Carbon, Nitrogen and Oxygen Stable Isotope Ratios in Chitin

Abstract

Stable isotope ratios in chitin are firmly imprinted during biopolymer biosynthesis and reflect dietary, metabolic, and environmental influences. Chitin is a chemically complex amino sugar biopolymer that also includes non-amino-sugar moieties with contrasting isotopic compositions. Reproducible N, C, O-stable isotope determinations should rely on a chemically purified chitin substrate with limited non-amino-sugar contributions. Insecta, Crustacea and Merostomata are not distinguished by systematic isotopic differences. δ13C, δ15N and δ18O values of arthropod chitins show a few ‰ variance within a single individual (lobster), among individuals within a population, along ecdysis (i.e., molting), and age, as long as growth is not accompanied by strong dietary or behavioral changes. Marine arthropod chitin averages 9.1‰ more 13C-enriched than terrestrial chitin. δ15N values of chitin become more positive with increasing trophic level. Although δ18Ochitin from a large array of aquatic crustacean species across many ecosystems expresses only weak overall correlation with δ18Owater and no clear dependence on temperature, a more careful selection of specific modern insect fauna or preserved insect chitin from sediments yields δ18Ochitin values that are useful for reconstructing the isotopic composition of paleoenvironmental water, and for constraining paleohumidity and paleotemperatures. Experimental oxic and anoxic heating and partial biodegradation of chitin in marine anoxic mud and in terrestrial oxic soils did not result in any significant C, N, O-isotopic shifts in the preserved amino sugar. Chemically preserved archeological chitin was found to be isotopically compatible with modern chitin from comparable environments. New analytical stable isotope techniques with reduced sample size requirements open opportunities to utilize geologically preserved chitin in paleoenvironmental studies.