Topical Collection: Groundwater-based agriculture in the Mediterranean

This essay introduces a collection of articles that explore the future of groundwater-based agriculture in the Mediterranean from an interdisciplinary perspective, in a context of declining water tables due to intensive groundwater use. The imminent crisis that many groundwater economies face due to very rapid and intense global change may have severe irreversible social, economic and environmental consequences, but could also be the opportunity to make a clear break with current agricultural development models and move towards more sustainable agricultural practices. The Mediterranean region is, therefore, an interesting case for the future of intensive groundwater use, as innovative ideas and practices may emerge and inspire similar groundwater-based agricultural systems around the world.     

ALMA as the ideal probe of the solar chromosphere

The very nature of the solar chromosphere, its structuring and dynamics, remains far from being properly understood, in spite of intensive research. Here we point out the potential of chromospheric observations at millimeter wavelengths to resolve this long-standing problem. Computations carried out with a sophisticated dynamic model of the solar chromosphere due to Carlsson and Stein demonstrate that millimeter emission is extremely sensitive to dynamic processes in the chromosphere and the appropriate wavelengths to look for dynamic signatures are in the range 0.8–5.0 mm. The model also suggests that high resolution observations at mm wavelengths, as will be provided by ALMA, will have the unique property of reacting to both the hot and the cool gas, and thus will have the potential of distinguishing between rival models of the solar atmosphere. Thus, initial results obtained from the observations of the quiet Sun at 3.5 mm with the BIMA array (resolution of 12″) reveal significant oscillations with amplitudes of 50–150 K and frequencies of 1.5–8 mHz with a tendency toward short-period oscillations in internetwork and longer periods in network regions. However higher spatial resolution, such as that provided by ALMA, is required for a clean separation between the features within the solar atmosphere and for an adequate comparison with the output of the comprehensive dynamic simulations.     

Geochemical assessment,mixing behavior and environmental impact of thermal waters in the Guelma geothermal system,Algeria

A study of thirteen geothermal springs located in the geothermal field of Guelma, northeastern Algeria, was conducted. Samples were collected during the period between January 2014 and February 2016. Geochemical processes responsible for the chemical composition of thermal and mineralized water were evaluated. The hydrochemical analysis shows that the thermal waters are characterized by the presence of two different chemical facies, the first type SO₄–Ca in the east, west and south of Guelma, the second type HCO₃–Ca in the south. This analysis also attributed to sodium, chlorides, and sulfates to an evaporitic terrigenous origin by the molar ratio Sr²⁺/Ca²⁺. The thermal spring waters from Guelma geothermal system have a meteoric origin, and all samples are immature with strong mixing between hot and shallow waters with 19–38.5% rate of mixing. The silica geothermometer shows that these thermal waters have a temperature varying from 84 to 122 °C and that the water came from a depth of 2100–3000 m through a fault system that limits the pull-apart basin of Guelma. Potential environmental effluent from thermal spas could pollute in both the irrigation and drinking waters, and which imposes danger on the health of the inhabitants of the region.

     

Transmission losses to alluvium and associated moisture dynamics in a semiarid ephemeral channel system in Southern Africa

Transmission losses through the bed of ephemeral rivers in arid and semiarid regions can account for a large proportion of the total amount of runoff generated upstream. Losses have typically been estimated by measuring discharge at two points in the channel system. This paper presents some results from a semiarid catchment in South Africa based on moisture observations of the alluvial material using neutron probe access tubes. Channel flow in this catchment is a very infrequent occurrence and consequently presents few opportunities to observe the processes of transmission loss. Only two events have occurred since the beginning of observations in this area (3 October, and 14 November, 1989). the available information is far from complete but suggests that some 75 per cent of the upstream flow in the first event and 22 per cent in the second event was lost to the alluvial material.     

