Thread: Methods for Archaeological Dating

I'm just really curious about the different ways we can date findings, and the reliability of each method. How, for instance, we go about determining that the pinkie toe bone of a monkey found in Africa is approximately 3 million years old, or some such thing like that. I do not want anyone to be bashing the styles of dating, nor claiming that they are all 'inaccurate'. It is beyond widely accepted that the methods (especially like radiometric) used are, to a large degree, accurate. I'm wondering how accurate, and how reliable, not whether or not they are accurate or reliable.

Most archaeological dating is on the order of thousands of years at most. I only point that out because you mentioned the "pinkie bone" of millions of years in age. But some of the techniques are similar in principle.

Radiocarbon (Carbon 14)

Organic samples can be dated up to 50,000 years but the margin for error increases the further back in time one goes. A sample 2,000 years old, for instance, might have a margin of error of +/- 250 years. This margin for error can be reduced by corroborating with other dating methods like dendrochronology.

Dendrochronology

This is tree-ring dating. The growth layers in trees are formed annually and can simply be counted and matched to known environmental changes. Correlations to tree-rings have been made to earthquakes, droughts, volcanic eruptions, etc. One can obtain results from charcoal where rings can still be visible. Bannister and Smiley (1955:179) stipulate four basic requirements for dendrochronology research:

• there must be trees that produce clearly defined annual rings as a result of a definite growing season
• tree growth must be principally dependent upon one controlling factor
• there must have been an indigenous prehistoric population that made extensive use of wood
• the wood must e well enough preserved so that it still retains tis cellular structure

Glacial Varve sequences

Melt water basins of retreating glaciers create annual deposits of thin clay laminae and go back to 20,000 years in some cases. While human cultural remains are rarely found in these layers, sites of human habitation can be relatively dated by comparing associated pollen and diatoms. This is a useful tool in corroborating other methods of dating.

Stratigraphy

When the absolute date of an object is known, artifacts and features in the same stratum or in strata that are bracketed by known dates can be assumed with confidence.

Patination analysis

The surface oxidation of artifacts or even petroglyphs and other rock art and the build up of a patina or desert varnish can be examined and a dated.

Seriation

Using chronological sequences of styles, types, or assemblages of types (cultures) can be useful to date a site, but existing sequences have to already be established. Ceramicists in the U.S. are very proficient, though, at looking at pot sherds in sites of the American southwest and determining what culture and period the site is from. The same idea can be applied to lithics, fish hooks, jewelry, etc.

Middens and garbage

Analysis of the rates of refuse accumulation can provide good information on how long a site is occupied and sometimes when.

Thermoluminescence

Ceramics can be examined to evaluate the amount of electrons remaining in the artifact since it was last fired and, thus, provide a time at which the ceramic/pottery was made.

Fission-track

Materials that are 1-2 million years old are easiest to be dated by this method, but it can be applied to younger materials, particularly uranium-bearing glass (either human or natural origin -like bottles or obsidian).

lexicostatic dating and glottochronology

This is a technique used to estimate the age of languages and the dates of divergence between two languages.


And many, many more.

References:

Bannister, B. and Smiley, T.L. (1955). Dendrochronology. In T.L. Smiley (ed.), Geochronology (pp. 177-195). University of Arizona Bulletin Series 26.[/list]

I remember watching a documentary about super volcanoes, and one of the demonstrations they used to show how widespread the eruption of (I think it was Yellowstone, could be wrong) the volcano was, was by evaluating the layer of volcanic 'debris' around the volcano, and comparing it to similar layers across the planet. the compositions were nearly identical, indicating that it must have come from a common source. They used that idea to show that the fossil remains stuck in that layer of strata was all the same age. If you don't know the age of an object in that layer of strata, is there a way to determine, say, the age of the strata itself? Or would you need to take some object contained therein, and date that?

Just trying to stimulate conversation and discussion, otherwise I'm sure I'd google anything I wanted more clarity on. (after all, isn't that the point of a forum?)

After erosion washes away layers they have to be put down somewhere. They settle to the bottom and as the earths crust changes, they become the new layers. Layers are formed by sedimentation of once dissolved or water carried materials. Igneous rocks and metamorphic rocks are made from melted rock (igneous, from the heat from the center of the earth) and heat and pressure (metamorphic rock) The layers of strata can be pushed up as the crust changes.

yes, but how do we know how old some specific layer of igneous rock is? What methods would we go about using to determine the date of a specific sedimentary layer?

Igneous rock can be dated radiometrically by looking at the radioactive isotopes present in minerals crystallized in the rock itself. This will give a date at the time the crystals were formed, which is the point at which the igneous rock cooled.

Other dating can be done on old rock that contains iron particles (paleomagnetic and archaeomagnetic dating). This is done by analyzing the directionality of the particles. They'll point to the magnetic north pole and, thanks to sea-floor spreading we have a very good sequence going back millions of years which shows the pattern of polar-flip on Earth which can be matched to the rock.

