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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.

During the Holocene (0-11.7 ka BP), the subtropical regions of Africa were characterized by very significant climatic changes. These changes were evidenced by variations in lake levels which reflect the balance between precipitation and evaporation (P-E) at the watershed scale. These climatic conditions are mainly associated with the dynamics of the African summer monsoon in relation with the location of the Intertropical Convergence Zone (ITCZ), which is modulated by summer insolation induced by the Earth's orbital parameters. This mechanism explains the wetter conditions observed from 11 ka BP to 5.5 ka BP in the Sahelo-Saharan zone. This period, called "Green Sahara" or "African Humid Period", was characterized by a green landscape, covered by grasslands and trees, dotted with numerous lakes, and incised by large river networks.Despite the numerous studies carried out on the African Humid Period, there is a scarcity of data for quantifying source and origin of precipitation. The Lake Chad Basin (BLT) is a key region for paleoclimatic research because of its position reflecting main tropical atmospheric mechanisms and its endorheic morphology amplifying climatic signals. More particularly, the crater palaeolakes of Trou au Natron (Pic Toussidé) and Era Kohor (Emi Koussi) in the Tibesti mountains offer unique sedimentary archives that may record the climatic history of the Sahara.This work aims to reconstruct the evolution of these crater palaeolakes, thanks to the oxygen isotopic composition of diatoms (18Odiatom), from the termination of the last deglaciation until the African Humid Period. The identification of the fossil diatom assemblages combined with the 18Odiatom values should give insights on the evolution of the limnological parameters, the relative depth, the chemistry, and the water isotopic composition of these palaeolakes.The measurement of 18Odiatom is carried out using the IR-laser fluorination-isotope ratio mass spectrometry technique at the CEREGE stable isotope laboratory, after dehydration under a flow of nitrogen gas. The diatoms are purified at the CEREGE micropaleontology laboratory after decarbonation and organic matter oxidation. The taxonomic determination of diatoms is carried at CEREGE (France) and at the University of N’Djamena (Chad).Preliminary results from the two Tibesti mountains records cover the Holocene wet period. They show significant variations in the d18Odiatom values and a distinct evolution of ecosystems as demonstrated by taxonomic assemblage of fossil diatoms. These results are compared with a reconstruction of the d18Odiatom from Lake Chad for the same period. This comparison evidenced substantial data for the reconstruction of the Holocene wet period, in terms of origin of water inflows in the basin, eg Tropical versus Mediterranean and lowland versus mountainous atmospheric processes, and on the reconstruction of the migration and the position of the ITCZ. These two questions are still speculative and will certainly provide data for global climate circulation models which are struggling to reproduce the climate in Saharan latitudes.

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In the Mediterranean area, the 4.2 ka BP event is recorded with contrasting expressions between regions. In the southern Alps, the high-altitude Lake Petit (Mercantour Massif, France; 2200 a.s.l.) offers pollen and diatom-rich sediments covering the last 4800 years. A multi-proxy analysis recently revealed a detrital pulse around 4200 cal BP due to increasing erosion in the lake catchment. The involvement of a rapid climate change leading to increasing runoff and soil erosion was proposed. Here, in order to clarify this hypothesis, we measured the oxygen isotope composition of diatom silica frustules (delta O-18(diatom)) from the same sedimentary core. Diatoms were analysed by laser fluorination isotope ratio mass spectrometry after an inert gas flow dehydration. We additionally enhanced the accuracy of the age-depth model using the Bacon R package. The delta O-18(diatom) record allows us to identify a 500-year time lapse, from 4400 to 3900 cal BP, where delta O-18(diatom) reached its highest values (> 31 parts per thousand). delta O-18(diatom) was about 3 parts per thousand higher than the modern values and the shifts at 4400 and 3900 cal BP were of similar amplitude as the seasonal delta O-18(diatom) shifts occurring today. This period of high delta O-18(diatom) values can be explained by the intensification of O-18-enriched Mediterranean precipitation events feeding the lake during the ice-free season. This agrees with other records from the southern Alps suggesting runoff intensification around 4200 cal BP. Possible changes in other climatic parameters may have played a concomitant role, including a decrease in the contribution of O-18-depleted Atlantic winter precipitation to the lake water due to snow deficit. Data recording the 4.2 ka BP event in the northwestern Mediterranean area are still sparse. In the Lake Petit watershed, the 4.2 ka BP event translated into a change in precipitation regime from 4400 to 3900 cal BP. This record contributes to the recent efforts to characterize and investigate the geographical extent of the 4.2 ka BP event in the Mediterranean area.

