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. Continental atmospheric relative humidity (RH) is a key climateparameter. Combined with atmospheric temperature, it allows us to estimatethe concentration of atmospheric water vapor, which is one of the maincomponents of the global water cycle and the most important gas contributingto the natural greenhouse effect. However, there is a lack of proxiessuitable for reconstructing, in a quantitative way, past changes ofcontinental atmospheric humidity. This reduces the possibility of makingmodel–data comparisons necessary for the implementation of climate models.Over the past 10 years, analytical developments have enabled a fewlaboratories to reach sufficient precision for measuring the triple oxygenisotopes, expressed by the 17O -excess ( 17O -excess = ln( δ 17O +  1) – 0.528  ×  ln( δ 18O +  1)), in water, water vapor and minerals. The 17O -excess represents an alternative to deuterium-excess forinvestigating relative humidity conditions that prevail during waterevaporation. Phytoliths are micrometric amorphous silica particles that formcontinuously in living plants. Phytolith morphological assemblages from soilsand sediments are commonly used as past vegetation and hydrous stressindicators. In the present study, we examine whether changes in atmosphericRH imprint the 17O -excess of phytoliths in a measurable way andwhether this imprint offers a potential for reconstructing past RH. For thatpurpose, we first monitored the 17O -excess evolution of soil water,grass leaf water and grass phytoliths in response to changes in RH (from 40to 100 %) in a growth chamber experiment where transpiration reached asteady state. Decreasing RH from 80 to 40 % decreases the 17O -excess of phytoliths by 4.1 per meg/% as a result of kineticfractionation of the leaf water subject to evaporation. In order to modelwith accuracy the triple oxygen isotope fractionation in play in plant waterand in phytoliths we recommend direct and continuous measurements of thetriple isotope composition of water vapor. Then, we measured the 17O -excess of 57 phytolith assemblages collected from top soilsalong a RH and vegetation transect in inter-tropical West and Central Africa.Although scattered, the 17O -excess of phytoliths decreases with RHby 3.4 per meg/%. The similarity of the trends observed in the growthchamber and nature supports that RH is an important control of 17O -excess of phytoliths in the natural environment. However, otherparameters such as changes in the triple isotope composition of the soilwater or phytolith origin in the plant may come into play. Assessment ofthese parameters through additional growth chambers experiments and fieldcampaigns will bring us closer to an accurate proxy of changes in relativehumidity.

biogeosciences

The triple oxygen isotope composition of phytoliths as a proxy of continental atmospheric humidity: insights from climate chamber and climate transect calibrations