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