Composition Of Rain Research Articles

What controls the isotopic composition of the African monsoon precipitation? Insights from event‐based precipitation collected during the 2006 AMMA field campaign

The stable isotopic composition of the tropical precipitation constitutes a useful tool for paleoclimate reconstructions and to better constrain the water cycle. To better understand what controls the isotopic composition of tropical precipitation, we analyze the δ18O and deuterium‐excess of the precipitation of individual events collected in the Niamey area (Niger) during the monsoon season, as part of the 2006 AMMA field campaign. During the monsoon onset, the abrupt increase of convective activity over the Sahel is associated with an abrupt change in the isotopic composition. Before the onset, when convective activity is scarce, the rain composition records the intensity and the organization of individual convective systems. After the onset, on the contrary, it records a regional‐scale intra‐seasonal variability over the Sahel, by integrating convective activity both spatially and temporally over the previous days.

Comparison of sources affecting chemical compositions of aerosol and rainwater at different locations in Turkey

In this study, concentrations of elements and ions were measured in atmospheric aerosol and rainwater samples collected at two rural stations on the Mediterranean and Black Sea coasts of Turkey. The relationship between chemical compositions of atmospheric aerosol and rainwater was investigated using receptor-oriented methods utilizing long-term data collected at the Antalya and Amasra stations in Turkey. Factor analysis performed on aerosol and rainwater data sets yielded that source types affecting chemical composition of rain and atmospheric particles were generally the same, but the contribution of each source type to rain and aerosol compositions showed differences. Potential source contribution function calculations were performed to determine if source regions affecting the chemical composition of rain and atmospheric particles were identical. The general pattern observed in distribution of source regions in rainwater and aerosol data sets showed some similarities at both stations, but there were substantial differences in detail. Source regions at the Antalya Station affecting the chemical composition of rainwater were local as compared to source regions affecting the composition of aerosol. The same difference was not observed in the Amasra Station. The source regions for crustal components of rainwater at the Antalya Station included western parts of Turkey, while European countries and North Africa were found to be the source regions affecting crustal components of aerosol at the Antalya Station. Potential source contribution function calculations indicated that major contributions to the anthropogenic components of aerosol at Antalya came mostly from the western parts of Turkey and European countries, while central Anatolia was the major source region contributing to anthropogenic components of rainwater at the Antalya Station. At the Amasra Station, the area north of the Black Sea and western parts of Turkey were the most important source regions affecting crustal and anthropogenic components for both aerosol and rainwater. It was observed that northern Europe, parts of Turkey, Ukraine, Russia, and some Balkan regions were the main source regions contributing to SO 4 2− concentrations both in rainwater and aerosol samples collected at both sampling sites.

Chemical composition of wet deposition in a Mediterranean site Athens, Greece related to the origin of air masses

The goal of this study is to determine the chemical composition of rain, in the wider region of Athens, Greece for the time period 1st September 2001 to 31st August 2002. Two model automatic rain samplers were installed in the Meteorological Station of Laboratory of Climatology (latitude: 37° 58′ N, longitude: 23° 47′ E) inside the Athens University Campus and in a site at Heraklio Attica, a northern suburb of Athens (latitude: 38° 03′ N, longitude: 23° 45′ E). The concentrations (μeq l−1) of the major cations (H+, Na+, K+, Ca2+ and Mg2+) and anions (Cl−, $$ {\text{NO}}^{ - }_{3} $$ , $$ {\text{HCO}}^{ - }_{3} $$ και $$ {\text{SO}}^{{2 - }}_{4} $$ ), as well as pH and conductivity of rain in 39 total samples were determined. The figures of pH range from 6.4 to 8.4 and conductivity from 8 to 207 μS cm−1. The analysis showed that Ca2+ ions are abundant within all examined samples, while $$ {\text{HCO}}^{ - }_{3} $$ and $$ {\text{SO}}^{{2 - }}_{4} $$ present the highest concentrations from the anions. In order to find out the origin of the air masses, the air mass back trajectories were calculated. Five sectors of the origin of air masses were revealed: the North, the South, the Local, the West and the East sector. Multivariate methods included Factor Analysis and Discriminant Analysis were applied to the examined ion concentrations and three main factors were extracted, which discriminated the ions according to their origin. The first group of ions is interpreted as the result of the anthropogenic activity, the second group represents the acidity–alkalinity independently of their source and the third one the marine influence.

