日本琵琶湖中微囊藻水华形成机理的再认识

Blooms of cyanobacteria are responsible for many problems in freshwater ecosystems. The massive growth of these microorganisms may limit the utilization of freshwater for human requirements since, apart from other problems, the production of toxic substances has been found to occur frequently during blooming periods. Ecologically, cyanobacterial blooms can modify dramatically the ecosystem through their low edibility within the food web and the huge primary production. Thus, saprobic processes are stimulated and the characteristics related to anaerobic conditions are also more extreme.Cyanobacterial blooms are many times explained as the consequence of the eutrophication of waterbodies. However, factors promoting bloom formation and ecological succession of cyanobacteria are not well understood yet.     

The Pleistocene hominid site of Ternifine, Algeria: New results on the environment, age, and human industries

A new multidisciplinary research program, started in 1981, provided new data on the stratigraphy, fauna, and human industries, as well as the first results on paleomagnetism and sedimentology, for the Ternifine site in Algeria, which yielded the earliest hominid remains known in North Africa. The fossils were deposited in a swamp or lake surrounded by a very open, dry environment. The lake was fed by artesian springs that raised the underlying Miocene sands. Although nothing suggests a camp or butchery site, we discovered the first undisputable bone artifact in this site, the earliest known in this part of Africa. According to paleontological data, 700,000 yr B.P. is a likely age for the Ternifine deposits, which is consistent with the paleomagnetic results.     

Transmission electron microscopy of experimentally deformed chalcopyrite single crystals

Two crystals of natural chalcopyrite, CuFeS₂, experimentally deformed at 200° C have been studied by means of transmission electron microscopy (TEM). The activated glide planes are (001) and {112}. The dislocations in (001) have the Burgers vector [110] and a predominating edge character. They are split into two colinear partials b=1/2[110] and can cross split into {112}. The dislocations in {112} consist of straight segments along low index lattice lines. They are often arranged in dipoles generating trails of loops. Few dislocations with b=1/2[ \(\overline {11} \) 1] and [1 \(\bar 1\) 0] are present and dislocations with b=[0 \(\bar 2\) 1] occur in low angle subgrain boundaries. From weak beam contrasts it is presumed that most of the dislocations gliding in {112} have b=1/2〈3 \(\overline {11} \) 〉. They are dissociated into up to four partials. Microtwins and different types of stacking faults in {112} also occur. Models of the dissociation of dislocations are discussed.     

Les stilpnomélanes

Chemical and physical datas for stilpnomelanes are given. A study of correlations between physical datas and chemical composition gives the following results: 1. There is a linear correlation between chemical composition and indices of refraction and birefringence (a mathematical expression of this correlation is given). 2. There is no linear correlation between chemical composition and specific gravity. Detailed datas (bibliography, chemical analysis, X-ray diffraction and so on) will be published later.     

Evidence for Non-Conservative Behaviour of Chlorine in Humid Tropical Environments

The knowledge of the biogeochemical cycle of chlorine (Cl)is important since this element is used as a tracer of geochemical and hydrological processes in oceanic or continental environments. More specifically, Cl can be used to correct surface water composition from atmospheric contribution in order to calculate precise chemical weathering rates in watersheds. Beyond the problem of potential Cl sources in a given watershed, which is directly related to the lithology, vegetation, and industrial activities, the Cl normalization is based on the assumption that this element behaves conservatively during surface processes (e.g., chemical weathering, adsorption/desorption processes).The purpose of the present study is to forecast the geochemical behavior of Cl in a forested ecosystem located under humid tropical environment.For this reason, we have analyzed the Cl (and also Ca and Na) concentrations ofsurface waters (rainwater, groundwater, river water) over a two-year period in the Nsimi–Zoetele watershed (Cameroon).The Cl mass balance for the watershed appears to be equilibrated over the studied period (1995–1996) but Cl behavior in Mengong River draining the watershed suggests a non-conservative behavior. Indeed, Cl concentrationsin the Mengong River are low during dry seasons and high during wet seasons, which is the reverse tendency to what is usually observed taking into account dilution and evaporation processes. As Cl concentrations in the Mengong River are lower than those measured in all the feeding reservoirs, Cl should be adsorbed onto the soils of the watershed. However, as the Cl mass balance is equilibrated over the whole-year, Cl should be adsorbed and releasedat a seasonal scale. The results we obtained for this small watershed were not generalized for a larger studied basin (i.e., Nyong River basin). Even if these results should be followed by further investigations, this study suggests that Cl normalization should be used with caution to avoid under- or over-estimation of chemical weathering rates.     

