OXYGEN ISOTOPE ANALYSIS OF BIOGENIC PHOSPHATE: AN APPLICATION TO THE EARLY EOCENE

Since its inception, the method used to determine the stable oxygen isotopic composition of biogenic phosphate (δ¹⁸O_bp) derived from an organic biogenic phosphate matrix has been through 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. Starting sample size, however, has mostly been prohibitively large for many applications, generally in the range of five to hundreds of milligrams. We have modified the procedure to use greatly reduced sample sizes (one to six milligrams of biogenic phosphate matrix), while retaining comparable or better precision with previous studies. 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 from a high-latitude site (Ellesmere Island), and Coryphodon teeth and gar scales from two mid-latitude sites (Colorado) have been analyzed with these smaller starting sample. Colorado specimens come from strata of Wasatchian age (Wa-7, NALMA), and correlate to the Early Eocene Thermal Maximum (EETM). This was a time 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 about 17 to 18°C, and a MAT high-latitude estimate of approximately 3.5°C. These temperatures corroborate previous estimates from similar localities and ages, and therefore show a similar temperature gradient as related to paleolatitude. This temperature gradient, approximately 0.4°C per degree latitude, is a relationship with a reduced slope compared to the present day gradient of about 0.6°C per degree latitude.