Selective separation of very fine particles at a planar air–water interface

Selective fine particle separation is a key unit operation in the mineral and related industries. In flotation, the capture of fine particles by bubbles is inefficient due to their low mass and momentum, which result in low particle–bubble collision efficiency. We demonstrate that it is possible to selectively separate a mixture of very fine hydrophobic graphite and hydrophilic quartz particles by direct contact with an air–water interface without a particle–bubble collision step involved. We demonstrate that it is possible to scale-up the process from a simple batch to a continuous process. Good selective separation of graphite from quartz gangue could be obtained under continuous conditions.     

Tectonic implications of new single zircon Pb-Pb evaporation data in the Lossogonoi and Longido ruby-districts,Mozambican metamorphic Belt of north-eastern Tanzania

Three single zircon Pb-Pb evaporation dating studies were performed on felsic orthogneisses and migmatites from the Longido and Lossogonoi ruby districts, Mozambique Belt of north-eastern Tanzania, in order to better constrain the geological setting of gemstone mineralizations. Igneous emplacement ages of protoliths ranging between 2636 and 2448 Ma document for the first time the presence of a Neoarchean to Lower Paleoproterozoic (Siderian) basement reworked in the Late Neoproterozoic Mozambique Belt of north-eastern Tanzania. This ancient crust of unknown dimension is well documented farther south, but also in south-eastern Kenya. A shearing event under high-grade amphibolite facies conditions, postdating the Pan-African metamorphic peak at 640 Ma and following nappes emplacement is demonstrated at ca. 610 Ma from metamorphic zircons of Lossogonoi district. In Lossogonoi district, ruby crystallizes during this last stage of deformation.     

Springtime changes in snow chemistry lead to new insights into mercury methylation in the Arctic

Seasonal snow is an active media and an important climate factor that governs nutrient transfer in Arctic ecosystems. Since the snow stores and transforms nutrients and contaminants, it is of crucial importance to gain a better understanding of the dynamics of contaminant cycling within the snowpack and its subsequent release to catchments via meltwater. Over the course of a two-month field study in the spring of 2008, we collected snow and meltwater samples from a seasonal snowpack in Ny-Ålesund, Norway (78°56′N, 11°52′E), which were analyzed for major inorganic ions and some organic acids, as well as total, dissolved, bioavailable mercury (THg, DHg, BioHg, respectively) and monomethylmercury (MMHg) species. We observe a seasonal gradient for ion concentrations, with surface samples becoming less concentrated as the season progressed. A significant negative correlation between BioHg and MMHg was observed in the snowpack. MMHg was positively and significantly correlated to methanesulfonate concentrations. Based on these results, we propose a new model for aerobic methylation of mercury involving species in the dimethylsulfoniopropionate cycle.     

Nitrogenase activity and estimates of nitrogen fixation by freshwater benthic blue‐green algae

The sandy substrate of Lake Taharoa (west coast, North Island, New Zealand ‐ 35°50'S, 173°41'E) is covered by communities of filamentous algae that extend from the exposed beach down to 21 m depth. The algae bind the sand to form crusts and mats which may break off as discrete plates. The dominant species are the blue‐greens Microcoleus, Nostoc, Phormidium, Lyngbya, Oscillatoria, Scytonema, Stigonema, Shizothrix, Calothrix, Dichothrix, Tolypothrix, and Anabaena, with occasional high concentrations of the desmid Cylindrocystis. Nitrogenase activity, measured by acetylene reduction, showed a wide range of rates (4–150 μmol C₂H₄ m^‐2h^‐1). Estimates of annual rates of nitrogen fixation by the Taharoa communities are comparable with those for periphytic blue‐green algae‐dominated systems reported elsewhere.     

Subtidal Galeolaria hystrix (Polychaeta: Serpulidae) reefs in Paterson Inlet,Stewart Island,New Zealand

Serpulid patch reefs of Galeolaria hystrix Mörch, 1863 were found in water depths of 9–16 m in Big Glory Bay, Paterson Inlet, Stewart Island, and here we report preliminary studies of these important habitat‐formers. This is the first observation of this species in subtidal patch reefs; 114 reefs were noted in a survey of 28 000 m². Most reefs were 1–5 m in diameter, and up to 1.5 m high. Up to 65% of the serpulid tubes were occupied by living G. hystrix during a mid‐winter diving survey; 64% of reefs observed were whole, whereas 36% were broken or dead. Radiometric dating of a basal specimen of reef carbonate showed it to be less than 50 years old. Production of high‐Mg calcite, ranging from 9 to 11 wt% MgCO3, by G. hystrix may be as much as 11 kg CaCO3 m^‐2y^‐1, but was not reflected in surrounding sediments, which were dominantly terrigenous muds. A rich reef fauna, both sessile and motile, was associated with the reefs. Further study of these unusual temperate reefs is strongly recommended.     

