The superficial sediments of the English Channel and its Western Approaches

The English Channel and its Western Approaches constitute a 700 km long epicontinental sea located in a temperate environment and in a tectonic setting where subsidence is minimal. A sedimentological survey of this region reveals quite distinct provinces: a central sector (Western Channel) where predominantly bioclastic sediments are widely represented, bounded by two mainly terrigenous-rich zones, the outer terrace of the Celtic Sea and the Eastern Channel. Lateral sedimentary variations of decreasing grain size are interpreted in terms of current velocity patterns. The various types of such sequences may relate to the degree of mixing of older lithoclastic sediments with the Holocene bioclastic supply. A scenario for the evolution of the recent sedimentation of this epicontinental sea is presented. Starting from a permanent marine zone—at least since the Würm period—that bounds the Bay of Biscay, the Holocene transgression progressed over the English Channel. In the Western Channel earlier terrigenous deposits were gradually overlapped by bioclastic sediments that originated on the pre-Mesozoic rocky substratum and which were particularly extensive off the Armorican Massif. In the subsequently submerged Eastern Channel the pre-Holocene clastic source has undergone comparatively less modification and still crops out in most of the area. With the opening of the Straits of Dover, at about 9000 yr BP, submersion was complete and new hydrodynamic conditions developed as the eustatic level stabilized. The present sediment distribution is in harmony with the hydrodynamic setting except on the southern rise of the Celtic Sea where both morphology and sedimentary patterns still largely reflect pre-Holocene conditions.     

Position of the palaeontological site Aven I des Abîmes de La Fage, at Noailles (Corrèze, France), in the European Pleistocene chronology

Mourer-Chauviré, C., Philippe, M., Quinif, Y., Chaline, J., Debard, E., Guérin, C. & Hugueney, M. 2003 (September): Position of the palaeontological site Aven I des Abîmes de La Fage, at Noailles (Corrèze, France), in the European Pleistocene chronology. Boreas , Vol. 32, pp. 521–531. Oslo. ISSN 0300–9483.
The palaeontological locality of Aven I des Abîmes de La Fage, at Noailles, Corrèze, France, is a limestone swallow hole filling which includes a rich fauna of micro- and macromammals, and one of the most complete Pleistocene avifaunas in the Palearctic. According to the mammal fauna and to the sedimentological study, the filling was previously assigned to the penultimate, Saalian, glaciation. U/Th datings made on a stalagmitic floor deposited in a lateral gallery on top of the filling confirm this previous attribution. Revised lists of the mammal and bird faunas are provided. The comparison of the La Fage fauna with that of other European localities, radiochronologically or biostratigraphically dated, leads to the conclusion that most of the fossiliferous layers were deposited during the Marine Isotope Stage (MIS) 8 (242 to 301 ka).     

Chemical effect of carbonaceous aerosols on the diurnal cycle of MBL ozone at a tropical site: measurements and simulations

During late austral summer and winter 1998, black carbon (BC) aerosols were monitored with an Aethalometer at 2 sites of La Réunion Island (Indian Ocean): Saint‐Denis, the main city and Sainte‐Rose, a quite uninhabited region situated at the east coast. BC concentration data at Saint‐Denis show a marked diurnal cycle, which may be primarily attributed to traffic. The background data found at night‐time display average BC concentrations, ranging from about 80 to 250 ng/m³ whereas during the day, BC concentrations increase by a factor of at least 4. In comparison, BC concentrations vary in the range of 10 to 60 ng/m³ at Sainte‐Rose. Ozone concentration was also measured at Saint‐Denis using a Dasibi photometer and found to be at significant levels (means: 16.5–23 ppbv in April and 28.5–34 ppbv in September). A noticeable increase of ozone concentrations during the day points out the build‐up of pollutants enhancing photochemical transformations. However, during traffic pollution peaks, ozone concentration displays systematic depletion. The comparison of ozone and BC measurements at both seasons points to some possible effects of heterogeneous interaction of ozone and its precursors with BC particles. These interactions were also simulated with a 0D time‐dependent chemistry model using conditions of a polluted site. The measured ozone concentration characteristics (mean concentration and range of variation) are well simulated in the presence of BC. Our model results show that at La Réunion Island adsorption of ozone and its precursors onto BC aerosol particles could be one of the important steps determining ozone concentration characteristics, especially in absence of photochemistry during night‐time.     

Quantitative mineralogical analysis of carbonate sediments by X-ray diffraction: a new, automatic method for sediments with low carbonate content

A new X-ray diffraction method has been developed whereby the weight percentages of aragonite and low and high-magnesium calcite are determined from the integrated peak areas of samples. Peak areas are measured by a step scanning method. The weight percentages of MgCO₃ in calcite are determined from the angular position of the calcite peak. This technique uses a direct calculation method which simplifies the preparation of the samples and the calibration processes and increases the quality of the results. The fully automatic method uses a desk-top computer to guide the diffractometer and to carry out the necessary calculations. Tests on precision and accuracy of the method indicate that results with less than ± 4% error (mineral %) and ± 0.6% error (MgCO₃%) are obtainable for all samples even those with a low (10%) carbonate content.