How Do They Do That?
Dating and Research Methods Used in Archaeology
Archaeologists use several methods to date artifacts and piece together the culture and natural environment of prehistoric Americans. Because archaeological dates can be affected by modern human contamination or natural erosion, the relationship of datable materials to other cultural artifacts is often debated within the scientific community.
Archaeological dates are usually represented as “years before present” (years BP) in radiocarbon-dated items (see definition below). One radiocarbon year is equal to about 1.03 calendar years.
- Radiocarbon Dating
This is one of the most valuable and widely used methods in the science community. This method estimates the amount of carbon remaining in an animal or plant to determine the approximate age. Only materials that were once living, or that were made from materials that were once living can be carbon-dated. For example, charcoal can be carbon-dated because the burned wood was once a living plant. Projectile points, however, cannot be directly carbon-dated because they’re made from non-living rocks. Charcoal and bones were the most important radio-carbon dated items from Wilson Butte Cave.
- Study of “Time Marker Artifacts” Archaeologists also estimate dates by studying the types of artifacts found at a site. Artifacts contain certain stylistic features that can be tied to specific time periods. For example, Clovis, Folsom, and Plano projectile points are “time markers” for the Paleoindian period.
Archaeologists determine the age of a cultural site by studying strata and assemblages to re-create the prehistoric environment and put the site in context. For example, if tools are found in the same strata as a hearth, they are most likely from the same time period. This method becomes less reliable if an area has been disturbed and the strata have become mixed together.
It is important to understand these definitions:
Strata: Distinct layers of a deposit.
Assemblage: Artifacts which appeared to be in physical association, occurring together in limited occupation areas, or at approximately the same relative depth within a major stratum.
Paleobotany is the study of ancient plants. Understanding the plants that grew in an area can tell us a lot about the animals that probably lived there, and how people survived. To identify the ancient plants, scientists try to identify the pollen and seeds that remain in different layers, or strata, of an archaeological site.
One type of paleobotany study is the Pollen Extraction Method. Archaeologists use this method to identify the pollen in different strata of an excavation. First, scientists soak their sample in hydrochloric acid and separate the resulting material with fine mesh screens. This material is dried and crushed, run through a centrifuge (a machine that quickly spins liquid samples and separates cells) three times, and treated again in hydrochloric acid. This process removes the original pollen from the source material. Then, a patient person counts the individual pollen grains through a microscope. A reference chart is used to identify the pollen grains.
Next Page: Meet the Archaeological Team
Activity- Stratigraphy and Crossdating
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"TIME MARKER" ARTIFACT-Desert Archaic
Tradition. Northern side-notched point from
the A layer.
DOING THE MATH for Radiocarbon Dating:
The radioactive decay of carbon-14 follows an exponential decay.
A quantity is said to be subject to exponential decay
if it decreases at a rate proportional to its value. Symbolically, this can be expressed as the following differential equation
, where N
is the quantity and λ is a positive number called the decay constant
The solution to this equation is:
where, for a given sample of carbonaceous matter:
N0 = number of radiocarbon atoms at t = 0, i.e. the origin of the disintegration time,
N = number of radiocarbon atoms remaining after radioactive decay during the time t,
λ = radiocarbon decay or disintegration constant.
Strata: Distinct layers of a deposit
Wilson Butte Cave Strata cross section