berangere: (jomon doki)
[personal profile] berangere posting in [community profile] archaeology
Today is the last day of Three Weeks For Dreamwidth. Almost all the prompts of the Frequently-Or-Not-So-Frequently-Asked-Questions in Archaeology Project have been answered.
"Almost" means this is not over yet. Here is the second part of the answer to the question from [personal profile] yvi concerning sciences and archaeology.

I would like to know what modern scientific methods (especially to do with molecular Biology, but feel free to mention others) are used in archeology and for what.

Last week I presented the links between archaeology and molecular biology. This post will be an overview of the other sciences used in archaeology. Well, an overview of all the sciences I managed to think about.

1- dating methods

   The most famous of dating methods may be radiocarbon dating.
When we (animals and plants) are alive, we absorb elements to... build ourselves and in those elements there is carbon. A part of the carbon is radioactive, it is called radiocarbon or 14C. Since it is radioactive, it disintegrates itself at a stable rate : its half-life is of 5730 years. It means that after 5730 years, half the number of the radiocarbon atoms would have disappeared.
While we are alive we absorb constantly the carbon so the rate of radiocarbon in our bodies is stable. Then we die. And we do not absorb carbon anymore, but the radiocarbon continue to disintegrate itself.
If you measure the rate of radiocarbon in bones or plant remains, you can determine from when this organism has stopped being supplied in radiocarbon (= when it died).

  The biases of the method :
  This method is based on the hypothesis that radiocarbon rate is stable compared to the other isotopes of carbon that we also absorb. We know that this is not the case. The rate of radiocarbon varies : it is created in the high atmosphere and seems to be linked to the cosmic rays we receive from space. But the variation is not that important so the dating with radiocarbon is quite reliable (quite).
Moreover, we have been able to calibrate the dates, to know the correction we have to apply to the date obtained. This calibration has been made thanks to other dating methods and particularly dendrochronology.
  This method can be used to date things up to about 60.000 years ago.

  This is terribly popular  in the medias, the films... You will often hear it was used to date things that are not organic at all, just because "radiocarbone dating" is cool, so let's use the name... This is also terribly expensive and I have never participated to an excavation that used it. Charcoal and other carbonic remains are generally really carefully managed during the excavations in case of a radiocarbon dating... that never occurs.

  Other dating methods based on the decay of radioactive elements include :

  The Potassium / Argon method
It is used to date the formation of rocks :  the rocks created during a volcanic eruption. The radioactive potassium transforms in argon with a half-life of 1,25 billion of years. The principle is the same as for the radiocarbon.
"Rocks ?" Would you say, "Why would archaeologists want to date rocks ? They are not geologists." We are not. But people live near volcanoes, they build villages, the volcano erupts and covers the village and ! We can date that eruption ! Yeah !

  The Uranium / Thorium method
It is used essentially to date corals : it has been used to calibrate the radiocarbon dating method.


Other dating methods !

Dendrochronology
  When a tree is alive, it grows of one ring every year. If you cut a tree now, you count the rings and you know its age.
Each year, the climatic conditions are different and the width of the ring is different, according to the climatic conditions. If you cut two trees from the same place and look at their rings, you'll notice that the sequence of "big rings - small rings" is the same.
NOW. You cut a tree now, and you can go back till to... let's say 150 years ago. This is tree A. You find an old timber in a farm built 100 years ago. This is tree B. If you look at tree A and tree B rings, a part of them are the same. And now I need a picture because my explanations are not clear.



  So, going from tree to tree, we managed to create charts that go back to prehistory ! And now that we have that chart, when we find a timber, we just have to look at its rings and find in the chart the sequence that corresponds. We need to have the last ring (called the aubier) to know the exact date when the tree was cut.
  Well preserved wood remains are quite rare, and when they are found, it is quite a big deal, so dendrochronology studies are generally held. Well, this is far less expensive than carbon dating.

Thermoluminescence
  Some minerals have the capacity to accumulate the energy of radioactivity that surrounds us. When those minerals are highly heated, they release this energy.
Now you take a pottery. The clay is composed of some of those minerals. You heat pottery when you bake it, releasing all the energy. From now on, it begins to accumulate it again. When you find this pottery 2000 years later, it is loaded in energy again. All you have to do is to heat it again and measure the energy liberated : you will know for how long it has been accumulated, so you will know the date when the pottery was baked.
It also works for fireplaces, ovens... and I have been taught it works for sand : you're in a cave where people live, there is sand on the ground. This sand is heated by the sun, which release the energy. Suddenly, the cave collapse on the ground, so people go and live elsewhere, and the sun can't heat the sand anymore. Then the energy begins to accumulate in the sand, and you can date it. I must say that I have never heard of a real case when this has been used on sand : it may be theoretical.

Obsidian hydration
  Obsidian is a really nice volcanic rock that is really perfect to be chipped. When you chip it, the superior level of the stone begins to hydrate itself. The more the time passes, the more the obsidian hydrates itself, so you can know when the tool was chipped.

Archaeomagnetism
  So, the Earth's magnetic field vary with time. When rocks solidify, some minerals in it register this "palaeomagnitism" : they are orientated according to the magnetic north of the time. We have managed to retrace this palaeomagnetic field up to 1000 BCE in France.
Now this registration also happens in heated clay. Of course, in pottery, it is useless : even if registered, the pottery has been moved after baked. But not the oven ! So you can date the ovens with this method.

  Really nice methods aren't they ? But to be honest, more than 95% of the archaeological sites are dated by the typology, would it be of the pottery, of the tools, of the houses... Well, there are countries like Japan where absolute datings methods are more common. But extraordinarily enough, the dates obtained are considered dubious if they are not correlated by typology. That's really strange.

2- Biology

  Unlike the previous ones, those are sciences that are really used in a daily basis on archaeological context. All they generally need is a microscope and solid knowings in the fields involved. Botany and zoology really are important in the study of the plaeoenvironment.

Botany
  Archaeologist use a few sciences in botany to analyze the remains they find, like anthracology (study of the charcoals to determine which type of wood it was), carpology (study of seeds and fruits), palynology (study of pollens). All those sciences are called "archaeobotany".

Zoology
  Zoology in a archaeological context is called "archaeozoology". It is used to analyze all the animals remains found.

3- Geology

  Micromorphology can be used to determine smaller divisions in stratigraphy.
  Talking about stratigraphy, this also is a geological science at the origin.
  Petrography or mineralogy can be used to determine the provenance of the clay used in potteries.
  Prospecting in archaeology generally consist in a group of slaves students walking on parallel lines and picking up anything that they can see on the ground (anything that seems archaeological, of course). Then you draw maps. Aerial prospecting, where you watch the ground from a plane at the beginning of summer ot in mid-winter to figure out structures is also quite developed, even if far more expensive. And, related to geology, there is electro-magnetic prospecting. It is called electrical resistivity tomography. It is based on the resistivity of the soil, that changes when there are structures inside. Resistivity affects electric or magnetic fields.  You send an electric current or a magnetic field in the soil and observe how it is changed when you receipt it.


  These are all the sciences I managed to think about. If someboby wants precisions concerning any field, questions can be asked in the comments, or a prompt can be submitted in the masterpost for the FONSFAQ, that will stay open indefinitely.
  The next prompt, about archaeology of buildings, will be answered during next week, even if Three Weeks For Dreamwidth will the over.

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