Triple oxygen isotopes are powerful tracers of hydrological processes, yet their variability in atmospheric water vapor and the underlying controls remain poorly understood. We present a one-year record of triple oxygen and hydrogen isotopes of atmospheric water vapor (delta 18OV, d-excessV, 17O-excessV) measured below, within and above a downy oak forest canopy at the AnaEE platform O3HP in the French Mediterranean. This vapor dataset is complemented by isotopic data of precipitation (delta 18OP, d-excessP, 17O-excessP) and groundwater, as well as monthly observations of stomatal conductance and transpiration. Seasonal variations in 17O-excessV and d-excessV likely reflect changing evaporative conditions at oceanic moisture sources. d-excessP and 17O-excessP show a similar seasonal pattern, enhanced by summer rain re-evaporation. However, no clear isotopic differences were observed in vapor or precipitation derived from different oceanic source regions and weather regimes, likely due to frequent mixing of multiple moisture sources. Diurnal variations in 17O-excessV and d-excessV reflect a combination of vegetation-related processes, including local evapotranspiration. However, the impact of evapotranspiration was not evident at daily timescales. Although precipitation often deviates from isotopic equilibrium with near-surface atmospheric water vapor at the event scale due to incomplete equilibration and rain re-evaporation, equilibrium water vapor reliably approximates the near-surface isotopic composition of atmospheric water vapor at annual scale. Our results highlight the potential of 17O-excess for understanding water exchange between the land and the atmosphere, regardless of climatic and vegetation conditions. They enhance the mechanistic interpretation of precipitation isotopes, which is essential for reliable paleoclimate reconstructions.

CM chondrites are the archetypal group of aqueously altered meteorites, preserving records of water-rock interactions on planetesimals. Here, we couple rock magnetic properties with modal mineralogy measured from 48 CM chondrites to explore aqueous alteration through its influence on the magnetic mineralogy of these meteorites. These magnetic properties reflect a complex mixture of Fe-Ni metal, pyrrhotite, and magnetite in varying proportions and grain sizes. As the degree of alteration increases, the relative abundance of metal and pyrrhotite decreases with respect to magnetite, which forms predominantly as very fine grains (tens to hundreds of nanometers)—smaller, on average, than in CI1 chondrites. However, there is no trend between degree of alteration and magnetic properties, arguing that aqueous alteration may not be the only process that influenced the magnetic mineralogy of CM chondrites. Our measurements also indicate that transient heating events may have induced detectable changes in this mineralogy. We find hints of these changes throughout our datasets, possibly explaining the absence of simple trends with extent of aqueous alteration and arguing that weak/short-duration heating may be widespread among CM chondrites. If so, the magnetic mineralogy of many CM chondrites reflects a subtle interplay between aqueous alteration and weak transient heating.

Vegetation changes during the late Holocene in central Africa, especially in the Republic of Congo, are characterized by transitions between forests and savannas. However, the variables (climatic, anthropogenic) behind these transitions are still poorly identified, leading to an ongoing debate around the drivers of the central African forest block opening. Moreover, in tropical regions, the vapour pressure deficit (VPD) that controls photosynthesis and transpiration, constitutes a primary driver of ecosystems primary production and dynamics. In this context, we use a new proxy of atmospheric relative humidity (RH), which coupled with temperature allow to estimate VPD, the 17O-excess (d’17O – 0.528 x d'18O) of phytoliths. A series of calibrations have shown that the 17O-excess of plant leaf water that, according to the Craig and Gordon model is controlled by RH during transpiration, is transferred to phytoliths. A quantitative relationship linking the 17O-excess of phytoliths and RH of the growing season applies to controlled and natural climatic conditions regardless of vegetation type and atmospheric temperature. We propose to combine this new proxy of past RH, with phytoliths morphology, a long-standing paleo-vegetation proxy, to compare past RH and vegetation changes at the same temporal and spatial scales. Phytoliths were extracted from the sediments of Lake Ngofouo, located in a forest-savanna mosaic zone and which record the last 2000 years. Phytoliths types were identified and the 17O-excess of bulk phytoliths samples were analysed. Our preliminary results show a decoupling between RH and vegetation changes. A transition from forest to savanna was identified between 1534-1505 BP, following an increase in fire activity ca. 1540 BP, which marks the beginning of the recurrence of fires in the landscape. During this period no change in RH was observed (high estimated RH ~80-90%). A forest-savanna mosaic thus emerged in the landscape at 1460 BP, characterized by high percentage of grass phytoliths, despite a still high RH (~80%). RH decreased after from 79 to 62% between 997-829 BP and then increased from 62 to 83% between 829-490 BP. During the same period, tree cover increased from 829 to 662 BP and then decreased from 662 to 490 BP which occurred at the same time as an increase in population density and fire activity. It seems that the RH increase probably triggered an increase in grass biomass and thus in available fuel sufficient for more frequent and/or larger fires, which might be responsible for the later decrease in tree cover. The transition from forest to savanna at lake Ngofouo was not associated with a change in RH and was probably the result of the fire regime change that happened before. Interestingly, the later increase in RH impacted the vegetation differently; first by an increase in tree cover, and then by an increase in fire activity that resulted in a lower tree cover. This highlights the potential and complicated feedback that might exist between climate, fire and vegetation.This study is part of the PAST-17 and Thresholds projects funded by the ANR and the European Union’s Horizon 2020 research and innovation program.

