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Repost: Dating Methods in Archeology (10th grade)

Posted in Archeology, Dating Methods, repost, school on December 17, 2009 by Savannah Logsdon-Breakstone

*Note: this was written in 10th grade as a project for my science class. This was in 2004. I am adding it here because I was amused to find it earlier this evening.

I. An Introduction

The issue of an object’s placement in time is integral to processing an archaeological find. Without having a method by which to place a find in time, The find is useless in any manner other than knowing that the object has existed at some point. Chronology is a fundamental issue in archaeology, informing researchers not only of when an object was made, but also how long a civilization took to hit certain periods of development.
For example, If we know that civilization X started making iron tools around 1,000 BCE, and around 3,000 BCE they began to make bronze tools, then we know that civilization X ‘s Bronze Age lasted approximately 2,000 years, and can compare the development of a civilization to it’s contemporaries. Or if civilization Y appeared in location A around 2,000 BCE, but all traces of civilization Y are gone by 100 CE, then we know how long that particular civilization lasted in that location, and can also identify it’s contemporaries. The importance of chronology is obvious in the above example.

When considering this, one wonders what the methods of divining an objects age are and exactly how accurate these methods are. There are many different methods, each with their own accuracy adjustment. Some methods will give you an exact calendar date, while others will only tell you when it was used in comparison to the surrounding objects on site. Some methods will use the object itself, and others might use the things associated with the object. Until fairly recently, most objects were dated according to guesses, relative location, and other objects that were previously dated either at it’s making (a year on a coin, for example) or by another archaeologist.

Though an archaeologist uses dating methods to discover the date of an object or event, the dates that are found are used to create a framework in which is placed information found on the site or that is already known. This construction of a supposed time-line is a major foundation for archaeology.

II. The Basics of Archaeological Dating

As stated above, there are several different methods that provide different types of dates. Those methods that date in relation to other objects on site are commonly referred to as relative dating methods. Relative dating is exactly what it sounds like: the date of an object according to it’s relation to other objects on the site. Generally, the first step in processing a site is using relative dating to order the timeline of the site. This generally will not allow the researcher to give an object or event a calendar date without also using absolute dating methods.
Absolute dating methods give a calendar estimate as to the age of the object, which helps to determine the length of time between one object dated absolutely and another. Using the example of civilization X again, say that after excavating a site, we find some early iron weapons on the first level of the site that pertains to civilization X. After going through a few levels, we reach the bottom level of our excavations. Here we find an early bronze weapon. By using an absolute dating method, we learn that the early iron weapon dates to about 1,000 BCE and that the bronze weapon dates to about 3,000 BCE. Therefore, we can speculate that civilization X ‘s Bronze Age lasted about 2,000 years.

The relative and absolute dating describes the scale of measurement in dating. When an age is determined, it is determined by either direct or indirect means. When researchers determine the date of an object directly, they use a method, usually an absolute method, to date the object itself. When they use indirect dating, they have another object from the same or a contemporaneous event that they have used a direct dating method on to determine the age of the object you need the date for. Unfortunately, this sort of dating is both less reliable and more time consuming then direct dating because you must first show that they are from the same (or contemporaneous) event.

Following are several different methods of dating objects and speculated events from an archaeological site.

III. Stratigraphy and Seriation

Stratigraphy and seriation are two relative methods of dating. Stratigraphy is the oldest method of dating used by archaeologists and is relatively simple. Seriation is a bit more complex to jot down, and involves having the ability to interpret statistics gathered from a site into a graph. Both are the simplest and possibly the oldest of the methods still used by archaeologists.

The word stratigraphy has two meanings. It can mean the arrangement of strata in a geographic (or, in our case, archaeological) deposit. Most archaeologists, however, call this stratification. When they say stratigraphy, they generally mean the study of the stratification.
The principals of superposition are the basis for stratification. The principals of superposition are that the layer at the top was made the most recent, while the layer on the bottom was made the longest time ago. However, this relatively simple idea isn’t as simple as it first appears. In order to be sure that this particular method is accurate, the researcher must take into consideration several variables. Some of these variables is past human disruption of the layers, natural disaster eroding away layers, and occupation.

Seriation basically means ordering, and was first developed and used by archaeologist Sir William Flinders-Petrie around 1899. The concept behind it is based on an assumption that cultural characteristics change over time, which is particularly obvious in the case of fads, typically in a specific pattern usually containing an introduction, a peak, and a decline in popularity period. By graphing the results of the find, one can usually find the point in the sequence in which an object was more popular, therefore placing a culture’s development or even to get the exact date of an object by the frequency of occurrence in a particular location. By graphing the information with horizontal bars centered along a vertical axis, you can observe the rise, peak, and decline of a particular object, typically forming a battle-ship shape. The graph is founded on different sorts of information, such as stylistic (Changing styles of a sort of object) or frequency (the proportion of certain objects in a deposit).

