1.8 Archaeomagnetism

Introduction

Archaeomagnetism is a method for dating fired materials and sediments from archaeological sites, based on changes of the Earth’s magnetic field in the past. The principles of the method are well established; see Linford (2006) and Zananiri et al. (2007). It has been used in Scotland from 1967 (Aitken and Hawley 1967) and is increasingly part of multi-method site chronologies. The strengths of archaeomagnetic dating are that it dates fired clay and stone, for example hearths, kilns, ovens and furnaces, which occur frequently on archaeological sites; it dates the last use of features, providing a clear link to human activity; it is cost effective and is potentially most precise in periods where other dating methods, e.g. radiocarbon dating, are problematic.

Principles and practicalities of the method

Archaeomagnetic dating is based on a comparison of the ancient geomagnetic field, as recorded by archaeological materials, with a dated record of changes in the Earth’s field over time in a particular geographical area, referred to as a secular variation curve. The geomagnetic field changes both in direction (declination and inclination) and in strength (intensity) and archaeomagnetic dating can be based on either changes in direction or intensity or a combination of the two. Dating by direction requires the exact position of the archaeological material in relation to the present geomagnetic field to be recorded, and so the material must be undisturbed and sampled in situ. Dating by intensity does not require in situ samples but is less precise and experimentally more difficult. The vast majority of UK studies are dating by direction, as intensity dating is not commercially viable at present

For archaeological material to be suitable for dating using magnetic direction it must contain sufficient magnetised particles, and an event must have caused these particles to record the Earth’s magnetic field. Many geologically derived materials e.g. soils, sediments, clays, contain sufficient magnetic minerals. There are primarily two types of archaeological events which may result in the Earth’s magnetic at a particular moment being recorded by archaeological material: heating and deposition in air or water. If materials have been heated to a sufficiently high temperature (>400°C) they may retain a thermoremament magnetisation, which reflects the Earth’s magnetic field at the time of last cooling. Suitable archaeological features would include hearths, kilns and other fired structures. Sediments may acquire a datable detrital remanent magnetisation from the alignment of their magnetic grains by the ambient field during deposition. Such an effect allows deposits in wells, ditches and streams to be dated. However, factors such as bioturbation and diagenesis of sediments can cause post-depositional disturbance of the magnetisation. In the UK archaeomagnetic dating can be applied to features expected to date from 1000BC to the present day, as this is the period covered by the secular variation curve. However, as discussed below the precision of the date obtained will vary according to the period.

Samples of robust fired materials are usually taken by attaching a 25mm flanged plastic reference button to a cleaned stable area of the feature using a fast setting epoxy resin or encasing part of the feature in plaster of Paris (Clark et al. 1988). Sediments and friable fired materials are sampled by insertion of 25mm diameter plastic cylinders. Magnetometers used are sufficiently sensitive for only small samples (c. 1cm3) to be required; approximately 15 samples are needed from each feature and it may be possible to select sampling location to minimise the visual impact if the feature is to be preserved. In the laboratory the remanent magnetisation of each sample is measured in a magnetometer and the stability of this magnetisation evaluated by alternating magnetic fields or thermal demagnetisation (Linford 2006).

Once a stable magnetic direction has been obtained, this is dated by comparing it with the secular variation curve showing changes in the Earth’s field over time (Clark et al. 1988; Zananiri et al. 2007). The secular variation curve is compiled from direct measurements of the field which extend back to AD1576 in Britain, and, prior to that, from archaeomagnetic measurements from features dated by other methods. There are a number of factors that will influence the precision of the dates obtained:

1. Differential recording of the field by different parts of the feature
2. Disturbance of the material after firing/deposition
3. Uncertainties in sampling and laboratory measurements
4. Precision of the secular variation curve itself
5. Uncertainties in the comparison of the magnetic direction with the secular variation curve
6. Spatial variation of the geomagnetic field

The precision of the secular variation curve varies with time and so the precision of the date obtained will depend on the archaeological period. As the geomagnetic field has occasionally had the same direction at two different times, it is also possible to obtain two or more alternative dates for a single archaeological event. In most case the archaeological evidence can be used to select the most likely of these. Given the number of contributing factors it is not possible to define the general precision of archaeomagnetic dates but there will be an error margin of at least ±50 years. It is important to note that, since the methods relies on the reliability of previously dated sites, the secular variation curve improves as more measurements become available. Features that cannot be dated or give broad age ranges now, may be datable in the future.

Applications of archaeomagnetic dating in Scotland

There have been a significant number of archaeomagnetic studies in Scotland, with 27 sites producing 103 dates. There are concentrated in the Iron Age, mainly reflecting the use of archaeomagnetic dating in the investigation of vitrified hillforts (Gentles 1989) and within long-running research projects. Such investigations have the advantage of being able to integrate archaeomagnetic studies with other methods to produce significant advances in archaeological understanding (e.g. Outram and Batt 2010). However, there is a dearth of archaeomagnetic dates from commercial archaeological investigations when compared with England. This may partly be due to lack of awareness of the method, but the main issue is the distribution of archaeomagnetic laboratories. There are no laboratories in Scotland; the English laboratories are currently based in Bradford, Liverpool, Lancaster, Isle of Man, Plymouth and Portsmouth. Because archaeomagnetic dating usually requires laboratory personnel to collect samples on site, the lack of Scottish laboratories makes the method both expensive and unable to respond rapidly, except within the framework of planned research excavations.

The distribution of the archaeomagnetic dates produced on Scottish material. Key to placemark colours: Orange-Bronze Age; Green-Iron Age; White-Early Medieval; Yellow-Medieval; Pink-multiperiod sites. ©Cathy Batt. Full details are available from http://www.brad.ac.uk/archaeomagnetism/

Emerging opportunities

It is clearly advantageous to raise the profile of archaeomagnetic dating in Scotland. The method has been shown to provide valuable archaeological information and supplement the suite of chronological tools available. In addition, the development of archaeomagnetic dating as a method, and the wider understanding of the Earth’s magnetic field, is hindered by the lack of data from Scotland. The main opportunities for progress are:

1. Increasing the level of awareness of the potential of archaeomagnetic dating within Scottish excavations.
2. The consideration of archaeomagnetic dating at the planning stages of major archaeological projects.
3. Targeting key periods and locations for intensive sampling of suitable material, most notably the Bronze Age, Medieval and Post-Medieval periods.
4. Training of personnel within Scotland to undertake sampling, increasing the availability of the method and decreasing costs.
5. Establishment of an archaeomagnetic dating facility within Scotland, ideally in association with other dating methods or within geophysics research laboratories with appropriate equipment.