Triple oxygen isotope analysis of bioapatite as tracer for diagenetic alteration of bones and teeth
Abstract The detection of diagenetic alteration is critical for palaeoclimate reconstruction that is based on the oxygen isotope composition of fossil bones and teeth. So far, no direct chemical proxy has been found to track diagenetic modification of the oxygen isotope ratios. Here, a new approach to identify diagenetic changes of δ18OPO4 values in skeletal apatite of small mammals by means of triple oxygen isotope analysis (16O, 17O and18O) is presented. Our method is based on the fact that inhaled air oxygen (O2) has an isotope anomaly on its rare isotope 17O. Inhaled air O2 is a major source of oxygen in small land-living mammals. A fraction of the anomaly is transferred via body water to skeletal apatite, where it can be detected by means of δ17O and δ18O analyses. The approach, considering the current analytical uncertainty, is restricted to small mammals with body masses ≤ 1 kg. This is due to the low specific metabolic rates of large mammals, resulting in a lower fraction of oxygen inhaled via breathing relative to oxygen from other sources in their body water. Remnant negative 17O anomalies derived from in vivo inhaled O2 have been detected in enamel bioapatite of Eocene to Miocene rodent teeth while dentine of the same teeth lacks significant 17O anomalies. This suggests preservation of the original phosphate oxygen isotope composition in enamel of these small mammal teeth. In contrast, 17O anomalies in dentine have been erased due to diagenetic alteration with isotopically normal diagenetic fluids. Triple oxygen isotope analysis of bioapatite thus seems to be a useful new proxy to directly detect diagenetic alterations of the δ18OPO4 values of small mammal teeth.