Origin and age of Paleoproterozoic conglomerates and sandstones of the Tarkwaian Group in Burkina Faso, West Africa

The Tarkwaian rocks of Burkina Faso overlie the Birimian Supergroup which is considered to be part of a juvenile crust formed during the Eburnean orogen. They comprise fluviatile pebbles (quartz, rhyolite, chert and schist), embedded in an acid volcaniclastic matrix. During a sinistral transpressive shearing they underwent a low-grade metamorphism characterized by a paragonitic muscovite assemblage formed during isoclinal folding. Fold axes are nearly horizontal, parallel to a stretching lineation and oriented 035–215°.A typologic study of zircons in the volcaniclastics, coupled with radiometric datings, enables the volcanism to be characterized and its age to be determined.The typologic study has shown that: (1) zircons from the rhyolitic pebbles, the matrix of the conglomerates and from the sandstone are basically the same; (2) the zircons' morphology characterizes a calc-alkaline to alkaline evolutionary trend implying three main episodes of volcanism; (3) the morphology of this zircon population is very similar to those usually observed in post-orogenic metasediments or sediments. The tectonic setting is one of intracontinental extension associated with rifting and crustal thinning in the course of a major period of crustal growth around 2.1 Ga. The continental volcaniclastics accumulated in the rift at the same time as the sediments.The radiometric study has shown that there are no reworked Archean rocks. The ages cover the period 2170 to 2124 Ma. Three main volcanic episodes may be defined 2170, 2150 and 2124 Ma; the last one was probably contemporaneous with the sedimentation. Deformation of Tarkwaian rocks took place between 2124 ± 9 Ma (the age of the youngest inherited zircon) and 1991 ± 12 Ma (the age of a post-Tarkwaian granite). This period, 2170 to 2124 Ma, fits well with the main Birimian crustal growth and the ages reported for the Tarkwaian rocks of Ghana. The same period of accretion is also recognized in South America.The Tarkwaian conglomeratic rocks of Burkina Faso do not have gold concentrations similar to those in Ghana and Guyana. With respect to the Tarkwaian rocks of Ghana, where gold is concentrated in the quartz pebbles, they also differ because: (1) there is a volcaniclastic contribution; (2) the zircons from the Tarkwaian rocks in Burkina Faso are euhedral, indicating that transport distances were short; and (3) they have been deformed by shearing.With respect to Guyana, the rocks are very similar and probably have the same origin; the short transport distance may not allow a secondary concentration of gold. These differences, at least, may help explain the lack of economic gold concentration in the Tarkwaian metasediments.     

Benthic primary production, respiration and remineralisation: in situ measurements in the soft-bottom Abra alba community of the western English Channel (North Brittany)

In situ measurements of ammonium and carbon dioxide fluxes were performed using benthic chambers at the end of spring and the end of summer in two soft-bottom Abra alba communities of the western English Channel (North Brittany): the muddy sand community (5 m, about 10% of surface irradiance) and the fine-sand community (19 m, about 1% of surface irradiance). High rates of ammonium regeneration were measured in the two communities at the end of summer (296.03±40.07 and 201.7±62.74 μmolN m⁻² h⁻¹, respectively) as well as high respiration rates (2.60±0.94 and 2.23±0.59 mmolC m⁻² h⁻¹, respectively). Significant benthic gross primary production (up to 6.11 mmolC m⁻² h⁻¹) was measured in the muddy sand community but no benthic primary production was measured in the fine-sand community. It suggests that microphytobenthic production values used in simulations previously published for these two communities were overestimated while values of community respiration were underestimated. The study confirms that this benthic system is heterotrophic and strengthens the idea that an important pelagic-benthic coupling is required for the functioning in such coastal ecosystems.     

