Summary
Highlights
The lecture transitions from relative dating to absolute dating, highlighting its importance for understanding chronological questions like the age of artifacts, contemporaneity of villages, and rates of cultural change. Absolute dating methods provide calendar years, making the past come alive with precise timelines.
Dendrochronology, or tree ring dating, is effective for dating objects up to 10,000 years old. It relies on the annual growth rings of trees, which vary in thickness based on climate. Andrew Douglas pioneered this method in 1913, developing master sequences to date ancient Southwestern pueblos with remarkable accuracy. The technique involves correlating ring patterns from archaeological wood samples with established master chronologies, though it's limited to non-tropical regions with distinct seasons and requires a complete master sequence linked to living trees.
Radiocarbon dating, often using Accelerator Mass Spectrometry (AMS), is a widely known archaeological dating technique suitable for organic materials from 50,000 years ago to around 1950. Developed by Willard Libby in 1949, it revolutionized archaeology by providing objective dates for wood, bone, and other organic matter. This method measures the ratio of carbon-14 to carbon-12, relying on the known decay rate of carbon-14. Calibration with tree rings and other data sources is crucial, as original assumptions about constant atmospheric carbon-14 levels were later found to be incorrect.
Trapped charge dating methods, like thermoluminescence (TL), extend dating capabilities back about 300,000 years. TL measures the accumulated radiation energy stored in materials like heated clay vessels or fired bricks, which is released as light when heated. Optically Stimulated Luminescence (OSL) uses laser technology to date quartz and feldspar grains in archaeological layers, effective for sites between 100 and 100,000 years old. These methods assume the material was heated to a high enough temperature to 'reset' the radiation clock.
Electron spin resonance (ESR) dating measures radiation-induced defects or trapped electrons in bone or shell samples without needing heat, making it a non-destructive method similar to TL. It's particularly useful for dating tooth enamel and bone, allowing investigators to date fossil fragments up to a million years old. ESR has crucial applications in studying early human evolution.
Potassium-Argon (K-Ar) and Argon-Argon (Ar-Ar) dating are used for much older geological periods, typically in volcanic rocks. Potassium-40, abundant in Earth's crust, decays into Argon-40 with a half-life of 1.3 billion years. By measuring the Argon-40 accumulated since the rock's formation, archaeologists can date associated human settlements, especially in volcanic regions like East Africa. More recent techniques use laser fusion to date single grains of volcanic ash for higher precision.
Archaeomagnetic dating utilizes changes in Earth's magnetic field over time. Clay materials heated to a high temperature record the direction and intensity of the magnetic field at that moment. By correlating this with known variations in Earth's magnetic field, structures with well-baked clay floors (like kilns or ovens) can be dated. However, its application is limited by the availability of well-maintained records of magnetic field variations, which typically only go back a few centuries or millennia.
Obsidian hydration dating is a method applicable across the entire span of human existence. When obsidian, a volcanic glass used for tools, is freshly fractured, its new surface absorbs water from the surroundings, forming a measurable hydration layer. The depth of this layer increases with time, indicating how long ago the tool was manufactured or used. This technique is highly accurate and widely used in regions like California and Mesoamerica, although it is a destructive method requiring a small sample of the obsidian.