Quantitative data are needed to constrain vegetation-hydroclimate in water cycle modelling. Here, we use the triple oxygen isotope composition (δ'18O and 17O-excess) of water compartments to track water transfers and mixing within the soil-plant-atmosphere continuum. At three AMMA-CATCH sites in Benin and Senegal we monitored the δ'18O and 17O-excess of precipitation, groundwater, soil water and plant water, as well as the 17O-excess of phytoliths, an indicator of atmospheric relative humidity. We found that : 1) the 17O-excess in precipitation is very stable over several years; 2) groundwater has δ'18O and 17O-excess values consistent with a multi-year recharge by modern precipitation; 3) the 17O-excess in soil water shows a limited contribution of evaporated water, despite high evaporation conditions, which has important implications for our knowledge of water transfers within soils; 4) extrapolating linear relationships between δ'18O and excess 17O-excess of leaf and stem water allows us to determine the origin of the water absorbed by the roots. At the savanna and dry forest sites, during the rainy season, grasses absorb soil water supplied by precipitation. In contrast, during the dry season, trees reach the perennial groundwater recharge. 5) the 17O-excess of grass and tree leaf water follow the dynamics of relative humidity; 6) the 17O-excess of grass phytoliths records daily relative humidity during the growing season. These results provide a solid basis for using the triple oxygen isotope composition of water and phytoliths to trace present and past water cycles at the soil-plant-atmosphere interface.This study was conducted in the framework of the HUMI-17 and PAST-17 projects supported by the ANR (ANR-17-CE01-0002-01 and ANR-22-CE01-0027-01), JA and CV have benefited from a Marie Sklodowska-Curie grant from the European Union (n°101063961 for JA and 101063961 for CV). TT acknowledge funds from FORMAS (Dnr 2021-00644), and the European Union under the Development Smart Innovation through Research in Agriculture (DeSIRA) Initiative (FOOD/2019/410-169).

Phytoliths are amorphous silica particles that precipitate within and between plant cells, and their fossilized morphological assemblages are widely used to reconstruct paleo-vegetation. The triple oxygen isotope composition of phytoliths, expressed by the 17O-excess, is a promising proxy to reconstruct atmospheric relative humidity (RH). However, fossil phytoliths in lake or peat sediments often coexist with diatom frustules and sponge spicules, whose oxygen isotope signatures contribute to the average isotopic composition of biogenic silica, biasing the RH reconstruction. In this case, it is necessary to separate or at least concentrate the phytoliths. We developed a filtration protocol for this purpose. We tested the protocol on 31 lacustrine and peat sedimentary samples from West and Central Africa, and these are the main results: • Phytolith concentrations increased in 23 samples, primarily due to the removal of long pennate diatom frustules and sponge spicules. Six samples showed no significant change in phytolith concentration, while two samples showed a decrease. • Twenty-nine samples achieved final phytolith proportions exceeding 40 % and a sensitivity analysis based on an isotope mass balance equation confirmed that these samples are suitable for 17O-excess measurements to reconstruct RH.

