🦕Intro to Paleoanthropology Unit 9 – Dating Methods in Paleoanthropology

Key Concepts and Terminology

• Dating methods determine the age of fossils, artifacts, and geological events
• Relative dating establishes the order of events without providing specific dates
• Absolute dating assigns specific ages or date ranges to events
• Stratigraphy studies the layering of rocks and their relative ages
• Chronology refers to the arrangement of events in order of their occurrence
• Half-life represents the time required for half of a radioactive isotope to decay
• Calibration curves are used to convert radiocarbon ages into calendar years
• Taphonomy investigates the processes that affect an organism after its death and until its discovery

Types of Dating Methods

• Relative dating methods determine the order of events without assigning specific ages
  • Stratigraphy analyzes the layering of sediments and rocks
  • Biostratigraphy uses fossils to establish relative ages
• Absolute dating methods provide specific ages or date ranges for events
  • Radiometric dating measures the decay of radioactive isotopes (carbon-14, potassium-argon)
  • Luminescence dating measures the accumulated radiation dose in minerals
• Chemical dating methods analyze chemical changes in materials over time
  • Amino acid racemization measures the ratio of amino acid isomers
• Incremental dating methods count annual growth rings or layers (tree rings, ice cores)

Relative Dating Techniques

• Stratigraphy is based on the principle of superposition
  • Layers of sediment are deposited in a time sequence, with the oldest on the bottom
• Law of superposition states that younger layers are deposited on top of older layers
• Cross-cutting relationships occur when a feature (fault, intrusion) cuts across older rocks
• Inclusions are fragments of one rock included within another, indicating the host rock is younger
• Fossil succession uses the appearance and disappearance of species to establish relative ages
  • Index fossils are distinctive species that existed for a relatively short time period

Absolute Dating Techniques

• Radiometric dating measures the decay of radioactive isotopes
  • Each isotope has a specific half-life, the time required for half of the original amount to decay
• Carbon-14 dating is used for organic materials up to ~50,000 years old
  • Living organisms absorb carbon-14 from the atmosphere, which decays after death
• Potassium-argon dating is used for rocks and minerals containing potassium
  • Potassium-40 decays into argon-40 with a half-life of 1.3 billion years
• Uranium-series dating measures the decay of uranium isotopes in carbonates and teeth
• Electron spin resonance (ESR) dating measures trapped electrons in tooth enamel and quartz

Radiometric Dating Methods

• Radioactive decay is a spontaneous process where unstable isotopes lose energy and transform into stable isotopes
• The rate of decay is constant and measured by the half-life
• The parent isotope decays into the daughter isotope at a predictable rate
• The ratio of parent to daughter isotopes is measured to calculate the age
• Closed system assumption states that no parent or daughter isotopes were added or removed since formation
• Contamination, leaching, and weathering can affect the accuracy of radiometric dating

Biostratigraphy and Chronostratigraphy

• Biostratigraphy uses fossils to correlate and date sedimentary rocks
• Index fossils are species with short lifespans and wide geographic distribution
• Appearance and disappearance of index fossils mark the boundaries of biostratigraphic units
• Chronostratigraphy divides geologic time into named units based on time
• Chronostratigraphic units are defined by lower and upper boundary stratotypes (GSSPs)
• Magnetostratigraphy uses reversals in Earth's magnetic field recorded in rocks to correlate and date sequences

Challenges and Limitations

• Preservation bias favors hard parts (bones, shells) over soft tissues
• Reworking and redeposition can mix fossils from different time periods
• Contamination by younger or older carbon can affect radiocarbon dating accuracy
• Calibration is needed to convert radiocarbon ages into calendar years
• Closed system assumption may not always be valid for radiometric dating
• Sampling errors and analytical uncertainties can affect the precision of dating methods
• Time resolution varies among different dating methods, limiting their applicability

Applications in Paleoanthropology

• Dating methods provide a temporal framework for human evolution and cultural development
• Radiometric dating of volcanic ash layers helps constrain the ages of hominin fossils
• Biostratigraphy and magnetostratigraphy are used to correlate fossil-bearing sediments across sites
• Chronometric dating methods (OSL, ESR) date the burial time of hominin remains and artifacts
• Radiocarbon dating is applied to organic materials associated with archaeological sites
• Dated fossils and artifacts help reconstruct past environments and climates
• Combining multiple dating methods improves the accuracy and precision of age estimates
• Establishing reliable chronologies is crucial for understanding the tempo and mode of human evolution