Oxygen isotopic determination of climatic variation using phosphate from beaver bone, tooth enamel, and dentine

The δ18O of Canadian beaver (Castor canadensis) teeth should reflect variations in the isotopic composition of the water in which the beavers live, as their incisors grow rapidly and continuously. We observe seasonal variations in phosphate δ18O using samples of enamel taken along the length of single teeth. In the spring the δ18O of the enamel being deposited gradually declines, reflecting a retarded input of δ18O depleted winter water. After mid-year, enamel δ18O is higher than average (as represented by the δ18O of bone phosphate from the same animal) and passes through a maximum in late summer or early fall. Overall, the amplitude of seasonal excursions in enamel δ18O (4‰) is much smaller than the expected summer-winter range in the δ18O of meteoric water (> 10‰). This is because hydrologic mixing processes, gradual admixing of environmental water with beaver body water, long-term plant growth, and oxygen inputs of relatively constant value (particularly atmospheric oxygen) tend to even out summer-winter differences in the δ18O of oxygen inputs to the beaver. The δ18O of bone from adult beavers was uniform at 11.9 ± 0.5‰ over the study area. Analyses of a Sangamon age giant beaver (Castoroides ohioensis) incisor from Hopwood Farm, Illinois, show a slightly larger 5.5‰ seasonal cycle of δ18O with an average enamel δ18O of 18‰. This suggests that average temperatures were warmer during the Sangamon than today and that seasonal temperature differences and/or relative humidity variations were larger.