Seasonal Variations in Triple Oxygen Isotope Ratios of Precipitation in the Western and Central United States.

Triple oxygen isotope ratios Δ'17O offer new opportunities to improve reconstructions of past climate by quantifying evaporation, relative humidity, and diagenesis in geologic archives. However, the utility of Δ'17O in paleoclimate applications is hampered by a limited understanding of how precipitation Δ'7O values vary across time and space. To improve applications of Δ'17O, we present δ18O, d-excess, and Δ'17O data from 26 precipitation sites in the western and central United States and three streams from the Willamette River Basin in western Oregon. In this data set, we find that precipitation Δ'17O tracks evaporation but appears insensitive to many controls that govern variation in δ18O, including Rayleigh distillation, elevation, latitude, longitude, and local precipitation amount. Seasonality has a large effect on Δ'17O variation in the data set and we observe higher seasonally amount-weighted average precipitation Δ'17O values in the winter (40 ± 15 per meg [± standard deviation]) than in the summer (18 ± 18 per meg). This seasonal precipitation Δ'17O variability likely arises from a combination of sub-cloud evaporation, atmospheric mixing, moisture recycling, sublimation, and/or relative humidity, but the data set is not well suited to quantitatively assess isotopic variability associated with each of these processes. The seasonal Δ'17O pattern, which is absent in d-excess and opposite in sign from δ18O, appears in other data sets globally; it showcases the influence of seasonality on Δ'17O values of precipitation and highlights the need for further systematic studies to understand variation in Δ'17O values of precipitation.