Woodland burial studies

Soil mineral weathering rates

The study

The mineralogical content of forest soils has a major influence on the calculations of both forest nutrient and pollutant budget studies. However, whilst it is possible to measure the mineralogical content within soil samples, their rates of decay and thus contribution to the soil chemistry is poorly understood.

To help address this lack of information, in 1997, 7 types of reference mineral samples were buried in a selection of different soil horizons at 10 of the Level II monitoring sites. The soil horizons were selected to provide a diverse range of chemical environments. At each site, replicate sets were buried to allow recovery at different time intervals. The buried minerals varied in their chemical stability, ranging from the highly soluble Gypsum, through Apatite, intermediate Micas minerals to the more stable Microcline and Tourmaline.

The results

The minerals were recovered after 1, 3, 5 and 10 years.

Differences in the rates of mineral loss were seen not only between sites, but also individual soil horizons. For some of the mineral types used, chemical analysis of soil solution collected from the monitoring sites by tension lysimeters, allowed geochemical modelling of the mineral’s saturation index (SI) within the soil solution. This saturation value can be used as an indicator of the mineral’s susceptibility to dissolution or precipitation within that soil environment.

Good correlations have been found for comparative predicted rates of loss from the model with actual weight losses of mineral. An example is shown below for the mineral ‘Tourmaline’.

Final analysis of the minerals recovered after the 10 year period of this project is ongoing.

Decay rates for the buried archaeological resource

The Woodland Burial Study is a collaborative project between archaeological conservators (led by Karla Graham from English Heritage) and Forest Research (Peter Crow - woodland geoarchaeologist: peter.crow@forestry.gsi.gov.uk).

Main study aims

To undertake the burial, recovery and analysis of a variety of modern analogue materials and develop methods for monitoring their rates of degradation.
To develop methodologies for monitoring the archaeological burial environment, directly correlating material degradation both with its immediate burial environment and the parallel soil mineral weathering study.
To identify any possible correlation between material degradation and the burial environment, and evaluate any potential influences on the preservation of archaeological remains in situ.

This work builds upon the research into the weathering rates of minerals found within forest soils (above).

A net bag containing horn and antler samples, ready for burial In 2005 sets of iron, copper, red deer antler, cattle horn and cotton samples were buried at three depths, down to one metre, within soil horizons already being monitored by Forest Research. The materials chosen represent a selection of materials found in archaeological artefacts and they range both in their degradation mechanisms (chemical, physical and biological) and their susceptibility to degradation.

Results to date

The first set of buried samples were recovered in 2006. Even after only six months, all the analogue materials display some differences between the soil horizons.

To date, the length of burial period and analyses are not sufficient to consider the effects of land use on the preservation of archaeological remains in situ. The last set of samples were retrieved in 2007 and the environmental monitoring data for the period will be analysed for trends during 2008.

Alongside this, geochemical modelling will be undertaken using the chemistry of the soil solutions (derived from the burial horizons) to simulate the reaction paths of the minerals in the analogue materials. It will provide an indication of the likely minerals that could occur in these burial conditions (and hence what material survives) and it will be compared to the actual results observed for the analogue materials. Such information may allow archaeologists to examine a simple soil solution and potentially predict what types of archaeological materials may survive on a site, or what may have been lost. In turn, this would guide the prioritisation of costly excavations and aid site interpretation.

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