INTERPRETING HIGH-RESOLUTION OXYGEN ISOTOPE BIOARCHIVES IN CONTINENTAL PALEOENVIRONMENTS

Interpretation of seasonal isotopic records preserved in ontogenetic growth histories of fossil organisms obligates paleoecologists and paleoclimatologists to associate compositions of shell material with more generally-embraced measures of ambient climate such as means and amplitudes of seasonal temperature variation. This requirement in turn necessitates an assessment of relations between lake and river surface water composition and that of meteoric precipitation in the same locality. To evaluate differences in surface water and meteoric composition, we have determined spatial variation in means and seasonal ranges of d¹⁸O for rivers and for meteoric precipitation across the contiguous United States.

Stable isotope compositions of meteoric precipitation are primarily dependent upon ambient temperature. Because local temperature primarily varies with season, we have derived annual means and seasonal amplitudes of compositional variation by determining best-fit sinusoids to water oxygen isotope data. These include compositions of meteoric precipitation from the IAEA/Global Network for Isotopes in Precipitation database and compositions of riverine water data from USGS open-file report 00-160.

Resultant values were gridded across the 48 states by applying an equation that describes variation in mean composition and amplitude of seasonal variation using latitude and altitude to a digital elevation model with a horizontal grid spacing of 30 arc seconds (GTOPO30). Residuals between compositions and amplitudes predicted by the latitude-altitude model and best-fit sinusoids from each station were then contoured and added back to the initial grid to yield best-estimate maps of means and seasonal amplitudes of variation for meteoric d¹⁸O and riverine d¹⁸O at any location in the United States.

Comparison of spatial variation in meteoric precipitation d¹⁸O and riverine d¹⁸O documents a strong, predictable relation between mean values. However, seasonal amplitudes of meteoric precipitation and surface water variation at the same locality are considerably different over different regions of the country, and reflect the regional importance of seasonal infiltration and evapotransporation.