International audience

Adverse natural conditions will, generally, induce gemmulation in freshwater sponges. Because of this environmental dependence, gemmoscleres are given exceptional value in taxonomic, ecological and paleoenvironmental studies. Other spicules categories such as microscleres and beta megascleres have received little attention with regard to their occurrence and function during the sponge biological cycle. Metania spinata, a South American species common to bog waters in the Cerrado biome, produces alpha and beta megascleres, microscleres and gemmoscleres. To detect the environmental factors triggering the production of all these kinds of spicules, the species annual seasonal cycle was studied. Artificial substrates were devised, supplied with gemmules and placed in Lagoa Verde pond which contained a natural population of M. spinata. Field monitoring was conducted for eight months in order to observe the growth of sponges and spicules formation. Samples of water were taken monthly for physical and chemical parameters determination. The appearance of the alpha megascleres was sequentially followed by that of microscleres, gemmoscleres and beta megascleres. The first ones built the new sponge skeleton, the last three were involved in keeping inner moisture in the sponge body or its gemmules. The water level, temperature and the silicon (Si) concentration in the pond were the most important factors related to this sequential production of spicules, confirming environmental reconstructions based on the presence or absence of alpha megascleres and gemmoscleres in past sediments.

International audience

International audience

14C AMS analysis of occluded carbon in phytoliths (phytC) is a promising dating tool for palaeoenvironmental and archaeological studies. In order to assess the accuracy of this method, different tests were recently carried out on large phytolith concentrates of phytC samples extracted from soils and harvested plants, in association with blank samples of SiO2 powder to check the absence of carbon contamination during the treatments. Despite this precaution, 14C values from recent harvested plants were inexplicably old (2 - 8 ka years BP). Nevertheless, we noticed that many chemical extraction protocols that were used did not lead to samples totally free of organic matter. In order to tackle this problem, and as a first step, the efficiency of common extraction protocols from the literature were tested. Samples were analyzed by SEM/EDX in order to assess the purity of the siliceous material following extraction. As a result of these tests, a new extraction protocol combining acid digestion, oxidation and dry ashing to acquire pure samples of phytoliths from harvested plants is proposed. In a second step, modern and well dated archaeological materials (harvested plants grown within a FACE experiment and plant residues from a 17th century French mummy) were analyzed in 14C-AMS. Results should allow either to demonstrate the reliability of 14C-AMS analysis of phytolith occluded carbon as a dating tool or trigger further investigations of possible sources of old occluded carbon in phytoliths if the 14C ages are still older than expected.

International audience; Over the past decades, analysis of occluded carbon in phytoliths (opaline silica mineral bodies that form in and between plant cells) has become a workhorse of paleoclimate and archaeological studies. Since different plant types exhibit distinctive phytolith morphologies, their assemblages are used in identifying vegetation histories or food culture adaptations. A few direct radiocarbon AMS measurements of phytoliths have been carried out, but these measurements are difficult due to the low concentrations of phytoliths in some plant species, and the small amount of C per phytolith (

