Marc Steinmann

Nd isotope evidence for the evolution of the paleocurrents in the Atlantic and Tethys Oceans during the past 180 Ma

The Nd isotopic composition of Atlantic and Tethys seawater, as deduced from marine phosphorites, varied considerably during the past 180 Ma. The early Tethys and Central Atlantic seawater from 180 to 160 Ma ago (Early-Middle Jurassic) had a Nd isotopic signature identical to that of the Pacific (eNd ≈ −6) suggesting that Pacific seawater entered the newly forming Tethys basin. However, with time continental runoff draining into the young basin became more important and led to a decrease in the Nd isotopic composition and, finally, a decoupling from the Pacific Ocean. During the late Early Cretaceous (120-90 Ma ago) Atlantic and Tethys seawater reached continental crust-like Nd isotopic composition values (eNd ≈ −10) which could reflect high weathering rates induced by the warm and humid climate at that time. In the time span between 80 and 50 Ma (Late Cretaceous-Early Tertiary) the Nd isotopic composition of both Tethys and Atlantic seawater strongly increased towards Pacific seawater values. In the case of the Atlantic Ocean, this change has been correlated with the opening of the South Atlantic, which enabled the more radiogenic Pacific seawater to travel westwards around the southern edge of Africa into the South and finally the North Atlantic. In the case of Tethys seawater we have to assume that, during this period, large masses of Pacific seawater entered the Tethys again directly through the Indian-Tethys seaway. The renewed importance of this seaway might be related to the Late Cretaceous-Early Tertiary first-order, global sea level highstand. The most abrupt change towards lower, more crust-like values occurred in the early Miocene (25-17 Ma ago) in the South and North Atlantic, as well as in the Tethys. During this period, the143Nd/144Nd isotope ratios dropped from ≈ 0.5124 (eNd ≈ −4.2) down to ≈ 0.5122 (eNd ≈ −8). This change indicates a diminishing influx of Pacif seawater into the Atlantic and Tethys Oceans and can be correlated with the breakdown of the circum-equatorial circulation patterns of the world oceans, due to plate tectonics. From 20 to 17 Ma ago (Miocene) the Nd isotopic composition of the Atlantic Ocean increased again and reached values as high as 0.5123 (eNd ≈ −6.6), whereas Tethys seawater remained at low values. This decoupling can be correlated with the incipient complete isolation of the Tethys; the Nd isotopic composition was henceforth controlled by the continental runoff from surrounding land masses. The major increase in the Nd isotopic composition observed in the Atlantic can be related to the opening of the Drake Passage and establishment of the circum-Antarctic current system. This allowed Pacific seawater to enter directly the South Atlantic around Antarctica. A renewed drop in the Nd isotopic composition of Atlantic seawater during the late Miocene can be related to the onset of North Atlantic Bottom Water formation and its subsequent upwelling.

Rare earth element behavior and Pb, Sr, Nd isotope systematics in a heavy metal contaminated soil

Abstract The aim of this study is to characterize the processes and phases which control migration and retention of rare earth elements (REE) in a heavy metal contaminated soil. In addition to concentration data, we used Pb, Sr and Nd isotopic compositions in order to distinguish between natural and anthropogenic trace metals and to characterize the phases leached away during the sequential extraction procedure. The samples were sequentially extracted in 3 steps with 1 N acetic acid (HAc), 1 N HCl and 1 N HNO 3 . The Pb isotope data showed that anthropogenic Pb had mainly been retained in the uppermost 10 cm by the organic matter of the topsoil. The 87 Sr/ 86 Sr ratios of the HAc extracts are almost constant and indicate that soil carbonate is derived from regionally outcropping carbonate-rich sediments. Most HCl and HNO 3 extracts have more radiogenic Sr isotopic compositions, but it is unclear whether this reflects a growing influence of anthropogenic or silicate-derived Sr. The depth distribution of the REE is mainly controlled by two different parameters: soil pH for the HAc extractable REE and Fe Mn oxides for the REE in the HCl and HNO 3 extracts. A part of the HNO 3 extractable REE was also bound to the organic matter of the topsoil. The REE concentrations in the HAc extractable phase increase with depth and increasing soil pH, which indicates that they are derived from the surface and hence are of anthropogenic origin. This is confirmed by 143 Nd/ 144 Nd isotope ratios which show a mixing between a natural end-member at the top and an anthropogenic end-member at the base of the profile. We assume that the anthropogenic REE were transported in dissolved form as carbonate complexes and then precipitated during downward migration as soil pH increased.