Seed rain during initial colonization of abandoned pastures in the premontane wet forest zone of southern Costa Rica

Understanding tropical succession requires insight into propagule availability, which constrains possible woody plant recruitment, yet seed rain composition in multiple post-agricultural sites has seldom been examined. We monitored seed rain for 60 wk in five abandoned pastures in southern Costa Rica, collecting a total of 1 140 688 seeds of 165 morphospecies. Most seeds (80.1%) arrived during the wet season. Species richness was highest in the wet season and greater in forest than in pasture. Seed rain density was greatest at the forest/pasture edge and decreased drastically just a few metres into pastures. In and near the forest, animal-dispersed seeds were more abundant than seeds dispersed by other means, while wind-dispersed taxa increased in relative importance at greater distances from the forest. Total seed input to pastures did not reflect size of adjacent forest fragments, although seed rain density varied more than threefold among sites. Among-site variation in density of regenerating woody seedlings was roughly proportional to among-site variation in seed rain. Morphospecies composition differed significantly among sites. Also, seed rain and woody plant colonists were rather dissimilar in composition, suggesting that while propagule availability is necessary for early woody plant establishment, it is a poor predictor of successional trajectory.

Seed transfer among bird-dispersed trees and its consequences for post-dispersal seed fate

Summary We investigated seed transfer, i.e. the seed movement away from a source canopy to areas beneath heterospecific canopies, among the ornithochorous tree species Taxus baccata, Ilex aquifolium and Crataegus monogyna in temperate secondary forests in NW Spain, by analysing the composition of multispecific seed rain beneath the canopy of each species, at four sites and for 2 years. To evaluate the consequences on seed fate, we estimated predation by rodents in manipulated seed rains, representing variable levels of relative proportion and total density for combinations of a preferred species paired with a less-preferred species. Seed rain under Taxus canopies was dominated by Taxus seeds, which occurred in low proportion under heterospecific canopies. Ilex seeds dominated the areas under Ilex but accounted for 20–40% of seeds under heterospecific trees. Crataegus seeds were not dominant in any of the microhabitats. The probability of being deposited beneath a heterospecific canopy was much higher for Ilex and Crataegus than for Taxus. The effects of seed rain composition on post-dispersal seed predation were species-specific. Taxus seeds experienced lower predation when occurring in a background of seeds dominated by heterospecific, Ilex or Crataegus, seeds. Crataegus seeds escaped predation more successfully in high-density patches, independently of seed clump composition. Predation on Ilex seeds was independent to both the density and the composition of seed clump. Seed transfer among heterospecific tree species may contribute to shape the template of propagule abundances from which forest will develop, by generating seed combinations favourable to escape from predation.

Influence of meteorological factors and polluting environment on rain chemistry and wet deposition in a rural area near Chimay, Belgium

