The Oseberg ship burial is one of the most famous Viking graves.
Viking funerals sometimes involved the burial of the body and treasure of the ruler along with a complete ship under a huge mound of earth.
At the time of the excavation in 1904, the queen's burial was estimated to date sometime after A.D. 850.
In recent years, however, dendrochronology was used to show that the burial was constructed earlier than that time.
Accurate dating is essential to document changes in human behavior over time.
The framework of things and events placed in time is called a chronology.
Only in the last 50 years that archaeologists have been able to answer this question with some accuracy.
Archaeologists use a number of different terms and abbreviations for dates.
Techniques for dating items from the past are either relative or absolute in terms of the age they provide.
Relative Dating Methods
Until about 1950, there was almost no way to reliably determine the age of the remains of early humans and their artifacts.
Dating was often done by association and the dates were relative.
Relative dating methods often use stratigraphic relationships to order older and younger materials.
Example: Pipe Stems
Historical records rarely tell us much about the lives of the people who were alive at that time.
Historic archaeology is able to fill in some of the gaps.
Various keys to time past have been developed on the basis of rapidly changing technology.
One of the more interesting keys to historic time comes from the classification of clay smoking pipes.
Hundreds of stem and bowl fragments of these pipes can sometimes be found at historic sites.
The size of the wire and thus the diameter of the whole became finer over time.
The shape of the pipe bowl also changed over time.
Reckoning Time
A calendar is a system for organizing time into repeatable and predictable units.
There are approximately 40 different calendars in use in the world today.
The earliest evidence for an awareness of time comes from 15,000 years ago.
By 3000 B.C. the Sumerians in Mesopotamia were using a lunar calendar.
The first Egyptian calendar had 365 days.
Maya Calendar
Most of the known calendrical systems have minor errors or inaccuracies, requiring some correction.
The Maya, however, appear to have developed an extremely exact system of time.
The Mayan calendar is based upon two cycles: a 260-day sacred year and the 365-day solar year.
Absolute Dating Methods
Absolute dating methods are used to assign a specific, calendar age to an object.
There are four major groups of absolute dating techniques commonly used in archaeology: accumulation of layers, radioactive decay, trapped charges, and magnetism.
Each of these methods is used with specific kinds of materials to obtain a date.
Accumulated layers take several forms.
Annual layers, or growth rings, form in trees and provide the basic data for dendrochronology.
Annual layers known as varves form in lake bottoms in arctic and subarctic regions and provide a kind of calendar of past climates but these layers are rarely related directly to archaeology.
Obsidian accumulates a layer of weathering which becomes thicker over time.
The principle of radioactive decay is well known.
Radioactive forms of elements like carbon, argon, and uranium decay over time into non-radioactive states.
The rate of decay, or half-life, is well known so that it is possible to determine how much time has passed since the material began to decay.
Trapped charge techniques involve the principle of accumulation of charges or records of events in material.
If the rate of accumulation is known, the number years elapsed in accumulation can be determined and provides the age of the artifact.
There are two kinds of magnetic dating techniques used in archaeology, involving changes in the position of the earth's magnetic pole.
Archaeomagnetism relies on the continual shifting of the location of the North Pole over the last several thousand years.
Magnetic polarity relies on reversals in the placement of the earth's magnetic pole.
Dendrochronology
One of the first methods to be used for absolute dating was based on the annual growth rings in trees.
Each ring has a darker and lighter part marking the slower and faster growth of the tree during the year.
Important climatic information is also recorded in the size and pattern of the annual growth rings.
Part of the distinctive sequence of rings from one tree can be overlapped with all or part of a sequence from another tree.
Dendrochronology can only be used in areas where substantial timbers and trees are preserved.
Example: Pueblo Bonito
The occupation of Pueblo Bonito has been precisely dated using dendrochronology.
Chaco Canyon lies in the arid mountains of northwestern New Mexico.
There were at least nine large towns of several hundred rooms each in Chaco Canyon after AD 900.
The largest and most impressive of these was Pueblo Bonito.
Tree-ring dates obtained from preserved wooden beams place the earliest building at Pueblo Bonito at AD 919.
Example: French Neolithic Lake Dwellings
Neolithic settlements have been found in the Alpine region of France.
These villages, once along the shoreline, were submerged sometime after their abandonment as the lakes grew in size and depth.
Archaeologists were able to date a series of settlements along the ancient lakeshore precisely using the tree ring sequence in the well-preserved construction timbers in the lake sediments.
