Abstract. Continental relative humidity (RH) is a key climate parameter, but there is a lack of quantitative RH proxies suitable for climate model–data comparisons. Recently, a combination of climate chamber and natural transect calibrations have laid the groundwork for examining the robustness of the triple oxygen isotope composition (δ′18O and 17O-excess) of phytoliths, that can preserve in sediments, as a new proxy for past changes in RH. However, it was recommended that besides RH, additional factors that may impact δ′18O and 17O-excess of plant water and phytoliths be examined. Here, the effects of grass leaf length, leaf development stage and day–night alternations are addressed from growth chamber experiments. The triple oxygen isotope compositions of leaf water and phytoliths of the grass species F. arundinacea are analysed. Evolution of the leaf water δ′18O and 17O-excess along the leaf length can be modelled using a string-of-lakes approach to which an unevaporated–evaporated mixing equation must be added. We show that for phytoliths to record this evolution, a kinetic fractionation between leaf water and silica, increasing from the base to the apex, must be assumed. Despite the isotope heterogeneity of leaf water along the leaf length, the bulk leaf phytolith δ′18O and 17O-excess values can be estimated from the Craig and Gordon model and a mean leaf water–phytolith fractionation exponent (λPhyto-LW) of 0.521. In addition to not being leaf length dependent, δ′18O and 17O-excess of grass phytoliths are expected to be impacted only very slightly by the stem vs. leaf biomass ratio. Our experiment additionally shows that because a lot of silica polymerises in grasses when the leaf reaches senescence (58 % of leaf phytoliths in mass), RH prevailing during the start of senescence should be considered in addition to RH prevailing during leaf growth when interpreting the 17O-excess of grass bulk phytoliths. Although under the study conditions 17O-excessPhyto do not vary significantly from constant day to day–night conditions, additional monitoring at low RH conditions should be done before drawing any generalisable conclusions. Overall, this study strengthens the reliability of the 17O-excess of phytoliths to be used as a proxy of RH. If future studies show that the mean value of 0.521 used for the grass leaf water–phytolith fractionation exponent λPhyto-LW is not climate dependent, then grassland leaf water 17O-excess obtained from grassland phytolith 17O-excess would inform on isotope signals of several soil–plant-atmosphere processes.
Modeling work of the isotopic composition of tree ring cellulose (δ18Ocell) 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 (Δ18Ovap_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 Δ18Ovap_eq can be explained by evaporation of small raindrops (from 1% to 5% of initial droplet mass). Positive Δ18Ovap_eq can result from vegetation transpiration with transpired water accounting for 14% to 29% to ambient water vapor. The low Δ18Ovap_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 δ18Ocell. A perfect agreement is shown between observed and calculated δ18Ocell provided that the isotopic composition of source water is significantly higher than the expected averaged isotopic composition of precipitation over the tree growing period.
Abstract. Continental atmospheric relative humidity is a major climate parameter whose variability is poorly understood by global climate models. Models' improvement relies on model–data comparisons for past periods. However, there are no truly quantitative indicators of relative humidity for the pre-instrumental period. Previous studies highlighted a quantitative relationship between the triple oxygen isotope composition of phytoliths, particularly the 17O excess of phytoliths, and atmospheric relative humidity. Here, as part of a series of calibrations, we examine the respective controls of soil water isotope composition, temperature, CO2 concentration and relative humidity on phytolith 17O excess. For that purpose, the grass species Festuca arundinacea was grown in growth chambers where these parameters were varying. The setup was designed to control the evolution of the triple oxygen isotope composition of phytoliths and all the water compartments of the soil–plant–atmosphere continuum. Different analytical techniques (cavity ring-down spectroscopy and isotope ratio mass spectrometry) were used to analyze water and silica. An inter-laboratory comparison allowed to strengthen the isotope data matching. Water and phytolith isotope compositions were compared to previous datasets obtained from growth chamber and natural tropical sites. The results show that the δ′18O value of the source water governs the starting point from which the triple oxygen isotope composition of leaf water, phytolith-forming water and phytoliths evolves. However, since the 17O excess varies little in the growth chamber and natural source waters, this has no impact on the strong relative humidity dependency of the 17O excess of phytoliths, demonstrated for the 40 %–80% relative humidity range. This relative humidity dependency is not impacted by changes in air temperature or CO2 concentration either. A relative humidity proxy equation is proposed. Each per meg of change in phytolith 17O excess reflects a change in atmospheric relative humidity of ca. 0.2 %. The ±15 per meg reproducibility on the measurement of phytolith 17O excess corresponds to a ±3.6 % precision on the reconstructed relative humidity. The low sensitivity of phytolith 17O excess to climate parameters other than relative humidity makes it particularly suitable for quantitative reconstructions of continental relative humidity changes in the past.