Because it is an important climate parameter, an accurate proxy is required for the reconstruction of past Relative Humidity (RH). Triple oxygen isotopes of grass-leaf phytoliths have a high potential for this purpose. A strong correlation between RH and O-17-excess of grass leaf phytoliths has been previously demonstrated from calibration experiments in growth chambers and at natural Sub-Saharan African and Mediterranean sites. Here we focus on the potential of 17 O-excess of grass leaf phytoliths as a proxy of RH across the North American Great Plains. We tested the relationship between the O-17-excess of naturally grown Calamovilfa longifolia phytoliths and RH for different periods of the grass growing season. The results show that O-17-excess of leaf phytoliths has a strong correlation with late-growing season (July-August) daytime RH, which varies from 29 to 72 % across North America. The correlation can be expressed as: O-17-excess leaf phytoliths = 4.14(+0.47) x RH (%) - 497(+26) (R-2 = 0.85, p-value<0.0001). To use the O-17-excess of bulk phytoliths buried in soils and sediments as a quantitative proxy of past RH, future calibration studies on soil phytoliths are needed to evaluate the contribution of non-transpiring phytoliths. The(17)O-excess of stem water, which was determined by O-17-excess of stem phytoliths, shows similar values to the O-17-excess of precipitation in North America without evaporation, which therefore does not affect the O-17-excess of leaf phytoliths in North America. These results demonstrate that the O-17-excess of phytoliths is a promising proxy for regional growing season RH.

Quantitative data are needed to constrain the feedback loops between vegetation and hydroclimate. In this study, the amplitudes of variations in the triple oxygen isotope composition of water at the soil-plant-atmosphere interface are measured in savanna and dry forest contexts in West Africa (Benin and Senegal). Comparison of in situ data and model estimates reveals the following: (a) The value of O-17-excess for reconstructing climate archives is confirmed, given its small variability in precipitation (a few per meg) compared to the very large magnitude of change in the O-17-excess of phytoliths (hundreds of per meg) in response to RH changes. (b) At the beginning of the dry season, the O-17-excess in soil water is lower than that of precipitation by only 30 per meg in the sandy-loam soils and 50 per meg in the sandy soil. This shows the limited contribution of evaporated water to bulk soil water and provides clues to constrain the complex hydrological functioning of soils; (c) The regression line connecting the triple oxygen isotope composition of water in the stems and leaves of grasses and trees can be used to determine the origin of the water absorbed by the roots. Semi-evergreen trees draw their water from the water table during the dry season, whereas grasses and semi-evergreen trees use surface water during the wet season. These original data open up new perspectives for the use of the triple oxygen isotope composition of water and phytoliths to better understand current and past hydrological cycles.

Atmospheric relative humidity (RH) and vapor pressure deficit (VPD, a combination of RH and temperature) are primary drivers of vegetation productivity and dynamics, especially in the tropics where air temperature changes are limited. Local RH and VPD conditions are also important factors used to predict the probability of fire occurrence and behavior because they control fuel moisture. RH is therefore a crucial climate variable that drives vegetation distribution and modifies disturbance regimes. Since the end of the last century, continental RH has been decreasing (and VPD increasing) resulting in increasing mortality in tropical trees. However, RH predictions from Earth System Models (ESMs) are poorly constrained and vary on a regional scale from one model to another, probably due to our poor knowledge of feedbacks between vegetation and the atmosphere. Reconstructing independently past vegetation and RH changes beyond the instrumental period is of intense interest in order to better understand interactions between vegetation and the RH and produce quantitative data available for comparison with ESMs outputs for past climates. A set of calibrations recently showed that the triple isotope composition of oxygen in phytoliths, which are biogenic silica particles formed in plants, expressed by the 17O-excess (d’17O – 0.528*d’18O), is a proxy of RH conditions prevailing during the plant water evaporation, independently of vegetation type. A quantitative relationship between 17O-excess of phytoliths and RH of the growing season was therefore established. Combining the study of phytoliths morphology, a commonly used indicator of past vegetation, and their 17O-excess signature, makes it possible to produce independent reconstructions of past vegetation and RH. Here, the triple oxygen isotope composition of sedimentary phytoliths from Lake Guiers (16°15′00”N, 15°50′00”W), located in the Sahelian area of Senegal, was analyzed by IR-laser-fluorination-IRMS. The morphological assemblages of phytoliths were also defined using optical microscopy and phytolith indices. The sediments were dated using 210Pb and radiocarbon, and record the last 4000 years. Modern soil and plant samples were also analyzed for phytoliths content and the associated RH was estimated using their 17O-excess signature. We provide reconstructions of regional changes in RH and vegetation over the last 4000 years, with a focus on the last 400 years due to a higher temporal resolution. We also studied the potential biases in the regional RH reconstructions due to the contribution of phytolith from local lakeshore vegetation (growing under higher RH conditions) and to the contribution of diatoms (produced in isotopic equilibrium with the lake water). This study is part of the HUMI-17, PAST-17 and Thresholds projects funded by the French National Research Agency (ANR) and the European Union’s Horizon 2020 research and innovation program. The coring was made possible thanks to the contribution of the OLAC (Office des LAcs et Cours d’eau du Sénégal).

