Archaeology ch 7

Sequence comparison

(Also known as cross-dating). Presumes the existence of past cultural connections, such as trade, so that the resemblances are not accidental. Very useful for building broad chronologies for a region.

Half-life of potassium-argon dating

1.31 billion years for this rare isotope

E.G. Squier and E.H. Davis

1848- determined the minimum age of mounds in the Mississippi River Valley from the age of the oldest trees growing on them. Reasoned that the tree would not be allowed to grow on them while they were in use. Oldest tree there was at least 300 y/o, so site must be at least 300 y/o.

Each method has some limits or built-in inaccuracies, thus:

A chronology based on several different methods and many dated samples is more reliable than a sequence based on a single method or only a few dated samples.

Obsidian hydration

A new-age determination technique. Based on the cumulative adsorption of water by volcanic glass. Over time, the absorbed water forms a hydration layer on the exposed surfaces of obsidian. The thickness of the layer increases over time, therefore, the layer can be used to determine how long the surface has been exposed if the rate of hydration is known.

Argon-argon dating

A refinement of the K-Ar method that allows several age determinations to be made from each sample, thus increasing the reliability of the date and producing more accurate results from smaller samples. IOW: refinement of K-Ar technique, measures the Argon isotopes ratios in mineral samples. More accurate than the K-Ar techniques.

Seriation

A relative dating method derived from these cultural regularities.

Stylistic seriation

A technique for ordering artifacts and attributes according to similarity in style. The more limited the source area of the artifacts in question, the more likely the seriation reflects the passage of time

Bone chemistry dating techniques

Allow the archaeologist to determine if bones found in the same matrix were deposited in the site at the same time, Bones in a deposit will lose components at a steady rate

Examples of chronological dating:

Arranging automobiles of different years in year order Clothing styles arranged the same way Different music styles

Uranium-series dating

Based on the half-lives of a few uranium isotopes (short enough to be useful to archaeology). Method can date deposits associated with caves, lakes, and springs. Calcite formations in a number of European Paleolithic caves have been dated by this technique

Potassium-argon dating (K-Ar)

Based on the radioactive decay of a rare isotope of potassium to form argon gas. Used principally to determine ages for geological formations that contain potassium.

Why is geochronology important?

Because the effects of long-term geological processes, such as glacial advance of retreat, or fluctuations in the land or sea levels, can be useful in dating archaeological remains.

Dendrochronology

Commonly called "tree-ring" dating is the best known method of directly determining absolute age for wood. Based on counting the annual growth rings in the cross-sections of cut trees.

To reconstruct the past:

Define time dimensions Determine which remains are from the same period and which are from different time periods.

Relative dating

Evaluating the age of one item relative to other items - for example, determining that artifact A is older than artifact B. Actual ages are not assigned to data.

Australopithecus

Mary and Louis Leakey found the remains that are included in the genus "Australopithecus" - an early hominid. Assigned the bones an age of about 1.75 million years.

Archaeological stratigraphy

May represent a combination of both behavioral and natural transformational processes.

Radiocarbon dating

Most important radiometric technique for archaeologists.

Frequency seriation

Orders the sequence of sites or deposits by studying the relative frequencies of their artifact. Based on the assumption that the frequency of each artifact type follows a predictable pattern, from the time of its origin to an expanding popularity

Absolute dating

Placing the age of a sample on an absolute time scale, usually a calendrical system. Assigns an age in years, but is seldom precise.

Modern dendrochronology

Refined the method to use a cross-link method to extend the aging sequence far beyond the life of a single tree. Tree-ring dating thereby provides a way to calibrate radiocarbon dates.

Horizontal stratigraphy

Relative sequence arranging sites in temporal order.

Archaeomagnetism

Relies upon the fact that the earth's magnetic field varies over time, shifting in the horizontal plane (expressed as declination angle) as well as vertically (expressed by the dip angle). The changing magnetic course can be extended back in time by analysis of mineral compounds, such as clay, and their ages computed.

Chronological sequence

Series of relative or absolute dates arranged in order of their ages. Enable archaeologists to reconstruct the order of ancient events.

Radiometric methods

Several age-determination techniques exploit the principle of radioactive decay, the transformation of unstable radioactive isotopes into stable elements. Most frequently provide indirect age determinations. Exploit the principle of radioactive defeat.

Fission-track dating

Similar to other radiometric dating methods, except instead of being based on the counts of daughter isotope particles, it is based on the number of fission tracks preserved in a mineral sample. Left by fission particles produced by the decay of trace elements like 238U as they pass through a mineral. The age of a given mineral deposit can be determined by counting the number of fission tracks it contains. Fission track dates have been consistent with argon-argon dating

Geochronology

The application by geologists of techniques the age of geological formations.

Stratigraphy

The archaeological interpretation of the significance of stratification. As long as the context of the stratified desposit is clear, the archaeologist can use stratigraphy to determine the relative age of the deposition of artifacts recovered at the site.

Problems of obsidian hydration

The hydration rate can vary with the composition of the obsidian. Different temperatures affect the hydration rate. ***Less reliable as a absolute dating technique than as a relative dating technique.

Battleship-shaped curves

The length of time and the degree of popularity (frequency) vary with each type, but when presented diagrammatically most examples form one or more lenslike patterns.

Half-life

The period required for one half of the unstable atoms to decay and form a stable daughter isotope.

Indirect dating

Uses analysis of material associated with the artifact, ecofact, or feature being studied to evaluate its age.

Direct dating

Uses analysis of the artifact, ecofact, or feature itself to arrive at its age.

Weakness of floral and fauna

Variations in temperature and humidity cause the loss and gains to vary from site to site.