Seismic expression of gas and gas hydrates across the western Black Sea

This study is a synthesis of gas-related features in recent sediments across the western Black Sea basin. The investigation is based on an extensive seismic dataset, and integrates published information from previous local studies. Our data reveal widespread occurrences of seismic facies indicating free gas in sediments and gas escape in the water column. The presence of gas hydrates is inferred from bottom-simulating reflections (BSRs). The distribution of the gas facies shows (1) major gas accumulations close to the seafloor in the coastal area and along the shelfbreak, (2) ubiquitous gas migration from the deeper subsurface on the shelf and (3) gas hydrate occurrences on the lower slope (below 750 m water depth). The coastal and shelfbreak shallow gas areas correspond to the highstand and lowstand depocentres, respectively. Gas in these areas most likely results from in situ degradation of biogenic methane, probably with a contribution of deep gas in the shelfbreak accumulation. On the western shelf, vertical gas migration appears to originate from a source of Eocene age or older and, in some cases, it is clearly related to known deep oil and gas fields. Gas release at the seafloor is abundant at water depths shallower than 725 m, which corresponds to the minimum theoretical depth for methane hydrate stability, but occurs only exceptionally at water depths where hydrates can form. As such, gas entering the hydrate stability field appears to form hydrates, acting as a buffer for gas migration towards the seafloor and subsequent escape.     

Modelling carbon cycling through phytoplankton and microbes in the Scotia—Weddell Sea area during sea ice retreat

An ecological model to calculate phytoplankton development and microbial loop dynamics in the marginal ice zone of the antarctic ecosystem has been established on the basis of physical and biological (phyto- and bacterioplankton biomass and activity and counting of two classes of heterotrophic nanoplankton) measurements carried out in the marginal ice zone of the Scotia-Weddell Sea sector of the Southern Ocean during sea ice retreat 1988 (EPOS 1 and 2 expeditions). Application of this model at latitudes where sea ice retreat occurs and in adjacent open sea and permanently ice-covered areas demonstrated that the marginal ice zone is a region of enhanced primary and bacterioplankton production. Combining the results of the phyto- and bacterioplankton models allowed the quantitative estimate of the carbon fluxes through the lower level of the planktonic food web of the Weddell Sea marginal ice zone during the sea ice retreat period. The resulting carbon budget revealed the quantitative importance of microbial and micrograzing processes in the pathways of net primary production, 71% of this latter being assimilated in the microbial food web. However, total net microbial food web secondary production contributed 28% of‘marginal ice zone produced’ food resources available to krill and other Zooplankton.     

On the use of long-term observation of water level and temperature along the shore for a better understanding of the dynamics: example of Toulon area,France

Ocean Dynamics - A dense network of instruments has been deployed within harbors along the Mediterranean coast, in the Toulon Metropole area, between the Hyères islands and the Sanary Bay in...     

Adakitic magmas in the Ecuadorian Volcanic Front: Petrogenesis of the Iliniza Volcanic Complex (Ecuador)

The Iliniza Volcanic Complex (IVC) is a poorly known volcanic complex located 60 km SSW of Quito in the Western Cordillera of Ecuador. It comprises twin peaks, North Iliniza and South Iliniza, and two satellite domes, Pilongo and Tishigcuchi. The study of the IVC was undertaken in order to better constrain the role of adakitic magmas in the Ecuadorian arc evolution. The presence of volcanic rocks with an adakitic imprint or even pristine adakites in the Ecuadorian volcanic arc is known since the late 1990s. Adakitic magmas are produced by the partial melting of a basaltic source leaving a garnet rich residue. This process can be related to the melting of an overthickened crust or a subducting oceanic crust. For the last case a special geodynamic context is required, like the subduction of a young lithosphere or when the subduction angle is not very steep; both cases are possible in Ecuador. The products of the IVC, made up of medium-K basaltic andesites, andesites and dacites, have been divided in different geochemical series whose origin requires various interactions between the different magma sources involved in this subduction zone. North Iliniza is a classic calc-alkaline series that we interpret as resulting from the partial melting of the mantle wedge. For South Iliniza, a simple evolution with fractional crystallization of amphibole, plagioclase, clinopyroxene, magnetite, apatite and zircon from a parental magma, being itself the product of the mixing of 36% adakitic and 64% calc-alkaline magma, has been quantified. For the Santa Rosa rhyolites, a slab melting origin with little mantle interactions during the ascent of magmas has been established. The Pilongo series magma is the product of a moderate to high degree (26%) of partial melting of the subducting oceanic crust, which reached the surface without interaction with the mantle wedge. The Tishigcuchi series shows two stages of evolution: (1) metasomatism of the mantle wedge peridotite by slab melts, and (2) partial melting (10%) of this metasomatized source. Therefore, the relative ages of the edifices show a geochemical evolution from calc-alkaline to adakitic magmas, as is observed for several volcanoes of the Ecuadorian arc.     

