Effects of chemical pretreatments on delta O-18 measurements, chemical composition, and morphology of chironomid head capsules

Stable oxygen isotope measurements on fossil chironomid head capsules from lake sediments show that these chitinous remains can be used to reconstruct past lake water delta O-18 and, indirectly, past climate change. We examined the impact of chemical pretreatment procedures on the chemical and stable oxygen isotope composition, and morphology of chironomid cuticles. Use of alkali, acids, and sodium chlorite alters the chemical composition and the morphological structure of chironomid cuticles by selective removal of chitin or proteins. Gas chromatograms of pyrolyzates show that NaClO2 causes deproteination, whereas the combined use of HCl and HF results in partial chitin removal. Head capsules pretreated with KOH contained both chitin- and protein-derived moieties, although the concentration of protein was reduced, especially after KOH treatment at high concentration (28%) and temperature (100A degrees C). Scanning electron microscopy confirmed that a proteinaceous matrix is still present in modern and fossil head capsules after KOH treatment. This matrix, however, is largely absent in head capsules pretreated with NaClO2. A change in the proportion of chitin and proteins in our samples was associated with differences in chironomid delta O-18 values. Our results suggest that deproteination results in a relative increase of chironomid delta O-18, whereas removal of chitin leads to decreased delta O-18 values. We therefore discourage the use of acids or prolonged (a parts per thousand yen1 h) exposure to hot alkali (70A degrees C) prior to chironomid delta O-18 analysis. Chitin purification by sodium chlorite causes significant weight loss, which may preclude down-core chironomid delta O-18 measurements. Caution and standardization are required when pretreating samples for chironomid delta O-18 analysis to ensure reliable, comparable, and reproducible results.