Electron microprobe U–Th–Pb monazite dating of the Transamazonian metamorphic overprint on Archean rocks from the Amapá Block, southeastern Guiana Shield, Northern Brazil

The Amapá Block, southeastern Guiana Shield, represents an Archean block involved in a large Paleoproterozoic belt, with evolution related to the Transamazonian orogenic cycle (2.26 to 1.95 Ga). High spatial resolution dating using an electron-probe microanalyzer (EPMA) was employed to obtain U–Th–Pb chemical ages in monazite of seven rock samples of the Archean basement from that tectonic block, which underwent granulite- and amphibolite-facies metamorphism. Pb–Pb zircon dating was also performed on one sample.Monazite and zircon ages demonstrate that the metamorphic overprinting of the Archean basement occurred during the Transamazonian orogenesis, and two main tectono-thermal events were recorded. The first one is revealed by monazite ages of 2096 ± 6, 2093 ± 8, 2088 ± 8, 2087 ± 3 and 2086 ± 8 Ma, and by the zircon age of 2091 ± 5 Ma, obtained in granulitic rocks. These concordant ages provided a reliable estimate of the time of the granulite-facies metamorphism in the southwest of the Amapá Block and, coupled with petro-structural data, suggest that it was contemporaneous to the development of a thrusting system associated to the collisional stage of the Transamazonian orogenesis, at about 2.10–2.08 Ga.The later event, under amphibolite-facies conditions, is recorded by monazite ages of 2056 ± 7 and 2038 ± 6 Ma, and is consistent with a post-collisional stage, marked by granite emplacement and coeval migmatization of the Archean basement along strike-slip shear zones.     

Hydrogeological modeling constraints provided by geophysical and geochemical mapping of a chlorinated ethenes plume in northern France

A methodological approach is described which combines geophysical and geochemical data to delineate the extent of a chlorinated ethenes plume in northern France; the methodology was used to calibrate a hydrogeological model of the contaminants’ migration and degradation. The existence of strong reducing conditions in some parts of the aquifer is first determined by measuring in situ the redox potential and dissolved oxygen, dissolved ferrous iron and chloride concentrations. Electrical resistivity imaging and electromagnetic mapping, using the Slingram method, are then used to determine the shape of the pollutant plume. A decreasing empirical exponential relation between measured chloride concentrations in the water and aquifer electrical resistivity is observed; the resistivity formation factor calculated at a few points also shows a major contribution of chloride concentration in the resistivity of the saturated porous medium. MODFLOW software and MT3D99 first-order parent–daughter chain reaction and the RT3D aerobic–anaerobic model for tetrachloroethene (PCE)/trichloroethene (TCE) dechlorination are finally used for a first attempt at modeling the degradation of the chlorinated ethenes. After calibration, the distribution of the chlorinated ethenes and their degradation products simulated with the model approximately reflects the mean measured values in the observation wells, confirming the data-derived image of the plume.     

Development of a high resolution runoff routing model, calibration and application to assess runoff from the LMD GCM

Large-scale runoff routing models (RRMs) are important as a validation tool for GCMs, and to close the hydrological cycle in fully-coupled climate models. The model RiTHM was developed to simulate the discharge of large rivers from the total runoff simulated by the LMD GCM. It uses a 1024×800 grid, nested in the 64×50 grid of the LMD GCM. The runoff simulated in a GCM grid cell is uniformly distributed over the underlying cells, where a series of two reservoirs accounts for the delay related to infiltration through the unsaturated zone and aquifers. The resulting riverflow is routed assuming pure translation along the drainage network, extracted with a GIS from a 5 min DEM. The transfer time from a cell to the outlet depends on topography, and on a basin-wide parameter, the time of concentration. RiTHM was calibrated in 11 river basins, using a realistic runoff forcing (computed by the land surface model SECHIBA from reanalyzed meteorological forcing). This led to a very satisfactory reproduction of observed hydrographs. The main problems were related to hydraulic processes neglected in RiTHM (reservoirs, diversion of riverflow because of flooding or irrigation). These results helped to validate SECHIBA, except for its snow processes, shown to be too simple. With the same parameters, RiTHM was also forced with runoff from the LMD GCM. This induced an important degradation of the simulated hydrographs, regarding both volume and timing. It was largely explained by errors in precipitation, and more generally climate, in the GCM. The direct calibration of RiTHM under the GCM-runoff forcing markedly improved the timing of simulated discharge, which could be interesting for land–atmosphere–ocean coupling. This work demonstrated that the usefulness of RRMs for GCMs strongly depends on their adequate calibration.     