This paper focuses on a detailed analysis of the effects of meteorological factors explaining the variability of rain composition. Inorganic composition of 113 individual rain events was measured from May 2002 to October 2005 at a rural site near Chimay, in the western part of the Belgian Ardennes. Original models were fitted for each studied ion (H +, Mg 2+, Ca 2+, K +, NH 4 +, Na +, Cl −, NO 3 − and SO 4 2−) to relate rain event concentration or wet deposition to the rainfall volume ( R), the length of the antecedent dry period (ADP), the volume of the previous event ( R prev) as well as to the mean wind speed and the prevailing wind direction during both the dry and the rainy periods. These variables explained from 32% (H +) to 69% (NO 3 −) of rain concentration variability. Concentrations decreased logarithmically with increasing R values except in case of H + for which a positive effect of rain volume on rain concentration was observed. ADP affected positively rain concentrations of all ions excluding K + and H + for which, respectively, a nonsignificant and a negative effect of this variable was observed. Increasing R prev strengthened the effect of the variable R on H +, Mg 2+, Ca 2+, Na +, NH 4 + and SO 4 2− concentrations while it softened the effect of ADP on NO 3 − concentrations. Wind speed and direction during dry and rainy periods explained together from 8% (K +) to 38% (Na +) of rain concentration total variability. R 2 coefficients of the wet deposition models ranged from 0.51 (K +) to 0.79 (SO 4 2−). For all ions, wet deposition increased significantly with increasing R values while the effects of the other variables were similar to those on concentrations. Wind conditions during dry and rainy periods explained from 4% (H +) to 24% (Na +) of wet deposition total variability. On an annual scale, the total dry period duration, the total rainfall volume as well as the shape of the distributions of the length of the antecedent dry periods and of the rain event volume are important parameters that influence annual wet deposition.

Late Holocene development of a mangrove ecosystem in southeastern Brazil (Itanhaém, state of São Paulo)

Most palynological studies of mangroves have been carried out in the Indo-Pacific region, but few have investigated these ecosystems along the southern Atlantic coast. This paper provides information on the palynology of a mangrove at Itanhaém, state of São Paulo, on the southeastern Brazilian coast. This mangrove occurs on microtidal flats adjacent to a fluvial-tidal channel mouth (Itanhaém River), part of an estuarine lagoon system established in the mid-Holocene. Pollen samples were collected from the surface and from bromeliad water in order to define the composition of modern pollen rain in the study area. The results show a strong presence of rain forest taxa in the mangrove pollen spectra, which differ from those obtained from northern Brazilian mangroves, which are characterized by higher percentages of the mangrove taxa Rhizophora and Avicennia. Based on palynological and 14C chronological analysis of a shallow core measuring 135 cm in depth, the Itanhaém mangrove already existed by 1300 years BP and began expanding about 1000 years BP, reaching its present extent sometime after 300 years BP. Its evolution is related to the sedimentary filling of the estuarine–lagoonal depositional system. Two factors may have controlled the development of this mangrove: the relative increase of intertidal areas due to this sedimentary filling and the decrease in wave action and increase in pelitic deposition in the intertidal zone related to the partial closing of the estuarine–lagoonal inlets. These processes were favored by the advance of bayhead deltas and by the development of sand spits as a part of the regression of the Holocene strandplain.

On the potential of 230Th, 231Pa, and 10Be for marine rain ratio determinations: A modeling study

The global distributions of the radionuclides 230Th, 231Pa, and 10Be are simulated with a biogeochemical ocean general circulation model. Sensitivity experiments for changes in the composition of the particle composition ratio (POC:CaCO3:BSi:clay, POC = particulate organic carbon, CaCO3 = calcium carbonate, BSi = biogenic silica) are carried out with and without biogeochemical feedback. The absolute water column concentrations of the radionuclides react significantly to changes in the particle rain composition. The radionuclide ratio in the sediment, however, is less sensitive to changes in the particle composition ratio. Still, selected areas could record composition ratio changes reliably for paleo reconstructions. Measurements of 230Th, 231Pa, and 10Be in the water column have the potential to monitor changes in the large scale CaCO3 production which may occur as a consequence of ocean acidification due to oceanic uptake of anthropogenic CO2.

Shrub invasion into subalpine vegetation: implications for restoration of the native ecosystem

