OXYGEN ISOTOPE DETERMINATION OF CLIMATE VARIATION IN NEBRASKA USING PHOSPHATE FROM MODERN AND FOSSIL BEAVER TOOTH ENAMEL

The d¹⁸O composition of Nebraska beaver (Castor canadensis) tooth enamel reflects variations in the isotopic composition of the surface water in which the beavers live, as their incisors grow continuously at rates of about 0.75 mm/day. In Nebraska, surface water d¹⁸O values typically track those of local precipitation (P) which are strongly controlled by temperature (T): to a good approximation following the relationship d¹⁸O_P=0.45 T(^oC) - 14.5. If the d¹⁸O-T relationship can be determined for modern beavers across Nebraska, it should be possible to estimate paleotemperatures from fossil beaver teeth collected from various geologic formations within the state. To test this approach, ninety-nine beaver teeth were obtained from Nebraska fur and wildlife depredation trappers from 2000 to 2002 to define the modern climate signal present within the teeth. On each tooth, the transverse enamel ridges, interpreted as daily growth increments, were counted and used to mark monthly zones along the outer convex portion of the tooth. Enamel samples were collected from across each zone, and the biogenic phosphate present in the enamel was extracted and analyzed to determine the d¹⁸O for each month represented on the tooth (typically a five to six month time span). Preliminary d¹⁸O data from the modern teeth show a seasonal trend of isotopically lighter values (-25 to 20 ^o/oo) in winter months, and heavier values (10 to 40 ^o/oo) in summer months. This trend results from seasonal atmospheric temperature variations and mimics the d¹⁸O trend of local precipitation; however, tooth values are generally more enriched. An average yearly value of 7.2 ^o/oo was calculated for all teeth analyzed in the study which is enriched by about 14.6 ^o/oo over the yearly weighted mean precipitation value of -7.4 ^o/oo. To identify paleoclimate changes across the state, the seasonal and yearly average d¹⁸O values from the modern teeth will be compared to d¹⁸O values of a number of fossil teeth (currently being analyzed). These include, for example, an 8 million year old Dipoides stirtoni from the Ogallala Fm., and a 2.5 million year old Castoroides idahoensis from the Long Pine Fm. Using the d¹⁸O-T relationship determined from the modern teeth, temperature estimates can be made for each fossil tooth and its corresponding age date, and the historical pattern of climate variation across Nebraska can be determined.