Sources of Cd, Cu, Ni and Zn in Portuguese coastal waters

In order to identify the major sources of trace metals (TM) in the Portuguese coastal waters, 58 surface water samples were collected during September 1988. The area sampled extended from the Tagus Estuary (down to a salinity of 25) to cape Ste Marie on the southern coast of Portugal. Dissolved metal concentrations in the fully marine waters ranged from 30 to 250 pM for Cd, 0.7–15 nM for Cu, 0.9–20 nM for Zn and 1.8–4.5 nM for Ni. Within the Tagus Estuary (salinity 25), concentrations increased to 3400 pM for Cd, 26 nM for Cu, 14 nM for Ni and 230 nM for Zn.The large-scale distribution of these metals is dominated by two strong continental sources, both probably linked to the exploitation of pyrite ores. In the Tagus Estuary, TM enrichments can be mostly attributed to a pyrite roasting plant located on the shore in front of Lisbon. Concerning the south Portuguese shelf waters, several hypotheses are proposed to explain their elevated metal concentrations. We particularly discussed the likely influence of the Tinto/Odiel rivers located 100 km eastward, an influence well known in the shelf waters of the Gulf of Cadiz. These rivers are extremely metal-rich because of acid mine tailings originating from their catchment. Between these two regions, upwelling of relatively metal-poor water largely contributes to the dilution of the continental inputs. Indeed, water exchanges on the shelf linked to the upwelling involve water fluxes 500 times higher than the Tagus River flow, and renew the coastal waters that are thus cleaned from terrestrial contamination. Contrary to many other upwelling systems in non-contaminated areas, the Portuguese upwelling does not act as a source of trace-metal enrichment of the continental margin waters.     

Geochemical characteristics of Yamadağı volcanics in central east Anatolia: an example from collision-zone volcanism

Neogene Yamadağı volcanic rocks consist of basaltic trachyandesite, trachyandesite, andesite, and dacite. The major- and trace-element chemistry indicates that the lavas are dominantly calc-alkaline and mildly alkaline in character, sodic in nature, and intermediate to acidic in composition. REE and trace-element patterns of volcanic rocks are similar to those typical of within plate magmatics. Volcanic rocks have low ⁸⁷Sr/⁸⁶Sr (0.70389–0.70633) and high ¹⁴³Nd/¹⁴⁴Nd ratios (0.51267–0.51276) and mostly plot within the mantle array of the isotope ratio diagram. The linear correlations among ⁸⁷Sr/⁸⁶Sr−¹⁴³Nd/¹⁴⁴Nd, SiO₂−⁸⁷Sr/⁸⁶Sr and SiO₂−¹⁴³Nd/¹⁴⁴Nd isotope ratios in the volcanics suggest that fractional crystallization combined with minor assimilation was an important process within the collision zone.     

Composition and sources of dissolved and particulate PAH in surface waters from the Rhone delta (NW Mediterranean)

PAH were determined in surface waters from the Rhone delta in winter and summer 1987. Both particulate and dissolved phases were analyzed by GC and GC/MS. Concentrations of major unsubstituted compounds associated with particles varied from a few to 20 ng l⁻¹, or from 0.42 to 6.0 μg g⁻¹. The seasonal variations reflected higher inputs in winter resulting from the presence of highly PAH-enriched particles. Tetra- and penta-cyclic PAH with MW 202 and 252 predominated, which reflected a significant origin from pyrolytic processes. A rapid decrease of the level of higher MW PAH was observed seawards in both seasons. Study of the tricyclic compounds and their alkylated homologues demonstrated a mainly fossil origin in winter, whereas in summer a high heterogeneity of particles was observed. PAH in the dissolved phase were found at concentrations up to 31 and 50 ng l⁻¹ in summer and winter, respectively. These levels were much higher than those encountered in the corresponding particulate phases. PAH with MW 178 and 202 predominated, and the alkylated homologues of the tricyclic compounds showed a major fossil orgin in winter and a mixed origin in summer. In winter dissolved PAH were transported through the delta into the marine environment without significant losses.     

Quantitative mapping of active mud volcanism at the western Mediterranean Ridge-backstop contact

Based on a new quantitative analysis of sidescan sonar data combined with coring, we propose a revised model for the origin for Mediterranean Ridge mud volcanism. Image analysis techniques are used to produce a synthetic and objective map of recent mud flows covering a 640 × 700 km² area, which represents more than half of the entire Mediterranean Ridge mud belt. We identify 215 mud flows, extruded during the last 37,000–60,000 years. This time period corresponds to the limit of penetration of the sonar, that we evaluate through geoacoustic modeling of the backscattered signal returned by the mud breccia-hemipelagites contact, and calibrate by coring. We show that during this period, at least 96% of the mud volume has been extruded at the Mediterranean Ridge-Hellenic backstop contact, the remaining being scattered over the prism. We suggest that the source is a Messinian (5–6 Ma) mud reservoir that remained close to the backstop contact, at variance with the classical transport-through-the-wedge model. A revised mud budget indicates that steady-state input is not needed. We propose that the source layer was deposited in deep and narrow pre-Messinian basins, sealed by Messinian evaporites, and finally inverted in post-Messinian times. Onset of motion of the Anatolia-Aegea microplate in the Pliocene resulted in change from slow to fast convergence, triggering shear partitioning at the edges of the backstop and basin inversion. Mud volcanism initiation is probably coeval with the latest events of this kinematic re-organization, i.e. opening of the Corinth Gulf and activation of the Kephalonia fault around 1–2 Ma.     

