Contamination: Refining Methods

“Contamination is around us all the time…” (Prof. Bert Roberts)

Numerous types of contamination can be introduced into archaeological records, but there are two main types that are particularly significant.

Intrusive Artefacts

The first concerns artefacts and their location within a particular layer. When an artefact is found in its original burial location it is called ‘in situ’ (from Latin meaning ‘in place’). However, during excavations there is often ‘contamination’ of layers by ‘intrusive’ artefacts from older or younger deposits. This can occur when artefacts are moved as a result of bioturbation (e.g. animal burrowing, anthropogenic activities, or natural post depositional processes of erosion and reworking). These can cause artefacts to be moved from one layer to another and, therefore, redeposited outside of their original context. In such situations, artefacts are called ‘intrusive’ (i.e. not in situ).

Contaminant Phenomena

The second type of contamination is connected with ecofacts. This is more difficult to discern, because it can often only be traced on a microscopic level. Great care therefore needs to be taken during excavations, throughout the process of sampling and recording, as well as afterwards, in the laboratory, to make sure that no intrusive material is introduced into the samples.

Contamination exists most commonly in sediments that form the bulk of the archaeological deposits, and that can be connected with a number of different phenomena, such as bioturbation (both animal, e.g. rodents, porcupines, ants, termites; and vegetation, e.g. roots), chemical diagenesis and mineral precipitation (e.g. flowstone formation). The occurrence of such contamination at any stage of the post-depositional history of the site can pose a major threat to reliably interpreting the stratigraphy and accuracy of the dating information. This is especially true for chronological methods that focus on dating sediment grains (e.g. OSL, IRSL) or small bits of charcoal (¹⁴C). Uranium-series dating of bone depends entirely on the post-burial migration of uranium into the bone, so its highly sensitive to the percolation of water.

Contamination is also of utmost concern during the recovery of ancient DNA, as well as microscopic botanical remains and residues, such as starches and lipids. Any contamination can very easily lead to false results.

New and Refined Methods

Many scientists are devoting their time to establishing new methods for targeting the best places to collect samples and improve sampling protocols and sample pre-treatments. Such methods aim to ensure that only in situ materials are analysed and that contamination is not introduced during later stages of analysis. Such ground-breaking work is currently being undertaken by geoarchaeological and archaeochemical teams working at Liang Bua, and by UOW PhD students working on the formation of natural deposits that are similarly affected by post-depositional processes (e.g. bioturbation by termites).

Conversation starter

• Why is there a need for new and refined methods?
• How did contamination impact on the initial dating of the Hobbit? (see Step 3.7 for clues)