International audience; Aim This study calibrates the relationship between phytolith indices, modern vegetation structure, and a climate parameter (AET/PET, i.e. the ratio of annual actual evapotranspiration to annual potential evapotranspiration), in order to present new proxies for long-term Quaternary climate and vegetation changes, and model/data comparisons. Location Sixty-two modern soil surface samples from West Africa (Mauritania and Senegal), collected along a latitudinal transect across four bioclimatic zones, were analysed. Methods Two phytolith indices are defined as normalized data: (1) humidity-aridity index [Iph (%) = saddle vs. cross + dumbbell + saddle], and (2) water stress index [fan-shaped index (Fs) (%) = fan-shaped vs. sum of characteristic phytoliths]. Vegetation structures are delimited according to Iph and Fs boundaries. Bootstrapped regression methods are used for evaluating the strength of the relationship between the two phytolith indices and AET/PET. Additional modern phytolith assemblages, from Mexico, Cameroon and Tanzania are extracted in order to test the calibration established from the West African samples. Accuracy of the AET/PET phytolith proxy is compared with equivalent pollen proxy from the same area. Results Characterization of the grass cover is accurately made through Iph. A boundary of 20 +/- 1.4% discriminates tall grass savannas from short grass savannas. Water stress and transpiration experienced by the grass cover can be estimated through Fs. AET/PET is accurately estimated from phytoliths by a transfer function: AET/PET = -0.605 Fs - 0.387 Iph + 0.272 (Iph - 20)(2) (r = 0.80 +/- 0.04) in the application domain (AET/PET ranging from 0.1 +/- 0.04 to 0.45 +/- 0.04). Phytolith and pollen estimate with similar precision (r(pollen) = 0.84 +/- 0.04) the AET/PET in the studied area. Conclusions This study demonstrates that we can rely on the phytolith indices Iph and Fs to distinguish the different grasslands in tropical areas. Moreover, a new phytolith proxy of AET/PET, linked to water availability, is presented. We suggest from these results that combining phytolith and pollen proxies of AET/PET would help to constrain this climate parameter better, especially when phytolith assemblages are dominated by Panicoideae and Chloridoideae C-4-grass phytoliths, are devoid of Pooideae C-3-grass phytoliths, and occur with a few tropical ligneous woody dicotyledon phytoliths. As AET/PET is a bioclimatic indicator commonly used in vegetation models, such a combination would help to make model/data comparisons more efficient.

International audience; Les résultats préliminaires obtenus pour la région d'Essaouira montrent que les phytolithes sont des traceurs potentiels du paysage végétal, notamment pour les monocotylédones.Par le calcul de l'indice phytolithique (Iph), les morphotypes associés à des taxons correspondant aux sous-familles de Poaceae sont en accord avec un milieu chaud et aride. L'assemblage moderne reflète avec fidélité les données bioclimatiques du terrain étudié.Les phytolithes de gymnospermes ne sont pas encore répertoriés, mais permettraient de fournir plus d'informations sur les taux de boisement et la densité du couvert arboré, à partir du rapport dicotylédones/Poaceae complété.Il reste à montrer, maintenant, si l'aridité relevée est anthropique ou climatique. Pour cela, un assemblage fossile doit être réalisé et comparé à celui-là.

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.

Continental relative humidity (RH) is a key-climate parameter. However, there is a lack of quantitative RH proxies suitable for climate model-data comparisons. Recently, a combination of climate chamber and natural transect calibrations laid the groundwork for examining the robustness of the triple oxygen isotope composition (δ18O, δ17O) of phytoliths as a new proxy for past changes in RH. However, it was recommended that besides RH, additional factors that may impact δ18O and δ17O of plant water and phytoliths be examined. Here, the effects of leaf anatomy, leaf development stage and day/night alternations are addressed from the growth of the grass species F. arundinacea in climate chambers. Plant water and phytoliths are analyzed in δ18O and δ17O. Silicification patterns are examined using light and scanning electron observation of phytoliths. The isotope data show the increasing contribution of evaporated epidermal water to the bulk leaf water, from sheath to proximal and apical leaf blade. However, despite this isotope heterogeneity, δ18O and δ17O of the bulk leaf water can be predicted by the Craig and Gordon model, in the given experimental conditions (high RH). Regarding phytoliths, their forming water (mainly epidermal) is, as expected, more impacted by evaporation than the bulk leaf water. This discrepancy increases from sheath to proximal and apical blade and can be explained by the steepening of the radial concentration gradient of evaporated water along the leaf. However, we show that because most of silica polymerizes in epidermal long cells of the apical blade of the leaves, the δ18O and δ17O of bulk grass phytoliths should not be impacted by the diversity in grass anatomy. The data additionally show that most of silica polymerizes at the end of the leaf elongation stage and at the transition towards leaf senescence. Thus, climate conditions at that time should be considered when interpreting δ18O and δ17O of phytoliths from the natural environment. At least, no light/dark effect was detected on the δ18O and δ17O signature of plant water and phytoliths of F. arundinacea. However, when day/night alternations are characterized by significant changes in RH, the lowest RH conditions favoring evaporation and silica polymerization should be considered when calibrating the phytolith proxy. This study contributes to the identification of the parameters driving the δ18O and δ17O of bulk grass phytoliths. It additionally brings elements to further understand and model the δ18O and δ17O of grass leaf water, which influences the isotope signal of several processes at the soil/plant/atmosphere interface.