The Upper Rhône Delta Sedimentary Record in the Arles-Piton Core: Analysis of Delta-Plain Subenvironments, Avulsion Frequency, Aggradation Rate and Origin of Sediment Yield

Results from analyses of the Arles‐Piton sediment core, retrieved from the apex of the Rhône Delta, highlight processes of Holocene deltaic construction controlled mainly by hydrosedimentary variability and channel avulsions. The alluvial suite was investigated for grain size, sedimentary structures, CaCO₃, organic matter, heavy minerals and chrono‐stratigraphy (¹⁴C and archaeological/historical dates). The study shows the succession of six facies associations: a distributary channel (before 6157‐5843 BC), a swamp (5719‐5530/4796‐4463 BC), a distal flood plain (5719‐5530/4796‐4463 BC), a distributary channel (4796‐4463/2900‐2503 BC), a proximal flood plain (2900‐2503 BC/AD 270‐290), and a crevasse splay (after AD 270‐290). Substantial changes in hydrodynamics are strongly linked to three channel avulsions (before 6157‐5843 BC, after 4796‐4463 BC and after 2900‐2503 BC). A correlation with the whole channel avulsion history of the Rhône Delta allowed us to propose an average rhythm of channel avulsion of c. 1450 years. From 5719‐5530 BC to AD 270‐290, the flood plain aggraded at the average rate of 2.5 mm/a. The aggradation rates were higher both in the proximal and distal flood plains, where sedimentation process is continuous. They were lower both in the active distributary channels, because of frequent truncation of the alluvial suite, and the abandoned channels where detritic inputs are minimum. The sediment supply arriving to the upper Rhône Delta was derived mainly from proximal source areas (Massif Central, Southern Alps) during the last 8000 years, except during the hydrological changes of Roman antiquity during which detritic inputs were derived firstly from the Northern Alps and Southern Alps, and secondly from the Massif Central.     

The multivariate (co)variogram as a spatial weighting function in classification methods

The multivariate variogram and the multivariate covariogram are used as spatial weighting functions for forming spatially homogeneous groups automatically. The groups are created after either deflating similarities between distant samples with the multivariate covariogram or by inflating dissimilarities between distant samples with the multivariate variogram. These approaches can be seen as generalization of the Oliver and Webster proposal. Two data sets show the efficiency of the two weighting functions when compared to the classical approach which does not take spatial information into account. In one case study, the weighting of similarities by the multivariate covariogram showed more interpretable results than the weighting of dissimilarities by the multivariate variogram.     

Modélisation de propagation d’écoulement entre lits mineur et majeur sur les fleuves Sénégal et Niger

In the Senegal River valley and Niger Inner Delta, the annual floods inundate a wide floodplain consisting of a complex network of lakes and channels, where topographic information needed by standard hydraulic models is difficult to obtain. To represent the flood propagation between mainstream and floodplain, we use a model designed for flood propagation in river mainstreams with flat bed and large overflow and without topographic data. Depending on the water level in the riverbed, the model calibrated on the levels observed at two stations gives the level in the floodplains and propagation time between stations. Several cases are tested for various types of hydraulic connections between mainstream and floodplain. The model could correctly reproduce the flood rise and fall in the floodplain, even for a lake connected by a single channel to the riverbed or in the case of a strong attenuation of the flood between very distant stations.     

Speciation and role of iron in cloud droplets at the puy de Dôme station

Iron is the most abundant transition metal in the atmosphere and can play a significant role in cloudwater chemistry where its reactivity is closely related to the partitioning between Fe(II) and Fe(III). The objective of this work is to determine the total iron content and the iron speciation in a free tropospheric site, and to understand which factors influence these parameters. We collected 147 samples of cloudwater during 34 cloud events over a period of four years at the puy de Dôme summit. Besides iron we measured other chemical compounds, solar radiation, physico-chemical and meteorological parameters potentially connected with iron reactivity. The total iron concentrations ranged from 0.1 to 9.1 μM with the major frequency occurring at low levels. The pH and presence of organic complexants seem to be the most significant factors connected with total dissolved iron; while the iron oxidation state seems to be an independent factor. Light intensity, presence of complexants or oxidants (H₂O₂) do not influence the Fe(II)/Fe(Total) ratio, that was quite constant at about 0.75. This could be due to the potential redox that forces the Fe(II)-Fe(III) couple to the reduced form or, more probably to the complexation by Natural Organic Matter, that can stabilize iron in its reduced form and prevent further oxidation. Our field measurements did not show the diurnal cycle observed in surface water and predicted by models of atmospheric chemistry. This result prompts a more careful review of the role of iron and, by analogy, all the transition metals in atmospheric liquid phase, often over-estimated in the literature.