The ability of plant communities to recover after non-native species invasion will depend upon the nature of their soil seed bank and seed rain characteristics. This study assessed changes in the soil seed bank and seed rain associated with the invasion of the non-native shrub Cytisus scoparius in subalpine vegetation. Soil seed bank and seed rain composition, density and richness were investigated at three areas of different stages of invasion: (i) recent (8–10 years), (ii) mature (15–16 years) and (iii) long-term (25 years). There were few changes in seed bank composition or richness regardless of invasion stage. By contrast, the seed rain composition, richness and density was substantially different within long-invaded areas. Very few seeds were able to colonise the dense barrier characteristic of larger, more mature C. scoparius stands. Some prominent herbs from the native vegetation were under-represented or absent from the seed bank, both in invaded and uninvaded areas. Laboratory germination experiments demonstrated that most native species germinate easily, which may imply a transient seed bank, rather than a persistent one. The majority of herbaceous and shrub species were capable of resprouting vegetatively. Therefore, regeneration appeared more reliant on the bud and tuber bank than a persistent soil seed bank. The dominance of graminoid species and C. scoparius rather than other herbaceous, shrub or tree species suggests that the regenerating vegetation will be dominated by grass species and/or C. scoparius. Hence, in areas where long-invaded C.␣scoparius stands are present the recovery of native subalpine vegetation maybe difficult. Recovery may only be possible through wind dispersal from the surrounding intact vegetation or through actively reseeding the area. This study highlights the importance of early intervention in invasive species management.

Biogeochemical cycling of nutrients in karstic catchments, southwestern China: Linkages to changes of eco-environments

In southwestern China, karst rocky desertification, a process of land degradation, takes place widely due to human impacts on the fragile karstic ecological systems, which includes soil erosion, extensive exposure of basement rocks, and drastic decrease in soil productivity. A study on biogeochemical cycling of nutrients in the karstic catchments has been carried out to understand the relationships between degradation of ecological system and nutrient cycling. Our studies on chemical and isotopic compositions ( 6 13C, 6 34S, 5 15N, 87Sr/S6Sr) of river, lake, rain, and ground water in the karstic catchments of southwestern China provided important information on the sources of major ions to them, chemical weathering rates of river basins, fluxes of dissolved materials from the drainage basins, and on their linkages to changes of ecological systems and environments. The weathering rates of carbonate rocks and the fluxes of nutrients from the river basins are high as compared with karstic terrains in the world. Comparative studies on the carbon isotopic composition of dissolved inorganic carbon in surface water of karstic and none-karstic catchments indicate that more dissolved inorganic carbon in the karstic rivers originated from respiration of soil organic carbon. The interrelationships between isotopic composition and chemical composition of the surface water suggest that the high weathering rates of carbonate rocks and large fluxes of carbon, sulfur, and nitrogen from the karstic river basins may be ascribed to the roles of sulfur and nitrogen cycling in the catchments. Weathering of carbonate rocks by sulfuric and nitric acids can give rise to a net source of CO2 to atmosphere through precipitation of carbonate minerals, which will change our view that weathering of carbonate rocks can neither subtract nor produce greenhouse gas in atmosphere from a long-term point of view. The sulfur and nitrogen isotopic compositions respectively of sulfate and nitrate ions in rain and ground and surface waters indicate important anthropogenic sources of these ions, in addition to natural origin. It is concluded that human activities can disturb the natural cycling of nutrients, and result in changes in ecological environments.

Rapid late Miocene rise of the Bolivian Altiplano: Evidence for removal of mantle lithosphere

Abstract The oxygen isotopic compositions of carbonates deposited in the northern Altiplano demonstrate a rapid change in late Miocene time, which we attribute to an increase in elevation. Rainfall samples over an elevation transect adjacent to the Altiplano of northern Bolivia display a systematic decrease in mean annual δ 18 O values and define a local δ 18 O vs. altitude gradient [R. Gonfiantini, M.-A. Roche, J.-C. Olivry, J.-C. Fontes, G.M. Zuppi, The altitude effect on the isotopic composition of tropical rains, Chem. Geol. 181 (2001) 147-167.], which we apply to paleo-meteoric water values determined from carbonates. Age constraints from magnetostragraphy and paleoelevation estimates suggest surface uplift of ∼2.5 to 3.5 km occurred between ∼10.3 Ma and 6.8 ± 0.4 Ma, when the Altiplano rose to its current elevation. Only the removal of dense eclogitic lower crust and mantle lithosphere can generate a change of this magnitude and rapidity. This change coincides with an ∼30% decrease in the rate of convergence between the South America and Nazca plates and the propagation of deformation into the eastern Subandean zone, which we infer to have resulted from the increase in the force per unit length applied by the elevated Andean plateau to the surrounding lowlands.

