THE OXYGEN ISOTOPIC INTEGRITY OF FOSSIL TURTLE BONE: AN EXAMPLE FROM THE PALEOCENE-EOCENE OF WYOMING

A difficulty in using the oxygen isotope composition (δ¹⁸O) of vertebrate bioapatite in paleoclimate studies is the need to distinguish variation from temporal changes in the δ¹⁸O of environmental waters from that due to temperature-dependent fractionation during biomineraliztion. One solution is multiple-taxon comparisons using data from coexisting homeothermic and heterothermic animals. Mammals precipitate bioapatite in isotopic equilibrium with body water at a constant temperature, but body water δ¹⁸O can deviate from local meteoric water due to species-specific dietary and physiological effects. Fossil emydid turtles have been suggested as a potentially useful proxy because 1) they have a dense fossil record; 2) they may be functional homeotherms due to behaviors such as basking, and therefore may precipitate bioapatite in a narrow temperature range; and 3) their aquatic habitat constrains the isotopic variability of dietary inputs. However, because turtles lack teeth and therefore tooth enamel, sampling must focus on bone, which is potentially more susceptible to diagenetic alteration due to smaller apatite crystals and greater porosity.

We tested the isotopic integrity of fossil turtle bone by examining the oxygen isotope composition of structural carbonate (δ¹⁸O_c) and phosphate (δ¹⁸O_p) in turtle bone across the Paleocene-Eocene boundary in the Clarks Fork Basin, Wyoming. Previous studies of soil carbonates and mammalian tooth enamel provide an extensive dataset for interpreting the turtle data. The range of fossil turtle bone δ¹⁸O_c values (16-28‰ VSMOW) is greater than that for paleosol carbonates and mammal teeth, suggesting that turtle bone carbonate is not pristine. A lack of covariation between δ¹⁸O_c and δ¹⁸O_p values supports this conclusion. The mean δ¹⁸O_p values, however, are statistically indistinguishable from those of the pantodont Coryphodon, and the range (9-15‰) is within that predicted from mammalian tooth enamel carbonate. One possible interpretation of this result is that the diagenetic δ¹⁸O_p value is within the range observed in presumably unaltered tooth enamel. Alternatively, fossil turtle bone δ¹⁸O_p values may be relatively unaltered and therefore suitable for paleoclimatic reconstruction in combination with data from known heterotherms.