The triple oxygen isotopic composition of precipitation in the western United States

Abstract:

Triple oxygen isotope variations in water and sedimentary materials have potential to be valuable paleoclimate proxies because ¹⁷O-excess, the deviation from an expected relationship between ¹⁸O/¹⁶O and ¹⁷O/¹⁶O ratios, is very sensitive to kinetic fractionation effects which will vary as a function of climate conditions. The majority of studies of ¹⁷O-excess in waters focus on high-latitude precipitation and ice; little is known about the variation in ¹⁷O-excess of precipitation from low- and mid-latitudes. Here we present δ¹⁸O, δD and δ¹⁷O data from weekly precipitation collections from the western United States. Among precipitation samples that range in δ¹⁸O from -26 to +5‰, ¹⁷O-excess averages 0.02 ± 0.02‰ (1σ) and ranges between -0.06 and +0.07‰. In the majority of locations, ¹⁷O-excess values of summer precipitation are considerably lower and more variable than ¹⁷O-excess of winter precipitation. For summer precipitation, there is a strong negative correlation between ¹⁷O-excess and δ¹⁸O and a weak, positive correlation with d-excess. We attribute both the lower and more variable summer ¹⁷O-excess values to the effects of re-evaporation of rainfall in warmer settings. However, in some regions, like western Oregon, there is little seasonal variation in ¹⁷O-excess and no trend between ¹⁷O-excess and d-excess. These data indicate that both season and climate regime are important factors in determining ¹⁷O-excess of precipitation and its relationship to both δ¹⁸O and d-excess. We use these data to show how measurements of ¹⁷O-excess can complement measurements of δ¹⁸O and δD in hydrologic studies, and how ¹⁷O-excess can be used as an additional constraint on the climate variables that drive variation of δ¹⁸O in sedimentary archives.