Triple oxygen isotopes in eggshell carbonate as a proxy of late Cenozoic CO2 and primary productivity

The triple oxygen isotope composition of biominerals is a promising recorder of changes in global primary productivity (GPP), as well as environment and animal physiology. However, application is largely unexplored, particularly in many regions of the world where eggshell is a common component of paleontological/archeological sites of the late Cenozoic. Ratites (ostriches and their relatives), as nonobligate drinkers, gain most of their oxygen from atmospheric O2 and leaves, meaning that their eggshell should carry a strong imprint of anomalous 17O in atmospheric O2 - which is ultimately related to GPP and CO2 levels. In this study, we analyzed fossil eggshells from different climate states of the late Cenozoic, notably the Middle Miocene, Late Miocene, and Last Glacial Maximum (LGM), as well as modern eggshell from the same region, southern Africa. We interpret our results using an existing animal body water model, which we modify for parameter estimation using Bayesian inverse methods. Our results show that the predominant signal in the triple isotopic composition of biomineral carbonate is that of global carbon cycling (i.e. the ratio of GPP:CO2), meaning that with further refinement, a relatively common material can be used to reconstruct changes in planetary primary productivity for periods beyond the ice core record. During the Middle Miocene, we find that GPP was significantly lower than today, at 95% confidence. Comparison with box-model output of the atmospheric O2 budget shows that Middle Miocene GPP levels dropped to below 60% of modern values, assuming that Middle Miocene CO2 levels were between 441 and 472 ppm (95% confidence). The result is discussed, with potentially important implications for future climate change.