H AND O ISOTOPE RATIOS OF HETEROTROPH TISSUES AS PALEOCLIMATIC AND PALEOECOLOGICAL PROXIES

The isotopes of hydrogen and oxygen are actively cycled and sorted within the hydrological cycle, assimilated by living autotrophic and heterotrophic organisms, and preserved in sediments within minerals and organic material. Isotope ratios of H and O (δ²H and δ¹⁸O) in many biogenic minerals (e.g., enamel carbonate) and autotroph tissues (e.g., cellulose) are well studied and widely used in paleoclimatological and paleoecological studies. Despite their ubiquity and the integral link to climate via the hydrological cycle, relatively little is known about how H and O isotope ratios of structural organic molecules in heterotroph tissues reflect the environment and ecology experienced by ancient organisms. New δ²H and δ¹⁸O data from laboratory experiments and natural samples representing heterotrophic organisms, ranging from the brine shrimp (Artemia franciscana) to humans, illustrates that δ²H and δ¹⁸O values, normally tightly coupled in hydrological systems, are strongly decoupled in some organic systems. The evidence suggests that the isotopic composition of H in structural organic proteins and carbohydrates in heterotophs is strongly linked to dietary H via inheritance of C-bound H through molecular routing and/or inheritance from substrate molecules used in biosynthesis. In contrast, organic O appears to exchange more completely with body water and hence is most strongly linked to the δ¹⁸O values of water ingested by obligate drinkers. Comparison of natural-system H and O isotope data across a number of tissue types and taxa provides examples of coupling and decoupling of these isotope systems. These results suggest that δ²H and δ¹⁸O values of fossil organic tissues may be uniquely useful indicators of both the dietary ecology of ancient organisms as well as robust proxies for the isotopic composition of paleo-waters.