Recharge processes in karstic systems investigated through the correlation of chemical and isotopic composition of rain and spring-waters

Four karstic springs in southern France, along with rainwater in the same area, were monitored during two hydrological cycles. Stable isotopic ratios (δ 18O and δD) allow the contribution of the rain waters from the previous periods (discretised as winter or summer period) to the spring water to be estimated. These computations indicate that heavy rainfall events during the autumn season contribute for 56 ± 7% and 68 ± 9% of the spring water discharge during the following winter and summer seasons, respectively. During the low flow period, residence time does not exceed 1 hydrological cycle, for a large part of the water. Stable isotopic ratios (δ 18O and δD) combined with Cl concentration allow the evapotranspiration coefficients to be estimated although the recharge surface is not known and hydrological balance can not be computed. Except for one spring, the evapotranspiration coefficients range from 0.54 to 0.38 (46–62% volume reduction), even during the high flow period. The short residence times, and the strong evapotranspiration coefficients whatever the period (winter or summer) are interpreted as the result of the major role of the epikarst reservoir in the karst recharge functioning over direct or diffuse infiltration.

Predictive models of copper runoff from external structures.

A general model for annual runoff rate predictions of total copper from naturally patinated copper on buildings at specific urban or rural sites of low chloride influence has been deduced from laboratory and field data. All parameters within the model have a physical meaning and include the average annual rain acidity (pH), the annual rain quantity and the geometry of a building in terms of surface inclination. In 70% of all reported annual runoff rates, the predicted values are within 30% from the observed values. The individual and interactive effect of rain composition in terms of pH, sulfate, chloride and nitrate concentration was investigated in immersion experiments in artificial rain water representative of urban and rural sites of Europe. The results show pH to have a dominating effect on patina dissolution, nitrate to have a small inhibiting effect, whereas no significant effect was seen for chloride and sulfate. In case pH data are not available, a model has been statistically deduced from field data by considering SO2 as influencing parameter, rather than pH. The predictability with the SO2 model is not as good as with the pH model i.e. the pH model should preferentially be used since it is a better predictor and all parameters within the model can be physically explained.

Isotopic study of lead transfer at the interface soil‐plants‐atmosphere

Soil, rock, plant and rainwater samples have been collected in the evergreen forest of Cameroon and analyzed for their 206Pb/207Pb and 208Pb/207Pb ratios and Pb concentrations. Plant isotopic ratios exhibit the same values than the rains collected at the same site and higher values than the ones observed for surface soils (litter and 20 cm deep). The isotopic composition of rains and plants is in agreement with a mixing model between an anthropogenic end‐member and a Saharan particle end‐member. This indicates that the enrichment of lead in rainwaters is anthropogenic in origin and that the presence of lead in plants is essentially due to the assimilation of atmospheric lead directly from their leaves and/or from their bark.

Chemical Composition and Acidity of Rain at the Gulf of Iskenderun, North-East Mediterranean

The purpose of the present work was to investigate the chemistry of rain in the Gulf of Iskenderun, North East Mediterranean, Turkey. The Gulf region has a large number of industries. Main industries existing in the region are iron and steel works, fertilizer plants, a cement plant, and several medium and mini size steel mills in addition to the international pipe line terminals. This study aims to apportion the local sources contributing to the overall pollution of the region. To this end a precipitation sampling program was started in January 2000, and over 48 precipitation samples were collected from each of 5 stations located at Iskenderun city center, Iskenderun industrial zone, Payas city center, the campus of Iskenderun Technical College and the campus of Mustafa Kemal University. Samples were analysed for pH, NO3+, Cl-. Ca, Al, Ba, Na, Cd, Co, Cr, Cu, Pb, Li, Mg, Mn, Ni, Zn, Fe, K. Concentrations of metal ions were determined by ICP-AES. NO3- ions and pH were determined by using NO3- selective electrode and pH meter, respectively.pH values of the collected samples at the industrial zone and at Payas city center, ranged between 5.02 and 7.38, respectively. NO3- and metal ions concentrations were highly variable. Concentrations of Ca and Fe ions were higher in the industrial zone and Payas city center. In the other three stations, concentrations of metal ions and NO3- ion were lower than that of industrial zone and the values of pH ranged between 6 and 7.4. The average pH values at Iskenderun Gulf showed that the precipitation was not acidic, because of the high concentration of Ca. The highest concentrations of Na and Cl ions were recorded in the University campus because the campus is located by the Mediterranean Sea.

