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The broad scope of isotopic applications of bone collagen ranges from modern ecological and physiological investigations to learning about living conditions of animals in the past (Ambrose & DeNiro, 1989; Chisholm et al., 1982; DeNiro & Epstein 1978, 1981; DeNiro & Weiner, 1988; Hare et al., 1991; Hedges & Law, 1989; Leyden et al., 2006; Lis et al., 2008; Reynard & Hedges, 2008). Bone collagen represents one of the best preserved proteins in fossilized animal remains that in some cases has been chemically preserved for up to 120,000 years (Bocherens et al., 1999). Collagen is protected from degradation by being encapsulated into the bone mineral bioapatite. In comparison to other biopolymers found in the archaeological record, bone collagen is relatively abundant, easily extracted, and offers a long-term record of the life of humans and animals (Collins et al., 2002). Stable isotope ratios in the living biomass directly relate to the stable isotope ratios of life-supporting substrates in the environment. Animals integrate stable isotopes into their biomass through the air we breathe, water we drink and food we eat. Despite the apparent permanence of mineral constituents of bone, the organic components undergo constant turnover through the life of a vertebrate and its collagen content represents a ‘running average’ of months, years, or a lifetime, depending on the specific bone and the animal. Bone collagen thus offers a valuable insight into the environmental conditions during part of or the entire life of an animal (LeeThorp, 2008; Tuross et al., 2008). Carbon and nitrogen stable isotopes of bone collagen have been in broad use in determining diets, paleodiets, trophic levels, and paleoenvironments associated with modern and fossilized bone samples (Ambrose & DeNiro 1989; Chisholm et al., 1982; Hare et al., 1991; Schwarcz, 2000; Walker & DeNiro, 1986; Walter & Leslie, 2009), while other isotopes are only recently finding useful applications. Hydrogen is unique among all elements in terms of the doubling of the atomic mass from the light hydrogen nuclide protium 1H to the less abundant, but heavier nuclide deuterium 2H (or traditionally also abbreviated as D). This large mass difference between the two stable hydrogen isotopes creates strong fractionation effects (i.e. the preferential use or participation of one of the two isotopes in chemical and physical processes) and generates an extremely wide natural isotopic range of hydrogen stable isotope ratios (Sessions et al., 1999). Available hydrogen stable isotope data from bone collagen suggest greater variability than the most of the earlier studies had presumed, a situation that would compromise the diagnostic value of individual forensic collagen δD values for many species.

Museum collections contain a wealth of insect remains originating from a wide geographic range, which can be used to investigate their utility as a proxy for environmental isotope ratios. Chitinous remains of insects such as beetles (Coleoptera) are chemically stable and their stable isotope composition is strongly related to that of environmental water in the period of cuticle formation. We present a dataset of chitin δD and δ18O in two genera of water beetles from a museum collection containing 40 locations for Helophorus (water scavenging beetles) and 48 locations for Hydroporus (predaceous diving beetles) that were selected from latitudes 27–82°N in North America. Only two genera were used to minimize inter-sample variation caused by species-specific differences in metabolic effects, feeding strategy, habitat, and life cycle. The isotopic composition of water beetle exoskeletons had a strong latitudinal trend (North–South) from −160 to +65 ‰ for δD and from 7 to 34 ‰ for δ18O, paralleling gradients of isotopes in precipitation. Strong relationships were observed between isotopic composition of beetles and modelled July precipitation (0.71 < R2 < 0.82, p < 0.001). The relationship between δD and δ18O in the beetle samples had a systematic offset from the global meteoric water line, which was likely caused by metabolic effects during chitin formation. The offset between δD values in beetles and in modelled precipitation was 33 ‰ larger, on average, for Hydroporus compared with Helophorus, suggesting fractionation of hydrogen isotopes during passage through the food chain. This trophic level effect was not observed for stable oxygen isotopes. Furthermore, the observed deviations between isotopic composition of water beetles and modelled precipitation at collection sites were not constant and indicated local hydrological deviations from modelled precipitation. The largest deviations were observed for sites in the Southern US and the Arctic that are highly evaporative and at sites in the Rocky Mountains and Coastal Mountains that were fed by snow melt. Our results indicated that the isotopic composition of water beetles from a museum collection was systematically related to δD and δ18O values of precipitation at the collection site.