Seasonal control of the Saint-Lawrence maximum turbidity zone by tidal-flat sedimentation

The core of the turbidity maximum zone in the Saint-Lawrence Estuary is located in the North Channel and oscillates in front of the large (3×10⁶ m²) intertidal flats and marshes of Cap Tourmente. It is shown that seasonal fluctuations in the intensity and the position of this core are mainly determined by suspended sediment exchanges between the channel and the marshes. Fine sediments, most of them found 20 km downstream in the channel off Cap Maillard in late winter and early spring, are advected upstream over the flats during the summer months by the tide. The deposition, favored by marsh plant growth, reaches 5×10⁵ metric tons in three months. A period of intense erosion, at a mean rate of 4,500 metric tons per tide, coincides with the destruction of the plant cover by migratory geese. The material removed fills up the Chenal de l’Île d’Orléans upstream and is flushed back into the water column during the next spring freshette. This rotating seasonal sediment circulation, although very localized, exerts a major influence on the distribution and storage time of suspended particles in the upper estuary.     

A comparison of total precipitation values estimated from measurements and a 1D cloud model

The purpose of this study is to establish a relation between observed total precipitation values and estimations from a one-dimensional diagnostic cloud model. Total precipitation values estimated from maximum liquid water content, maximum vertical velocity, cloud top height, and temperature excess are also used to provide an equation for the total precipitation prediction. Data for this study were collected in Istanbul during the autumns of 1987 and 1988. The statistical models are developed with multiple regression technique and then comparatively verified with independent data for 1990. The multiple regression coefficients are in the range of 75% to 80% in the statistical models. Results of the test showed that total precipitation values estimated from the above techniques are in good agreement, with correlation coefficient between 40% and 46% based on test data for 1990.     

A detailed study of the duality relation for the least squares adjustment in euclidean spaces

The Euclidean spaces with their inner products are used to describe methods of least squares adjustment as orthogonal projections on finite-dimensional subspaces. A unified Euclidean space approach to the least squares adjustment methods “observation equations” and “condition equations” is suggested. Hence not only the two adjustment solutions are treated from the view-point of Euclidean space theory in a unified frame but also the existing duality relation between the methods of “observation equations” and “condition equations” is discussed in full detail. Another purpose of this paper is to contribute to the development of some familiarity with Euclidean and Hilbert space concepts. We are convinced that Euclidean and Hilbert space techniques in least squares adjustment are elegant and powerful geodetic methods.     

Production and pelagic dissolution of biogenic silica in the Southern Ocean

Vertical distributions of particulate silica, and of production and dissolution rates of biogenic silica, were determined on two N-S transects across the Pacific sector of the Antarctic Circumpolar Current during the austral spring of 1978. Particulate silica profiles showed elevated levels in surface water and near the bottom, with low (35–110 nmol Si · 1^?1) and vertically uniform values through the intervening water column. Both the particulate silica content of the upper 200 m and the production rate of biogenic silica in the photic zone increased from north to south, reaching their highest values near the edge of the receding pack ice. A significant, but variable, fraction (18–58%) of the biogenic silica produced in the surface layer was redissolving in the upper 90–98 m. Net production of biogenic silica in the surface layer (production minus dissolution) was proceeding at a mean rate of ca. 2 mmol Si · m^?2 · day^?1. This is ca. 4 times greater than the most recent estimate of the mean accumulation rate of siliceous sediments beneath the ACC. We estimate, based on mass balance, that the mean dissolution rate of biogenic silica in subsurface water column in the Southern Ocean is 1.2–2.9 mmol Si · m^?2 · day^?1.     

Les Facies de Transgression du Trias sur la Bordure Nord-est du Massif Central Français

Résumé La transgression triasique est suivie d'Est en Ouest sur la bordure du Massif Central français. Les formations sont de plus en plus récentes vers l'Ouest, indépendamment de leur faciès. Les limites du Buntsandstein et du Trias supérieur semblent locales et d'origine tectonique, tandis que la limite du Ladinien supérieur parait plus générale et correspond à la période où le caractère marin de la transgression sur les massifs hercyniens était le plus franc.Le début de la transgression aniso-ladinienne est daté pour l'Anisien par des empreintes de Reptiles et pour les faciès les plus marins du Ladinien par des Foraminifères et des pollens. A cette période, des microfaunes mésogéennes de mer ouverte apparaissent sur les bordures.Dans les faciès de transgression réduits, le Trias est constitué de grès fins silteux à pseudomorphoses de sel gemme et empreintes de Reptiles, étalés par une mer pelliculaire sur une surface plate. Le socle sous-jacent a fourni peu de matériel clastique et les courants sont orientés du Nord au Sud. Les évaporites sont plus concentrées sur la bordure que plus au large, ce qui implique une certaine aridité pendant toute la période de la transgression.Les faciès de bordure les plus avancés sur le socle sont formés d'une dolomie paralique supratidale déposée dans des lagunes évaporitiques. Sur les massifs hercyniens, des fractures et des failles ouvertes ont permis des circulations auxquelles sont liés des remplissages carbonates et des minéralisations.

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Conditions of the triassic transgression are analysed from East to West across the eastern border of the Central Massif. Formations directly overlying the basement get younger and younger westwards, without relation with the environment. Buntsandstein and upper Triassic boundaries appear to be syntectonic local limits. The upper Ladininian boundary seems to be more general and connected with the main period of the marine environment for the transgression on the variscan basement rocks. The anisoladinian transgression beginns during the anisian age as indicated by fooprints, while ladinian age is supported by mesogean foraminifera and palynology during the climax of the transgression, in an open sea environment.Silty fine sandstones with footprints and salt casts constitute the triassic transgression facies on the flattened border of the Central Massif. The underlying basement supplied only a little arenaceous material and the currents are oriented from Nord to South. Evaporites are more concentrated at the boundary of the transgression than towards the basin, what means aridity.Paralic supratidal dolomite constitutes the most external deposits at the transgression limit, in a syntectonic environment. Dolomite infillings and ore deposits are connected with water circulations along faults and fractures.

