Analytical Methods:Lab Carbonate

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Carbonate Analysis
Carbonates can indicate past human activity as they are contained in ash and lime-based building materials. They are also precipitates as a result of soil-forming processes so care needs to be taken over their interpretation.

Questions
Carbonate analysis can be used to identify certain anthropogenic inputs, such as ash and lime-based building materials in the soil. Carbonate minerals may also develop as a result of soil formation processes, hence carbonate analysis may be used to investigate buried soils or to identify the effects of post-depositional carbonate crystal or nodule formation in archaeological deposits.

Case studies of the use of carbonate analysis in archaeological studies include:

Sampling
Bulk samples are required for carbonate analysis. If the aim is to estimate the amount of anthropogenic carbonate inputs to a deposit, a known volume of soil should be sampled.

Samples should not be stored wet in sealed bags as the accumulation of carboxylic acid from biological respiration can attack the carbonates. Air-dry soils for storage, but make a note of the percentage change in weight on drying (% moisture content).

Analysis
Carbonate determinations are usually made on air-dried soils that have been sieved to less than 2 mm. Quantitative analysis involves either:
 * Measuring the amount of carbon dioxide produced when the sample is mixed with 3M HCl. This technique requires the use of a calcimeter to collect and measure the CO2 evolved (Avery and Bascomb, 1974), and should be used when the concentration of carbonate is expected to be low (Crowther, 2006).
 * Measurement of the amount of hydrochloric acid required to react with the carbonate in the sample (e.g. Rowell, 1994). This is a slightly easier method of analysis, involving only a burette for the titration rather than a calcimeter. More details of this method can be found here.

Data and interpretation
The data is expressed as the percentage calcium carbonate equivalent in the soil, whilst in fact other carbonates may also have been present and contributed to the analysis.

Naturally occuring carbonates, such as limestone, dolomite and shell, will also contribute to the total carbonate content of the soil and their potential presence should be considered when interpreting the data. Carbonates may also be precipitated from the ground water in hard water areas.

Related Techniques

 * Field carbonate identification
 * Soil pH

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