Effect of anion composition of simulated acid rain on nutrient behavior in reclaimed saline soils

The purpose of this study is to demonstrate the relative influence of the anion composition of simulated acid rain on the behavior of nutrients in three reclaimed saline soils. Reclaimed saline soil samples used for this study were collected from surface soils of the Saemangeum and Koheung soils in Korea and the Hachirogata soil in Japan. The Hachirogata soil behaved as a high variable charge soil which was derived from sediments and reclaimed from the deposits of the lake Hachirogata, while the Saemangeum and Koheung soils behaved as permanent charge soils with the cation exchange capacity (CEC) being largely independent of the pH. The simulated acid rain at pH 3.0 was prepared with nitrate-to-sulfate mol ratios of 0.75 : 0.25, 0.5 : 0.5, 0.25 : 0.75, and control (pH 5.7). The influence of the anion composition of simulated acid rain on the changes in the contents of nutrients in reclaimed saline soils was as follows: The soil organic matter content decreased from 36.8 to 24.1 g kg−1 for the Saemangeum soil, from 29.0 to 19.8 g kg−1 for the Koheung soil, and from 137.7 to 85.6 g kg−1 for the Hachirogata soil, respectively: The CEC and point of zero charge (PZC) values in the Saemangeum and Koheung soils were not significantly different, whereas in the Hachirogata soil the CEC decreased by about 15% and the PZC decreased by 1.1 unit. The content of ammonia nitrogen decreased by about 20 to 30%. The content of nitrate nitrogen increased by about 10% for the Hachirogata soil but there was no appreciable difference in the case of the Saemangeum and Koheung soils. The contents of Al-P and Ca-P were not significantly different from the original values in the untreated soil, whereas that of Fe-P decreased by about 30%. The content of inositol phosphate decreased by about 35% in the Hachirogata soil but did not change appreciably in the case of the Saemangeum and Kohueng soils. Different mechanisms were involved in the behavior of anions and cations between the soils with predominant aluminosilicates and the soil rich in hydrous oxides of iron and aluminum. Also, the contents of most of the nutrients changed considerably with the increase of the nitrate content of the simulated acid rain.

Analysis of rain quality data from the South African interior

AbstractRain acidity may be ascribed to emissions from power station stacks, as well as emissions from other industry, biomass burning, maritime influences, agricultural influences, etc. Rain quality data are available for 30 sites in the South African interior, some from as early as 1985 for up to 14 rainfall seasons, while others only have relatively short records. The article examines trends over time in the raw and volume weighted concentrations of the parameters measured, separately for each of the sites for which sufficient data are available. The main thrust, however, is to examine the inter‐relationship structure between the concentrations within each rain event (unweighted data), separately for each site, and to examine whether these inter‐relationships have changed over time. The rain events at individual sites can be characterized by approximately eight combinations of rainfall parameters (or rain composition signatures), and these are common to all sites. Some sites will have more events from one signature than another, but there appear to be no signatures unique to a single site. Analysis via factor and cluster analysis, with a correspondence analysis of the results, also aid interpretation of the patterns. This spatio‐temporal analysis, performed by pooling all rain event data, irrespective of site or time period, results in nine combinations of rainfall parameters being sufficient to characterize the rain events. The sites and rainfall seasons show patterns in these combinations of parameters, with some combinations appearing more frequently during certain rainfall seasons. In particular, the presence of the combination of low acetate and formate with high magnesium appears to be increasing in the later rainfall seasons, as does this combination together with calcium, sodium, chloride, potassium and fluoride. As expected, sites close together exhibit similar signatures. Copyright © 2002 John Wiley & Sons, Ltd.