<div> <div> <div> <p>Modelling of the oxygen isotopic composition (&#948;<sup>18</sup>O) of tree-ring cellulose rely on the isotopic equilibrium assumption between the atmospheric water vapor and the tree source water, which is frequently assimilated to integrated precipitation. We explore the veracity of this assumption based on observations collected (&#948;<sup>18</sup>O of rain, rivers, leaves, tree-rings) or monitored (&#948;<sup>18</sup>O of water vapor) during a field campaign in R&#237;o Negro province, Argentina, in late summer 2017 (February-March). We examine, firstly, how the &#948;<sup>18</sup>O of water vapor deviate from the equilibrium with precipitation and, secondly, what is the impact of the isotopic equilibrium assumption on the calculation of the isotopic composition of tree-ring cellulose.</p> <p>For oxygen, the isotopic disequilibrium between rain and vapor range between -2.0 and 4.1&#8240;. Rain drops re-evaporation during their fall, evaporation of soil water and vegetation transpiration (resulting in transpired water accounting for 14 to 29% of ambient water vapor) could produce this disequilibrium. The small value of the disequilibrium at the study site is likely due to the high level of relative humidity (from 70 to 96%) favoring the isotopic diffusive exchanges between the two water phases and thus promoting the isotopic equilibrium.</p> <p>A perfect agreement between observed and calculated isotopic composition of cellulose is obtained if the source water is assumed to be in isotopic equilibrium with the measured water vapor. This hypothetical source water has a significantly higher &#948;<sup>18</sup>O than the expected averaged isotopic composition of precipitation over the growing period or than the groundwater (river value). The veracity of the hypothesis of the isotopic equilibrium between water vapor and source water in tree-ring paleoclimate studies is discussed in light of these results.</p> </div> </div> </div>
<p>The oxygen isotope signature of leaf water is used to trace several processes at the soil-plant-atmosphere interface. During photosynthesis, it is transferred to the oxygen isotope signature of atmospheric CO<sub>2</sub> and O<sub>2</sub>, which can be used for reconstructing past changes in gross primary production. The oxygen isotope signature of leaf water additionally imprints leaf organic and mineral compounds, such as phytoliths, used as paleoclimate and paleovegetation proxies when extracted from sedimentary materials.</p><p>Numerous experimental and modelling studies were dedicated to constrain the main parameters responsible for changes in the &#948;<sup>18</sup>O of leaf water. Although these models usually correctly depict the main trends of <sup>18</sup>O-enrichment of the leaf water when relative humidity decreases, the calculated absolute values often depart from the observed ones by several &#8240;. Moreover, the &#948;<sup>18</sup>O of leaf water absorbed by plants is dependent on the &#948;<sup>18</sup>O value of meteoric and soil waters that can vary by several &#8240; at different space and time scales. These added uncertainties make our knowledge of the parameters responsible for changes in the &#948;<sup>18</sup>O of leaf water and phytoliths flawed.</p><p>Changes in the triple oxygen isotope composition of leaf water, expressed by the <sup>17</sup>O-excess, are controlled by fewer variables than changes in &#948;<sup>18</sup>O. In meteoric water the <sup>17</sup>O-excess varies slightly as it is weakly affected by temperature or phase changes during air mass transport. This makes the soil water fed by meteoric water and the atmospheric vapour in equilibrium with meteoric water changing little from a place to another. Hence the <sup>17</sup>O-excess of leaf water is essentially controlled by the evaporative fractionation. The latest depends on the ratio of vapor pressure in the air to vapor pressure in the stomata intercellular space, close to relative humidity. Leaf water evaporative fractionation can lead to <sup>17</sup>O-excess negative values that can exceed most of surficial water ones.</p><p>Here we present the outcomes of several recent growth chamber and field studies, for the purpose of i) refining the grass leaf water and phytoliths &#948;<sup>18</sup>O and <sup>17</sup>O-excess modelling, ii) assessing whether the &#948;<sup>18</sup>O and <sup>17</sup>O-excess of grass leaf water can be reconstructed from phytoliths, and iii) examining the precision of the <sup>17</sup>O-excess of phytoliths as a new proxy for past changes in continental atmospheric relative humidity. Atmospheric continental relative humidity is an important climate parameter poorly constrained in global climate models. A model-data comparison approach, applicable beyond the instrumental period, is essential to progress on this issue. However, there is currently a lack of proxies allowing quantitative reconstruction of past continental relative humidity. The <sup>17</sup>O-excess signature of phytoliths could fill this gap.</p>
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