THE VERTEBRATE TRIPLE OXYGEN ISOTOPE CO2 BAROMETER: PROGRESS AND CHALLENGES (Invited Presentation)

The vertebrate biomineral triple oxygen isotope pCO₂ barometer proposed by Pack et al. (2013) (Geochim. Cosmochim. Acta 102, 306-317) is based on the premise that body water and hence biominerals of land-dwelling vertebrates contains a small fraction of oxygen derived from atmospheric O₂ (through respiration), and that the triple oxygen isotope composition of atmospheric O₂ is sensitive to atmospheric pCO₂ and global gross primary productivity (GPP). A significant challenge with this barometer is dealing with the variable ‘dilution’ of the atmospheric O₂ signal relating to differing water-use strategies amongst vertebrates. In other words, water-conserving animals will have a larger relative fraction of O₂-derived oxygen than will animals with high water-intake requirements. Here we further elaborate on the ‘environmental physiology isotope concordance’ (EPIC) approach for addressing this biological signal. In short, the EPIC approach makes use of endmember body water triple oxygen isotope models for the most water-conserving (e.g., kangaroo rat, ostrich), and water-promiscuous (e.g., hippopotamus, elephants) physiologies. Each model results in a predicted relationship between biomineral Δ¹⁷O and atmospheric O₂ Δ¹⁷O. For any given assemblage of contemporaneous vertebrates, there should be a concordant atmospheric O₂ Δ¹⁷O value (or range of values) which, when propagated through the endmember body water models, can predict all of the measured biomineral Δ¹⁷O values. No assumptions about the water-balance ecophysiology of extinct animals are required, but greater the range of water-balance physiologies sampled in the contemporaneous assemblage, the more narrowly-constrained will be the estimate of atmospheric O₂ Δ¹⁷O, and hence pCO₂. We summarize published and unpublished vertebrate Δ¹⁷O data spanning the past 160 million years and use the EPIC approach to interpret these in terms of atmospheric pCO₂. We will also address challenges that must be overcome for full realization of the vertebrate triple oxygen isotope pCO₂ method, including addressing variability in past GPP, and minimizing sample size requirements.