Analytical Methods:Bioturbation

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Questions
Earthworms are responsible for burying finds at the ground surface and filling voids below stones and within the soil through their casting activities. They create vertical burrows into which they may drag organic matter from the surface and they mix (turbate) the upper soil layers destroying depositional startigraphy and creating the granular structure typical of topsoils. An assessment of earthworm activity, therefore, can provide insights into the likely alteration and loss of stratigraphic information as a result of bioturbation, or be used to help assess the degree to which a buried deposit is sealed and it’s suitability for dating.

Samples
Field fresh samples or exposed sections of soil are required for assessing earthworm numbers. See the analytical methods below.

Analysis
There are three main ways of assessing earthworm activity:

1. Field assessment of casts, tunnels, and soil structure
A general assessment of earthworm activity may be made by:
 * Counting the number of earthworm casts in a given area at the soil surface,
 * Assessing of the numbers of tubular earthworm tunnels and stone-lined chambers in a section (these tend to be made by deep living earthworms with permanent burrows),
 * Assessing the strength of granular structural development in soils (formed by topsoil dwelling earthworms), or
 * Checking for the presence of a worm sorted layer of small stones at a depth of between 15 and 40 cm.

2. Measurement of earthworm numbers in a known volume of soil
Measure out an area 30 cm x 30 cm and as quickly as possible dig out the soil to a depth of 30 cm. Place the spoil on a sheet and hand count the number of worms recovered.

3. Measurement of earthworm numbers using a disclosing solution
N.b. This technique causes harm to earthworms by breaking down the mucus coating that keeps them hydrated. Use sparingly and wash down area and any worms with clean water afterwards.

Mix 1 tablespoon of liquid soap in one gallon water and pour it over an area of ground 1m2. Leave for 5 minutes and then count the number of earthworms that have come to the surface.

Data and interpretation
Earthworms have three basic lifestyles.
 * Epigeic earthworms feed and live in the organic litter layer at the soil surface and form no permanent burrows.
 * Endogeic earthworms live and feed in the organo-mineral topsoil, they are responsible for the bioturbation that forms the homogenised soil A horizon (topsoil) and the formation of worm sorted layers. They can be very destructive to archaeological stratigraphy, though their activity is usually restricted to the upper 15-40 cm of soil. As archaeological deposits accumulate over time, however, their effects may be found throughout a section.
 * Anecic earthworms live at depth in the soil and have permanent deep burrows allowing them to go to the surface at night to feed and mate. These earthworms are usually responsible for the deep vertical burrows sometimes seen in archaeological sections, which sometimes end in small chambers lined with gravel.

Each of the different methods identifies a different set of earthworms. Disclosing solution usually provides the most complete estimate of earthworm numbers. Physical removal and hand counting tends to recover only the epigeic and endogeic earthworms, however it is these that are usually the most destructive to archaeological stratigraphies. Physical removal and disclosing fluid only provide information about current numbers of earthworms. Changing environmental conditions and sediment properties could have caused significant differences in bioturbation intensity in the past.

Earthworm activity varies markedly during the year this can affect the results of counting earthworm casts, physically counting worms and using disclosing solution. Burrows and structural development give a broader assessment of earthworm activity over a period of time, however, it is usually difficult to say what period of time they may have formed over.

Related techniques
Laboratory technique - Counting earthworm granules

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