Isotopic analysis on nanogram quantities of carbon from dissolved insect cuticle: a method for paleoenvironmental inferences

Rationale: Carbon isotope (δ¹³C) data from arthropod cuticles provide invaluable information on past and present biogeochemical processes. However, such analyses typically require large sample sizes that may mask important variation in δ¹³C values within or among species. Methods: We have evaluated a spooling-wire microcombustion (SWiM) device and isotope ratio mass spectrometry (IRMS) to measure the δ¹³C values of carbon dissolved from the cuticle of chitinous aquatic zooplankton. The effects of temperature, pH, and reaction time on the δ¹³C values of acid-dissolved bulk cuticle and purified chitin fractions obtained from a single species of chironomid from four commercial suppliers were assessed. These results were compared with baseline δ¹³C values obtained on solid cuticle using conventional EA (elemental analyzer)/IRMS. Results: The results indicate differential, time-dependent dissolution of chitin, lipid and protein fractions of cuticle concomitant with slow depolymerization and deacetylation of chitin. Isotopic offsets between dissolved bulk head capsules and a purified chitin fraction suggest the contributions of other isotopically lighter components of the bulk head capsules to bulk chitin extracts. The SWiM/IRMS δ¹³C results obtained on dissolved cuticle using a treatment of 4 N HCl at 25 °C for 24 h produced generally stable δ¹³C values, large sample/blank CO₂ yields and a positive correlation with conventional EA/IRMS results on unprocessed cuticle. Conclusions: The SWiM/IRMS system offers a reliable method to determine δ¹³C values on nanogram quantities of carbon from dissolved insect cuticle, thus reducing sample size requirements and providing new opportunities to use δ¹³C variation among/within species for reconstructing paleo-biogeochemical processes.