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Zusammenfassung Die Triastransgressionsschichten werden von Osten nach Westen am Nordrand des französischen Zentralmassivs beschrieben. Die auf dem Grundgebirge auflagernden Basisschichten werden, unabhängig von ihrer Faziesausbildung, nach Westen stets jünger. Die Begrenzungen der Ablagerungen des Buntsandsteins und der oberen Trias werden nur lokal, wahrscheinlich in Abhängigkeit von synsedimentären, tektonischen Strukturen angetroffen. Die Beckenausdehnung am Top des oberen Ladiniums ist dagegen zusammenhängend zu erkennen. Sie entspricht den eindeutigeren marinen Transgressionsverhältnissen auf dem variskischen Sockel.Die einsetzende Transgression des Aniso-Ladiniums wird durch Reptilienfährten als anisisch und die stärker marinen Ablagerungen durch Foraminiferen und Pollen als ladinisch datiert. In diesem Zeitabschnitt dringen offenmarine, mesogäische Mikrofossilien bis zum Beckenrand vor.Die geringmächtigen Transgressionsablagerungen bestehen aus siltigen Feinsanden mit Steinsalzpseudomorphosen und Reptilienfährten. Diese Sande wurden von einem sehr seichten Meer auf dem stark eingeebneten Grundgebirge verteilt. Das Grundgebirge hat nur sehr wenige klastische Abtragungsprodukte geliefert. Die Strömungen waren von Norden nach Süden ausgerichtet. Evaporite treten stärker in den randlichen Gebieten als zum offenen Becken hin auf. Dies zeigt auf eine ausgeprägte Aridität während der gesamten Transgressionszeit hin.Die am weitesten zum Festland hin vorgeschobenen Transgressionssedimente bestehen aus paralischen, subpratidalen Dolomiten, die sich in evaporitischen Lagunen bildeten. Die in den offenen Spalten und Verwerfungen des Grundgebirges zirkulierenden Lösungen führten zu Dolomitausfällungen und zu schichtgebundenen Vererzungen.

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Grain-size-sensitive flow and shear-stress enhancement at the brittle–ductile transition of the continental crust

Localized shear zones along low-angle normal faults have been identified in regions of extension at the brittle-ductile transition of the continental crust. The possibility of the strain localizing at a depth of 10 km is interpreted here as a consequence of an increase in the equivalent shear stress applied to the flow of the lower crust. This enhancement of the flow stress is seen as a prerequisite for the triggering of brittle deformation mechanisms leading to strain localization. The lower crust rheology used to examine this stress increase is strain-rate, temperature and grain-size dependent, due to the coupling of dislocation and diffusion creep. The model structure proposed consists of a top layer, the upper crust, gliding rigidly above a bottom layer, the lower crust, which deforms in simple shear. During a short time interval (1400 years), the equivalent shear stress is found to increase by a factor of up to 3 (67 MPa for anorthite and 17 MPa for quartz). For anorthite, this stress could explain the activation of a Mohr-Coulomb failure with a friction coefficient of 0.2, which is reasonable at the depth of 10 km. Dislocation creep is activated during a rapid change in the prescribed velocity, whereas diffusion creep dominates if the velocity is held constant, highlighting the importance of grain-size sensitivity for lower crustal rheology.     

Controls on Quaternary coastal evolution of the east-northeastern coast of Brazil: roles of sea-level history, trade winds and climate

East-northeastern Brazil has a wave-dominated, micro- to meso-tidal coast, lying entirely within the southern Atlantic trade wind belt. Integration of geologic mapping, radiocarbon dating and vibracoring data shows that the Quaternary coastal evolution of this area was controlled by three major factors: (1) sea-level history; (2) trade winds; and (3) climate change.

Sea-level history. Along the east-northeastern coast of Brazil, relative sea level has fallen approximately 5 m during the last 5000 y. Correlation of this sea-level history with the evolution of beach-ridge, lagoonal and coastal plain deposits shows that: (1) sea-level rise favours the formation of barrier island—lagoonal systems and the construction of intralagoonal deltas; (2) sea-level lowering is not conductive to barrier island formation. Rather, lagoons and bays become emergent and beach-ridge plains rapidly prograde.

Trade winds. Sediment dispersal systems along the coastal zone of east-northeastern Brazil have been highly persistent since Pleistocene time, as deduced from beach-ridge orientation. This persistence results from the fact that sediment dispersal in wave-dominated settings is ultimately controlled by atmospheric circulation which, for the east-northeastern coast of Brazil is associated with the South Atlantic high-pressure cell. The remarkable stability of this cell through time, has allowed the accumulation of extensive beach-ridge plains at the longshore drift sinks located along the coast.

Climate change. Effects of Quaternary climate changes on coastal sedimentation are twofold. Climate changes may affect rainfall patterns, thus exerting an important control on coastal dune development. Along the coast of northeastern Brazil, active coastal dunes are only present in those areas in which at least four consecutive dry months occur during the year. Mapping of these areas has shown that dune development during the Holocene has been episodic, these episodes being probably controlled by variations in rainfall patterns associated with climate changes. Secondly, despite its overall stability, the position of the high-pressure cell has experienced small shifts in position during the Holocene in response to climate changes. Changes in wind direction associated with these shifts have induced modifications in the coastal dispersion system, which are recorded in the strandplains as small truncations in the beach-ridge alignments.

These results have important implications in understanding accumulation of ancient sandstone shoreline sequences.