Oxygen isotopes in biogenic silica (δ18OBSi) from lake sediments allow for quantitative reconstruction of past hydroclimate and proxy-model comparison in terrestrial environments. The signals of individual records have been attributed to different factors, such as air temperature (Tair), atmospheric circulation patterns, hydrological changes, and lake evaporation. While every lake has its own local set of drivers of δ18O variability, here we explore the extent to which regional or even global signals emerge from a series of paleoenvironmental records. This study provides a comprehensive compilation and combined statistical evaluation of the existing lake sediment δ18OBSi records, largely missing in other summary publications (i.e. PAGES network). For this purpose, we have identified and compiled 71 down-core records published to date and complemented these datasets with additional lake basin parameters (e.g. lake water residence time and catchment size) to best characterize the signal properties. Records feature widely different temporal coverage and resolution, ranging from decadal-scale records covering the past 150 years to records with multi-millennial-scale resolution spanning glacial–interglacial cycles. The best coverage in number of records (N = 37) and data points (N = 2112) is available for Northern Hemispheric (NH) extratropical regions throughout the Holocene (roughly corresponding to Marine Isotope Stage 1; MIS 1). To address the different variabilities and temporal offsets, records were brought to a common temporal resolution by binning and subsequently filtered for hydrologically open lakes with lake water residence times < 100 years. For mid- to high-latitude (> 45° N) lakes, we find common δ18OBSi patterns among the lake records during both the Holocene and Common Era (CE). These include maxima and minima corresponding to known climate episodes, such as the Holocene Thermal Maximum (HTM), Neoglacial Cooling, Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). These patterns are in line with long-term air temperature changes supported by previously published climate reconstructions from other archives, as well as Holocene summer insolation changes. In conclusion, oxygen isotope records from NH extratropical lake sediments feature a common climate signal at centennial (for CE) and millennial (for Holocene) timescales despite stemming from different lakes in different geographic locations and hence constitute a valuable proxy for past climate reconstructions.

Vegetation changes during the late Holocene in central Africa, especially in the Republic of Congo, are characterized by transitions between forests and savannas. However, the variables (climatic, anthropogenic) behind these transitions are still poorly identified, leading to an ongoing debate around the drivers of the central African forest block opening. Moreover, in tropical regions, the vapour pressure deficit (VPD) that controls photosynthesis and transpiration, constitutes a primary driver of ecosystems primary production and dynamics. In this context, we use a new proxy of atmospheric relative humidity (RH), which coupled with temperature allow to estimate VPD, the 17O-excess (d’17O – 0.528 x d'18O) of phytoliths. A series of calibrations have shown that the 17O-excess of plant leaf water that, according to the Craig and Gordon model is controlled by RH during transpiration, is transferred to phytoliths. A quantitative relationship linking the 17O-excess of phytoliths and RH of the growing season applies to controlled and natural climatic conditions regardless of vegetation type and atmospheric temperature. We propose to combine this new proxy of past RH, with phytoliths morphology, a long-standing paleo-vegetation proxy, to compare past RH and vegetation changes at the same temporal and spatial scales. Phytoliths were extracted from the sediments of Lake Ngofouo, located in a forest-savanna mosaic zone and which record the last 2000 years. Phytoliths types were identified and the 17O-excess of bulk phytoliths samples were analysed. Our preliminary results show a decoupling between RH and vegetation changes. A transition from forest to savanna was identified between 1534-1505 BP, following an increase in fire activity ca. 1540 BP, which marks the beginning of the recurrence of fires in the landscape. During this period no change in RH was observed (high estimated RH ~80-90%). A forest-savanna mosaic thus emerged in the landscape at 1460 BP, characterized by high percentage of grass phytoliths, despite a still high RH (~80%). RH decreased after from 79 to 62% between 997-829 BP and then increased from 62 to 83% between 829-490 BP. During the same period, tree cover increased from 829 to 662 BP and then decreased from 662 to 490 BP which occurred at the same time as an increase in population density and fire activity. It seems that the RH increase probably triggered an increase in grass biomass and thus in available fuel sufficient for more frequent and/or larger fires, which might be responsible for the later decrease in tree cover. The transition from forest to savanna at lake Ngofouo was not associated with a change in RH and was probably the result of the fire regime change that happened before. Interestingly, the later increase in RH impacted the vegetation differently; first by an increase in tree cover, and then by an increase in fire activity that resulted in a lower tree cover. This highlights the potential and complicated feedback that might exist between climate, fire and vegetation. This study is part of the PAST-17 and Thresholds projects funded by the ANR and the European Union’s Horizon 2020 research and innovation program.

