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

Wednesday, 17 December 2014
Shuning Li1, Naomi E. Levin1, J Renee Brooks2 and Jeffrey M Welker3, (1)Johns Hopkins Univ-EPS, Baltimore, MD, United States, (2)Environmental Protection Agency Corvallis, Western Ecology Division, Corvallis, OR, United States, (3)University of Alaska Anchorage, Department of Biological Sciences, Anchorage, AK, United States
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.