OXYGEN ISOTOPIC COMPOSITION OF FOSSIL HORSE TOOTH PHOSPHATE AS A RECORD OF CONTINENTAL PALEOCLIMATE

Oxygen isotopic, elemental, and X-ray data are presented for a suite o f 24 fossil horse teeth from Nebraska ranging in age from 18.2 to 8.5 Ma, to test the use of delta(18)O of enamel phosphate (delta(18)O(PO4) ) as a quantitative record of continental climate. Modern equid teeth were analyzed to estimate a relationship between delta(18)O(PO4) and e nvironmental water. Multiple samples of seven different fossil species from Burge Quarry, a similar to 12 Ma attritional fossil deposit, ind icate that diagenetic overprints exist but can be detected by decrease d P concentration and increased crystallinity relative to modern ename l. Isotopic variation for the pristine samples from Burge Quarry is +/ - 1.5 parts per thousand (1 sigma, n=9), which may represent the resol ution of the procedure within a stratigraphic horizon. There are no ap parent correlations with body size, hypsodonty, or phylogeny. A range of 7 parts per thousand in delta(18)O(PO4) occurs over the 10 m.y. int erval. A trend towards depleted delta(18)O(PO4) of about 4 parts per t housand corresponds to a depletion of up to 6 parts per thousand in de lta(18)O of precipitation between 18.2 and 8.5 Ma, but the range of va riation at Burge is large relative to the climate signal. Our results demonstrate that delta 6(18)O(PO4) should be useful in quantitatively reconstructing Cenozoic continental paleoclimate on 10(6)-year timesca les. Isotopic variation due to taphonomic bias and the terrestrial roc k record will likely obscure higher-order climate signals.