Palaeozoic oolitic ironstone of the French Armorican Massif: a chemical and structural trap for orogenic base metal–As–Sb–Au mineralisation during Hercynian strike-slip deformation

In the Saint-Aubin-des-Chateaux quarry (Armorican Hercynian belt, western France), an epigenetic hydrothermal alteration affects an oolitic ironstone layer intercalated within the Lower Ordovician Grès armoricain Formation. The hydrothermal overprint produced pervasive and massive sulphidation with stratoid pyritised lenticular bodies within the oolitic ironstone layer. These sulphide lenses are spatially associated with strike-slip faults and extend laterally from them. After the massive sulphidation stage (Fe–As, stage 1), subsequent fracturing allowed the deposition of base metals (stage 2) and Pb–Sb–Au (stage 3) parageneses in veins. The dominant brittle structures are vertical extension veins, conjugate shear veins and strike-slip faults of various orders. All these structures are filled with the same paragenetic sequence. Deformation analysis allows the identification of structures that developed incrementally via right-lateral simple shear compatible with bulk strain affecting the Central Armorican Domain. Each increment corresponds to a fracture set filled with specific parageneses. Successive hydrothermal pulses reflect clockwise rotation of the horizontal shortening direction. Geothermometry on chlorite and arsenopyrite shows an input of hot hydrothermal fluids (maximum of 390–350°C) during the main sulphide stage 1. The subsequent stages present a marked temperature drop (300–275°C). Lead isotopes suggest that the lead source is similar for all hydrothermal stages and corresponds to the underlying Neo-Proterozoic basement. Lead isotope data, relative ages of deformation and comparison with neighbouring deposits suggest that large-scale fluid pulses occurred during the whole Hercynian orogeny rather than pulses restricted to the late Hercynian period. The vicinity of the Hercynian internal domain appears as a key control for deformation and fluid flow in the oolitic ironstones, which acted as a chemical and structural trap for the hydrothermal fluids. The epigenetic mineralisation of Saint-Aubin-des-Chateaux appears to be very similar to epigenetic sulphidation described in banded iron formation-hosted gold deposits.     

Seston fatty acid composition and copepod RNA:DNA ratio with respect to the underwater light climate in fluvial Lac Saint-Pierre

The relationship between the underwater light availability at different wavelengths (from 351 to 700?nm) and the fatty acid (FA) composition of seston, as well as the trophic transfer of fatty acids from producers to consumers and its influence on copepod growth condition, were investigated throughout fluvial Lac Saint-Pierre (Québec, Canada). Seston and zooplankton were collected at 11 sampling sites located within distinct water masses discriminated according to their underwater spectral characteristics. Diffuse light attenuation coefficients (K_d(??)) varied among sampling sites and wavelengths (??) and were negatively correlated to seston composition in some essential fatty acids. Particularly, the relationships between K_d(??) and the seston concentration in 20:5n3 and 22:6n3 differed and were wavelength dependent, being stronger for ?? close to the absorption maxima of chlorophyll a, suggesting a potential link with photosynthetic processes. The concentrations of 16:1n7, 18:3n3 and 20:5n3 in copepods were strongly correlated to those in the seston, which points towards the trophic transfer of these fatty acids between primary producers and herbivorous consumers. Moreover, the growth condition of copepods, as expressed by their RNA:DNA ratio, was correlated to the concentrations of 16:1n7, 18:3n3 and 20:5n3 in the seston and in copepods. Our field study sheds light on the potential importance, yet to be precised, of specific wavelengths as a driver of Lac Saint-Pierre??s productivity through their influence on fatty acids composition of seston and its nutritional quality for primary consumers.