Repeated thermal resetting of phengites in the Mulhacen Complex (Betic Zone, southeastern Spain) shown byAr/Ar step heating and single grain laser probe dating

This study reports the results of the first⁴⁰Ar/³⁹Ar combined induction furnace and laser probe dating of phengites from the Mulhacen HP/LT metamorphic complex in the Betic Cordilleras, southern Spain. Laser step heating and spot fusion analyses on different halves of a split single grain were made with a continuous laser probe. Spot fusion analysis resulted in ages of about 30–31 Ma in the core and ages as low as 25–26 Ma in the rim. Laser step heating on the other half of the grain gave a spectrum with apparent ages increasing from about 25 Ma to 29.5 Ma. The age spectrum and the decreasing ages towards the rim of the grain may imply that resetting essentially occurred by volume diffusion of radiogenic⁴⁰Ar due to late stage reheating resulting from extensional tectonics. Ages around 30 Ma in the core of the grain are interpreted as minimum estimates of the cooling age of the main tectono-metamorphic phaseD₂.

Induction furnace step heating on phengite separates from mica schists and one gneiss resulted in two types of age spectra. Type I spectra show monotonously rising apparent ages from14.5 ± 1.9 Ma to20.7 ± 0.2 Ma, and in a second sample from16.9 ± 0.7 to29.7 ± 0.2 Ma. Type II spectra are characterized by plateaus of14.4 ± 0.1 Ma (the gneiss sample),17.3 ± 0.1 Ma and17.6 ± 0.1 Ma. Type II spectra show low temperature apparent ages significantly below the plateau age, implying resetting subsequent to initial cooling. Modelling of the age spectra demonstrated that the plateau ages are possibly the result of strong resetting (75–85% of argon loss) of an older isotope system. Total fusion of a number of phengite single grains from marbles taken close to type II mica schists yielded ages of15.4 ± 1.2 Ma and17.0 ± 0.7 Ma. The observed repeated resetting is coeval with major volcanic activity in basins adjacent to the metamorphic ranges, pointing to a resetting by advective fluid transport related to volcanism.     

Plissements Quaternaires Dans la Région de Gafsa et le Sud Tunisien

Résumé L'Atlas tunisien est affecté, dans la zone de Gafsa, de plissements quaternaires post-acheuléens et anté-moustériens. La succession stratigraphique se termine par des dépÔts continentaux qui, débutant avec le sommet du Miocène, sont surmontés en continuité de sédimentation par une série congloméatique renfermant des silex taillés de l'Acheuléen. Cet ensemble a été plissé en pli faillé avec flanc sud accentué et affaissé (pendages de 70 à 80 degrés). Au complexe plissé se superposent des alluvions horizontales dont la haute terrasse est datée par des industries à silex du Moustérien. Cette orogénèse s'intègre dans la géologie régionale du Sud tunisien où des faits analogues sont connus en plusieures localités.     

Unites structurales de la marge continentale sous-marine de Cannes a Menton d'apres la sismique refraction

Offshore seismic refraction profiles permitted us to characterize the extent of the geological formations which prolong the principal units of the Provence and Alpes Maritimes southwards and to establish their respective relationships. The concept of a “Var graben” is confirmed and defined. A magma chamber, probably in relation with the volcanic deposits of the Cap d'Ail, has been disclosed and coincides with the positive magnetic anomaly observed in this area.     

Sediment community metabolism associated with continental shelf hypoxia, Northern Gulf of Mexico

Net fluxes of respiratory metabolites (O₂, dissolved inorganic carbon (DIC), NH₄ ⁺, NO₃ ^?, and NO₂ ^?) across the sediment-water interface were measured using in-situ benthic incubation chambers in the area of intermittent seasonal hypoxia associated with the Mississippi River plume. Sulfate reduction was measured in sediments incubated with trace levels of³⁵S-labeled sulfate. Heterotrophic remineralization, measured as nutrient regeneration, sediment community oxygen consumption (SOC), sulfate reduction, or DIC production, varied positively as a function of temperature. SOC was inversely related to oxygen concentration of the bottom water. The DIC fluxes were more than 2 times higher than SOC alone, under hypoxic conditions, suggesting that oxygen uptake alone cannot be used to estimate total community remineralization under conditions of low oxygen concentration in the water column. A carbon budget is constructed that compares sources, stocks, transformations, and sinks of carbon in the top meter of sediment. A comparison of remineralization processes within the sediments implicates sulfate reduction as most important, followed by aerobic respiration and denitrification. Bacteria accounted for more than 90% of the total community biomass, compared to the metazoan invertebrates, due presumably to hypoxic stress.