Triple oxygen isotopes (17O-excess) of water are useful to trace evaporation at the soil–plant–atmosphere interface. The 17O-excess of plant silica, i.e., phytoliths, inherited from leaf water, was previously calibrated in growth chambers as a proxy of atmospheric relative humidity (RH). Here, using a model–data approach, we examine the parameters that control the triple oxygen isotope composition of bulk grass leaf water and phytoliths in natura, at the O3HP experimental platform located in the French Mediterranean area. A grass plot was equipped to measure for 1 year, all environmental and plant physiological parameters relevant for modeling the isotope composition of the grass leaf water. In particular, the triple oxygen and hydrogen isotope composition of atmospheric water vapor above the grass was measured continuously using a cavity ring-down spectrometer, and the grass leaf temperature was monitored at plot scale using an infrared (IR) radiometer. Grass leaves were collected in different seasons of the year and over a 24 h period in June. Grass leaf water was extracted by cryogenic vacuum distillation and analyzed by isotope ratio mass spectrometry (IRMS). Phytoliths were analyzed by IR–laser fluorination–IRMS after chemical extraction. We showed that the traditional Craig–Gordon steady-state model modified for grass leaves reliably predicts the triple oxygen isotope composition of leaf water during daytime but is sensitive to uncertainties on the leaf-to-air temperature difference. Deviations from isotope steady state at night are well represented in the triple oxygen isotope system and predictable by a non-steady-state model. The 17O-excess of phytoliths confirms the applicability of the 17O-excessphyto vs. RH equation established in previous growth chamber experiments. Further, it recorded average daytime RH over the growth period rather than daily RH, related to low transpiration and silicification during the night. This model–data approach highlights the utility of the triple oxygen isotope system to improve the understanding of water exchange at the soil–plant–atmosphere interface. The in natura experiment underlines the applicability of 17O-excess of phytoliths as a RH proxy.

The climate of the African Holocene Humid Period (AHHP) is reconstructed in the Tibesti Volcanic Massif (TVM) in the central Sahara from well-preserved diatomaceous deposits in the two crater palaeolakes of Trou au Natron at Pic Toussidé and Era Kohor at Emi Koussi. The two records cover the period from ∼9500 to 4500 cal yr BP. Climate and palaeoenvironmental changes during this period were inferred from diatom assemblages, interpretation of variations in their oxygen isotope composition (δ18Odiatom), reconstruction of lake water conductivity from diatom-based transfer functions, and estimation of the lake water balance (Evaporation/Inflow ratio, E/I). Our findings provide evidence for two distinct lacustrine episodes. During the early to mid-Holocene transition, low δ18Odiatom values, high percentages of planktonic diatoms, low lake water conductivity and a positive water balance (E/I < 1) suggest wet conditions, which were likely related to the optimum of the AHHP. From the mid-to late Holocene transition, an aridification trend is revealed by increasing δ18Odiatom values, high percentages of benthic diatoms and a negative water budget (E/I > 1), occurring as early as 6500 cal yr BP and intensifying after 5300 cal yr BP. Moreover, our data show on average a decrease in precipitation amounts of ∼35% between the peak and the end of the AHHP in the Tibesti region. This timing of the AHHP in the mountainous Tibesti is consistent with the aridification of the central Sahara recorded at lowland sites, which has mainly been related to the southward retreat of the Intertropical Convergence Zone (ITCZ) and the associated African monsoonal rainfall belt, following the gradually declining summer insolation that led to the termination of the AHHP. Our results prove the existence of Holocene lakes in the TVM craters that developed contemporaneously with the lakes of the Chadian basin and the Libyan Sahara. On a broader scale, our data share similar hydroclimatic patterns with studies from the eastern and northern Sahara.