IV. Other Relative Dating Methods

There are a couple of other forms of relative dating worth mentioning before we move on to absolute dating.

Cross-dating I mentioned previously. Cross-dating is basically indirectly dating an object. This was the only real method of dating an object in early archaeology. To use cross-dating, you must have two objects that you can prove are from the same event. One of these objects must already have a date given to it. If you then have a date for the one object, you can usually assume that the other object is from around the same date.

Pollen analysis uses the concept of relative abundance. Pollen of a certain plant will go through certain cycles of abundance. By calculating the abundance of each type of pollen found on an object, a researcher can calculate the date by placing the object at the date that the abundance of pollen on the object matches others with similar abundances.

V. Dendrochronology

Dendrochronology is an absolute dating method by which one observes the rings of a tree. This method is based on the idea that, in temperate climates, trees produce growth rings due to wood growth, the rings made by a layer of small cells paired with a layer of large cells. In years with more optimal growing conditions, the cells of each sort tend to be larger, and the cells tend to be smaller in poorer growing conditions. Trees of the same species in the same area generally will have the same pattern in rings.

By observing the growth rings of an area and matching them with progressively older pieces of wood, researchers can create a master chronology. By studying the results of this master chronology and comparing it to rings on a wooden object, they can determine the age of the object by placing it in the master chronology.

To get the sample of wood from the object, a small diameter core sample is typically extracted. On objects that are brittle from age, the wood is generally preserved first. Then the counting and measuring of rings is generally performed and then compared to the master growth ring.

For dendrochronology to be accurate, only tree species that show rings that are sensitive to the seasons can be used- most commonly pines, firs, oaks, and juniper. Also, the wood must be well enough preserved to still have the outer growth rings. And, as with all dating methods, the context of the object(s) is of vast importance- if the wood was taken from an older structure to build the one that is being studied, then the age will be inaccurate.

VI. Overview of Radiometric Dating

Radiometric dating is based on the concept of atrophy. Atrophy is a process of slow decay, usually at a fixed rate. Everything decays, but some elements decay at a more steady rate then others. The most commonly used elements are Carbon-14, Potassium-40, and Uranium-238. Most of the best elements to use in radiometric dating are those that are most commonly found in nature.

Though the exact decay of an element cannot be predicted, those elements with a stable average rate are ideal. The rate of decay in elements is put in the terms of half-lives, or “the amount of time it takes for half of the atoms in a sample of the radioactive isotope to decay to a stable form” (Anthropology Department, University of British Columbia). When you compare the amount of the original element to the amount of what is produced when it decays, then matching it to the element’s half-life, a reasonably accurate estimate of how long it has been decaying can be determined.

Some of the more common radiometric dating methods are radiocarbon, potassium-argon, fission track, and thermoluminescence. I will go more into each of these later. Each of these have a formula applied to them which results in an age for the object in question.

VII. Radiocarbon Dating

One of the more commonly used of the radiometric dating methods is radiocarbon dating, commonly referred to as “Carbon-14 Dating”. Radiocarbon dating was discovered in 1949 by a team of scientists at the University of Chicago. It is now the most commonly used and most publicized form of absolute dating. Radiocarbon dating was developed around the rate of decay of carbon-14, which is created in the upper atmosphere and is then oxidized into CO2 which is consumed by most living things through respiration. Carbon-14 is constantly decaying, converting into nitrogen-12, but in living organisms, is replenished until it stops performing respiration. Most living things (or things that were made from once living things, such as wood) can be dated using radiometric dating because of this.

As previously stated, radiometric (and hence, radiocarbon) dating methods use the half-life of an element to determine the age of an object. The half life of carbon-14 is 5730 years on average. By comparing the remaining and the current amounts of carbon-14, one can estimate how long ago the object stopped living. When reporting a radiocarbon measurement, the measurement is stated as x years B.P. (Before Present), present being 1950. As with all methods of dating, an error factor must be calculated into an equation before finally releasing a date to the public.

