The oxygen isotope composition (δ18O) of apatite from mammalian tooth enamel can be used to infer the δ18O value of ingested water, which is in turn related to that of precipitation stored in surface reservoirs. Therefore, the δ18O value of phosphate from fossil tooth enamel can be used to infer the δ18O value of these reservoirs in the past. In this paper, tooth enamel from a semiaquatic mammal taxon (Coryphodon) collected from five early Eocene localities in North America is used to construct patterns in δ18O values of river water for this time period.

At all localities, the δ18O value of river water (δ18Or) is estimated to have been higher during the early Eocene relative to present-day North American rivers, although the δ18O vs. latitude gradient was shallower during the Eocene. Higher δ18Or values are consistent with warmer Eocene air masses being able to hold more water vapor and with an increase in the poleward transport of both moisture and latent heat. The regular decrease in δ18Or with latitude indicates that global atmospheric circulation patterns and hydrological transport were not much different from those of the present, although the shallower δ18O vs. latitude gradient during the Eocene may reflect regional differences in precipitation, evaporation, and river recharge.

At a more regional scale, the δ18O value of river water can provide insight into topographic relief during the early Eocene. In the case of intermontane basins of Wyoming, differences in average δ18Or values between basins indicate that Laramide mountain relief was on the order of 475 m. It is suggested that anomalously low δ18Or values reported previously do not provide unambiguous evidence for permanent snow at higher elevations and may instead reflect brief episodes of cooler winters and/or altered atmospheric circulation patterns.

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