Oxygen isotope analysis of biogenic phosphate: An application to the Early Eocene Thermal Maximum

Logan Hackett, Colorado School of Mines, Dept. of Geology and Geological Engineering, Golden, CO, USA, [email protected]u

The principal method used to determine the stable oxygen isotopic composition of biogenic phosphate (δ¹⁸O_bp) is by precipitation and analysis of silver phosphate (Ag₃PO₄).  Procedural improvements over time have focused on replacing HNO₃ with HF as the dissociation agent, and dilution of the HPO₄⁺ -containing supernatant prior to precipitation of Ag₃PO₄, in both cases providing more precise data. However, since the inception of this procedure, the amount of biogenic phosphate used as starting sample has been in the range of five to hundreds of milligrams. My development research shows that using greatly reduced sample sizes (one to six milligrams of starting material) results in comparable precision to previous techniques. This change in procedure provides new opportunity for δ¹⁸O_bp analysis, where biogenic phosphate samples previously considered too small or too prized can now be analyzed.

Samples for isotopic analysis in this study include large herbivorous mammals and gar from the Eocene of North America. Tapiromorph tooth enamel samples from a high-latitude site, and Coryphodon and gar scales from separate mid-latitude sites have been analyzed with these new, smaller starting sample sizes. Specimens come from strata of Wasatchian age (NALMA), correlating to the Early Eocene Thermal Maximum (EETM). This time is hypothesized to have been, globally, one of the warmest periods in the last 65 my. Data reported here show Mean Annual Temperature (MAT) estimates for mid-latitudes to range from ~18 to 19°C, and a MAT high-latitude estimate of ~4.5°C from continental North America.  These data confirm previous estimates of mid- to high-latitude warming.