There are two common ways to go about dating an object using the radiocarbon dating method. The first and most conventional is done by first converting the material into a gas or liquid state and is then placed on a counter that measures radioactive activity to find the rate of decay. The formula for dating an object by this method is:

t=[In (Nf/No)/(-0.693)]x t1/2

In being the natural logarithm, Nf/No being the percent of carbon-14 in the object in comparison to the percentage in living tissue, and t1/2 being the half-life of carbon-14. The half life of carbon-14 is 5,700 years, so in a object with 10% carbon-14 compared to living tissue:

t=[In (0.10)/(-0.693)]x 5,700 years

t=[(0.10)/(-0.693)]x 5,700 years

t=[3.323]x 5,700 years

t=18,940 years old

The above example is taken directly from ‘s article on Carbon-14 dating, as is the formula ( Currently, the oldest date that can be accurately dated using this technique is 40,000-50,000 years.

The other technique of radiocarbon dating uses an accelerator mass spectrometry, also known as AMS, method to count radiocarbon amounts directly rather then measuring the decay rate. This allows for less of a sample to be taken from the object being dated, thus causing less damage to the object.

VIII. Other forms of Radiometric Dating

There are several other forms of radiometric dating that are worth mentioning before we get to other forms of absolute dating. These are potassium-argon, fission-track, and thermoluminescence.

Potassium-argon dating is based on the fact that potassium-40 decays into argon-40, and that in the case of a volcanic eruption, the only argon contained in the rock will have been produced since the rock cooled. By using samples that trapped all of the argon gas produced and measuring it then comparing it to the potassium in the sample, one can calculate an approximate age for an object. However, potassium-argon dating is one of the lest reliable of the radiometric dating methods.

Fission-track dating is based on spontaneous fission of uranium-238. Though most uranium decays into a stable lead, occasionally the atom splits in two at a high speed, damaging the surrounding structure. By observing an object under a microscope, one can see tracks left by this “spontaneous fission”. Uranium rarely goes through this process, but there is a fixed rate at which it does occur. At the time when uranium is made in an object, it has no fission decay, so by counting the fission tracks left behind, one can get an age for the object in question.

Thermoluminescence is a very new technique and is yet to become widely used in the field, though it has a lot of potential due to the fact that it can date heat-altered artifacts and has a larger effective range then the more commonly used methods. Instead of measuring the rate of radioactive decay, thermoluminescence looks at the “amount of radiation trapped within crystalline materials that have the property of storing this energy within the microscopic crystal lattice fabric” (Anthropology Department, University of British Columbia). This energy is released when the object is rapidly heated to about 500 degrees celsius, reaching a flash point at which all electrons are freed and the energy is emitted as a flash of light. The greater the light, the greater the age of the object.

IX. Other Absolute Dating Methods: Obsidian Hydration

Obsidian hydration is based on the fact that a fresh surface of obsidian will absorb water around it. The water seeps into the stone to form a hydration layer, containing 3.5% water, which increases the density of the layer to allow it to be observable under a polarized light microscope.

To get an estimate, the object in question is submitted to a lab where a small section of the object is removed and then ground to fit on a microscope slide, where the hydrogen band is measured. Though this method holds a lot of promise, obsidian hydration isn’t fully understood, making it slightly more unreliable and even a tad unscientific, as the factors cannot be fully controlled in a manner that is needed to make an accurate assumption.

X. Conclusion

As we have seen, there are many ways to date objects found on an archaeological site, some more accurate then others, but all with error margins. There are two different sorts of dating- absolute dating, where we can set a calendar date, and relative dating, where we can place an object in time relative to other objects found on site. There are two methods to go about dating objects: direct, where we find the date from the object itself, and indirect, where we find the date from other objects proven to be contemporary.

There were four types of relative dating covered in this paper: stratigraphy, seriation, cross-dating, and pollen analysis. Stratigraphy deals with where objects are relative to each other in the paper. Seriation deals with the rise, peak, and fall in the popularity of an object. Indirect dating and cross-dating are basically the same thing, both being a method of finding an object that is dated that is assumed contemporary. The last was pollen analysis, which compares the amount of pollen on an object to another object’s amount of pollen.

Six absolute methods were also mentioned: dendrochronology, radiocarbon dating, potassium-argon dating, fission track dating, thermoluminescence, and obsidian hydration. Four of these, fission track. radiocarbon, potassium-argon dating, and thermoluminescence, are radiometric dating methods, dealing with radioactive elements and their decay. For example, radiocarbon dating uses carbon-14, a very common radioactive element found in nearly all living things. On the other hand, dendrochronology deals with tree-rings and compares the rings in an object to a chart of growth rings for the area, thereby giving an approximate age. And last of all, obsidian hydration dating deals with the rate at which water (H2O) evaporates on the surface of obsidian.

Placement in time is a very important part of archaeology. With out it, we would not know what came first- Mesopotamian or Babylonian civilization (it’s Mesopotamian, in case the reader was wondering). And without methods by which we can place an object in time, the idea of archaeology would be futile, and development in understanding our pasts and ourselves would be useless.