9.3 Paleomagnetism and Other Dating Techniques

Paleomagnetism and dating techniques are crucial tools in unraveling Earth's past. By studying magnetic properties in rocks and using various dating methods, scientists can determine the age of fossils and geological formations, providing a timeline for evolutionary history.

These techniques, from geomagnetic reversals to amino acid racemization, each have unique strengths and limitations. Understanding their applications and constraints is essential for accurately interpreting the geological record and piecing together the puzzle of human origins.

Principles of Paleomagnetism and Dating Techniques

Principles of paleomagnetism

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• Earth's magnetic field exhibits dipole nature and undergoes periodic reversals
• Magnetic minerals in rocks align with Earth's magnetic field during formation preserving magnetic orientation
• Paleomagnetism process measures magnetic properties in rocks and compares with known magnetic field changes
• Application to sedimentary layers involves deposition of magnetic particles in sediments sequentially recording magnetic field changes
• Dating fossils accomplished by associating with dated sedimentary layers bracketing fossil age using layers above and below

Geomagnetic reversals for dating

• Geomagnetic reversals occur when Earth's magnetic field completely flips at irregular intervals averaging every 200,000-300,000 years
• Reversals caused by changes in Earth's core dynamics and complex interactions of magnetic field components
• Magnetic polarity time scale records past reversals compiled from multiple sources
• Dating applications correlate rock sequences with known reversal patterns determining relative ages
• Magnetostratigraphy studies magnetic properties in stratigraphic sequences constructing local magnetic polarity sequences

Additional Dating Techniques and Evaluation

Alternative dating techniques

• Amino acid racemization (AAR) measures conversion of L-amino acids to D-amino acids over time dating organic materials (shells, teeth) up to 1 million years
• Electron spin resonance (ESR) measures trapped electrons in crystalline materials dating tooth enamel, quartz, and carbonates from 10,000 to 2 million years
• Thermoluminescence (TL) measures light emitted by heated crystalline materials dating pottery, burnt flint, and sediments up to 500,000 years
• Optically stimulated luminescence (OSL) measures light emitted by stimulated quartz grains dating sediments and archaeological materials up to 350,000 years
• Uranium-series dating uses decay of uranium isotopes to thorium and lead dating cave deposits, corals, and bones up to 500,000 years

Strengths vs limitations of dating methods

• Paleomagnetism widely applicable to sedimentary rocks providing relative dating over long time periods but requires undisturbed sequences and may be affected by local magnetic anomalies
• AAR applicable to wide range of organic materials relatively simple and inexpensive but sensitive to temperature and environmental conditions requiring calibration
• ESR non-destructive technique applicable beyond radiocarbon dating range but requires complex calibration and may be affected by environmental radiation
• TL and OSL applicable to inorganic materials useful for archaeological sites but require careful sample collection and may be affected by incomplete bleaching
• Uranium-series dating offers high precision for suitable materials applicable to wide time range but requires closed system conditions limited to specific materials