Transfer of rare earth elements (REE) from natural soil to plant systems: implications for the environmental availability of anthropogenic REE

Background and aimsRare Earth Elements (REE) are widely used to trace natural geochemical processes. They are also increasingly used by man (electronics industry, medicine, agriculture) and therefore considered as emerging pollutants. The present study documents REE mobility in non-polluted natural soil-plant systems in order to characterize their environmental availability for future anthropogenic pollution.MethodsThe study is based on a field approach in non-polluted natural sites with contrasting geological environments (limestone, granite, and carbonatite) and highly variable REE contents.ResultsREE concentrations in soils do not directly reflect bedrock concentrations, but depend largely on pedogenetic processes and on the mineralogy of bedrock and soil. The soils of all sites are with respect to bedrock enriched in heavy REE. The REE uptake by plants is not primarily controlled by the plant itself, but depends on the concentration and the speciation in the soil and the adsorbed soil water pool.ConclusionsREE uptake by plant roots are linked with those of Fe. Roots absorb preferentially the light REE. Before translocation, REE are retained by the Casparian strip leading to much lower concentrations in the aerial parts. The transport of the REE within the xylem is associated with the general nutrient flux.

Trace element and isotopic evidence for REE migration and fractionation in salts next to a basalt dyke

Abstract Neodymium and Sr isotopic compositions and the rare earth elements (REE) distribution patterns have been determined in salts adjacent to a basaltic dyke along 2 parallel horizontal profiles. The salts, originally consisting of carnallite (KMgCl 3 · 6H 2 O), have been transformed during basalt intrusion mainly into halite (NaCl) and sylvite (KCl) by fluids saturated in NaCl. The Sr isotope data suggests that much more fluid penetrated the upper than the lower horizon. The Nd isotope data shows that in the upper horizon, where fluid flow was stronger, Nd is essentially derived from the basalt. In contrast, in the lower horizon a strong salt Nd component is present. The REE data document in both horizons is a strong depletion of Ce, Pr, Nd, Sm and Eu with increasing distance from the basalt. This depletion of the light rare earths (LREE) is stronger in the upper horizon where fluid flow was stronger. The authors suggest that this REE fractionation is more likely due to precipitation of LREE-enriched accessory minerals such as apatite, than to differential REE solubility caused by selective REE complexation. This finding is of interest for REE behaviour in brines in general, and for the behaviour of radioactive REE and actinides in a salt repository for high-level nuclear waste in particular.

Dynamics of copper and zinc sedimentation in a lagooning system receiving landfill leachate.

This study characterises the sediment dredged from a lagooning system composed of a settling pond and three lagoons that receive leachates from a municipal solid waste (MSW) landfill in France. Organic carbon, carbonate, iron oxyhydroxides, copper (Cu) and zinc (Zn) concentrations were measured in the sediment collected from upstream to downstream in the lagooning system. In order to complete our investigation of sedimentation mechanisms, leachates were sampled in both dry (spring) and wet (winter) seasonal conditions. Precipitation of calcite and amorphous Fe-oxyhydroxides and sedimentation of organic matter occurred in the settling pond. Since different distributions of Zn and Cu concentrations are measured in sediment samples collected downstream in the lagooning system, it is suggested that these elements were not distributed in a similar way in the leachate fractions during the first stage of treatment in the settling pond, so that their sedimentation dynamics in the lagooning system differ. In the lagoons, it was found that organic carbon plays a major role in Cu and Zn mobility and trapping. The presence of macrophytes along the edges provided an input of organic matter that enhanced Cu and Zn scavenging. This edge effect resulted in a two-fold increase in Cu and Zn concentrations in the sediment deposited near the banks of the lagoons, thus confirming the importance of vegetation for the retention of Cu and Zn in lagooning systems.

