FOSSIL BONE AS A PALEOSOL AND PALEOCLIMATE PROXY

The CO₃ component of fossil bone apatite is thought to recrystallize and alter chemically and isotopically on time-scales similar to that of the formation of soils. Therefore, C- and O- isotope compositions should serve as a paleosol and paleoclimate proxy. This hypothesis was tested and confirmed with mid- to late-Cenozoic fossils from Oregon and Idaho, albeit with some complications: (1) Isotope compositions of fossil bone CO₃ do not correlate directly with compositions of syn-stratal paleosol CO₃, but rather correlate better with a combination of syn-stratal and next-higher paleosol compositions. This suggests that bone CO₃ alters more slowly than formation of paleosol CO₃. (2) Aridity confers strong positive deviations to δ¹⁸O values, and must be corrected for based on correlations with increased δ¹³C values. Corrected bone composition corresponds well with the known mid- to late-Cenozoic isotope record in Oregon, confirming its potential as a paleoclimate proxy. Bone CO₃ compositions across the Eocene-Oligocene transition (EOT), in the WY-SD-NE region provide additional paleoclimate insight (Zanazzi et al., this meeting). In Nebraska, ~200 bone fragments from a ~100 meter section straddling the EOT provide 20-40 kyr temporal resolution. These data show an abrupt, 1.65±0.28‰ (±2σ) increase in bone CO₃ δ¹⁸O at the EOT, with no resolvable change to δ¹³C (–0.15±0.19, ±2σ). Tooth enamel isotope compositions from before and after the EOT show no change to δ¹⁸O (±~0.5 permil), indicating that local water compositions remained constant. The increase in bone δ¹⁸O therefore reflects an abrupt drop in mean annual temperature (MAT): -8±3 °C (±2σ). Bone and tooth compositions from before and after the EOT may be used to infer absolute MAT, albeit with greater uncertainty because of calibration uncertainties for tooth enamel vs. water: latest Eocene = 27±8 °C, earliest Oligocene = 17±8 °C. Although bone clearly must alter in situ in soils, alteration rates for fossils in fact remain poorly known. Estimates include ~0.1 yrs for unusual soft tissue preservation, 1-1000 yrs for fossil teeth (Kohn, this meeting), and 10000-50000 yrs for archeological bone. Better understanding of fossilization processes will ultimately improve interpretation of fossil bone compositions.