As you are no doubt aware, ScienceBlogs has started a new multiblog series on science basics. Several people have suggested I do a few posts on dating techniques, such as radiocarbon, which I am happy to do. Before I get started on that (I was planning on doing several other techniques as well) I would like to throw this out for your consideration.
Rock art has been a bit of a difficult thing to date. There are various relative dating techniques, such as the analysis of the evolution of various motifs, or excavating below the panels. Beginning in the 1980’s several new techniques were invented to try to get a handle on absolute dates. One of these, applied to petroglyphs, is called cation-ratio dating. The idea is that rocks develop a thin coating of varnish through time. This coating is derived from clay sized particles of dust that are fixed to rocks via microbes. This varnish is deposited in small laminations and it’s chemistry changes through time based on climatic factors. Over time certain trace elements (mobile cations) leach out, leaving the relatively immobile cations. The ration of these two can be calibrated to allow for age determinations. Some groups were able to replicate the technique while others failed to do so. Through the 1990’s further research has been done on CR and at this point, it is about as reliable as, and precise, as, say, ceramic typologies.
A second technique uses accelerator mass spectrometry to date small bits of organic matter that gets trapped in or under the rock varnish. This technique was developed in the late 1980’s, but by 1992 some discrepancies between control sample ages and empirical results. Research into the problem indicated that the trapped organics sometimes included older or younger contaminants. Consequently the age estimates tended to be inconsistent.. Current research is looking at determining which carbon type is necessary and determining the environmental conditions the technique will work under.
There are a couple of points to take away from this. First, initial applications of new techniques are apt to be based on simplistic assumptions that have to be revised by further research. This can involve refining our knowledge about how the process works (I will get back to this when I discuss radiocarbon dating) or learning what types of materials the techniques can be used on. Second, the fact that the wrinkles of a given method have been worked out does not mean that research on that method ceases. For example, the way radiocarbon dating is applied today is quite a bit different and more sophisticated than in Libby’s day. There are a number of journals out there devoted to scientific research into various methods. Some cover a wide variety of methods, some, such as Radiocarbon are devoted to specific techniques. Consequently, before you rush to apply whatever pet objection you may have to dating techniques, you may want to check the journals to see if the problem is being researched or has been resolved already.
Filed under: Archaeology