To clarify a point implied in Skinwalker's post: if we have dated two volcanic rocks, then we know the age range of the sediments between them. Also, a knowledge of deposition rates for different types of sediments allows a reasonable estimate of where within that range the specific age lies.

got it. sedimentary rocks aren't topsoil, right? They would be formed differently, though the decomposition of rocks formed deeper within the earth, right? Assuming the assertions I made are right, how does, say, a dinosaur, get into the sedimentary layer from the topsoil layer? is there an idea of how much time it takes for the bones to become encased within the layer the inevitably rest in and fossilize? from what I understand, each individual layer is pretty thin, right? How do we measure which layer, if the bone (say the skull of a large animal) is lying in more than one sedimentary layer? If I'm wrong, please don't hesitate to correct me

Topsoil is rarely preserved in the geological record. (An exception is the seat earth found in coal sequences.) Your hypothetical dinosaur bone could be preserved in one of a number of different sediments. It could be a gravel patch on the inner bend of a river meander; it might be fine silt on the flood plain of a river; it could be on mudflats at the mouth of a delta. There are many more possibilities.

What is necessary is that the sediment in which the dinosaur will be preserved is covered up (quite quickly) by other sediments. This may occur as the result of flooding, sea level rise, or sinking land. Flooding is commonplace within all river systems; sea level rises are very common on a geological time scale (as are the retreats) and sinking of portions of land masses are oftne a conseqence of plate tectonics.

Some sedimentary layers are, as you suggest, quite thin, but others can be feet thick. It will depend upon the specifics of the environment what is actually deposited, in terms of character and thickness.

I hope this helps. Please ask further questions to clarify any points.

It most certainly DOES help, thank you. But I must ask, your examples are near water sources; are those the only places we will generally find fossils? Do places, say, higher in the mountains, for the sake of argument lets say the Himalayan Range, also have preserved fossils in layers of the rock? Or is that not even a real possibility. I guess any fossil would have to settle in a valley between the mountains and be subject to sedimentary burial there, right? Are there other ways for a fossil to be preserved and dated, if it's in a place where sedimentary buildup isn't happening?

How about the desert. Are there any places where the sediment gathers without the use of water carrying it to it's final destination? Are there other methods for sediment to be deposited? Aside from volcanic eruption, how does a mass of decomposed, shredded rock get on top of a fossil, if it's not near water?

Originally Posted by Arcane_Mathamatition

But I must ask, your examples are near water sources; are those the only places we will generally find fossils?

Think throught the logic. We find fossils in sediments. The commonest mode of sedimentary deposition is from water. There are some exceptions: loess deposits are wind blown dust and sand, desert dunes are moved by the wind and deposited in a dry environment; glaciers may not only erode, but deposit boulder clay. Ash deposits from volcanic eruptions can be effective at trapping animals. However, most sediments are deposited, or precipitated in water.

Originally Posted by Arcane_Mathamatition

Do places, say, higher in the mountains, for the sake of argument lets say the Himalayan Range, also have preserved fossils in layers of the rock?

Fossils are found close to the summit of Mount Everest and in many other mountain ranges around the world. At the time those sediments were deposited they were near, at, or below sea level. Subsequent mountain building have raised them to their present altitutde. When they formed it was in lagoons, or on beaches, or in lakes, or deltas, etc

Originally Posted by Arcane_Mathamatition

I I guess any fossil would have to settle in a valley between the mountains and be subject to sedimentary burial there, right?

No, for the reasons noted above. The valleys are a very recent feature (geologically speaking).

In general if there is no sedimentation then there is nothing to bury the dead animals, so fossilisation will not be able to proceed. One exception I can think of (and its not truly an exception) could occur in karst country. This is a limestone area where caves have been formed by the dissolving some of the carbonate. The the roofs of some of these caves may collapse. Animals may then fall into the resultant hole and be covered by subsequent collapse. (Creationists have tried to claim the jumble of bones found in geologically much older sediments is evidence for the Flood. )

talking about cave deposits : absolute dating is possible (with the necessary caveats) using uranium series and electro-spin resonance

for a more modern approach, if we have a small society on, say, a small island in the pacific, where there happens to be no clay, and the society gets whipped out by a tsunami, for example. Is there a way to date the finds made on that island? being that they would be relatively new, I'm assuming carbon-14, but I know there must be someother methods to help that along, just as way of being 'more' sure as to when the event happened.

This new method seems promising, but not fully validated yet.

http://news.bbc.co.uk/2/hi/uk_news/s...st/8058185.stm

By the way, I believe the term archaeology refers only to the study of human history and prehistory, not to dinosaur fossils etc.

In relation to molten rock, I believe that K40/Ar40 dating is often used.

Potassium 40 is an isotope that slowly decays to Argon 40. The rate of decay is known, so that the ratio between Potassium 40 and Argon 40 tells you a date. What date?

Since Argon is a gas, when the rock is liquid, any Argon leaves the rock (bubbles out). Thus, if we test solidified lava, we know that any Argon 40 in the lava has appeared since it was liquid. The amount of Potassium 40 compared to Argon 40 tells us how long.

If lava outflows from a volcano onto another rock stratum, we can use this method to date the lava, back to when it was liquid. The rock statum under the lava is hence older than the lava. So the K40/Ar40 dating tells us a minimum age for that stratum.

Originally Posted by Bunbury

By the way, I believe the term archaeology refers only to the study of human history and prehistory, not to dinosaur fossils etc.

Quite correct this is a very common mistake. The study of Ancient life is Paleontology, while the study of human cultures through history.

how about the study of anything buried, be it ancient or fresh? is there a blanket term for the study of what is in the past, regardless of the age?

Originally Posted by Arcane_Mathematician

how about the study of anything buried, be it ancient or fresh? is there a blanket term for the study of what is in the past, regardless of the age?

A good general term "could" be Geology. but that generally only covers geologic processes. While the methodology is similar the different areas of study are not generally considered to be in the same fields.