Continental atmospheric relative humidity (RH) is a key climate parameter. Combined with atmospheric temperature, it allows us to estimate the concentration of atmospheric water vapor, which is one of the main components of the global water cycle and the most important gas contributing to the natural greenhouse effect. However, there is a lack of proxies suitable for reconstructing, in a quantitative way, past changes of continental atmospheric humidity. This reduces the possibility of making model–data comparisons necessary for the implementation of climate models. Over the past 10 years, analytical developments have enabled a few laboratories to reach sufficient precision for measuring the triple oxygen isotopes, expressed by the 17O-excess (17O-excess = ln (δ17O + 1) – 0.528 × ln (δ18O + 1)), in water, water vapor and minerals. The 17O-excess represents an alternative to deuterium-excess for investigating relative humidity conditions that prevail during water evaporation. Phytoliths are micrometric amorphous silica particles that form continuously in living plants. Phytolith morphological assemblages from soils and sediments are commonly used as past vegetation and hydrous stress indicators. In the present study, we examine whether changes in atmospheric RH imprint the 17O-excess of phytoliths in a measurable way and whether this imprint offers a potential for reconstructing past RH. For that purpose, we first monitored the 17O-excess evolution of soil water, grass leaf water and grass phytoliths in response to changes in RH (from 40 to 100 %) in a growth chamber experiment where transpiration reached a steady state. Decreasing RH from 80 to 40 % decreases the 17O-excess of phytoliths by 4.1 per meg/% as a result of kinetic fractionation of the leaf water subject to evaporation. In order to model with accuracy the triple oxygen isotope fractionation in play in plant water and in phytoliths we recommend direct and continuous measurements of the triple isotope composition of water vapor. Then, we measured the 17O-excess of 57 phytolith assemblages collected from top soils along a RH and vegetation transect in inter-tropical West and Central Africa. Although scattered, the 17O-excess of phytoliths decreases with RH by 3.4 per meg/%. The similarity of the trends observed in the growth chamber and nature supports that RH is an important control of 17O-excess of phytoliths in the natural environment. However, other parameters such as changes in the triple isotope composition of the soil water or phytolith origin in the plant may come into play. Assessment of these parameters through additional growth chambers experiments and field campaigns will bring us closer to an accurate proxy of changes in relative humidity.

