Characterization of low-molecular-weight organic acids isolated from the Toarcian argillite pore water (Tournemire site,France)

Bacterial activity in the near-field environment of radioactive waste disposal facilities in deep argillaceous rocks is currently under investigation. Bacterial development could be enhanced by the availability of low-molecular-weight organic acids (LMWOA) dissolved in the pore water of clayey rocks. This study firstly aimed at isolating and characterizing the LMWOA of the Toarcian argillite from the Tournemire Underground Research Laboratory (France, Aveyron). It also aimed at assessing the disturbing effects that could be induced by a disposal facility on the type and release of LMWOA (exposure to oxygen, alkaline pH, temperature increase). Under the experimental conditions investigated at 60 °C, the mean dissolved organic carbon (DOC) is about 15 mg L⁻¹ with a LMWOA proportion of 35 %. The main LMWOA are composed of formate, fumarate, propionate, acetate, and lactate. DOC is mainly influenced by both temperature and pH increases, whereas the LMWOA type nature and release are more specifically affected by the exposure to oxygen.     

Geochemical and oxygen isotope signatures of mantle corundum megacrysts from the Mbuji-Mayi kimberlite,Democratic Republic of Congo,and the Changle alkali basalt,China

Oxygen isotope signatures of ruby and sapphire megacrysts, combined with trace-element analysis, from the Mbuji-Mayi kimberlite, Democratic Republic of Congo, and the Changle alkali basalt, China, provide clues to specify their origin in the deep Earth. At Mbuji-Mayi, pink sapphires have δ¹⁸O values in the range 4.3 to 5.4‰ (N = 10) with a mean of 4.9 ± 0.4‰, and rubies from 5.5 to 5.6‰ (N = 3). The Ga/Mg ratio of pink sapphires is between 1.9 and 3.9, and in rubies, between 0.6 and 2.6. The blue or yellow sapphires from Changle have δ¹⁸O values from 4.6 to 5.2 ‰, with a mean of 4.9 ± 0.2‰ (N = 9). The Ga/Mg ratio is between 5.7 and 11.3. The homogenous isotopic composition of ruby suggests a derivation from upper mantle xenoliths (garnet lherzolite, pyroxenite) or metagabbros and/or lower crustal garnet clinopyroxenite eclogite-type xenoliths included in kimberlites. Data from the pink sapphires from Mbuji-Mayi suggest a mantle origin, but different probable protoliths: either subducted oceanic protolith transformed into eclogite with δ¹⁸O values buffered to the mantle value, or clinopyroxenite protoliths in peridotite. The Changle sapphires have a mantle O-isotope signature. They probably formed in syenitic magmas produced by low degree partial melting of a spinel lherzolite source. The kimberlite and the alkali basalt acted as gem conveyors from the upper mantle up to the surface.     

Paleolimnological investigations of anthropogenic environmental change in Lake Tanganyika: VII. Carbonate isotope geochemistry as a record of riverine runoff

Evaporation dominates the removal of water from Lake Tanganyika, and therefore the oxygen isotope composition of lake water has become very positive in comparison to the waters entering the lake. The surface water in Lake Tanganyika has remained relatively unchanged over the last 30 years with a seasonal range of +3.2 to +3.5 VSMOW. Water from small rivers entering the lake seems to have a ¹⁸O value between –3.5 and –4.0, based on scattered measurements. The two largest catchments emptying into the lake deliver water that has a ¹⁸O value between these two extremes. This large contrast is the basis of a model presented here that attempts to reconstruct the history of runoff intensity based on the ¹⁸O of carbonate shells from Lake Tanganyika cores. In order to use biogenic carbonates to monitor changes in the ¹⁸O of mixing-zone water, however, the oxygen isotope fractionation between water and shell carbonate must be well understood. The relatively invariant environmental conditions of the lake allow us to constrain the fractionation of both oxygen and carbon isotope ratios. Although molluskan aragonitic shell ¹⁸O values are in agreement with published mineral-water fractionations, ostracode calcite is 1.2 more positive than that of inorganic calcite precipitated under similar conditions. Ostracode shell ¹⁸O data from two cores from central Lake Tanganyika suggest that runoff decreased in the first half of this millennium and has increased in the last century. This conclusion is poorly constrained, however, and much more work needs to be done on stable isotope variation in both the waters and carbonates of Lake Tanganyika. We also compared the ¹³C of shells against predicted values based solely on the ¹³C of lake water dissolved inorganic carbon (DIC). The ostracode Mecynocypria opaca is the only ostracode or mollusk that falls within the predicted range. This suggests that M. opaca has potential for reconstructing the carbon isotope ratio of DIC in Lake Tanganyika, and may be a useful tool in the study of the history of the lakes productivity and carbon cycle.     

