Jean Riotte

U-Th-Ra Fractionation During Weathering and River Transport

The potential of radioactive disequilibria as tracers and chronometers of weathering processes has been recognised since the 1960’s (e.g., Rosholt et al. 1966; Hansen and Stout 1968). This interest results from the dual property of the nuclides of the U and Th radioactive series (1) to be fractionated during water-rock interactions and (2) to have radioactive periods of the same order of magnitude as the time constants of many weathering processes and chemical transfers to ground and river waters. Therefore, the study of radioactive disequilibria in surface environments should help to bring information about the nature, the intensity but also the time-scale of the water-rock interactions produced during weathering and related chemical transfers. These different properties have justified many of the studies on U-Th series in weathering profiles and river waters. Rosholt (1982), Scott (1982) and Osmond and Ivanovich (1992) gave a synthesis of the studies of U-series in weathering and surface hydrology up to the 1980’s and the 1990’s respectively. Here, we present the main directions taken in these domains over the last decade. They were partly stimulated by the analytical developments made in the measurement of the medium half-life nuclides of 238U series (i.e., 234U-230Th-226Ra) in the mid-1980’s, namely, the use of thermal ionisation mass spectrometry (TIMS) (e.g., Chen et al. 1986; Edwards et al. 1987; Cohen et al. 1991; Chabaux 1993;Chabaux et al. 1994) and more recently the use of MC ICP-MS (Turner et al. 2001; Robinson et al. 2002) for U and Th isotope analysis. Details of these developments are given in this volume (Goldstein and Stirling 2003). Compared to the radioactive counting methods previously used, the new techniques permit (1) a reduction by one order of magnitude or more of the size of the sample required for …

(234U/238U) activity ratios in freshwaters as tracers of hydrological processes: the Strengbach watershed (Vosges, France)

Abstract (234U/238U) and 87Sr/86Sr isotopic ratios, as well as major and trace (U, Ba, Sr, Rb) element concentrations were analyzed in dissolved loads of the Strengbach stream (Vosges, France) in order to constrain the sources of U isotopes (234U–238U) to river waters. The variations of the (234U/238U) activity ratios along the stream indicate a clear dependence between the (234U/238U) activity ratios of the surface waters and the different types of rocks forming the watershed, with near equilibrium values for the waters draining granites and high (234U/238U) disequilibria (1.4) for those flowing over carbonates. The high (234U/238U) disequilibria are related to a supply of 234U-enriched groundwaters located within the carbonate rocks. The (234U/238U) activity ratios of the waters at the outlet of the catchment collecting the sources of the stream on the granitic lithology, decrease from 1.02 to 0.96 when the discharge of the stream increases. Such a tendency requires mixing between a water body enriched in 234U which weathered the granitic bed rock at secular equilibrium, and a water with a (234U/238U) activity ratio below unity representing a mobilization of U from material that has already been weathered. Comparison of the geochemical characteristics of waters collected during the year and those collected during a flood event, reveals the involvement of two different “weathered” end-members, depending on the hydrological conditions: during the year, the dissolved U transported by the river originates from bed-rock and deep horizons of the weathering profile, whereas a significant part of U, during the flood event, is supplied by superficial horizons of soils, probably complexed by organic colloids. These results outline the potential of (234U/238U) activity ratios to investigate hydrological processes and emphasize that their use as tracers of rock weathering should be made cautiously.

Isotopic tracing of the dissolved U fluxes of Himalayan rivers: implications for present and past U budgets of the Ganges-Brahmaputra system

