Diatoms are unicellular siliceous algae that live in most aquatic environments (lakes, rivers, estuaries)
Their value to palaeoenvironmental and geoarchaeological studies relate to their robust silica ‘frustule’ that is often preserved in sedimentary archives. Diatoms also have distinct frustule morphologies that are often species specific, which assists in identification. When combined with the fact that many diatoms have restricted environmental niches in which they thrive, the analysis of diatom assemblages from sedimentary archives can provide a window into past landscapes.
If multiple samples can be analysed through a sedimentary sequence, this provides the opportunity to assess changes in depositional environments over time. For example, diatom signals can reveal changes in relative sea-level in coastal lowlands, the identification of former aquatic settings that have since been infilled, eutrophication of freshwater bodies, lake level changes over time etc.
PERCS can provide guidance with regards suitable sampling strategies depending on the types of deposits under investigation. This in itself can maximise the palaeoenvironmental and geoarchaeological potential of a site through avoiding the evaluation of deposits likely to be of limited value.
PERCS provides a two stage process when undertaking pollen investigations
Diatom assessment is essentially an ‘assessment of potential’, whereby an initial evaluation of the samples is undertaken. If present, key taxa are identified, their relative abundance is noted and key lifeform and ecological affiliations can be identified (fresh, brackish, marine taxa etc). Provisional interpretations can be achieved at assessment stage and, if preservation is good enough and the deposits are of significance to the client (in terms of age, associated archaeological and/or environmental relevance) recommendations can be made to take the samples and associated sequence to full analysis
Diatom full analysis involves a much greater study of the microfossil assemblage previously identified. The diatom count is increased in order to maximise the assemblage preserved and to record rare species perhaps not initially encountered at assessment stage. The preservation of the diatom frustules is also evaluated, to ensure the asemblages are a fair reflection of the environment at the time of deposition (and post-depositional processed have biased any such assemblages). Any other evidence present in the samples is also noted in order to maximise the interpretative potential of the samples