Oxygen isotope studies of potassic volcanic rocks of the Roman Province,Central Italy

The ¹⁸O/¹⁶O ratios of rocks and coexisting minerals were measured for 93 samples of leucite-bearing lavas, pyroclastics, and related volcanic rocks from the Quaternary Roman Co-Magmatic Province, Italy. The ¹⁸O values were found to generally increase northward in the sequence: Ischia (5.8 to 7.0); Somma-Vesuvius and Phlegrean Fields (7.3 to 8.3); Alban Hills (7.3 to 8.7); M. Sabatini (7.3 to 9.7); Vico Volcano (7.4 to 10.2); and M. Vulsini (8.1 to 11.7). The northward increase in ¹⁸O parallels a similar increase in ⁸⁷Sr/⁸⁶Sr, and these data indicate that the Roman magmas have interacted strongly with high-¹⁸O continental crust. A marked increase in ¹⁸O occurs just north of Rome where the Roman Province begins to overlap the calc-alkaline, oversaturated Tuscan Magmatic Province. Therefore, some of the observed ¹⁸O/¹⁶O and ⁸⁷Sr/⁸⁶Sr enrichments in the Roman magmas may have been facilitated by direct mixing with the high-¹⁸O Tuscan magmas or because the high-¹⁸O country rocks underwent widespread heating during a couple of million years of Tuscan igneous activity. Although many of the Roman magmas underwent fractional crystallization without appreciable change in ¹⁸O, contamination has produced a correlation between ¹⁸O and SiO₂ content at several of the volcanic centers; thus the trachytes are typically higher in ¹⁸O than the undersaturated rocks. The major features of the oxygen isotope data can be explained in terms of a simple two-component mixing model in which one end-member was a primary, strongly undersaturated magma derived from the upper mantle, with ¹⁸O+6, ⁸⁷Sr/⁸⁶Sr0.704 to 0.705, and SiO₂<44wt.%. However, none of the analyzed samples have these values, as they have all been contaminated to some extent. The closest approach is found in some of the leucitepyroxenite ejecta from the Alban Hills. The second end-member, derived from the continental crust, had a variable composition with ¹⁸O+12 to +20, ⁸⁷Sr/⁸⁶Sr0.712 to 0.720, and SiO₂65wt.%, and it mixed in much greater proportions in the volcanoes north of Rome than in those of the Alban Hills or the Naples area. The widespread interactions between the Roman magmas and the continental crust are probably due to (1) the fact that such low-SiO₂ magmas always have a very strong tendency to interact with quartz-bearing rocks of the continental crust, and (2) in Italy, these magmas were emplaced into a tectonically very active area containing poorly consolidated sedimentary rocks, and in the northern part of the belt there had been a prior history of extensive calc-alkaline igneous activity.Publication of the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, Contribution Number 2501.     

Retrograde mineral and fluid evolution in high-pressure metapelites (Schistes lustrés unit, Western Alps)

Fluid inclusions have been analysed in successive generations of syn-metamorphic segregations within low-grade, high-pressure, low-temperature (HP–LT) metapelites from the Western Alps. Fluid composition was then compared to mass transfer deduced from outcrop-scale retrograde mineral reactions. Two types of quartz segregations (veins) occur in the `Schistes lustrés' unit: early blueschist-facies carpholite-bearing veins (BS) and retrograde greenschist-facies chlorite-bearing veins (GS). Fluid inclusions in both types of segregations are aqueous (no trace of dissolved gases such as CO₂, CH₄, N₂), with significant differences in density and composition (salinity). BS fluids are moderately saline fluids (average 9.1 wt% eq. NaCl) characterized by a chronological trend towards more dilute composition (from 15 down to 0 wt% eq. NaCl), whereas GS fluids have a very constant salinity of ∼3.7 wt% eq. NaCl. Both types of inclusions were continuously reset to lower densities along the retrograde path, until a temperature of ∼300 °C. Mass-balance calculations, together with fluid inclusion data, suggest that GS fluids result from the mixing between two fluid sources: one initial, early metamorphic, moderately saline HP fluid and a second nearly pure water fluid provided by the breakdown of carpholite. Estimates of the amount of water released by carpholite breakdown result in a dilution of the interstitial fluid phase (from 10 to 2.5–4 wt% eq. NaCl) consistent with the actual shift of the fluid composition. Alkali elements required for the formation of the GS chlorite + phengite assemblage after carpholite could be locally provided by HP phengite. This is taken as an indirect evidence that, during the generation of both BS and GS fluids, mixing with externally derived fluids may have been very limited. The location, amount and constant composition of the less saline GS fluids appear to be related to an interconnected porosity at the time of inclusion formation. Received: 19 October 1998 / Accepted: 19 July 2000     