U activity ratios have been measured in the dissolved loads of selected rivers from the Himalayan range, in Central Nepal, and from the Bangladesh, as well as in some rain waters. A few European and Asian rivers have also been analyzed for their U activity ratios. The data confirm the negligible effect of rainwater on the budget of dissolved U in river waters. The results also indicate that rivers on each Himalayan structural unit have homogeneous and specific U isotope compositions: i) (234U/238U) activity ratios slightly lower than unity in the dissolved load of the streams draining the Tethyan Sedimentary Series (TSS); ii) values slightly higher than unity for waters from the High Himalaya Crystalline (HHC) and the Lesser Himalaya (LH); iii) systematically higher (234U/238U) activity ratios for waters from the Siwaliks. Thus, U activity ratios, in association with Sr isotopic ratios, can be used to trace the sources of dissolved fluxes carried by these rivers. Coupling of U with Sr isotope data shows (1) that the U carried by the dissolved load of the Himalayan rivers mainly originates from U-rich lithologies of the TSS in the northern formations of the Tibetan plateau; and (2) that the elemental U and Sr fluxes carried by the Himalayan rivers at the outflow of the highlands are fairly homogeneous at the scale of the Himalayan chain. Rivers flowing on the Indian plain define a different trend from that of the Himalayan rivers in the U-Sr isotopic diagram, indicating the contribution of a specific floodplain component to the U and Sr budgets of the Ganges and the Brahmaputra. The influence of this component remains limited to 10 to 15 percent for the U flux, but can contribute 35 to 55% of the Sr flux. The variations of the Sr and U fluxes of the Ganges-Brahmaputra river system in response to climatic variations have been estimated by assuming a temporary cut off of the chemical fluxes from high-altitude terrains during glacial episodes. This scenario would significantly decrease the dissolved U flux of the Ganges-Brahmaputra river system and increase its U activity ratio. Such a climatic dependence of the Himalayan U flux could induce a periodic variation of the mean U activity ratio of the world rivers on glacial-interglacial timescales

Chemical weathering of basaltic lava flows undergoing extreme climatic conditions: the water geochemistry record

This study was dedicated to the early stage of the weathering of historic basaltic flows located in Mount Cameroon. The combination of high relief (i.e. 0 to 4071 m) and high rainfall range (i.e. 1.8 to 12 m/year) lead to strong climatic contrast. Spring and rivers were sampled all around the volcano. We report here the basic chemistry of the waters as well as strontium and uranium isotopic ratios. The combination of the molar proportions of solute obtained with the modal amounts of the minerals in the basalts gives a prediction of what should be the relative molar concentrations of major compounds in the weathering waters issuing from Mount Cameroon. The measured Alkalinity/Si and Mg/Si ratios are higher than the calculated ones while the measured Ca/Si ratio is equal to the calculated value. We suggest that the Si-poor waters of Mount Cameroon are due to biological pumping, trapping of Si in Fe-silicate minerals such as Si containing ferrihydrite and Si interaction with bacterial cell wall leading to the formation of allophane type minerals which were observed in Mount Cameroon soil profiles. Calcium uptake by plants explains the lower Ca/alkalinity ratios measured in the water samples. The water–rock ratio (R) calculated from the strontium isotopic compositions of the water samples, ranges from 29,452 to 367,450. The calculated weathering rates (WR) range from 1 to 20 mm/ky and from 1 to 103 mm/ky for high and low elevations, respectively, and agree with both the thickness and the age of paleosoils found in the same area and with previously published estimates from coupled reaction-transport models. This difference emphasizes the role of vegetation and rainfall at lower elevations as compared to what happens at high elevations.

Multimetallic contamination from Zn-ore smelter: solid speciation and potential mobility in riverine floodbank soils of the upper Lot River (SW France)

The former Zn-ore smelting activity in Viviez (Aveyron, France) resulted in multimetallic contamination of the upper Lot River system (SW France). This study addresses for the first time the metals/metalloids mobility in impacted riverside fluvial soils due to reducing conditions during long-term flooding events. Six impacted riverside fluvio-soils were sampled along the Riou Mort and Lot rivers. Their levels of contamination decrease with the distance from the contamination source. Higher enrichment factors (EF) relative to French average soil metallic content occur 1 km far from the contamination source and vary from 3 for Sn, to 5 for As, 9 for Sb, 27 for Pb, 40 for Zn and 63 for Cd. At 20 km downstream from the contamination source, EF still reach a value of 3 for Cd and Zn. A micro-physical mineralogical characterisation of contaminated soils revealed a variety of metal-bearing phases: metallic compounds, oxides and sulphides, Fe oxides, glass and silicates as well as coke. Sequential chemical extraction experiments were conducted on the most contaminated soil in order to identify which mineral phases were the best candidates to release metals/metalloids. At the end of experiments more than 70 % of total Cd and As, 55 % of Zn and Sb, and 40 % of Pb and Sn were extracted from the soil. Only Cd appeared as readily mobilised, with 33 % of its total amount being extracted in the first steps of experiments. Under reducing conditions, iron oxyhydroxides, franklinite and multimetallic oxides are the best candidates, in decreasing order of importance, for metals/metalloids release from the soil.