Programa de Pós-Graduação em Evolução Crustal e Recursos Naturais. Departamento de Geologia. Escola de Minas, Universidade Federal de Ouro Preto. Esponjas de água doce (Porifera, Demospongiae) são produtoras de estruturas de biosílica denominadas espículas, com extraordinário valor taxonômico, e que podem apresentar-se em mais de uma categoria, conforme a espécie, tendo reconhecida utilização como marcadores ambientais. As populações dessas esponjas vivem em condições ambientais específicas e mudanças nestes ambientes provocam respostas que se traduzem na produção das diferentes categorias de espículas. As espículas são depositadas e preservam-se no sedimento de ambiente límnico após a morte da esponja. Quando há grandes concentrações de espículas no sedimento, estes constituem espongilitos. Os depósitos de espongilitos no noroeste de Minas Gerais (Brasil) são extensos, ocorrendo em aproximadamente 80 lagoas circulares ou semicirculares, e ocupam uma área de cerca de 45 Km2. Os espongilitos representam recurso mineral importante na indústria cerâmica. O processo de formação dos espongilitos dessa região ocorre há pelo menos 28 mil anos, havendo ainda hoje populações de esponjas que continuamente contribuem para os depósitos de espículas. Para compreender a formação dos depósitos, é necessário estudar o ciclo de vida dos organismos que os formam. O conhecimento ecológico relacionado às esponjas, bem como a caracterização das diferentes espículas e a sua relação com o ambiente em que se formaram, trazem informações fundamentais sobre as condições paleoambientais de deposição dos sedimentos formadores de espongilitos. Tais informações podem indicar se havia disponibilidade constante de água, ou se ocorriam eventos de estiagem. Biosílicas de diatomáceas e fitólitos têm sido intensamente estudadas através de técnicas de análises de isótopos de oxigênio, que mostram uma relação termodependente com o valor de sua composição isotópica menos o valor da composição isotópica da água em que se formou. Estas técnicas permitem determinar as condições térmicas durante a formação dos depósitos no passado através das biosílicas sedimentadas. Neste sentido, testar a técnica de análise de isótopos de oxigênio em sílica biogênica de esponjas visa à sua validação como marcadora da paleotemperatura se o fracionamento isotópico em espículas ocorre em equilíbrio com a água da sua formação. A Lagoa Verde, no noroeste de Minas Gerais onde a espécie Metania spinata (Carter, 1881) ocorre atualmente em abundância, foi selecionada para este trabalho, sendo esta espécie a que mais contribuiu para a formação dos depósitos de espongilitos. Tanto os estudos ecológicos da esponja nesta região, como as técnicas de isótopos de oxigênio, tiveram por objetivo contribuir para a interpretação paleoambiental durante a formação dos espongilitos. O estudo ecológico de M. spinata, que produz quatro categorias de espículas ao longo do seu ciclo biológico (megascleras alfa e beta, microscleras e gemoscleras) determinou a época de eclosão das gêmulas, o desenvolvimento dos espécimes, a formação das gêmulas e a morte sazonal das esponjas. À partir de um dado momento do ciclo de vida, as esponjas amostradas apresentaram uma sequencia temporal de síntese de diferentes categorias de espículas, relacionadas com a precipitação de chuvas, concentração de silício, nível d’água da lagoa, temperatura d’água e balanço entre a entrada e saída de água no sistema calculada de forma indireta (precipitação – evapotranspiração). As diferentes funções de cada espícula colaboram com as paleointerpretações onde há sedimentos com diferentes composições espiculares. Os testes de análise isotópica de oxigênio nas espículas das esponjas apresentaram resultado inverso (+0,3‰/ºC) aos estabelecidos para diatomáceas e fitólitos (-0,2 a - 0,4‰/ºC). O resultado sugere um fracionamento cinético, de causa biológica. Entretanto, a relação entre a composição isotópica das espículas com a composição isotópica da água do momento da amostragem é diretamente proporcional e incita atenção (R2=0,8, p

Despite the suspected biological imprint on the terrestrial silicon (Si) cycle, plant contribution to the Si continental reservoir is poorly quantified. Within the soil-plant system, aqueous silicon (H4SiO40) can be retrieved from soil solution by plant uptake, clay formation and adsorption onto secondary oxides, or leached out and transferred to stream waters. Two approaches are very promising to trace Si within the soil-plant cycle: the Si stable isotopes and Ge/Si ratio. This thesis aims at quantifying the Si isotopic fractionation induced by plants and soil processes in both controlled (in vitro) and natural conditions (in situ). The model used is a tropical soil-plant system involving a Si-accumulating plant (banana, Musa acuminata Colla, cv Grande Naine) cropped on soils derived from basaltic ash but differing in weathering stage (Cameroon, West Africa). Si isotopic compositions in the different compartments of the soil-plant system were measured by MC-ICP-MS in dry plasma mode with external Mg doping. The analytical method required specific developments, and was validated by an inter-laboratory comparison of reference materials. Plant root uptake of Si, and Si transport within the plant induce an isotopic fractionation quantified in vitro and measured in situ. The plant Si isotopic signature is influenced by soil weathering stage, precisely by soil contents of clay and iron oxide. Soil clay-sized fractions sequestrate light Si isotopes following abiotic fractionating processes of mineral weathering and clay formation, leaving a residual solution enriched in heavy Si isotopes. Biogenic Si, enriched in light Si isotopes, constitutes a Si source for clay formation. Clay-sized Fe-oxides concentrate with increasing weathering and soil development. They selectively adsorb light Si isotopes by surface complexation of monomeric H4SiO40. Silicon transport within the plant produces a Ge/Si fractionation involving Ge sequestration in roots whereas weathering and clay formation induce a selective Ge sequestration in clay minerals, producing an increased Ge/Si ratio with increasing weathering, in excellent agreement with Si isotopic data. In the soil-plant system studied, Si stable isotopes thus trace three major continental processes: plant uptake and phytolith formation, sequestration of Si in clay minerals, and adsorption of Si by Fe-oxides. These biotic and abiotic processes all lead to a progressive enrichment of pore waters in heavy Si isotopes. (AGRO 3) -- UCL, 2008