Neogene–Quaternary structures in the border zone between Alps, Dinarides and Pannonian Basin (Hrvatsko zagorje and Karlovac Basins, Croatia)

Analysis of Neogene-Quaternary structures from seismic lines, surface measurements and geological-mapping is presented from the border zone between the Alps, Dinarides and Pannonian Basin. First, Early Miocene extension was possibly characterised by ENE directed extension. It was partly synchronous with NW-SE shortening. Second, Middle Miocene extension was possibly characterised by NW-SE to WNW-ESE directed extension. Again, this event was followed by a new generation of thrusts related to end-Sarmatian shortening. The last, Late Miocene E-W to WNW-ESE directed extension was followed by a final shortening that created major, map-scale folds, basement pop-ups and inverted former basins. Geometry, onlap and thickness patterns of the youngest syn-tectonic basin fill indicate that this last, N-S to NW-SE directed shortening started in Late Pontian and continued up to the present time. When taking into account the wider surrounding area, it seems that the structures related to this latest shortening are arranged in often perpendicular directions, centred at the eastern end of the Periadriatic lineament. To explain this fan-like pattern of synchronous shortenings a kinematic model is proposed combining counter-clockwise rotation with north- or northwestward shift of the Dinaridic block with respect to the more stable Alpine buttress.     

Selective feeding of the oystercrassostrea gigas fed on a natural microphytobenthos assemblage

In estuarine ecosystems, microphytobenthos resuspended by tidal currents often represents a large part of the food supply available to bivalves which feed selectively. This study investigated the feeding behavior of the Pacific oysterCrassostrea gigas (Thunberg) relative to a natural microphytobenthic assemblage and the effect on the structure of this assemblage. Oysters were fed only benthic microalgae collected on the intertidal mud flats of Bourgneuf Bay (France) at a suspended particulate matter concentration above the threshold of pseudofeces production. All species in the assemblage were endemic diatoms characteritic of tidal mud flat environments. Four dominant ones, which were all solitary cells with spear-shaped frustules [Navicula ammophila (Grunow),Navicula rostellata (Kützing),Plagiotropis lepidoptera (Kuntze), andStaurophora amphioxys (Mann)] represented more than 95% of the 16 species involved. Analysis of feeding processes showed that the retention of the four main diatoms was not significantly different, but that two species, the smallest (N. ammophila [22×4 μm]) and the largest (P. lepidoptera [60×15 μm]), were preferentially ingested. The study of post-ingestive selection revealed that these two species were also preferentially digeted, i.e., preferentially directed into the digestive diverticulum, when they passed through the gut ofC. gigas. Cell size and shape did not appear to account for pre-ingestive and post-ingestive selection. The composition of the assemblage was significantly modified by oyster filtration, although the retention rates of the four main species were not significantly different. The composition of microalgae in pseudofeces and feces as a result of pre-ingestive and post-ingestive selection differed from that in seawater.     

