Chemical composition influences δ18O of chironomid larvae head capsules - Implications for climate reconstructions

Abstract

Oxygen isotopes of chironomid head capsules in lake sediments have become an important proxy for climatereconstructions, especially for high-latitude regions. Various chemical pretreatments have been developed toremove organic and inorganic contaminants and to clean the head capsules prior to isotope analysis, but so farthere is no standardised routine method to assess the purity of the isolated chitin. We present a dataset of oxygenisotope analyses of head capsules from seven lakes in southern South America. The head capsules were assignedto three different taxonomic groups of Chironomidae (Chironominae, Orthocladiinae, Tanypodinae) and toCeratopogonidae. Orthocladiinae exhibited on average higher isotopic fractionation relative to lake water thanthe other taxonomic groups, although these differences were not statistically significant. Elemental analysesdemonstrate that the oxygen content and molar N/O ratio of the head capsules have a larger influence on oxygenisotopic composition than the host water isotopic composition suggesting a strong influence of incompletedeproteination on oxygen isotope values of chironomid head capsules. This is backed up by an experimentalapproach with differently pretreated chironomids, which exhibits a strong correlation with N/O ratio presum-ably due to various degrees of deproteination. According to the N/O ratio, pure chitin was obtained after removalof the minerogenic fraction with a mixture of 10 % HF and 5 % HCl (16 h, 20 ◦C) and subsequent treatment with5 % NaOH (2 h) and 7 % NaClO2 (10 h). In conclusion, we recommend using the N/O ratio for purity control anda critical assessment of different pretreatment methods to obtain pure chitin in future studies.