Fluid evolution of the late Archaean Rämepuro gold deposit in the Ilomantsi greenstone belt in eastern Finland

The late Archaean (ca. 2.7 Ga) Ilomantsi greenstone belt hosts a large number of small mesozonal gold occurrences. The Rämepuro deposit is spatially related to a feldspar porphyry dyke which has intruded the contact between metagreywackes and intermediate metavolcanic rocks. It consists of gold-bearing quartz-tourmaline-sulphide veins located mainly within an intensively altered and sheared zone about 20-30 m in width. Two types of fluid inclusions were distinguished in the quartz veins: (1) H₂O-CH₄ (Й equiv. wt% NaCl), and (2) H₂O-CO₂ (⢬ equiv. wt% NaCl). The two compositionally different fluid-inclusion types occur in separate veins. The compositions of decrepitate residues indicate that the type 1 fluid inclusions consist predominantly of Na, S, Ca, Cl, and in lesser proportions of K and Fe. The residues of the type 2 fluid inclusions are dominated by Na, Ca, Cl and minor K. For the type 1 and type 2 fluid inclusions, microthermometric experiments indicate average homogenisation temperatures of 310-350 and 220-250 °C respectively. Both types of fluid inclusions contain 1-3 anisotropic solids (Ca/Mg carbonate?, nahcolite?, tourmaline?). Sulphide daughter minerals occur only in the type 1 fluid inclusions. This fluid type is also often associated with short trails of sulphide inclusions which suggest mobilisation and redistribution of ore constituents. Some type 1 fluid inclusions display distinct morphological features ("implosion textures") related to post-trapping re-equilibration. The presence of the fluid-inclusion re-equilibration textures in some auriferous quartz veins and their absence in other gold-bearing quartz veins suggest an age difference between them. This conclusion is also supported by the compositionally different fluid regimes associated with the different quartz veins, indicating a change from reducing (CH₄) to a more oxidising (CO₂) fluid environment. The fluid-inclusion evidence suggests two separate gold mineralisation events, one related to late Archaean regional metamorphism, the other to Palaeoproterozoic thrusting and metamorphism.     

The distribution and flux of particulate matter in the Bideford River Estuary,Prince Edward Island,Canada

The temporal and spatial distribution of total and organic particulate matter is investigated in the Bideford River estuary. Particulate matter is homogenously distributed in both the water column and the surface sediment, due to high rates of resuspension and lateral transport. The measured mean sedimentation rate for the estuary is 183·5 g of particulate matter m^?2 day^?1, of which more than half is due to resuspension.The surface sediment of the estuary is quantitatively the dominant reservoir of organic matter, with an average of 902·5 g of particulate organic carbon (POC) m^?2 and 119·5 g of particulate organic nitrogen (PON) m^?2. Per unit surface area, the sediment contains 450 times more POC and 400 times more PON than the water column. Terrestrial erosion contributes high levels of particulate matter, both organic and inorganic, to the estuary from the surrounding watershed. Low rates of sediment export from the estuary result in the accumulation of the terrigenous material. The allochthonous input of terrigenous organic matter masks any relationship between the indigenous plant biomass and the organic matter.In the water column, a direct correlation exists between the organic matter, i.e. POC and PON, concentration and the phytoplankton biomass as measured by the plant pigments. Resuspension is responsible for the residual organic matter in the water column unaccounted for by the phytoplankton biomass.The particulate content of the water column and the surface sediment of the estuary is compared to that of the adjacent bay. Water-borne particulate matter is exported from the estuary to the bay, so that no significant differences in concentration are noted. The estuarine sediment, however, is five to six times richer in organic and silt-clay content than the bay sediment. Since sediment flux out of the estuary is restricted, the allochthonous contribution of terrigenous particulate matter to the bay sediment is minor, and the organic content of the bay sediment is directly correlated to the autochthonous plant biomass.     

Mise en évidence et analyse d'une structure atlasique ennoyée au front de la Chaîne alpine tunisienne

The eastern Tunisian Atlas shows major subsurface faults: the Kairouan–Sousse Fault (FKS), to the north, and the El Hdadja fault (FEH), to the south. The FKS is an inherited structural trend active since Late Cretaceous times. This fault is an eastern splay of the Chérichira–Labeïd fault. It separates a large northern diapiric structure (Ktifa Diapir) from a subsident domain (the Kairouan–El Hdadja rim-syncline), with a pull-apart configuration to the south. The latter area, which appears to be an inherited weakness zone at the range border, has recorded a series of tectonic events that characterizes the Alpine structural development in Tunisia. To cite this article: S. Khomsi et al., C. R. Geoscience 336 (2004).     

