PHYTOLITHS : QUANTIFICATION
For each slide representing a given sample, researchers tend to randomly count out a certain amount of phytoliths that represent the phytolith assemblage. This is defined by Piperno (1988) as "... the tabulation and quantification in percentages, absolute numbers, or ratios of all morphological variants observed in a sample". The minimum number of phytoliths counted that are considered to be representative of a slide (and therefore of a sample) differs quite widely:
- 100-200 (Piperno 1988);
- 250 (Madella 1998);
- 200-300 (Piperno 2006);
- 300 (Bobrova and Bobrov 1997);
- 500 (Lentfer and Boyd 1999);
- Number of phytoliths for each 300-400 mineral grains (Golyeva 1997);
- Three counts (Pearsall 1989): "quick scanning" (200 individuals), "short-cell scanning" (for smaller grass phytoliths; 200 individuals) and finally "diagnostic scanning" (500 individuals);
- 750 to 1800 "... in order to discuss at least 200 short-cell morphotypes for each sample" (Barboni et al. 1999).
It is clear from this list that the amount of phytoliths counted in each slides differs depending on the goals of the research. There is a general agreement that a good number (a statistically meaningful number) is between 200 and 300 individuals and this is supported by accumulation curves that show that at around 200 individuals the curve starts to flatten (Zurro, in preparation). Researchers dealing with palaeoenvironmental reconstructions often count a greater amount of phytoliths to be able to discuss at least 200 meaningful morphotypes (e.g. grass short cells), like in the works of Barboni et al. (1999).
For archaeological samples, Piperno (1998) recommends that after counting a statistically meaningful number of phytoliths to be between 200 and 300, a quick scan is carried out to assess the presence of rare, but often very important, morphologies. These morphologies are mostly related to plants with low phytolith production or low incidence in the settlement.
According to the work done by van der Veen & Fieller (1982) on charred seeds, the required sample size depends on four variables (that are still valid when counting phytoliths):
- the number of phytoliths in the target population (for phytoliths this should be considered as infinite);
- the proportion in which the particular phytolith type occurs;
- the accuracy, in absolute terms, that is required;
- the chance of achieving that accuracy.
The following table was created using the values from an example according to van der Veen & Fieller (1982, Table 4). Each phytolith type in a phytolith assemblage would probably account for around 20% of that assemblage.
- For a phytolith type with a proportion of 20% in the target population (the phytolith assemblage) and with an accuracy of 5% and a 95% chance of obtaining that accuracy there is the need to count 246 phytoliths in the slide.
- For a phytolith type with a proportion of 20% in the target population (the phytolith assemblage) and with an accuracy of 5% and a 98% chance of obtaining that accuracy there is the need to count 346 phytoliths in the slide.
These values are very similar to those routinely used in phytolith analysis (see above) and if a quick scan is also added most or all of the rare forms will be observed.
Calculating phytolith concentrations
The calculation of phytolith concentration is an important step in the interpretation of archaeological phytolith assemblages. The total number of phytoliths present in a preparation can be estimated by counting all phytoliths in a subsample of known weight or volume, or by adding an exotic particle to the prepartion (e.g. Lycopodium spores, microscopic glass beads, etc.) and then counting the phytoliths to an arbitrary sum. However, when the mineralogy of the deposits is varied and/or diagenesis is an important taphonomical processes volumes or weights can not be compared directly and therefore there is the need of a better standardization. Albert & Weiner (2001) proposed to calculate the number of phytolith per gram of the sediment's Acid Insoluble Fraction (AIF). This is the mineralogical fraction that survives -during the extraction procedure- the digestion of organic matter and the attack by Hydrochloric and Nitric acids.
- Albert, R. M. and Weiner, S. 2001. Study of phytoliths in prehistoric ashy layers from Kebara and Tabun caves using a quantitative approach. In Meunier, J. D. and Colin, F. (eds.) Phytoliths: Applications in Earth Sciences and Human History. Lisse: Balkema, 251-266.
- Barboni, D., Bonnefille, R., Alexandre, A. and Meunier, J. D. 1999. Phytoliths as paleoenvironmental indicators, West Side Middle Awash Valley, Ethiopia. Palaeogeography, Palaeoclimatology, Palaeoecology 152:87-100.
- Bobrova, E. and Bobrov, A. 1997. Phytoliths in soils: Species, composition, distribution along a soil profile, and value as environmental indicators. In Pinilla, A., Juan-Tresserras, J. and Machado, M. J. (eds.) The State of the Art of Phytoliths in Soils and Plants. Madrid: CSIC, 1-14.
- Golyeva, A. A. 1997. Contents and distribution of phytoliths in the main types of soils in Eastern Europe. In Pinilla, A., Juan-Tresserras, J. and Machado, M. J. (eds.) The State of the Art of Phytoliths in Soils and Plants. Madrid: CSIC, 15-22.
- Lentfer, . J. and Boyd W. E. 1999. An Assessment of Techniques for the Deflocculation and Removal of Clays from Sediments Used in Phytolith Analysis. Journal of Archaeological Science 26:31-44.
- Madella, M., Powers-Jones, A. H. and Jones, M. K. 1998. A Simple Method of Extraction of Opal Phytoliths from Sediments Using a Non-Toxic Heavy Liquid. Journal of Archaeological Science 25:801-803.
- Pearsall, D. M. 1989. Paleoethnobotany: A handbook of procedures. 470 pp. New York: Academic Press.
- Piperno, D. 1988. Phytolith analysis: An archaeological and geological perspective. 280 pp. New York: Academic Press.
- Piperno, D. 2006. Phytoliths. A Comprehensive Guide for Archaeologists and Paleoecologists. 238 pp. Lanham, New York, Toronto, Oxford: AltaMira Press (Rowman & Littlefield).
- van der Veen, M. and Fieller, N. 1982. Sampling seeds. Journal of Archaeological Science 9:287-298.