Geochemical behaviour of dissolved trace elements in a monsoon-dominated tropical river basin, Southwestern India

The study presents a 3-year time series data on dissolved trace elements and rare earth elements (REEs) in a monsoon-dominated river basin, the Nethravati River in tropical Southwestern India. The river basin lies on the metamorphic transition boundary which separates the Peninsular Gneiss and Southern Granulitic province belonging to Archean and Tertiary–Quaternary period (Western Dharwar Craton). The basin lithology is mainly composed of granite gneiss, charnockite and metasediment. This study highlights the importance of time series data for better estimation of metal fluxes and to understand the geochemical behaviour of metals in a river basin. The dissolved trace elements show seasonality in the river water metal concentrations forming two distinct groups of metals. First group is composed of heavy metals and minor elements that show higher concentrations during dry season and lesser concentrations during the monsoon season. Second group is composed of metals belonging to lanthanides and actinides with higher concentration in the monsoon and lower concentrations during the dry season. Although the metal concentration of both the groups appears to be controlled by the discharge, there are important biogeochemical processes affecting their concentration. This includes redox reactions (for Fe, Mn, As, Mo, Ba and Ce) and pH-mediated adsorption/desorption reactions (for Ni, Co, Cr, Cu and REEs). The abundance of Fe and Mn oxyhydroxides as a result of redox processes could be driving the geochemical redistribution of metals in the river water. There is a Ce anomaly (Ce/Ce*) at different time periods, both negative and positive, in case of dissolved phase, whereas there is positive anomaly in the particulate and bed sediments. The Ce anomaly correlates with the variations in the dissolved oxygen indicating the redistribution of Ce between particulate and dissolved phase under acidic to neutral pH and lower concentrations of dissolved organic carbon. Unlike other tropical and major world rivers, the effect of organic complexation on metal variability is negligible in the Nethravati River water.

Multielementary (Cd, Cu, Pb, Zn, Ni) Stable Isotopic Exchange Kinetic (SIEK) Method To Characterize Polymetallic Contaminations

A new method is proposed to precisely and simultaneously quantify the exchangeable pool of metals in soils and to describe its reactivity at short- and long-term. It is based on multielementary Stable Isotopic Exchange Kinetics (multi-SIEK), first validated by a comparison between two monoelementary radioactive ((109)Cd*, (65)Zn*) IEK experiments, a mono- ((106)Cd) and multi- ((62)Ni, (65)Cu, (67)Zn, (106)Cd, (204)Pb) SIEK. These experiments were performed on a polluted soil located near the Zn smelter plant of Viviez (Lot watershed, France). The IEK results obtained for Cd and Zn were consistent across the experiments. (109)Cd*, (65)Zn* IEK, and multi-SIEK were then applied on 3 non- and moderate impacted soils that also provided consistent results for Cd and Zn. Within these experimental conditions, it can be concluded that no competition occurs between Cd, Zn, and the other metals during SIEK. Multi-SIEK results indicate that the isotopically exchangeable pool of Ni, Zn, and Cu are small (E(Ni), E(Zn), and E(Cu) values up to 17%) whatever the pollution degree of the soils considered in this study and whatever the duration of the interaction. On the contrary, Cd displays the highest E values (from 35% to 61% after 1 week), and E(Pb) displays a maximum value of 26% after 1 week. The multi-SIEK provides useful information on metal sources and reactivity relationship. Ni would be located in stable pedogenic phases according to its very low enrichment factor. The low E(Zn) and E(Cu) are consistent with location of Zn and Cu in stable phases coming from tailings erosion. Though Pb enrichments in soils may also be attributed to tailings particles, its larger exchangeable pool suggests that the Pb-bearing phases are more labile than those containing Zn and Cu. The high mobility of Cd in upstream soils indicates that it has been mostly emitted as reactive atmospheric particles during high temperature ore-treatment.