Low temperature hydrolysis of laboratory tholins in ammonia-water solutions: Implications for prebiotic chemistry on Titan

Laboratory tholins react rapidly in 13 wt% ammonia-water at low temperature, producing complex organic molecules containing both oxygen and altered nitrogen functional groups. These reactions display first-order kinetics with half-lives between 0.3 and 14 days at 253 K. The reaction timescales are much shorter than the freezing timescales of impact melts and volcanic sites on Titan, providing ample time for the formation of oxygenated, possibly prebiotic, molecules on its surface. Comparing the rates of the hydrolysis reactions in ammonia-water to those measured in pure water [Neish, C.D, Somogyi, A., Imanaka, H., Lunine, J.I., Smith, M.A., 2008a. Astrobiology 8, 273-287], we find that incorporation of oxygen into the tholins is faster in the presence of ammonia. The rate increases could be due to the increased pH of the solution, or to the availability of new reaction pathways made possible by the presence of ammonia. Using labeled ¹⁵NH₃ water, we find that ammonia does incorporate into some products, and that the reactions with ammonia are largely independent of those with water. A related study in confirms water as the source of the oxygen incorporated into the oxygen containing products.     

The effects of mid-Phanerozoic environmental stress on bryozoan diversity, paleoecology, and paleogeography

Evidence of sustained environmental degradation associated with the end-Guadalupian, end-Permian, and end-Triassic extinctions has been inferred from numerous geochemical and sedimentological studies, but the long-term impacts of this extinction-associated stress on the evolutionary trajectories of marine invertebrates have not been explored. An examination of the diversity, extinction, paleoenvironmental range, and geographical distribution of marine stenolaemate bryozoans during the Permian to Jurassic interval provides striking new evidence of the taxonomic and ecological influence of these mid-Phanerozoic extinctions on one of the most abundant components of the Paleozoic Fauna. Elevated bryozoan extinction rates during the Late Permian and Late Triassic were coupled with major changes in their habitats. Bryozoans gradually disappeared from deep-water offshore settings during the Late Permian and from nearshore and offshore settings during the Late Triassic. Re-colonization of these environments in the wake of each crisis was delayed but coupled with increases in global generic diversity. The taxonomic effects of the end-Guadalupian extinction were milder than previously described, even though ecologically bryozoans were becoming restricted to nearshore settings. The end-Permian mass extinction remained the largest for bryozoans, drastically reducing global and assemblage generic diversity and triggering a permanent change in their paleoenvironmental preferences from nearshore to mid-shelf settings. The 285 Myr dominance of stenolaemate bryozoans ended during the Late Triassic when all but one order (Cyclostomata) became extinct, initiating a taxonomic switch between stenolaemate and gymnolaemate bryozoans. Moreover, spatio-temporal variations in the paleoenvironmental history of bryozoans imply that Late Permian and Late Triassic marine environmental instability resulted largely from some stressful deep-water phenomenon. High extinction rates in nearshore environments in the Late Permian provide a link between marine and terrestrial/atmosphere extinction-related perturbations.     

Formation of a terraced fan deposit in Coprates Catena, Mars

We have studied a terraced fan deposit with unique characteristics located within a trough of Coprates Catena. The fan has an average length of 6.8 km, and is approximately 44 km² in area and 18 km³ in volume. The fan's broad contributing valley is approximately 35 km long and it noticeably increases in depth about 12.8 km before it intersects the trough, where a rounded knickpoint marks the transition between flat-floored upstream and V-shaped downstream cross-sections. A 14-km-long channel with no apparent source enters the contributing valley from the south. A much smaller sinuous channel has incised along a smaller V-shaped valley in the uppermost eastern portion of the fan deposit. We explored several possible origins for the terraced fan, including mass wasting, volcanic flow, alluvial fan, and delta. We propose that water sourced from volcanic melting of ice eroded and transported material along the contributing valley. This material was then deposited as a delta in a lake within the trough. The concentric terraces are most likely the result of shoreline or ice cover erosion during drops in lake level. A light-toned layered deposit to the east of the fan deposit along the floor of the trough may represent a sedimentary unit formed during the terminal stages of the lake. Although other terraced fans have been identified on Mars, the Coprates Catena fan is unique because it has many more terraces and its surface was incised by a channel and associated valley. The identification of several other valleys to the east suggests that volcanic melting of volatiles during the Hesperian Period created favorable conditions for water flow along the plains in this region.     

Homogénéisation et validation des corrections de terrain gravimétriques jusqu'à la distance de 167 km sur l'ensemble de la FranceUpdating and validating the French gravity terrain corrections out to a distance of 167 km

For regional and national study purposes, there is a high need for updating the terrain corrections (TC) in the French gravity database. We have recomputed the TC for all the French gravity stations from 50 m out to a distance of 167 km. We compute the TC with a flat-top-prism algorithm and three DEM with grid spacing of 50, 250 and 1000 m, used in the zones 53 m/3 km, 3 km/10 km et 10 km/167 km, respectively. Analysing the DEM/station Δz and comparing our results to the ones previously obtained in the Alps area, we estimate the accuracy of our TC to be better than 1 mGal. To cite this article: G. Martelet et al., C. R. Geoscience 334 (2002) 449–454.