Can radiocarbon dating chironomid head capsules increase chronological accuracy in lake sediments?

Interpreting past environmental change from lake sediments is becoming increasingly important for understanding past, present and future global change. In order to understand rates of environmental and climatic change, and compare findings from different sedimentary sources accurate dating of these sequences is fundametal. Currently most lake sediment sequences are dated by radiocarbon. These dates can be subject to relatively large errors and dating different material can give different ages. Thus these uncertainties are carried forward into interpretation of the environmental reconstructions. We aim to reduce these uncertainties. Radiocarbon dating most lake sediments is problematic due to old carbon entering the lake from surrounding soils. We therefore need to find a method whereby this is minimised or eliminated entirely. One potential method which gets round this problem is to radiocarbon date chironomid (non-biting midge) larval head capsules. Chironomid larvae are often the dominant bottom living invertebrates (after oligochaetes) present in freshwater ecosystems. The head capsules are made from a robust polymer called chitin, and are preserved in lake sediments. Two previous studies have shown that by radiocarbon dating chironomid head capsules, and comparing this to dating the organic lake mud, younger dates are produced by the chironomids. This suggests that the organic mud contains old carbon, and therefore that the chironomid dates are more accurate. However, as yet no tests have been undertaken on this hypothesis. We aim to radiocarbon date samples of chironomid head capsules from a range of recent Icelandic lake sediments of known age including both a chitosan extract (recently developed) from head capsules and the whole head capsules (bulk chitin) themselves. We will then assess the radiocarbon concentrations of the samples, and attempt to match them to known atmospheric radiocarbon concentrations from the recent past, a period with a very clear radiocarbon 'spike' due to bomb testing. This will allow us to test the accuracy of using chironomids as a dating tool, i.e. how well their radiocarbon concentration (age) matches when they died and became incorporated into the lake sediment. If this accuracy can be demonstrated then it means by dating the chironomids we can tell excactly when changes indicated by other material in the lake sediments were taking place. The results from these analyses will allow us to examine (1) Whether either the chitosan extract or bulk chitin samples provide accurate age estimates when compared to their known age; (2) Whether the chitosan extracts are always younger than the bulk extracts, highlighting (as yet unknown) dating errors arising from reliance on bulk chitin samples; (3) The likely value of chironomid dating against other forms of radiocarbon dating for lake sediments. It is hypothesised that careful dating using these new protocols will provide an important innovative approach to deriving chronologies from a wide range of sedimentary archives, many of which are at present difficult to date.