Analytical Methods:Field OM/Mn
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Contents |
[edit] Field-based organic matter and manganese determination.
Based on the methods described in the USDA Field book for describing and sampling soils (Soil Survey Staff, 2002).
| Scale | A quick, field-based analysis to confirm the presence of organic matter and/or manganese. |
| Questions | Context and feature specific questions: Is this black material charcoal or manganese-oxide? Are these coatings organic?, Are these mottles and concentrations formed from manganese and hence likely to be post-depositional? |
| Samples and Storage | Fresh soil/sediment. |
| Time and Cost | A very quick, low-cost field technique. |
| General Comments | Does not require specialist knowledge. At best a semi-quantitative analysis, where quantitative data is required post-excavation laboratory analysis is required. |
[edit] Questions
This analysis is used to confirm the presence or absence of organic matter and/or manganese in a specific context or feature. It does not provide quantitative data on the percentage soil content nor will it show the presence of charcoal.
Because this is a very quick, cheap, non-specialist technique it is good for everyday on-site determinations to help guide excavation. For example it can be used to:
- Determine whether coatings on pores, stones and aggregates contain organic matter (this may indicate that organic matter has washed down through the profile and hence the context may not be sealed for dating).
- Differentiate between charcoal and nodules of manganese.
- determine whether colour differences are due to Mn and Fe/Mn mottles and concentrations and hence may be due to post-depositional waterlogging rather than necessarily indicating a different depositional context.
[edit] Method
[edit] Required equipment and chemicals1
- 3-4% H2O2 (Hydrogen Peroxide)
- Dropping dispensing bottle
Take a small sample (ca. 1 g) of the deposit and apply a drop or two of hydrogen peroxide. Watch and listen for the spped and strength of a reaction as bubbles of oxygen gas are produced (effervescence) in the presence of organic matter and manganese.
| Effervescence class | Visible effects |
|---|---|
| Noneffervescent | No bubbles form |
| Very slightly effervescent | Few bubbles form |
| Slightly effervescent | Numerous bubbles form |
| Strongly effervescent | Bubbles form a low foam |
| Violently effervescent | Bubbles form a thick foam |
Alternatively the hydrogen peroxide can be applied to a particular soil feature or coating of interest.
[edit] Data and interpretation
Manganese oxides react rapidly with hydrogen peroxide, whilst some forms of organic matter will react slowly with hydrogen peroxide. When a sample contains both manganese oxides and charcoal the manganese reaction will mask the reaction with organic matter.
The chemical reaction can be used to confirm the absence or presence of Manganese oxides or organic matter (not charcoal), it doesn't provide information on the percentage soil content. Laboratory analysis is needed for this.
[edit] Safety
1 Always consult chemical safety data sheets and follow appropriate health and safety protocols before handling or using chemicals.
Many chemical saftey data sheets are available here from this Oxford University based database.
[edit] Related techniques
Laboratory organic matter analysis