Assessing the contribution of electrical resistivity tomography (ERT) and self-potential (SP) methods for a water well drilling program in fractured/karstified limestones

ERT and SP investigations were conducted in carbonate rocks of the Dinant Synclinorium (Walloon Region of Belgium) to find suitable locations for new water wells in zones with little hydrogeological data. Since boreholes information needed to be representative of the area, large fractured zones were searched for the drillings. Large ERT profiles (320 to 640 m) allowed us to image the resistivity distribution of the first 60 m of the subsurface and to detect and characterize (in terms of direction, width and depth) fractured zones expected to be less resistive. Data errors, depth of investigation (DOI) indexes and sensitivity models were analyzed in order to avoid a misinterpretation of the resulting images. Self-potential measurements were performed along electrical profiles to complement our electrical results. Some negative anomalies possibly related to preferential flow pathways were detected. A drilling campaign was conducted according to geophysical results. ‘Ground truth’ geological data as well as pumping tests information gave us a way to assess the contribution of geophysics to a drilling program. We noticed that all the wells placed in low resistivity zones associated with SP anomalies provide very high yields and inversely, wells drilled in resistive zones or outside SP anomalies are limited in terms of capacity. An apparent coupling coefficient between SP signals and differences in hydraulic heads was also estimated in order to image the water table.     

The lamprophyres of Afyon stratovolcano,western Anatolia,Turkey: description and genesis

The Afyon stratovolcano exhibits lamprophyric rocks, emplaced as hydrovolcanic products, aphanitic lava flows and dyke intrusions, during the final stages of volcanic activity. Most of the Afyon volcanics belong to the silica-saturated alkaline suite, as potassic trachyandesites and trachytes, while the products of the latest activity are lamproitic lamprophyres (jumillite, orendite, verite, fitztroyite) and alkaline lamprophyres (campto-sannaite, sannaite, hyalo-monchiquite, analcime–monchiquite). Afyon lamprophyres exhibit LILE and Zr enrichments, related to mantle metasomatism.     

Radio observations of Comet 9P/Tempel 1 before and after Deep Impact

Comet 9P/Tempel 1 was the target of a multi-wavelength worldwide investigation in 2005. The NASA Deep Impact mission reached the comet on 4.24 July 2005, delivering a 370-kg impactor which hit the comet at 10.3 km s⁻¹. Following this impact, a cloud of gas and dust was excavated from the comet nucleus. The comet was observed in 2005 prior to and after the impact, at 18-cm wavelength with the Nançay radio telescope, in the millimeter range with the IRAM and CSO radio telescopes, and at 557 GHz with the Odin satellite. OH observations at Nançay provided a 4-month monitoring of the outgassing of the comet from March to June, followed by the observation of H₂O with Odin from June to August 2005. The peak of outgassing was found to be around between May and July. Observations conducted with the IRAM 30-m radio telescope in May and July 2005 resulted in detections of HCN, CH₃OH and H₂S with classical abundances relative to water (0.12, 2.7 and 0.5%, respectively). In addition, a variation of the HCN production rate with a period of 1.73±0.10 days was observed in May 2005, consistent with the 1.7-day rotation period of the nucleus. The phase of these variations, as well as those of CN seen in July by Jehin et al. [Jehin, E., Manfroid, J., Hutsemékers, D., Cochran, A.L., Arpigny, C., Jackson, W.M., Rauer, H., Schulz, R., Zucconi, J.-M., 2006. Astrophys. J. 641, L145-L148], is consistent with a rotation period of the nucleus of 1.715 days and a strong variation of the outgassing activity by a factor 3 from minimum to maximum. This also implies that the impact took place on the rising phase of the “natural” outgassing which reached its maximum ≈4 h after the impact. Post-impact observations at IRAM and CSO did not reveal a significant change of the outgassing rates and relative abundances, with the exception of CH₃OH which may have been more abundant by up to one order of magnitude in the ejecta. Most other variations are linked to the intrinsic variability of the comet. The Odin satellite monitored nearly continuously the H₂O line at 557 GHz during the 38 h following the impact on the 4th of July, in addition to weekly monitoring. Once the periodic variations related to the nucleus rotation are removed, a small increase of outgassing related to the impact is present, which corresponds to the release of ≈5000±2000 tons of water. Two other bursts of activity, also observed at other wavelengths, were seen on 23 June and 7 July; they correspond to even larger releases of gas.