The effect of diagenesis on oxygen isotope ratios of biogenic phosphates

Abstract

The effect of diagenesis on the oxygen isotope composition of ancient biogenic phosphates was evaluated by measuring different types of material from Middle Triassic localities in the Southern Alps. Dentin, bone and tooth enamel of different taxa, hosted by different rock types and metamorphosed to differing degrees, were analyzed for their δ18O and δ13C values using a new microanalytical GC-IRMMS technique. The δ18O values of the majority of bone and bulk tooth samples were 18 to 19 permil (SMOW), with three samples below this range and one close to 20 permil. In contrast, samples of tooth enamel were 19.4 to 20.7 permil, significantly higher than bone and bulk tooth. Without the enamel data, the 18 to 19 permil data could be misinterpreted as primary. Only when the higher δ18O values of the enamel are considered, is it apparent that the bulk data have undergone diagenesis, and temperature estimates are ∼5°C lower than would otherwise be calculated. Alteration of bulk tooth is also seen in a Miocene shark tooth, where δ18O values of enamel are 3 permil higher than those of bulk tooth. Although enamel is routinely analyzed for paleoclimate work of herbivorous mammals, bulk bone, teeth, or shells are almost exclusively used for older marine samples. The results of the present study indicate that bulk data should be treated with caution.