Limnic Responses to Increased Effective Humidity during the 8200 cal. yr BP Cooling Event in Southern Sweden

Highly resolved sediment stratigraphic and stable isotope records from a lake in south-central Sweden give evidence of relatively humid summers during the wide-spread cold event at c. 8200 cal. BP. A transient lake-level rise led to increased catchment erosion followed by enhanced phytoplankton production and disturbance of the Chara-dominated algal community. An associated increase in the deposition of organic material resulted in hypolimnetic oxygen deficit and methane formation as inferred from elevated sulphur content of the sediments and strong depletion in ¹³C of bulk organic material. In combination with dry and cold winter conditions as revealed by other proxy records, these data suggest the development of a strongly seasonal climate in northwestern Europe in response to altered atmospheric circulation at this stage. Enhanced zonal circulation with frequent cyclonic activity and increased effective humidity probably characterized the summer climate of southern and central Scandinavia, consistent with conditions inferred for continental Europe. In contrast, the winter climate was dominated by blocking high-pressure circulation, giving rise to cold and dry conditions.     

Geochemistry of coral from Papua New Guinea as a proxy for ENSO ocean–atmosphere interactions in the Pacific Warm Pool

A Porites sp. coral growing offshore from the Sepik and Ramu Rivers in equatorial northern Papua New Guinea has yielded an accurate 20-year history (1977–1996) of sea surface temperature (SST), river discharge, and wind-induced mixing of the upper water column. Depressions in average SSTs of about 0.5–1.0 °C (indicated by coral Sr/Ca) and markedly diminished freshwater runoff to the coastal ocean (indicated by coral δ¹⁸O, δ¹³C and UV fluorescence) are evident during the El Niño – Southern Oscillation (ENSO) events of 1982–1983, 1987 and 1991-1993. The perturbations recorded by the coral are in good agreement with changes in instrumental SST and river discharge/precipitation records, which are known to be diagnostic of the response of the Pacific Warm Pool ocean–atmosphere system to El Niño. Consideration of coastal ocean dynamics indicates that the establishment of northwest monsoon winds promotes mixing of near-surface waters to greater depths in the first quarter of most years, making the coral record sensitive to changes in the Asian–Australian monsoon cycle. Sudden cooling of SSTs by 1°C following westerly wind episodes, as indicated by the coral Sr/Ca, is consistent with greater mixing in the upper water column at these times. Furthermore, the coral UV fluorescence and oxygen isotope data indicate minimal contribution of river runoff to surface ocean waters at the beginning of most years, during the time of maximum discharge. This abrupt shift in flood-plume behaviour appears to reflect the duration and magnitude of northwest monsoon winds, which tend to disperse flood plume waters to a greater extent in the water column when wind-mixing is enhanced. Our results suggest that a multi-proxy geochemical approach to the production of long coral records should provide comprehensive reconstructions of tropical paleoclimate processes operating on interannual timescales.     

Amoeboid olivine aggregates and related objects in carbonaceous chondrites: records of nebular and asteroid processes

