Stable isotopic analysis of fossil chironomids as an approach to environmental reconstruction: state of development and future challenges

Abstract

Remains of chironomid larvae, especially their strongly sclerotized head capsules, can be found abundantly and well preserved in most lake sediment records. These remains mainly consist of chitin and proteins and, since their chemical composition does not seem to be strongly affected by decompositional processes, they can be used to develop palaeoenvironmental reconstructions based on their stable isotopic composition. Here we review available stable isotope studies based on fossil chironomids and indicate future research necessary to further develop this still relatively new research approach. Efforts to produce stable isotope records based on fossil chironomids have mainly examined the elements H, N, C, and O. They have focussed on (1) developing the methodology for preparing samples for isotopic analysis, (2) laboratory studies cultivating chironomid larvae under controlled conditions to determine the factors affecting their stable isotopic composition, (3) ecosystem-scale studies relating stable isotopic measurements of fossil chironomid assemblages to environmental conditions, and (4) developing first down-core records describing past changes in the stable isotopic composition of chironomid assemblages. These studies have shown that chemical sample pretreatment may affect the isotopic composition for some elements. Laboratory runs suggest that the diet of the larvae influences their stable isotopic composition for H, N, C and O, whereas stable isotopes in the ambient water also strongly influence their oxygen and to a lesser extent hydrogen isotopic composition. These experiments also indicate only minor offsets between the nitrogen and carbon isotopic composition of chironomid soft tissue and the fossilizing head capsules, whereas for hydrogen and oxygen this offset remains to be explored. Though few datasets have been published, the available ecosystem studies and developed down-core sediment records indicate that stable isotopes in chironomid remains have the potential to provide reconstructions of past climatic change (H, O) and insights into past food web structure, methane production and pollution of lake ecosystems (N, C). Future efforts will be necessary to develop these approaches including more detailed analyses of the effects of sample pretreatment on stable isotope measurements on chitinous fossils, more extensive laboratory studies constraining the effects of external factors (e.g., isotopic composition of food and ambient water, temperature) on stable isotopes in chironomid larvae, and surveys exploring seasonal changes in the isotopic composition of chironomid larvae and assessing how this seasonality influences fossil assemblages. Finally, multi-site field studies relating chironomid ?D, ?15N, ?13C and ?18O to parameters such as ?18O of precipitation, air and water temperatures, and nutrient and greenhouse gas concentrations in lakes will be necessary to assess the extent to which these stable isotopic approaches can provide quantitative reconstructions of parameters of interest for palaeoclimatological and palaeoenvironmental research.

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