The corrosion of basaltic dykes in evaporites: Ar–Sr–Nd isotope and rare earth elements evidence

Abstract The corrosion of basaltic dykes from central Germany which intruded during the Tertiary into rock and potash salts of Upper Permian age (Zechstein) has been studied. The corrosion behavior of these glassy basalts can serve as a natural analogue for the long-term corrosion expected for nuclear waste glasses in a salt repository. Our data demonstrate mobility and fractionation of the rare earth elements (REE) during a postintrusive circulation of salt brines. The processes controlling this behavior of the REE were dissolution and reprecipitation of phosphates. The K–Ar data document the synintrusive assimilation of a salt phase followed by a postintrusive fluid circulation in the peripheral parts. This circulation removed the assimilated salt and imported highly radiogenic salt Ar which was integrated into newly formed sheet silicates. The central part of the dyke was not affected by this circulation and could retain a mantle gas phase. The Sr isotope data document a synintrusive assimilation of salt Sr and a strong exchange with salt fluids during the postintrusive corrosion which was strongest in a zone next to the chilled dyke margins. This locally stronger alteration is probably due to different cooling and shrinking rates of the basalt at the margins and in the core of the dyke which led to the formation of a highly permeable fractured zone.

Strongly fractionated REE patterns in salts and their implications for REE migration in chloride-rich brines at elevated temperatures and pressures

Rare earth element (REE) distribution patterns have been determined in a salt profile adjacent to a basaltic intrusion. The REE concentrations in the salts are in the ppb range, which required an enrichment prior to ICP-MS analysis. The results document a strong REE fractionation with increasing distance from the basalt contact which could be due to REE-chloride solution complexation. The observed fractionation occurred at elevated temperatures and pressures and indicates a higher mobility of Ce, Pr, Nd, Sm and Eu. This finding is of interest for basic research and for the disposal of nuclear waste in salt formations.

Comparative Study of the Physicochemical Response of Two Karst Systems During Contrasting Flood Events in the French Jura Mountains

This paper presents preliminary results from two karst systems belonging to the “Jurassic Karst” observatory in the French Jura Mountains. The sites are characterized by localized and diffuse recharge. Physicochemical monitoring was performed at the karst outlet (springs), as well as in the unsaturated zone (cave and epikarstic spring). During two contrasting flood events, water level, temperature, electrical conductivity, dissolved organic carbon, turbidity, and dissolved oxygen were recorded at high frequency and compared. These preliminary results allow to propose a conceptual model for both sites. It was possible to distinguish specific autogenic and allogenic recharge mechanisms and to characterize the respective contribution of the saturated and unsaturated zones.

Basaltic dykes in evaporites: a natural analogue

Abstract We present rare earth element (REE) data of basalt and salt samples from central Germany where basaltic dykes of Tertiary age crosscut Upper Permian rock and potash salt. The glassy rims of the dykes can be considered as a natural analogue for the corrosion of nuclear waste glass in a salt repository, whereas the REE data from the salt can serve as an analogue for radionuclide migration in salt next to a leaking nuclear waste repository because the light rare earths (LREE) have a geochemical behavior similar to that of some actinides. Our basalt data demonstrate mobility and fractionation of the REE during postintrusive circulation of salt brines. The processes controlling this behavior of the REE were dissolution and reprecipitation of phosphate minerals. The salt data show that a small portion of the REE has left the basalt during postintrusive fluid circulation and migrated into the salt where a strong depletion of the LREE can be observed with increasing distance from the basalt contact. This fractionation is most probably due to precipitation of LREE-enriched accessory minerals such as apatite. In analogy to this, a similar behavior might be expected from actinides such as Am and Cm, which would in the case of a leaking salt nuclear waste repository probably be immobilized when phosphate minerals are present in the backfill material.