Stable carbon and oxygen isotopic variations in modern Rabdotus land snail shells in the southern Great Plains, USA, and their relation to environment

Variations of stable isotopic ratios of carbon (13C/12C) and oxygen (18O/16O) were investigated in modern shells of two species of Rabdotus land snails (R. dealbatus and R. alternatus) in the southern Great Plains. Geographic variation in relation to climate and vegetation, microgeographic variation, variability among individuals, and detailed records of seasonal variations within individual shells were studied. Stable carbon isotopic ratios in shell carbonate are primarily a function of the isotopic composition of the diet of the snails, as represented by the isotopic composition of shell organic matter. This in turn reflects the presence or absence of CAM (Crassulacean Acid Metabolism) or C4 plants. Vegetation density may have a small effect on the carbon isotope ratios. Microgeographic variation (samples within 25 to 300 m) is greater than that seen across different climatic regions and points to very local control of isotopic variations, predominantly related to vegetation. Seasonal variations, as assessed through serial analysis of individual shells (up to 35 samples per shell), may provide a means for distinguishing between isotopic influences of perennial CAM vs. annual C4 plants. Carbon isotopic variations in time-series of shells from a site provide a means of reconstructing temporal changes in environment and climate.Oxygen isotopic values of shell carbonate are uniform across the region and also show no significant microgeographic variation. The oxygen isotopic composition appears to be mainly a function of the rainwater isotopic composition, with no direct influence of rainfall amount or evaporative effects. The δ18O values are only 2‰ enriched relative to estimated equilibrium with rainwater. Variability is low (SD of 0.8‰ among sites), so the isotopic composition of fossil Rabdotus shells can provide a precise record of changes in the isotopic composition of rain over time.

Major ion chemistry of groundwaters in the Continental Terminal water table of southwestern Niger (Africa)

Numerous chemical analyses undertaken during the last 4 a in the Niamey area of the Continental Terminal groundwaters (SW Niger, Africa) reveal large seasonal variations and high NO 3 content (up to 2.87 meq/l). The positive correlation between conductivity and depth to the water table indicates that the chemical change is associated with the recharge. In the shallow wells, this highly mineralised recharge reflects the rain composition, variably concentrated by evaporation processes.

Modelling the continental effect of oxygen isotopes over Eurasia

δ18O in CO2 attracts attention for its ability to decipher the gross fluxes of terrestrial CO2 exchange. This capacity can only be exploited if one knows the impact of the biology and the water isotope cycle on the isotopic composition of atmospheric CO2. To study the processes, we built an integrated global model of δ18O in atmospheric CO2 that calculates 18O in the water cycle pools, the CO2 and the inherent δ18O-CO2 fluxes and transports these in the atmosphere. Within the framework of the European Project EUROSIBERIAN CARBONFLUX, we investigated the 18O processes on the continent of Eurasia north of 40 °N. We show that there is a large impoverishment in the water isotopic composition of rain in the continental interior but this is significantly reduced during the growing season. We validated the model directly by comparing it to Net Ecosystem Exchange (NEE) measurements from eddy-flux towers and indirectly by comparing it to atmospheric measurements of CO2 and δ18O-CO2 at 3000 m a.s.l. The model reproduces well the seasonal cycle of CO2 and the phase for δ18O-CO2 but it underestimates the amplitude of that species. Our model further predicts a large diurnal rectifier effect that is different in CO2 than in δ18O-CO2 in eastern Siberia due to very low or even negative leaf discrimination. We analyse the longitudinal gradient in leaf discrimination which reflects primarily the gradient in leaf water isotopic composition. The leaf water δ18O-H2O gradient is itself determined 25% by the meridional gradient in source water and 75% by the change of relative humidity with longitude.