Amoeboid olivine aggregates (AOAs) are the most common type of refractory inclusions in CM, CR, CH, CV, CO, and ungrouped carbonaceous chondrites Acfer 094 and Adelaide; only one AOA was found in the CB_b chondrite Hammadah al Hamra 237 and none were observed in the CB_a chondrites Bencubbin, Gujba, and Weatherford. In primitive (unaltered and unmetamorphosed) carbonaceous chondrites, AOAs consist of forsterite (Fa_<2), Fe, Ni-metal (5-12 wt% Ni), and Ca, Al-rich inclusions (CAIs) composed of Al-diopside, spinel, anorthite, and very rare melilite. Melilite is typically replaced by a fine-grained mixture of spinel, Al-diopside, and ±anorthite; spinel is replaced by anorthite. About 10% of AOAs contain low-Ca pyroxene replacing forsterite. Forsterite and spinel are always ¹⁶O-rich (δ^17,18O∼−40‰ to −50‰), whereas melilite, anorthite, and diopside could be either similarly ¹⁶O-rich or ¹⁶O-depleted to varying degrees; the latter is common in AOAs from altered and metamorphosed carbonaceous chondrites such as some CVs and COs. Low-Ca pyroxene is either ¹⁶O-rich (δ^17,18O∼−40‰) or ¹⁶O-poor (δ^17,18O∼0‰). Most AOAs in CV chondrites have unfractionated (∼2-10×CI) rare-earth element patterns. AOAs have similar textures, mineralogy and oxygen isotopic compositions to those of forsterite-rich accretionary rims surrounding different types of CAIs (compact and fluffy Type A, Type B, and fine-grained, spinel-rich) in CV and CR chondrites. AOAs in primitive carbonaceous chondrites show no evidence for alteration and thermal metamorphism. Secondary minerals in AOAs from CR, CM, and CO, and CV chondrites are similar to those in chondrules, CAIs, and matrices of their host meteorites and include phyllosilicates, magnetite, carbonates, nepheline, sodalite, grossular, wollastonite, hedenbergite, andradite, and ferrous olivine.Our observations and a thermodynamic analysis suggest that AOAs and forsterite-rich accretionary rims formed in ¹⁶O-rich gaseous reservoirs, probably in the CAI-forming region(s), as aggregates of solar nebular condensates originally composed of forsterite, Fe, Ni-metal, and CAIs. Some of the CAIs were melted prior to aggregation into AOAs and experienced formation of Wark-Lovering rims. Before and possibly after the aggregation, melilite and spinel in CAIs reacted with SiO and Mg of the solar nebula gas enriched in ¹⁶O to form Al-diopside and anorthite. Forsterite in some AOAs reacted with ¹⁶O-enriched SiO gas to form low-Ca pyroxene. Some other AOAs were either reheated in ¹⁶O-poor gaseous reservoirs or coated by ¹⁶O-depleted pyroxene-rich dust and melted to varying degrees, possibly during chondrule formation. The most extensively melted AOAs experienced oxygen isotope exchange with ¹⁶O-poor nebular gas and may have been transformed into magnesian (Type I) chondrules. Secondary mineralization and at least some of the oxygen isotope exchange in AOAs from altered and metamorphosed chondrites must have resulted from alteration in the presence of aqueous solutions after aggregation and lithification of the chondrite parent asteroids.     

The granite hosted gold deposit of Moulin de Chéni (Saint-Yrieix district,Massif Central,France): petrographic,structural, fluid inclusion and oxygen isotope constraints

The Moulin de Chéni orogenic gold deposit is the only granite-hosted deposit of the Saint-Yrieix district, French Massif Central. It occurs in 338±1.5 Ma-old peraluminous leucogranites and is characterized by intense microfracturing and bleaching of the granite in relation to pervasive sulfide crystallization. Formation of quartz veins and gold deposition occurred in two successive stages: an early mesozonal stage of quartz-sulfide (Fe-As-S) deposition, usually devoid of gold and a late epizonal stage of base metal and gold deposition. Both stages postdate peak metamorphism and granite intrusion. The genesis of the deposit is the result of four successive fluid events: (1) Percolation of aqueous-carbonic metamorphic fluids under an assumed lithostatic regime of 400–450 °C, at a maximum depth of 13 km; (2) Formation of the main quartz lodes with coeval K-alteration and introduction of As and S from aqueous-carbonic fluids percolating along regional faults. Arsenopyrite and pyrite deposition was linked to the alteration of Fe-silicates into K-feldspar and phengite at near-constant iron content in the bulk granite. Temperature was similar to that of the preceding stage, but pressure decreased to 100–50 MPa, suggesting rapid uplift of the basement up to 7.5 km depth; (3) The resulting extensional tectonic leads to the deposition of gold, boulangerite, galena and sphalerite in brecciated arsenopyrite and pyrite from aqueous fluids during a mixing process. Temperature and salinity decrease from 280 to 140 °C and 8.1 wt% eq. NaCl to 1.6 wt% eq. NaCl, respectively; (4) Sealing of the late fault system by barren comb quartz which precipitated from dilute meteoric aqueous fluids (1.6 wt% eq. NaCl to 0.9 wt% eq. NaCl) under hydrostatic conditions at 200–150 °C.Editorial handling: B. Lehmann     

Rhinocerotid tooth enamel 18O/16O variability between 23 and 12 Ma in southwestern France

The relationship between the oxygen isotope ratio of mammal tooth enamel and that of drinking water was used to reconstruct changes in the Miocene oxygen isotope ratio of rainfall (meteoric water ${\delta }^{18}$O_MW). These, in turn, are related to climatic parameters (temperature, precipitation and evaporation rate). ${\delta }^{18}$O values of rhinocerotid teeth from the Aquitaine Basin (southwestern France) suggest a significant climatic change between 17 and 12 Ma, characterized by cooling together with precipitation increase, in agreement with other terrestrial and oceanic records. To cite this article: I. Bentaleb et al., C. R. Geoscience 338 (2006).     

