Field Analysis:Has this deposit been bioturbated
From SASSA
SASSA Home Page ⇒ Field Analysis Home Page ⇒ Field Interpretation ⇒ Has this deposit been affected by bioturbation?
Contents |
[edit] Has this deposit been affected by bioturbation?
Bioturbation is the mixing of soils and sediments by animals, usually earthworms. The result can be the loss of stratigraphic information as different deposits become homogenised into one unit. Identifying the effects of bioturbation can be useful for assessing the potential loss of archaeological information.
[edit] Indicators of bioturbation
When attempting to identify the effects of bioturbation in the field a checklist of features that can be looked for/considered are:
- A favourable environment for earthworms:
- Organic or organo-mineral deposit.
- An unsealed surface and a deposit that is not too compressed or hard.
- A soil that is neither poorly or excessively drained.
- A loose, granular soil structure.
- Mixed artefact assemblage.
- Bands of coarse inclusions in a mixed fine groundmass of soil.
- Channel pores infilled with material of the same composition as the surrounding soil or of an adjacent context.
- Worm sorted layer of pea-sized stones at a depth of about 30 cm from the modern or former groundsurface.
- A gradual or diffuse lower context boundary unless the underlying context is cemented, compacted or otherwise impenetrable.
- Randomly oriented stones and artefacts.
- Any pans or sedimentary structures should be fractured and disturbed.
[edit] Uncertainties in identifying bioturbation
Uncertainties in identifying bioturbation often arise from:
- Seemingly granular structures in sediments with a high gravel (fine pebble) sized component. True granular structures are formed by aggregates of sand, silt and clay.
- The loss of granular structures following burial can result in the secondary formation of a structureless or blocky soil. However, in some cases gently tapping the larger aggregates may cause them to fall apart and reveal the remnants of a granular structure.
- Changing water tables, soil densities and the post-burial formation of pans and other soil features that can give the impression of an undisturbed soil horizon unsuitable for earthworms.
[edit] How does the SASSA Field Tool make this interpretation?
A printable recording sheet for SASSA's 'Has this deposit been affected by bioturbation?' interpretation tool can be found here
A high score tends to confirm that this context / deposit has been disturbed by bioturbation. In such a case you should consider whether bioturbation may have led to the loss of stratigraphic information and whether this may represent a buried soil horizon. To score highly you need:
- A moderately drained soil. This question accounts for 10% of the maximum total score.
- An open loamy soil texture with an organic or organo-mineral composition. This question accounts for 10% of the maximum total score.
- A uniform soil texture. This question accounts for 5% of the maximum total score
- A granular soil structure. In buried soils compression can overprint blocky or other structures, if you gently tap these larger structures they may fall open to reveal the former granular structure. This question accounts for 20% of the maximum total score.
- A moderately or very porous soil in which the dominant voids are tubular and contain infillings. This question accounts for 15% of the maximum total score
- An absence or strong disturbance of sedimentary structures. This question accounts for 10% of the maximum total score
- Gradual or diffuse context boundaries that are straight or wavy in shape. These questions account for 30% of the maximum total score.
If SASSA suggests this deposit may have been affected by bioturbation, you may also want to try the "Is this a buried soil?" interpretation tool.
[edit] Follow-on analyses
In the field an assessment of current eartworm numbers can be made as follows:
Post-excavation laboratory analysis may also help to confirm the actions of bioturbation.