3-D magnetic inversion and satellite imagery for the Um Salatit gold occurrence,Central Eastern Desert,Egypt

The Um Salatit is a gold occurrence situated in the Central Eastern Desert of Egypt. The studied area is characterized by an intensive imbrication zone of serpentinites and metavolcanic rocks. The integrated use of aeromagnetic and spectral reflectance remote sensing data has proved effective for mapping geology related to ore deposits. Spectral reflectance maps have been produced to discriminate various rocks, such as serpentinites, gabbros, and metavolcanic rocks. 3-D inversion of aeromagnetic data acquired over the Um Salatit study area produced a 3-D magnetic susceptibility model showing magnetic bodies surrounded by less magnetic host rock. The magnetic features are elongated ENE-WSW parallel to the major thrust fault and mapped the Um Salatit serpentinized rocks. Remote sensing data allow investigating surficial geological features and mapping the mineralized areas. Remote results, in conjunction with 3-D inversion of aeromagnetic data, demonstrate that gold occurrence in the study area was effectively restricted to the highly magnetic zone interpreted as host rocks.     

Gravity field and interior of Rhea from Cassini data analysis

The Cassini spacecraft encountered Rhea on November 26, 2005. Analysis of the Doppler data acquired at and around closest approach yields the mass of Rhea and the quadrupole moments of its gravity field with unprecedented accuracy. We obtained which corresponds to a density of . Our results for J₂ and C₂₂ are (7.947±0.892)×¹⁰⁻⁴ and (2.3526±0.0476)×¹⁰⁻⁴, respectively. These values are consistent with hydrostatic equilibrium. From the value of C₂₂, we infer the non-dimensional moment of inertia C/MR²=0.3721±0.0036. Our models of Rhea's interior based on the gravity data favor an almost undifferentiated satellite. A discontinuity between a core and a mantle is possible but not required by the data. Models with a constant silicate mass fraction throughout the body cannot account for the determined quadrupole coefficients. The data exclude fully differentiated models in which the core would be composed of unhydrated silicates and the mantle would be composed of pure ice. If the mantle contains 10% in mass of silicates, the core extends to 630 km in radius and has a silicate mass fraction of 40%. A continuous model in which the silicates are more concentrated toward the center of the body than in the outer layers is allowed by the gravity data but excluded by thermal evolution considerations. The one model that fits the gravity data and is self-consistent when energy transport and ice melting are qualitatively considered is an “almost undifferentiated” Rhea, in which a very large uniform core is surrounded by a relatively thin ice shell containing no rock at all.     

10 years of RXTE monitoring of anomalous X-ray pulsar 4U 0142+61: long-term variability

We report on 10 yr of monitoring of the 8.7-s Anomalous X-ray Pulsar 4U 0142+61 using the Rossi X-Ray Timing Explorer (RXTE). This pulsar exhibited stable rotation from 2000 until February 2006: the RMS phase residual for a spin-down model which includes ν, , and is 2.3%. We report a possible phase-coherent timing solution valid over a 10-yr span extending back to March 1996. A glitch may have occurred between 1998 and 2000, but it is not required by the existing data. We also report that the source’s pulse profile has been evolving in the past 6 years, such that the dip of emission between its two peaks has been getting shallower since 2000, almost as if the profile is recovering to its pre-2000 morphology, in which there was no clear distinction between the peaks. These profile variations are seen in the 2–4 keV band but not in 6–8 keV. Finally, we present the pulsed flux time series of the source in 2–10 keV. There is evidence of a slow but steady increase in the source’s pulsed flux since 2000. The pulsed flux variability and the narrow-band pulse profile changes present interesting challenges to aspects of the magnetar model. This work was supported by the Natural Sciences and Engineering Research Council (NSERC) PGSD scholarship to R.D. F.P.G. holds a National Research Council Research Associateship Award at NASA Goddard Space Flight Center. Additional support was provided by NSERC Discovery Grant Pgpin 228738-03 NSERC Steacie Supplement Smfsu 268264-03, FQRNT, CIAR, and CFI. V.M.K. is a Canada Research Chair.