Method for Stochastic Inverse Modeling of Fault Geometry and Connectivity Using Flow Data

This paper focuses on fault-related uncertainties in the subsurface, which can significantly affect the numerical simulation of physical processes. Our goal is to use dynamic data and process-based simulation to update structural uncertainty in a Bayesian inverse approach. We propose a stochastic fault model where the number and features of faults are made variable. In particular, this model samples uncertainties about connectivity between the faults. The stochastic three dimensional fault model is integrated within a stochastic inversion scheme in order to reduce uncertainties about fault characteristics and fault zone layout, by minimizing the mismatch between observed and simulated data.     

High-pressure behaviour of Ca-rich C 2 /c clinopyroxenes along the join diopside-enstatite (CaMgSi2O6-Mg2Si2O6)

 An in situ high-pressure (HP) X-ray diffraction investigation of synthetic diopside and of the Ca_0.8Mg_1.2Si₂O₆ clinopyroxene (Di₈₀En₂₀) was performed up to respectively P=40.8 and 15.1 GPa, using high brilliance synchrotron radiation. The compression of the cell parameters is markedly anisotropic, with β_b ⋙ β_c > β_a > β_asinβ for any pressure range and for both diopside and Di₈₀En₂₀. The compressibility along the crystallographic axes decreases significantly with pressure and is higher in Di₈₀En₂₀ than in diopside. The β cell parameter decreases as well with pressure, at a higher rate in Di₈₀En₂₀. The cell volume decreases at almost the same rate for the two compositions, since in diopside a higher compression along a* occurs. A change in the mechanism of deformation at P higher than about 5–10 GPa is suggested for both compositions from the analysis of the strain induced by compression. In diopside at lower pressures, the deformation mainly occurs, at a similar rate, along the b axis and at a direction 145° from the c axis on the (0 1 0) plane. At higher pressures, instead, the deformation occurs mostly along the b axis. In Di₈₀En₂₀ the orientation of the strain axes is the same as in diopside. The substitution of Ca with Mg in the M2 site induces at a given pressure a higher deformation on (0 1 0) with respect to diopside, but a similar change in the compressional behaviour is found. Changes in the M2 polyhedron with pressure can explain the above compressional behaviour. A third-order Birch-Murnaghan equation of state was fit to the retrieved volumes, with K=105.1(9) GPa, K′=6.8(1) for diopside and K=107.3(1.4) GPa, K′=5.7(3) for Di₈₀En₂₀; the same equation can be applied for any pressure range. The elasticity of diopside is therefore not significantly affected by Mg substitution into the M2 site, in contrast to the significant stiffening occurring for Ca substitution into Mg-rich orthopyroxenes. Received: 3 January 2000 / Accepted: 21 May 2000     

Modélisation physique de l'intrusion d'eau de mer dans un aquifère karstique : cas de l'Almyros d'Héraklion (Crète)

Coastal aquifers are subjected to natural seawater intrusion. Laboratory experiments were made to model the hydraulic conditions of saltwater inflow into a porous karstic conduit through which the freshwater flows. The conceptual model is based on the functioning scheme of the coastal karst system Almyros of Heraklion, Crete. The experiments show that seawater intrusion in a karst conduit can be diffuse, does not always lead to density separation of freshwater and seawater, and is not necessarily related to a conduit-network directly connected to the sea. To cite this article: B. Arfib, J. Ganoulis, C. R. Geoscience 336 (2004).     

Temporal variability in predator presence around a fin fish farm in the Northwestern Mediterranean Sea

Recently, aquaculture has generated worldwide interest as a result of the overexploitation of wild stocks combined with a growing international demand for fish and seafood products. Wild fish attracted to the marine fish farms, together with the presence of the farmed fish, are powerful attractants to predators that normally feed on similar or identical fish stocks in nature. This 9‐year study describes for the first time in Mediterranean waters the temporal variability of mammalian and avian predators in a coastal fin fish farm. In all, 99 months (1062 days during 36 consecutive seasons) were spent in the field. By examining the results of this study, it is clear that species as seagulls, shags, bottlenose dolphins and grey herons (considered to cause economic loss in aquaculture owing to direct predation) interact regularly with the fish farm. Although bottlenose dolphins and grey herons were not the most important of all predator species, predatory interactions with the fish farm occurred with what seems to be increasing regularity. Another result observed is the possible bottlenose dolphins’ attraction caused by the harvesting operations in the fish farm. The fish farm offers an alternative food source for predators; hunting at fish farms usually requires less effort on the part of the predator, and becomes a more attractive option than hunting wild fish over wide ranges. During the period of this study, individually identified dolphins feeding were regularly observed feeding on discarded fish from fish farm workers during harvesting operations, supporting the possibility that some individuals are habituated to this food supply. Based on the evidence presented in this paper, it is recommended that strategies for the management of both the aquaculture industry and marine mammal populations should take the results of this study into consideration.     

