Geoarchaeology:Dating
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[edit] Dating Soils and Sediments
Dating is as critical to understanding soil formation and sedimentary deposition as it is to archaeology, and many of the same issues and problems apply. Here the theoretical problems of dating soils and sediments are discussed rather than a detailed critique of the many available dating methods. However, a summary table outlining some of the main techniques, the age range they cover, and the types of material that can be dated is provided here.
Dating technqies can be split into two groups:
- Relative dating techniques - Basically depend on establishing stratigraphic or chronometric relationships between units based on their relative position, the biological or artefact assemblages they contain, or evidence of relative weathering and ageing of artefacts.
- Absolute dating techniques - Provide a statistical estimate of the absolute numerical age of the material being dated. Absolute dating techniques include:
- Radiocarbon dating
- Varves and tree-rings (dendrochronology)
- Potassium-Argon dating
- Lead-210 dating
- Luminescence dating
[edit] Problems of dating soils and sediments
When dating sediments the source of the organic matter and the likelihood of any age discrpancy between the material and the sediments in which they are found need to be considered. For example, sediments can contein organic materials eroded and redeposited from older sediments, in this case the organic matter will be considerably older than the sediments with which it is associated.
When dating soils the same problem of reworking must be considered, but dating soil is also more problematic as they do not accumulate in the same way as sediments. Organic and inorganic material added to the top of a soil does not usually remain there (except peat / histosols). Instead it is usually worked into the soil by a series biological, physical and / or chemical processes. This means that a soil sample will contain a mixture of particles each potentially dating to any period within the history of the soils development; hence, The dynamics or organic matter and mineral grain inclusion in the soil is an important factor in the measured date. The measured age of organic matter, or mineral grains, has to be regarded as the minimal age for the length of soil formation, whilst the bulk soil organic carbon age is regarded as the mean residence time (MRT) (Wang et al., 1996).
Bulk soil samples or individual mineral grains or fragments of charcoal recovered from the body of a soil are generally of low value for dating. Materials recovered from the interface between a sealed buried soil and its overburden may give a better estimate of the date at which the soil was sealed, but again it is possible these materials have had an extended residency time within the soil and may be preferable to date materials firmly associated with the deposit immediately overlying the soil.
[edit] Sealed contexts
Another potential problem in the dating of both sediments and soils is contamination. Roots, bioturbation, the washing of organic matter and fine mineral material down through the profile and the incorporation of old carbon from coal, carbonate minerals and dissolved carbonates in hard water areas are all potential problems. Ideally for dating contexts should be sealed - perfectly sealed between other sediment layers - such that they and the organic materials and artefacts they contain. For the reasons outlined above surface soils are never sealed contexts. Evidence that a context is not perfectly sealed may include:
- Roots and rhizomes penetrating from the layers above.
- Vertical tubular voids, sometimes terminating in a gravel lined chamber, that are formed by the earthworm Lumbricus terrestris. These worms live in deep permanent burrows and drag organic matter from the surface down their tunnels to feed.
- Animal burrows, often infilled with loose, granular soil.
- Coatings of clay, iron or organic matter on void walls, stones, and aggregate surfaces, caused by the washing of material down through the profile.
[edit] External websites
- Kevin Greene's Archaeology: an introduction, online companion, Dating the Past
- Wikipedia: dating methodology
- Canadian Archaeology Radiocarbon Database
[edit] References
Wang, Y., Amundson, R., and Trumbore, S. (1996) Radiocarbon dating of soil organic matter. Quaternary Research, 45, 282-288.
