Archaeological Dating and Early Human Migration

Absolute dating methods provide specific numerical ages for archaeological materials and events, allowing for a precise chronology.

This section outlines various architectural components, likely from a diagram or structural analysis:

Dendrochronology is a precise dating method based on the annual growth rings of trees.

Principle of Growth: Trees respond directly to their environment.
- In years with high precipitation, trees grow faster, producing thicker annual rings.
- In years with less precipitation, growth is slower, resulting in thinner annual rings.
Building Chronologies: Scientists construct tree-ring chronologies by starting with living trees and then finding progressively older wood specimens.
- Archaeological wood is crucial, as its outer rings are matched with the inner rings of more-recent specimens, creating an overlapping sequence.
- This process allows for extending chronologies back thousands of years. Examples mentioned include wood cross sections from years such as 1798, overlapping with 1871, and further overlapping with 1924, extending to 2013.

The map illustrates the geographical distribution of major prehistoric cultures in the American Southwest and Northern Mexico:

Anasazi: Located primarily in the Four Corners region (Utah, Nevada, Colorado, Arizona, New Mexico). Key sites include Mesa Verde and Chaco Canyon.
Hohokam: Centered in present-day Arizona. Key sites include Pueblo Grande and Casa Grande.
Mogollon: Found in parts of Arizona, New Mexico, and extending into Northern Mexico. Key sites include Gila Cliff Dwellings, Paquime, and Cuarenta Casas.

Radiocarbon dating, or C^{14} dating, is the most widely used dating method globally.

Isotopes of Carbon: Carbon exists in three main isotopes:
- C^{12} (common carbon)
- C^{13} (rare carbon)
- C^{14} (radioactive carbon)
Formation of C^{14}:
- Cosmic radiation bombards the upper atmosphere, producing neutrons.
- These neutrons collide with Nitrogen-14 (N^{14}) atoms, causing a nuclear reaction (neutron capture) that transmutes N^{14} into radioactive Carbon-14 (C^{14}).
Absorption by Living Organisms: All living organisms continuously absorb all three isotopes of carbon (including C^{14}) from the atmosphere (e.g., through photosynthesis in plants, and consumption by animals).
Radioactive Decay after Death:
- Upon the death and burial of an organism (e.g., wood, bone), the absorption of carbon ceases.
- The unstable C^{14} isotope begins to decay back into N^{14} through a process called beta decay, emitting a beta particle (electron or positron) from its nucleus.
- The rate of this decay is constant and measurable, with a half-life of approximately 5,730 years.
- By measuring the remaining C^{14} in a sample, scientists can determine the time elapsed since the organism's death.

Radiocarbon results are initially given in Radiocarbon Years Before Present (BP) and require calibration to convert them into calendar years.