Compressive fluctuations in the solar wind and their polytropic index

Magnetohydrodynamic compressive fluctuations of the interplanetary plasma in the region from 0.3 to 1 AU have been characterized in terms of their polytropic index. Following Chandrasekhar’s approach to polytropic fluids, this index has been determined through a fit of the observed variations of density and temperature. At least three different classes of fluctuations have been identified: (1) variations at constant thermal pressure, in low-speed solar wind and without a significant dependence on distance, (2) adiabatic variations, mainly close to 1 AU and without a relevant dependence on wind speed, and (3) variations at nearly constant density, in fast wind close to 0.3 AU. Variations at constant thermal pressure are probably a subset of the ensemble of total-pressure balanced structures, corresponding to cases in which the magnetic field magnitude does not vary appreciably throughout the structure. In this case the pressure equilibrium has to be assured by its thermal component only. The variations may be related to small flow-tubes with approximately the same magnetic-field intensity, convected by the wind in conditions of pressure equilibrium. This feature is mainly observed in low-velocity solar wind, in agreement with the magnetic topology (small open flow-tubes emerging through an ensemble of closed structures) expected for the source region of slow wind. Variations of adiabatic type may be related to magnetosonic waves excited by pressure imbalances between contiguous flow-tubes. Such imbalances are probably built up by interactions between wind flows with different speeds in the spiral geometry induced by the solar rotation. This may account for the fact that they are mainly found at a large distance from the sun. Temperature variations at almost constant density are mostly found in fast flows close to the sun. These are the solar wind regions with the best examples of incompressible behaviour. They are characterized by very stable values for particle density and magnetic intensity, and by fluctuations of Alfvénic type. It is likely that temperature fluctuations in these regions are a remnant of thermal features in the low solar atmosphere. In conclusion, the polytropic index appears to be a useful tool to understand the nature of the compressive turbulence in the interplanetary plasma, as far as the frozen-in magnetic field does not play a crucial role.     

A SEM-based method to determine the mineralogical composition and the particle size distribution of suspended sediment

A robust method for characterizing the mineralogy of suspended sediment in continental rivers is introduced. It encompasses 3 steps: the filtration of a few milliliters of water, measurements of X-ray energy dispersive spectra using Scanning Electron Microscopy (SEM), and robust machine learning tools of classification. The method is applied to suspended particles collected from various Amazonian rivers. A total of more than 204,000 particles were analyzed by SEM-EDXS (Energy Dispersive X-ray Spectroscopy), i.e. about 15,700 particles per sampling station, which lead to the identification of 15 distinct groups of mineralogical phases. The size distribution of particles collected on the filters was derived from the SEM micrographs taken in the backscattered electron imaging mode and analyzed with ImageJ freeware. The determination of the main mineralogical groups composing the bulk sediment associated with physical parameters such as particle size distribution or aspect ratio allows a precise characterization of the load of the terrigenous particles in rivers or lakes. In the case of the Amazonian rivers investigated, the results show that the identified mineralogies are consistent with previous studies as well as between the different samples collected. The method enabled the evolution of grain size distribution from fine to coarse material to be described in the water column. Implications about hydrodynamic sorting of mineral particles in the water column are also briefly discussed. The proposed method appears well suited for intensive routine monitoring of suspended sediment in river systems.