U-Th-Pb systematics of precambrian carbonate rocks: Dating of the formation and transformation of carbonate sediments

The current state in the Pb-Pb dating of the formation and epigenesis of Precambrian carbonate sediments has been reviewed. The geochemistry of admixture elements (U, Th, Pb, Rb, Sr, Mn, and Fe) constituting carbonate minerals and the redistribution of the elements in the course of early diagenesis and epigenesis have been considered. The advantages of choosing samples for Pb-Pb dating on the basis of geochemical criteria similar to those applied for Sr-isotope stratigraphy and the potentialities of applying different methods of decontamination of specimens, analyzed from surface contamination and possible epigenetic carbonate phases, were illustrated. By the example of a series of carbonate formations, distinctions in susceptibility of U-Pb and Rb-Sr systems of carbonate minerals to secondary transformations have been demonstrated. The prospects for using the method of step solution allowing noncogenetic carbonate generations to be separated and thus the accuracy and reliability of Pb-Pb dates to be upgraded were considered.     

Large bio-geographical shifts in the north-eastern Atlantic Ocean: From the subpolar gyre, via plankton, to blue whiting and pilot whales

Pronounced changes in fauna, extending from the English Channel in the south to the Barents Sea in the north-east and off Greenland in the north-west, have occurred in the late 1920s, the late 1960s and again in the late 1990s. We attribute these events to exchanges of subarctic and subtropical water masses in the north-eastern North Atlantic Ocean, associated with changes in the strength and extent of the subpolar gyre. These exchanges lead to variations in the influence exerted by the subarctic or Lusitanian biomes on the intermediate faunistic zone in the north-eastern Atlantic. This strong and persistent bottom-up bio-physical link is demonstrated using a numerical ocean general circulation model and data on four trophically connected levels in the food chain – phytoplankton, zooplankton, blue whiting, and pilot whales. The plankton data give a unique basin-scale depiction of these changes, and a long pilot whale record from the Faroe Islands offers an exceptional temporal perspective over three centuries. Recent advances in simulating the dynamics of the subpolar gyre suggests a potential for predicting the distribution of the main faunistic zones in the north-eastern Atlantic a few years into the future, which might facilitate a more rational management of the commercially important fisheries in this region.     

Hercynian polyphase tectonics in the northeast French Massif Central: the closure of the Brévenne Devonian–Dinantian rift

 Field and laboratory structural studies show that the Devonian–Dinantian units of the northeast French Massif Central experienced a complex and contrasting tectonic–metamorphic evolution during the Hercynian orogeny. The structural analysis of the pre-Middle Visean Brévenne–Violay–Beaujolais rocks, in the Loire area, shows a polyphase tectonic evolution associated with greenschist to amphibolite facies metamorphism. The first event, D1, probably occurred in Early Tournaisian or Latest Devonian times. It is responsible for the flat-lying regional foliation and the NW/SE- to N/S-trending lineation. It is well observed in the Violay group and corresponds to the NW-vergent emplacement of the Late Devonian units upon their gneissic basement, represented by the Affoux gneisses. The second event, D2, is responsible for the NE/SW- to E/W-trending lineation. To the south, D2 deformation is locally reworked by the Grand-Chemin dextral wrench fault, around 345–350 Ma ago. This polyphase deformation is also found in several Devonian–Dinantian areas of the NE Massif Central, but not in Morvan. This tectonics corresponds to the Tournaisian closure, by northward thrusting and subsequent intracontinental deformation, of the oceanic Brévenne–Violay–Beaujolais rift which opened in Devonian times in a back-arc setting. Received: 4 September 1998 / Accepted: 27 May 1999     

Variability of ozone in the marine boundary layer of the equatorial Pacific Ocean

This study examines the processes controlling the diurnal variability of ozone (O₃) in the marine boundary layer of the Kwajalein Atoll, Republic of the Marshall Islands (latitude 8° 43′ N, longitude 167° 44′ E), during July to September 1999. At the study site, situated in the equatorial Pacific Ocean, O₃ mixing ratios remained low, with an overall average of 9–10 parts per billion on a volume basis (ppbv) and a standard deviation of 2.5 ppbv. In the absence of convective storms, daily O₃ mixing ratios decreased after sunrise and reached minimum during the afternoon in response to photochemical reactions. The peak-to-peak amplitude of O₃ diurnal variation was approximately 1–3 ppbv. During the daytime, O₃ photolysis, hydroperoxyl radicals, hydroxyl radicals, and bromine atoms contributed to the destruction of O₃, which explained the observed minimum O₃ levels observed in the afternoon. The entrainment of O₃-richer air from the free troposphere to the local marine boundary layer provided a recovery mechanism of surface O₃ mixing ratio with a transport rate of 0.04 to 0.2 ppbv per hour during nighttime. In the presence of convection, downward transport of O₃-richer tropospheric air increased surface O₃ mixing ratios by 3–12 ppbv. The magnitude of O₃ increase due to moist convection was lower than that observed over the continent (as high as 20–30 ppbv). Differences were ascribed to the higher O₃ levels in the continental troposphere and weaker convection over the ocean. Present results suggest that moist convection plays a role in surface-level O₃ dynamics in the tropical marine boundary layer.     

Supersonic jet formation and propagation in x-pinches

Observations of supersonic jet propagation in low-current x-pinches are reported. X-pinches comprising of four 7.5 ??m diameter tungsten wires were driven by an 80 kA, 50 ns current pulse from a compact pulser. Coronal plasma surrounding the wire cores was accelerated perpendicular to their surface due to the global J×B force, and traveled toward the axis of the x-pinch to form an axially propagating jet. These jets moved towards the electrodes and, late in time (??150 ns), were observed to propagate well above the anode with a velocity of 3.3±0.6×10⁴ m/s. Tungsten jets remained collimated at distances of up to 16 mm from the cross point, and an estimate of the local sound speed gives a Mach number of ??6. This is the first demonstration that supersonic plasma jets can be produced using x-pinches with such a small, low current pulser. Experimental data compares well to three-dimensional simulations using the GORGON resistive MHD code, and possible scaling to astrophysical jets is discussed.     

Paleoecology and radiocarbon dating of the Pleistocene megafauna of the Brazilian Intertropical Region

During the Pleistocene a fauna composed of large (biomass > 44 kg) and giant mammals (biomass > 1000 kg) that are usually associated with open environments lived in the Brazilian Intertropical Region. We present here new information concerning the paleoecology and chronology of some species of this megafauna. Carbon isotope analyses were performed for a better understanding of the paleoecology of the species Eremotherium laurillardi (Lund, 1842), Notiomastodon platensis (Ameghino, 1888) and Toxodon platensis (Owen, 1849). The δ¹³C data allow attributing a generalist diet to these species, which varied according to the kind of habitat in which they lived. In more open habitats all species were grazers; in mixed habitats E. laurillardi and T. platensis were mixed feeders, and N. platensis was grazer; and in more closed habitats all species were mixed feeders.