3 resultados para Non-negative sources

em Archimer: Archive de l'Institut francais de recherche pour l'exploitation de la mer


Relevância:

30.00% 30.00%

Publicador:

Resumo:

Nickel isotope ratios were measured in ores, fly ash, slags and FeNi samples from two metallurgical plants located in the Goiás State, Brazil (Barro Alto, Niquelândia). This allowed investigating the mass-dependent fractionation of Ni isotopes during the Ni-laterite ore smelting and refining. Feeding material exhibits a large range of δ60Ni values (from 0.02 ± 0.10 ‰ to 0.20 ± 0.05 ‰, n=7), explained by the diversity of Ni-bearing phases, and the average of δ60Nifeeding materials was found equal to 0.08 ± 0.08‰ (2SD, n=7). Both δ60Ni values of fly ash (δ60Ni = 0.07 ± 0.07‰, n=10) and final FeNi produced (0.05 ± 0.02 ‰, n=2) were not significantly different from the feeding materials ones. These values are consistent with the very high production yield of the factories. However, smelting slags present the heaviest δ60Ni values of all the smelter samples, with δ60Ni ranging from 0.11 ± 0.05 ‰ to 0.27 ± 0.05 ‰ (n=8). Soils were also collected near and far from the Niquelândia metallurgical plant, to evaluate the potential of Ni isotopes for tracing the natural vs anthropogenic Ni in soils. The Ni isotopic composition of the non-impacted topsoils developed on ultramafic rocks ranges from -0.26 ± 0.09 ‰ to -0.04 ± 0.05 ‰ (n=20). On the contrary, the Ni isotopic composition of the non-ultramafic topsoils, collected close to the plant, exhibit a large variation of δ60Ni, ranging from -0.19 ± 0.13 ‰ up to 0.10 ± 0.05 ‰ (n=4). This slight but significant enrichment in heavy isotopes highlight the potential impact of smelting activity in the surrounding area, as well as the potential of Ni isotopes for discerning anthropogenic samples (heavier δ60Ni values) from natural ones (lighter δ60Ni values). However, given the global range of published δ60Ni values (from -1.03 to 2.5 ‰) and more particularly those associated to natural weathering of ultramafic rocks (from -0.61 to 0.32‰), the use of Ni isotopes for tracing environmental contamination from smelters will remain challenging.

Relevância:

30.00% 30.00%

Publicador:

Resumo:

Authigenic carbonate deposits have been sampled with the remotely operated vehicle ‘MARUM-QUEST 4000 m’ from five methane seeps between 731 and 1823 m water depth along the convergent Makran continental margin, offshore Pakistan (northern Arabian Sea). Two seeps on the upper slope are located within the oxygen minimum zone (OMZ; ca. 100 to 1100 m water depth), the other sites are situated in oxygenated water below the OMZ (below 1100 m water depth). The carbonate deposits vary with regard to their spatial extent, sedimentary fabrics, and associated seep fauna: Within the OMZ, carbonates are spatially restricted and associated with microbial mats, whereas in the oxygenated zone below the OMZ extensive carbonate crusts are exposed on the seafloor with abundant metazoans (bathymodiolin mussels, tube worms, galatheid crabs). Aragonite and Mg-calcite are the dominant carbonate minerals, forming common early diagenetic microcrystalline cement and clotted to radial-fibrous cement. The δ18Ocarbonate values range from 1.3 to 4.2‰ V-PDB, indicating carbonate precipitation at ambient bottom-water temperature in shallow sediment depth. Extremely low δ13Ccarbonate values (as low − 54.6‰ V-PDB) point to anaerobic oxidation of methane (AOM) as trigger for carbonate precipitation, with biogenic methane as dominant carbon source. Prevalence of biogenic methane in the seepage gas is corroborated by δ13Cmethane values ranging from − 70.3 to − 66.7‰ V-PDB, and also by back-calculations considering δ13Cmethane values of carbonate and incorporated lipid biomarkers. These calculations (Δδ13Cmethane–carbonate, Δδ13CANME–methane, Δδ13CMOX–methane) prove to be useful to assess the carbon stable isotope composition of seeping methane if this has not been determined in the first place; such an approach represents a useful tool to reconstruct fluid composition of ancient seeps. AOM is also revealed by lipid biomarkers of anaerobic methane oxidizing archaea such as crocetane, pentamethylicosane (PMI), and sn2-hydroxyarchaeol strongly depleted in 13C (δ13C values as low as − 127‰ V-PDB). Biomarkers of sulphate-reducing bacteria are also abundant, showing slightly less negative δ13C values, but still significantly 13C-depleted (average values as low as − 101‰). Other bacterial biomarkers, such as bacteriohopanepolyols (BHPs), hopanols, and hopanoic acids are detected in most carbonates, but are particularly common in seep carbonates from the non-OMZ sites. The BHP patterns of these carbonates and their low δ13C values resemble patterns of aerobic methanotrophic bacteria. In the shallower OMZ sites, BHPs revealed much lower contents and varying compositions, most likely reflecting other sources than aerobic methanotrophic bacteria. 230Th/U carbonate ages indicate that AOM-induced carbonate precipitation at the deeper non-OMZ seeps occurred mainly during the late Pleistocene-Holocene transition, i.e. between 19 and 15 ka before present, when the global sea level was lower than today.

Relevância:

30.00% 30.00%

Publicador:

Resumo:

Otoliths are calcified structures located in Osteichthyes’ inner ear that are involved in audition and balance. Their morphology is used as an indicator of various ecological processes or properties. This application requires identifying the endogenous and exogenous factors that act simultaneously as sources of shape variation. This thesis aims at detecting and quantifying the relative contributions of directional asymmetry and diet to otolith shape variation at the intra-population level. Directional asymmetry between left and right otoliths was found in flat-fishes, the blind-side otolith being always longer and larger, whereas it was negligible in round-fishes. However, asymmetry amplitude never exceeded 18 %, which suggests evolutionary canalization of otolith shape symmetry. A correlation between global diet and otolith was detected in 4 species studied in situ. Diet composition contributed more than food amount to morphological variation and affected otolith shape both globally and locally. An experimental study on sea bass (Dicentrarchus larbrax) showed that diet composition in terms of essential polyunsaturated fatty acids at larval stage affects otolith morphogenesis during juvenile stage without impacting on individuals’ somatic growth. This result suggests a direct effect of diet on otolith shape and not an indirect one through the somatic-otolith growth relationship. This effect disappeared at later stages, morphogenetic trajectories converging back to a similar shape, which suggests ontogenetic canalization of otolith shape.