Isotopic Equilibrium Between Precipitation and Water Vapor in Northern Patagonia and Its Consequences on δ 18 O cellulose Estimate

PENCHENAT, TIPHAINE; VIMEUX, FRANÇOISE; DAUX, VALÉRIE; CATTANI, OLIVIER; VIALE, MAXIMILIANO; VILLALBA, RICARDO; SRUR, ANA; OUTREQUIN, CLÉMENT

Journal of Geophysical Research: Biogeosciences

American Geophysical Union

Año: 2020 vol. 125

2169-8953

Modeling work of the isotopic composition of tree ring cellulose (18Ocell) relies on the isotopicequilibrium assumption between atmospheric water vapor and tree source water, frequentlyassimilated to integrated precipitation. Here, we explore the veracity of this assumption basedon observations collected during a field campaign in Río Negro province (Argentina) inFebruary-March 2017. We examine how the observed isotopic composition of water vapordeviates from equilibrium with precipitation. This deviation, named isotopic disequilibrium(18Ovap_eq), is low (between -2.0 and 4.1?) and a significant relationship is observed betweenthe isotopic composition of water vapor and its expected value at equilibrium. Negative18Ovap_eq can be explained by evaporation of small raindrops (from 1 to 5% of initial dropletmass). Positive 18Ovap_eq can result from vegetation transpiration with transpired wateraccounting for 14 to 29% to ambient water vapor. The low 18Ovap_eq at the study site may bedue to the high level of relative humidity (from 70 to 96%) favoring isotopic diffusiveexchanges between the two water phases and thus promoting the isotopic equilibrium. Weexamine the impact of the isotopic equilibrium assumption on the calculation of 18Ocell. Aperfect agreement is shown between observed and calculated 18Ocell provided that the isotopiccomposition of source water is significantly higher than the expected averaged isotopiccomposition of precipitation over the tree growing period.