12 Oct Geochemical and physical methods in geoarchaeology
Geochemical analysis is an integral part of quality geoarchaeological research (Macphail and Goldberg, 2018; Karkans and Goldberg, 2019). However, the type of the analysis depends on the specific information we want to obtain. This means that, for example, if the aim of the research is to determine the distribution of elements related to potential mining within a site, multi-element analysis and determination of trace elements is appropriate. These analyses can be performed quickly, but at the same time only indicatively, by the already mentioned X-ray fluorescence (see the XRF analysis presentation). However, more detailed values can be obtained only by laboratory methods. For this purpose, it is best to use, for example, ICP EOS for macroelement detection. At the same time, it is necessary to determine the types of extracts in which the samples are prepared for measurement. The so-called total extract is used as standard, in which the carbonates in the sample are dissolved in 20% HCl. The second type of a standardly used leachate, thanks to which the information about the elements accessible to plants can be obtained, is so-called Mehlich III (Mehlich, 1984).
If the goal of geochemical analysis is, for example, the distribution of light elements within an object (for example, phosphorus), it is appropriate to use direct laboratory detection of phosphorus. The measurement is again performed on the ICP EOS, but differs in the methods of sample preparation, which means that the resulting values are not statistically affected by the detection limits of the instrument. At the same time, it is possible to determine the differences between organic, inorganic and total phosphorus, in other words, the differences in a representation of several sources of phosphorus.
Another example of the use of laboratory chemical analysis can be the determination of organic carbon, total nitrogen or sulphur contents. Total carbon can be determined relatively accurately, for example using the PRISMACS analyser. If the sample is free of organic carbon, the amount of inorganic carbon can also be determined by the LECO analyser.
Part of the multidisciplinary assessment of the site is the application of soil acidity (pH) measuring (Grant et al., 1996), magnetic susceptibility measuring or the sample grain size determination. Magnetic susceptibility is usually measured by the Agico MFK1-FA Kappabridge instrument, at one operating frequency, f1 = 976 Hz with the AC field amplitude 200 A/m, when the total MS is converted to mass.
Grain size analyses are performed with a CILAS 1190 LD laser granulometer, which can determine the range from approx. 0.04 to 2,500 μm. The determination takes place after measuring a sample, which has been previously dispersed. The degree and method of dispersion always depends on a specific scientific issue and usually takes place only in KOH or HCl, if it is necessary to measure the granularity without secondary carbonates or after dispersion in H2O2 if it is expedient to determine the granularity independent of organic content. The data are usually presented in three fractions: clay (up to 2 micrometres), dust (2–63 micrometres) and sand (63–2,000 micrometres), but these fractions can be singled out in much more detail for possible interpretations.
It a frame of the ERCA Centre the geochemical and physical methods in geoarchaeology are guaranteed by Associate Professor Lenka Lisá, PhD., from the Geological Institute of the Czech Academy of Sciences.
References:
Grant, R., Laubel, A., Kronvang, B., Andersen, H.E., Svendsen, L.M., Fuglsang, A., 1996: Loss dissolved and particulate phosphorus from arable catchments by subsurface drainage. Water Research, 30, 2633–2642.
Karkanas P.T., Goldberg, P., 2019: Reconstructing Archaeological Sites: Understanding the Geoarchaeological Matrix. Willey Blackwell.
Macphail, R.I., Goldberg P., 2018: Applied Soils and Micromorphology in Archaeology. Cambridge University Press.
Mehlich A., 1984: Mehlich 3 soil test extractant: A modification of Mehlich 2 extractant, Communications in Soil Science and Plant Analysis, 15(12), 1409-1416, DOI: 10.1080/00103628409367568