Controls of DSi in streams and reservoirs along the Kaveri River, South India

There is an increasing body of evidence showing that land use may affect the concentration and flux of dissolved silica (DSi) and amorphous, biogenic Si particles (ASi/BSi) in surface waters. Here, we present a study of riverine waters collected within the Kaveri River Basin, which has a long history of land occupation with +43% population increase in the watershed during the last 30 years associated with agricultural practices including canal irrigation from reservoirs and, more recently, bore well pumping. We report total dissolved solids (TDS) and suspended material (TSM) for 15 river stations and 5 reservoirs along the Kaveri itself and its main tributaries sampled during pre-monsoon, monsoon and post-monsoon periods in 2006 and 2007. The TDS in the Kaveri River globally increases from the upper reaches (humid to sub-humid climate) to the lower reaches (semi-arid climate), and at a given station from monsoon (M) to hot season (HS). The DSi concentrations range from 129 μmol L(-1) (M) to 390 μmol L(-1) (HS) in the main Kaveri stream and reaches up to 686 μmol L(-1) in the Shimsha River (HS). Our results indicate that DSi and the main solutes of the Kaveri River have not drastically changed since the last 30 years despite the population increase. The pollution index of Van der Weijden and Pacheco (2006) ranges from 13% to 54% but DSi does not seem to be affected by domestic wastewater. ASi is mostly composed of diatoms and phytoliths that both play roles in controlling DSi. We suggest that DSi and ASi delivered to the cultivated areas through irrigation from reservoir may have two important consequences: increasing Si bioavailability for crops and limiting Si flux to the ocean.

Controls of Soil Spatial Variability in a Dry Tropical Forest

We examined the roles of lithology, topography, vegetation and fire in generating local-scale (<1 km2) soil spatial variability in a seasonally dry tropical forest (SDTF) in southern India. For this, we mapped soil (available nutrients, Al, total C, pH, moisture and texture in the top 10cm), rock outcrops, topography, all native woody plants ≥1 cm diameter at breast height (DBH), and spatial variation in fire frequency (times burnt during the 17 years preceding soil sampling) in a permanent 50-ha plot. Unlike classic catenas, lower elevation soils had lesser moisture, plant-available Ca, Cu, Mn, Mg, Zn, B, clay and total C. The distribution of plant-available Ca, Cu, Mn and Mg appeared to largely be determined by the whole-rock chemical composition differences between amphibolites and hornblende-biotite gneisses. Amphibolites were associated with summit positions, while gneisses dominated lower elevations, an observation that concurs with other studies in the region which suggest that hillslope-scale topography has been shaped by differential weathering of lithologies. Neither NO3−-N nor NH4+-N was explained by the basal area of trees belonging to Fabaceae, a family associated with N-fixing species, and no long-term effects of fire on soil parameters were detected. Local-scale lithological variation is an important first-order control over soil variability at the hillslope scale in this SDTF, by both direct influence on nutrient stocks and indirect influence via control of local relief.

The roots of the drought: Hydrology and water uptake strategies mediate forest‐wide demographic response to precipitation

1. Drought-induced tree mortality is expected to increase globally due to climate change, with profound implications for forest composition, function and global climate feedbacks. How drought is experienced by different species is thought to depend fundamentally on where they access water vertically below-ground, but this remains untracked so far due to the difficulty of measuring water availability at depths at which plants access water (few to several tens of metres), the broad temporal scales at which droughts at those depths unfold (seasonal to decadal), and the difficulty in linking these patterns to forest-wide species-specific demographic responses. 2. We address this problem through a new eco-hydrological framework: we used a hydrological model to estimate below-ground water availability by depth over a period of two decades that included a multi-year drought. Given this water availability scenario and 20year long-records of species-specific growth patterns, we inversely estimated the relative depths at which 12 common species in the forest accessed water via a model of water stress. Finally, we tested whether our estimates of species relative uptake depths predicted mortality in the multi-year drought. 3. The hydrological model revealed clear below-ground niches as precipitation was decoupled from water availability by depth at multi-annual scale. Species partitioned the hydrological niche by diverging in their uptake depths and so in the same forest stand, different species experienced very different drought patterns, resulting in clear differences in species-specific growth. Finally, species relative water uptake depths predicted species mortality patterns after the multi-year drought. Species that our method ranked as relying on deeper water were the ones that had suffered from greater mortality, as the zone from which they access water took longer to recharge after depletion. 4. Synthesis. This research changes our understanding of how hydrological niches operate for trees, with a trade-off between realized growth potential and survival under drought with decadal scale return time. The eco-hydrological framework highlights the importance of species-specific below-ground strategies in predicting forest response to drought. Applying this framework more broadly may help us better understand species coexistence in diverse forest communities and improve mechanistic predictions of forests productivity and compositional change under future climate.