Short-term variability in isotopic composition of precipitation: A case study from the Midwestern United States


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Abstract

Oxygen and deuterium isotopes in precipitation were analysed to define local isotopic trends in Iowa, US. The area is far inland from an oceanic source and the observed averages of δ18O and δ D are −6.43‰ and −41.35‰ for Ames, −7.53‰ and −51.33‰ for Cedar Falls, and −6.01‰ and −38.19‰ for Iowa City, respectively. Although these data generally follow global trends, they are different when compared to a semi-arid mid-continental location in North Platt, Nebraska. The local meteoric water lines of Iowa are δ D = 7.68 δ18O + 8.0 for Ames, δ D = 7.62 δ18O + 6.07 for Cedar Falls, and δ D = 7.78 δ18O + 8.61 for Iowa City. The current Iowa study compares well with a study conducted in Ames, Iowa, 10 years earlier. The differences between Iowa and Nebraska studies are attributed to a variable climate across the northern Great Plains ranging from sub-humid in the east to semi-arid in the west. Iowa being further east in the region is more strongly influenced by a moist sub-humid to humid climate fed by the tropical air stream from the Gulf of Mexico. The average d-excess values are 10.06‰ for Ames, 8.92‰ for Cedar Falls and 9.92‰ for Iowa City. Eighty seven percent of the samples are within the global d-excess range of 0‰ and 20‰. The results are similar to previous studies, including those by National Atmospheric Deposition Programs and International Atomic Energy Agency. It appears that the impact of recycled water or secondary evaporation on δ18O values of area precipitation is minimal.

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