TRIPLE OXYGEN ISOTOPES IN TERRESTRIAL BIOMINERALS: A NEW TOOL FOR ENVIRONMENTAL AND ECOLOGICAL RECONSTRUCTION (Invited Presentation)

The triple oxygen isotope composition (¹⁷O/¹⁶O relative to ¹⁸O/¹⁶O) of biominerals has received little attention until recently, in part because of analytical roadblocks, and in part because of the assumption that little new information would be gained beyond that provided by ¹⁸O/¹⁶O. Analytical advances now permit triple oxygen isotope measurements with sufficient precision to resolve environmentally and ecologically meaningful signals. In terrestrial organisms such as mammals, birds, and mollusks, isotope mass balance models predict that the triple oxygen isotope composition of body water (and hence biominerals) should be sensitive to evaporation, ingestion of evaporated waters (including leaf waters), and the proportion of body water derived from metabolism. The model predictions are generally borne out by data from modern animals. In African vertebrate communities, triple oxygen isotopes resolve animals deriving a large proportion of water from leaf-eating (e.g., giraffes, ostrich) from those requiring surface waters as a source of drinking water (e.g., hippos, elephants). In a sample of northeastern U.S. birds, wild birds are significantly different from domestic birds, again largely reflecting the influence of evaporated, plant-derived waters available to wild birds. Lacustrine mollusks record the isotopic composition of lake waters, which in turn reflect the extent to which lake waters are influenced by evaporation. The most anomalous triple oxygen isotope compositions that we observe are for late Jurassic dinosaur eggshells. These values are interpreted to reflect the contribution of metabolic water to dinosaurian body water at time when atmospheric O₂ had an anomalous triple oxygen isotope composition due to elevated CO₂ levels, lowered global primary productivity, or a combination of these. Key challenges to the widespread use of triple oxygen isotopes continue to be the bespoke nature of the analytical equipment, the time-consuming extraction procedure, and the large amount of material required (~10 mg of carbonate; up to ~150 mg of bioapatite). Nevertheless, as a community we should strive to overcome these challenges, as the triple oxygen isotope composition can provide a level of insight into environment and ecology well beyond that provided by ¹⁸O/¹⁶O alone.