Monthly precipitation isoscapes (δ18O) of the United States: Connections with surface temperatures, moisture source conditions, and air mass trajectories

This research presents the most spatially complete data set depicting average monthly delta O-18 precipitation values across the United States (isoscapes). Average monthly isoscapes were calculated by analyzing 5 consecutive years of precipitation samples collected at 73 collection sites from the United States Network for Isotopes in Precipitation. Three patterns across the United States emerged from the analysis: (1) the most O-18-depleted precipitation was not confined to the higher elevations of the Rocky Mountain region but were also found in the upper Great Lakes; (2) clear latitudinal gradients were observed in the Mississippi River valley: steeper during cold months than during warm months; and (3) coastal latitudinal gradients were significantly shallower than continental gradients. The average monthly isoscapes were significantly correlated with surface temperatures, particularly in winter but less so in summer. The Rayleigh distillation model, however, produced uniformly steeper monthly isotope temperature coefficients (delta O-18 parts per thousand/degrees C) than those calculated from empirical delta O-18 values. Our analysis suggests that differences in model/observation isotope temperature coefficients result from the contiguous United States having two primary moisture sources: the Gulf of Mexico and the northeast Pacific Ocean regions. A significant proportion of precipitation sourced from the Gulf of Mexico (northeast Pacific Ocean) occurs in warm (cool) regions resulting in more (less) isotopic depletion than if all moisture had originated from one source for the entire United States. These two primary precipitation source scenarios result in a smaller variation in oxygen isotopic variation for the distribution of temperatures in a given month, yielding lower than predicted isotope temperature coefficients. The results of this study highlight the importance of understanding environmental factors affecting regional precipitation isotope values, particularly if one wishes to use isotopic analysis for paleoclimate interpretation or as a measure of a modern day hydrologic process.