Premium
Isotopic Equilibrium Between Precipitation and Water Vapor in Northern Patagonia and Its Consequences on δ 18 O cellulose Estimate
Author(s) -
Penchenat Tiphaine,
Vimeux Françoise,
Daux Valérie,
Cattani Olivier,
Viale Maximiliano,
Villalba Ricardo,
Srur Ana,
Outrequin Clément
Publication year - 2020
Publication title -
journal of geophysical research: biogeosciences
Language(s) - English
Resource type - Journals
eISSN - 2169-8961
pISSN - 2169-8953
DOI - 10.1029/2019jg005418
Subject(s) - water vapor , precipitation , transpiration , evaporation , relative humidity , chemistry , stable isotope ratio , humidity , isotope , atmospheric sciences , thermodynamics , geology , meteorology , physics , biochemistry , photosynthesis , organic chemistry , quantum mechanics
Modeling work of the isotopic composition of tree ring cellulose (δ 18 O cell ) relies on the isotopic equilibrium assumption between atmospheric water vapor and tree source water, frequently assimilated to integrated precipitation. Here, we explore the veracity of this assumption based on observations collected during a field campaign in Río Negro province (Argentina) in February–March 2017. We examine how the observed isotopic composition of water vapor deviates from equilibrium with precipitation. This deviation, named isotopic disequilibrium (Δ 18 O vap_eq ), is low (between −2.0‰ and 4.1‰) and a significant relationship is observed between the isotopic composition of water vapor and its expected value at equilibrium. Negative Δ 18 O vap_eq can be explained by evaporation of small raindrops (from 1% to 5% of initial droplet mass). Positive Δ 18 O vap_eq can result from vegetation transpiration with transpired water accounting for 14% to 29% to ambient water vapor. The low Δ 18 O vap_eq at the study site may be due to the high level of relative humidity (from 70% to 96%) favoring isotopic diffusive exchanges between the two water phases and thus promoting the isotopic equilibrium. We examine the impact of the isotopic equilibrium assumption on the calculation of δ 18 O cell . A perfect agreement is shown between observed and calculated δ 18 O cell provided that the isotopic composition of source water is significantly higher than the expected averaged isotopic composition of precipitation over the tree growing period.