Dissolution is one among several taphonomical processes that may bias paleoenvironmental, paleoclimatic or taxonomic interpretation of phytolith assemblages. To improve our understanding of dissolution on grass phytoliths, we studied systematic changes of surface features, morphotype assemblages, and dissolution rates of phytoliths extracted from two grass species Hyparrhenia involucrata (Panicoideae), Nastus borbonicus (Bambusoideae), one soil from La Réunion Island (approximate mean age < 800 yr), and three paleosols from Ethiopia (approximate age of 4.4 million years). We used heavy-liquid to extract phytoliths, and 1% Na2CO3 to perform partial dissolution experiments. Physicochemical surface properties, morphotypes, and assemblages were analyzed using optical and scanning electron microscopy, laser diffraction, and X-ray diffractometry. Our results show that 1) phytoliths from different grass species may have different dissolution rates: phytoliths from the leaves of Hyparrhenia involucrata (Panicoideae) are more prone to dissolution than those from Nastus borbonicus (Bambusoideae). 2) Silicon (Si) released by phytolith assemblages (i.e., phytolith dissolution rate) decreases as follows: plant > soil > paleosol. 3) Dissolution leads to cavity formation on phytolith surfaces and disappearance of fragile silica particles. 4) Partial dissolution does not significantly change percentages of common grass phytolith morphotypes in a given assemblage. These results provide a benchmark for assessing the reliability of paleoenvironmental reconstructions using grass phytolith assemblages from buried soils and sediments.

RATIONALE Producing robust high frequency time series of raw atmospheric water vapor isotope data by laser spectrometry requires accurate calibration. Especially, the chemical composition of the analyzed sample gas can cause isotope bias. This study assesses the matrix effect on calibrated δ17 O, δ18 O, δ2 H, 17 O-excess, and d-excess values of atmospheric water vapor. METHODS A Picarro L2140-i cavity ring-down spectrometer combined with an autosampler and a vaporizer is used to analyze δ17 O, δ18 O, δ2 H, 17 O-excess and d-excess of two water standards. Isotope data obtained using synthetic air and dry ambient air as carrier gas at water mixing ratios ranging from 2000 to 30000 ppmv are compared. Based on the results, atmospheric water vapor measurements are calibrated. The expected precision is estimated by Monte Carlo simulation. RESULTS The dry air source strongly impacts raw isotope values of the two water standards, but has no effect on the mixing ratio dependency functions. When using synthetic air, δ17 O, δ18 O and 17 O-excess of calibrated atmospheric water vapor are overestimated by 0.6 ‰, 0.7 ‰, and 217 per meg, respectively, while δ2 H and d-excess are underestimated by 1.5 ‰ and 7.3 ‰. Optimum precisions for the calibrated δ17 O, δ18 O, δ2 H, 17 O-excess and d-excess values and 12-min integration time are 0.02 ‰, 0.03 ‰, 0.4 ‰, 14 per meg and 0.4 ‰, respectively. CONCLUSIONS In light of the obtained results, recommendations for the calibration of atmospheric water vapor isotope measurements are presented. The necessity to use dry ambient air as dry air source when running the standards for calibration is pointed out as a pre-requisite for accurate atmospheric water vapor 17 O-excess and d-excess measurements.

Abstract. The triple oxygen isotope composition of phytoliths (17O-excessphyto) can provide key information on past atmospheric relative humidity (RH) over land. Here, we examined how leaf-to-air temperature gradients and changes in the silica polymerization rate in response to stomatal conductance influence the interpretation of 17O-excessphyto in terms of RH. Further, we assessed the reliability of a theoretical isotope model of leaf water evaporation to predict the triple oxygen isotope composition of leaf water on diurnal and seasonal scale. For this purpose, we monitored a grass plot within a natural Mediterranean woodland for one year. We measured in particular the isotope composition of atmospheric water vapor and plot-scale grass leaf temperatures – two variables that are often only estimated. Grass leaf blades were collected in different seasons and over a 24-hour period for leaf water and phytolith isotope analysis. We found that the steady state model reliably predicts the triple oxygen isotope composition of leaf water during daytime but remains sensitive to uncertainties on the leaf-to-air temperature difference. Deviations from isotope steady state at night are well represented by the non-steady state model. In our study, the 17O-excessphyto best reflects average daytime RH over the growth period, rather than daily RH. Average daytime leaf-to-air temperature gradients of less than 2 °C introduce an insignificant bias to the RH estimate. The results also confirm the established triple oxygen isotope fractionation factors between phytoliths and leaf water. The findings of this study help to better understand how to interpret 17O-excessphyto of fossil phytolith assemblages in terms of past RH.