Investigation of early eocene water-vapor transport and paleoelevation using oxygen isotope data from geographically widespread

摘要

The oxygen isotope composition (delta~(18)O) of apatite from mammalian tooth enamel can be used to infer the delta~(18)I value of ingested water, which is in turn related to that of precipitation stored in surface reservoirs. Therefore, the delta~(18)O value of phosphate from fossil tooth enamel can be used to infer the delta~(18)O 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 delta~(18)O values of river water for this time period. At all localities, the delta~(18)O value of river water (delta~(18)O_r) is estimated to have been higher during the early Eocene relative to present-day North American rivers, although the delta~(18)O vs. latitude gradient was shallower during the Eocene. Higher delta~(18)O_r 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 delta~(18)O_r with latitude indicates that global atmospheric circulation patterns and hydrogical transport were not much different from those of the present, although the shallower delta~(18)O vs. latitute gradient during the Eocene may reflect regional differences in precipitation, evaporation, and river recharge. At a more regional scale, the delta~(18)O 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 delta~(18)O_r values between basins indicate that Laramide mountain relief was on the order of 475 m. It is suggested that anomalously low delta~(18)O_r 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.