The French data provide a great deal of information.
Land use and degradation and the response of population to crowding, and rapid changes in human population were examined.
The period of study was between 3180 BC and 2950 BC.
Radiocarbon Dating
Willard Libby announced the first age determinations from radioactive carbon in 1949.
The key to this procedure involves the principle of radioactive decay.
Carbon (abbreviated as C) is a chemical element with several isotopes.
Unstable radioactive isotopes in various materials decay into stable isotopes over a known period of time.
All living things absorb both stable carbon (primarily 12C) and its radioactive isotope (14C) throughout their lifetime.
The proportion of 12C and 14C remains constant in an organism until its death when the intake of fresh carbon stops.
The rate of decay for 14C has a half-life of about 5730 years.
The limit of radiocarbon dating is around 40,000 years ago.
There are a number of minor corrections that are made to radiocarbon dates to improve their accuracy.
Accelerator Mass Spectrometer (AMS) Dating
Determining the amount of 14C remaining in prehistoric materials is not an easy task.
Until recently, a sample of known weight was carefully cleaned and then burned to create a pure gas of carbon dioxide.
Several grams of organic material were normally required to produce enough gas for counting.
AMS are very large instruments that use magnets and sensitive collectors to separate and count individual carbon atoms.
Now, less than 0.01 g of sample is needed.
Science in Archaeology: Early Agriculture
A useful example of the development of radiocarbon dating comes from the 1970s and 1980s.
Archaeologists working near the Nile River in southern Egypt discovered a few grains of barley in a fireplace at a Late Paleolithic site called Wadi Kubbaniya.
Conventional radiocarbon dates were made on charcoal from the site and indicated the startlingly old age of 18,240 to 17,130 years ago.
The AMS date for one of the pieces of barley from the site came out as 4850 ± 150 years before present.
Example: The Shroud of Turin
The Shroud of Turin is a religious relic that many people believe was used to wrap the body of Christ.
Samples of the Shroud were dated using AMS.
Dates from control samples, taken from objects of known dates confirmed the accuracy from the labs.
Accelerator Mass Spectrometer measurements provided a calendar age range of AD 1260 - 1390.
Calibration
The radiocarbon dating of tree rings of known age has shown that Carbon-14 levels have changed over time.
Radiocarbon dates underestimate the actual age of a sample.
To make up for this error, dates in radiocarbon years are now corrected, or calibrated, to calendar years.
There is another correction that must be made for samples of certain kinds of plants or marine organisms.
A similar correction needs to be made for plants, and animals that eat those plants.
Radiopotassium Dating
Radiopotassium, or potassium-argon, dating is of crucial importance for determining the age of the earliest human remains.
The technique has been used to measure the age of the oldest rocks on the planet.
The process requires rather newly formed volcanic rocks or ash deposits.
One isotope of potassium has a half-life of approximately 1.3 billion years.
Example: Laetoli — Our First Steps
All of the evidence for our early ancestors before two million years ago comes from Africa.
Until 1970 there was relatively little evidence for the earliest human ancestors other than a few skulls and pieces of bone.
In the last thirty years or so, however, discoveries in Central and East Africa have reshaped and redefined our family tree.
The earliest human ancestors show definite indications of habitual bipedalism.
The most dramatic evidence for this new posture comes not from the fossil bones, however, but from actual footprints at the site of Laetoli in Tanzania.
The footprints were discovered by Mary Leakey in 1976.
Radiopotassium dating was used to establish at date between 3.6 and 3.8 m.y.a.
Protecting the Past: The Laetoli Footprints
After the excavation of the footprints, they were reburied for preservation.
Unfortunately the area has seen the growth of acacia trees and the spread of tree roots into the zone of footprints.
The final step in the conservation process involved the careful reburial of the entire area, with steps taken to inhibit the return of vegetation.
Thermoluminescence Dating
Thermoluminescence (TL)can be used to date samples up to 500,000 years ago.
It can be used to date burned flint and other burned materials that are common at archaeological sites.
A disadvantage is the large error factor associated with the dates because of uncertain assumptions in the method.
TL is the name for the physical process in which a mineral glows when heated.
All buried materials are constantly exposed to radiation from naturally occurring radioactivity and from cosmic rays that pass through the earth.
A small portion of this radiation is trapped as energy in crystalline materials in the form of thermoluminescence.
When heat is re-applied to the crystal in the laboratory, the energy in the crystal is released in the form of light, causing the material to glow.
The amount of TL released is measured and used to calculate how much time has passed.