Evaluation of oxygen isotopes in carbonate as an indicator of lake evolution in arid areas: The modern Qinghai Lake, Qinghai–Tibet Plateau

The oxygen isotopic composition of carbonate in lakes has been used as a useful indicator in Palaeolimnological research, and has made some important contributions to our understanding of lacustrine systems. For modern lakes in arid or cold areas, however, there are few data available to test the effect of lake salinity and temperature on the oxygen isotopic composition of various carbonate sources such as ostracod, bulk carbonate, and fine-grained carbonate (< 60 μm). Here we examined the oxygen isotopic composition of ostracods, bulk carbonate, and fine-grained carbonates, as well as that of coexisting water from Lake Qinghai and the smaller surrounding lakes and ponds on the Qinghai–Tibet Plateau. Our investigation highlights three key effects. First, the oxygen isotopic composition of ostracods, bulk carbonate, and fine-grained carbonate in the lakes and ponds shows a clear response to lake water δ¹⁸O values, and these vary with water salinity. The relationship between lake water δ¹⁸O and salinity is not only dominated by the evaporation/freshwater input ratios, but is also controlled by the distance to the mouth of the major rivers supplying to the lake. Second, the ostracod, bulk carbonate, and fine-grained carbonate show similar isotopic change trends in the study area, and oxygen isotopic differences between ostracods and authigenic carbonate may be explained by the different water temperatures and very small ‘vital offsets’ of ostracods. Finally, the effect of water depth on temperature leads to increasing δ¹⁸O values in carbonates as water depth increases, both in benthic ostracods living on the lake bottom, as well as in bulk carbonate precipitated at the water surface.For arid, high-altitude Lake Qinghai, our results suggest that variations in the δ¹⁸O values of carbonate in Lake Qinghai are mainly controlled by the oxygen-isotope ratio of the lake water changing with water salinity. As a secondary effect, increasing water depth leads to cooler bottom and surface water, which may result in more positive δ¹⁸O values of ostracod and bulk carbonate.     

Oxygen isotopes composition of sapphires from the French Massif Central: implications for the origin of gem corundum in basaltic fields

Alluvial and colluvial gem sapphires are common in the basaltic fields of the French Massif Central (FMC) but sapphire-bearing xenoliths are very rare, found only in the Menet trachytic cone in Cantal. The O-isotope composition of the sapphires ranges between 4.4 and 13.9‰. Two distinct groups have been defined: the first with a restricted isotopic range between 4.4 and 6.8‰ (n = 22; mean δ¹⁸O = 5.6 ± 0.7‰), falls within the worldwide range defined for blue-green-yellow sapphires related to basaltic gem fields (3.0 < δ¹⁸O < 8.2‰, n = 150), and overlaps the ranges defined for magmatic sapphires in syenite (4.4 < δ¹⁸O < 8.3‰, n = 29). A second group, with an isotopic range between 7.6 and 13.9‰ (n = 9), suggests a metamorphic sapphire source such as biotite schist in gneisses or skarns. The δ¹⁸O values of 4.4–4.5‰ for the blue sapphire-bearing anorthoclasite xenolith from Menet is lower than the δ¹⁸O values obtained for anorthoclase (7.7–7.9‰), but suggest that these sapphires were derived from an igneous reservoir in the subcontinental spinel lherzolitic mantle of the FMC. The presence of inclusions of columbite-group minerals, pyrochlore, Nb-bearing rutile, and thorite in these sapphires provides an additional argument for a magmatic origin. In the FMC lithospheric mantle, felsic melts crystallized to form anorthoclasites, the most evolved peraluminous variant of the alkaline basaltic melt. The O-isotopic compositions of the first group suggests that these sapphires crystallized from felsic magmas under upper mantle conditions. The second group of isotopic values, typified for example by the Le Bras sapphire with a δ¹⁸O of 13.9‰, indicates that metamorphic sapphires from granulites were transported to the surface by basaltic magma.