Effects of chemical pretreatments on δ18O measurements, chemical composition, and morphology of chironomid head capsules

Abstract

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 d18 O 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 NaClO 2 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 (100°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 NaClO 2 . A change in the proportion of chitin and proteins in our samples was associated with differences in chironomid

Stable oxygen isotopes in chironomid and cladoceran remains as indicators for lake‐water δ18O

“…The method was extensively tested with standards to assess the minimum weight at which analyses were affected by a systematic error (Verbruggen et al 2010a). Based on these trials, we aimed for a minimum sampling weight of 50 mg, and a sample weight of 40 mg was considered to be the absolute minimum necessary for measurements.…”

“…The d 18 O value of organic and inorganic matter produced within lakes will reflect past changes in lake-water d 18 O. Oxygen isotopic analyses of selected sediment components can therefore provide insights into past changes in climatic conditions (Leng and Marshall 2004;Verbruggen et al 2010b). Traditionally, studies examining d 18 O in lake sediment records have focused mainly on carbonates (Siegenthaler and Eicher 1986;von Grafenstein et al 2000).…”

“…Similarly, cladoceran remains can be found in large abundances in lake sediment records (Korhola and Rautio 2001). Chitinous invertebrate exoskeletons consist predominantly of biomacromolecules such as chitin and protein (Verbruggen et al 2010a) that contain a significant amount of oxygen. The d 18 O of exoskeletal remains can therefore provide information on the past d 18 O of the lake water in which these organisms lived.…”

Taxon-specific δ13C analysis of chitinous invertebrate remains in sediments from Strandsjön, Sweden

“…These include remains of primary producers such as Potamogeton (Herzschuh et al 2010;Turney 1999) and traces of organic matter within diatom valves (Barker et al 2013;Hurrell et al 2011) as well as of secondary producers such as aquatic invertebrates (for example, cladocerans (Perga 2010) and chironomids (Wooller et al 2008)). Chitinous invertebrate remains largely consist of proteins and chitin and are chemically robust and relatively resistant to microbial degradation, especially if buried under anoxic conditions (Verbruggen et al 2010). They can be identified and associated with a particular group of organisms.…”

Stable carbon isotopes of invertebrate remains: do they reveal past methane release from lakes?

“…These problems can be partly circumvented by analysing the chitinous remains of selected invertebrate taxa, such as exoskeleton fragments of aquatic insects, resting eggs of planktonic Crustacea or resting stages of moss animals (bryozoans) ; Chapter 1 and 4). These remains largely consist of a composite of proteins and chitin and are chemically robust and resistant to microbial degradation, especially if buried in anoxic lake sediments (Verbruggen et al 2010c). They can by identified under the microscope and associated with a particular group of organisms.…”