source rather than the sword's metallurgy.
  • The "Old Wood Problem":
  • Radiocarbon assays determine the year of biological death of the wood tissue, not the cultural event of its combustion or structural use.
  • In cold or arid environments (e.g., the High Arctic), dead wood preserves without rotting for centuries to millennia.
  • Example: An Arctic hearth assemblage featuring cultural artifacts typologically dated to approximately $1{,}000\text{ years}$ ago yielded a radiocarbon date exceeding $2{,}000\text{ years}$ because inhabitants burned drift timber that had died a thousand years prior to occupation.

Thermoluminescence (TL) Dating and Material Authentication

• Physical mechanism of thermoluminescence:

  • Absolute dating method targeting inorganic materials with crystalline lattice structures (e.g., pottery clay, terracotta figurines, quartz sediments, burnt flint).
  • Naturally occurring radioisotopes in clay and soils (uranium, thorium, potassium-40 in feldspars) undergo radioactive decay, releasing ionizing radiation.
  • Escaping ionizing electrons become trapped in structural defects (electron traps) within the crystalline lattices of quartz and feldspar minerals.
  • Trapped electron accumulation functions like fish caught in a net, accumulating steadily over time.

• Thermal zeroing and laboratory measurement:

  • Thermal zeroing event: Heating crystalline materials to approximately $400^\circ\text{C}$ to $500^\circ\text{C}$ (over $1{,}000^\circ\text{F}$) relaxes the crystalline lattice, causing all trapped electrons to escape in the form of light photons.
  • In ceramic production, kiln firing releases all geological luminescence, resetting the internal isotopic clock to zero.
  • Accumulation phase: After cooling, the ceramic re-traps electrons continuously emitted by internal radioisotopes and environmental background radiation throughout its period of use and deposition.
  • Laboratory detection: Technicians heat a small sample scraping (or apply targeted laser vaporization), measuring the emitted photons using sensitive photomultiplier tubes to calculate the duration since the initial firing.

• Effective analytical ranges:

  • Potassium feldspars in ceramics/pottery: Accurately dates samples from $50\text{ years}$ up to $20{,}000+\text{ years}$.
  • Quartz grains: Extends dating capabilities up to approximately $250{,}000\text{ years}$.
  • Burnt flint and specialized mineral matrices: Extends up to $500{,}000\text{ years}$.

• Museum authentication and forgery detection:

  • Thermoluminescence apparatus is compact and non-destructive enough to require only minimal scrape samples from the base of artifacts.
  • Art market context: Forgers manufacture imitation antiquities and attempt to display them in museums to establish false provenance, subsequently marketing them at inflated valuations.
  • Case study: An ostensibly authentic, crudely styled ceramic figurine was identified as genuine ($1{,}500\text{ years}$ old) via TL testing, while an aesthetically refined counterpart was proven to be a modern forgery; the crude manufacturing details represented authentic period technique rather than modern amateur fabrication.
  • Burnt lithic dating: Prehistoric flint tools inadvertently dropped or discarded into ancient hearths achieve the requisite $400^\circ\text{C}$ to $500^\circ\text{C}$ threshold, resetting their crystal lattices and allowing direct dating of Paleolithic campfires.

Questions and Discussion

• Question: Does cooking in ceramic vessels introduce thermal contamination or alter thermoluminescence dates?
  • Response: Standard culinary boiling and cooking temperatures do not attain the necessary $400^\circ\text{C}$ to $500^\circ\text{C}$ ($> 1{,}000^\circ\text{F}$) threshold required to zero the crystalline lattice; therefore, domestic food preparation does not reset the clock or induce analytical errors.
  • Caveat: Even in hypothetical scenarios where a vessel were subjected to extreme sustained heating sufficient to zero the sample, the resulting thermoluminescence assay would date the final high-temperature event rather than invalidate the physical mechanism.