A new tomographic image of the Pyrenean lithosphere from teleseismic data

A new tomographic model of the Pyrenean lithosphere is determined down to 200 km depth from teleseismic P and PKP travel times, with a lateral resolution of 0.25°. Compared to previous models, two important improvements are 1) a larger number of stations with a more even distribution, in particular to the west of the range, and 2) the introduction, before inversion, of crustal corrections inferred from previous refraction and reflection experiments. This last point is crucial because a strong Moho jump (up to 20 km) is present at the North Pyrenean Fault, the former boundary between Eurasian and Iberian plates. The comparison of the models obtained with and without crustal corrections reveals the strong contamination of the models by the crust down to 100 km depth. In the uncorrected model, a large strip with negative P-velocity anomalies, previously interpreted as subduction of lower crust, is observed. It disappears in the corrected model. Moreover, the introduction of crustal corrections allows us to reveal short wavelength heterogeneities which were hidden by the crustal signal.An attempt is made to relate the heterogeneities revealed by the tomographic model with the tectonic history of the Pyrenees, in particular with the Alpine orogeny. The Alpine phase includes an extensive episode with generation of the thin continental crust and possibly the opening of an oceanic sea floor, and then a compressive stage. In our model, no signature of an oceanic subducted slab could be detected all along the range, a result which rules out the opening of a large oceanic floor before the compressive stage. A subduction of continental crust is possible but, due to the transformation of lower crust into eclogite at depth, it can not be detected by seismological methods, whereas it was observed from electrical and gravity data. To the East of the range, large heterogeneities with low velocities are ascribable to the Neogene extension related to the rotation of the Corso–Sardo block and the opening of the Gulf of Lion. A prominent high velocity anomaly extending down to 200 km in eastern-central Pyrenees could possibly be interpreted as a detached piece of the Tethys slab. In north of Iberia outside the range, deep (down to 200 km) low velocity structures oriented N130°E are probably related to Hercynian orogeny.     

Functioning of a coastal karstic system with a submarine outlet,in southern Spain

A conceptual model of the functioning of a complex coastal karst aquifer in southern Spain is presented. The system has well developed conduits below sea level and is connected to the Mediterranean Sea. It discharges through two conduits 12 m below sea level, 20 km apart. The Moraig conduit is the main outlet; the discharge to the sea is brackish. The Toix conduit only discharges to the sea during heavy floods and lets in seawater the rest of the time. During the 1999–2000 hydrological year, both conduits were monitored by flowmeters and electrical conductivity-temperature probes. On the basis of the collected data, the hydrological relationship between the functioning of the system and the sea was characterised. The conceptual model assumes the existence of a huge reservoir called an “aquifer reservoir” which is supplied by (1) freshwater from rainfall and (2) seawater flowing into the Toix conduit. In addition, during heavy rainfall events, fast infiltration brings considerable amounts of freshwater into the Moraig conduit. This is typical of a “by-pass” mechanism. Salinity and flow rates were simulated with the use of a rainfall-discharge and a rainfall-salinity model. The simulation of flow and salinity time series can be used for water management purposes.     

Lateral stirring of large-scale tracer fields by altimetry

Ocean surface fronts and filaments have a strong impact on the global ocean circulation and biogeochemistry. Surface Lagrangian advection with time-evolving altimetric geostrophic velocities can be used to simulate the submesoscale front and filament structures in large-scale tracer fields. We study this technique in the Southern Ocean region south of Tasmania, a domain marked by strong meso- to submesoscale features such as the fronts of the Antarctic Circumpolar Current (ACC). Starting with large-scale surface tracer fields that we stir with altimetric velocities, we determine ‘advected’ fields which compare well with high-resolution in situ or satellite tracer data. We find that fine scales are best represented in a statistical sense after an optimal advection time of ～2 weeks, with enhanced signatures of the ACC fronts and better spectral energy. The technique works best in moderate to high EKE regions where lateral advection dominates. This technique may be used to infer the distribution of unresolved small scales in any physical or biogeochemical surface tracer that is dominated by lateral advection. Submesoscale dynamics also impact the subsurface of the ocean, and the Lagrangian advection at depth shows promising results. Finally, we show that climatological tracer fields computed from the advected large-scale fields display improved fine-scale mean features, such as the ACC fronts, which can be useful in the context of ocean modelling.     

Diversity and bioactivity of actinomycetes from Signy Is- land terrestrial soils,maritime Antarctic

The Antarctic represents a largely untapped source for isolation of new microorganisms with potential to produce bio- active natural products. Actinomycetes are of special interest among such microorganisms as they are known to produce a large number of natural products, many of which have clinical, pharmaceutical or agricultural applications. We isolated, characterized and classified actinomycetes from soil samples collected from different locations on Signy Island, South Orkney Islands, in the maritime Antarctic. A total of 95 putative actinomyeete strains were isolated from eight soil samples using eight types of selective isolation media. The strains were dereplicated into 16 groups based on morphology and Amplified Ribosomal DNA Restriction Analysis （ARDRA） patterns. Analysis of 16S rRNA gene sequences of representatives from each group showed that streptomy- cetes were the dominant actinomycetes isolated from these soils; however, there were also several strains belonging to diverse and rare genera in the class Actinobacteria, including Demetria, Glaciibacter, Kocuria, Marmoricola, Nakamurella and Tsukamurella. In addition, screening for antibacterial activity and non-ribosomal peptide synthetase genes showed that many of the actinomycete strains have the potential to produce antibacterial compounds.     

Test of tectonic models by great circle Rayleigh waves

Summary. A large set of published great circle Rayleigh wave phase velocities in the period range 125–350s is used to compare three recent tectonic models (Okal, Lévêque, Jordan). Prior to any regionalization, the symmetry property of the great circle integrals is used to obtain a lower limit of the signal/noise ratio in the data. It turns out that the signal is responsible for at least 30 per cent of the data variance in the period range 175–300s.
A standard regression method is applied for computing the'pure path'velocities and the model efficiency is derived from a variance analysis. It is shown that, even at great depth, none of the three models explains more than 60 per cent of the energy due to the long-wavelength lateral heterogeneities (λ6500 km).
The three models have nearly the same efficiency for explaining the short-period data ( T ∼ 125s). Between 200 and 300s, the higher performance of Okal's model indicates that it is important to separate the subduction zones from the other orogenic zones. By perturbing the lateral extension of the subduction zones, it comes out that they constitute on both sides of the subducting slabs wider anomalies than often assumed, suggesting large downgoing flows. On the contrary, the effect of surface features such as marginal seas are restricted to a close region in front of the trenches.
Finally, the anomalous ellipticity values deduced directly from great circle data are partly explained by a coupling between tectonics and ellipticity.     

The Pb-Sr-Nd isotope geochemistry of some recent circum-Mediterranean granites

Pb, Sr and Nd isotopic compositions have been analyzed in recent granites from Northern Africa, Northern Italy and Greece. Lead isotope compositions of K-feldspars are rather homogeneous, and cluster close to the modern lead of Stacey and Kramers (1975) but with slightly higher²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb ratios. The Cyclades samples, however, have higher²⁰⁶Pb/²⁰⁴Pb ratios. Addition of mantle-derived lead was probably very limited, which supports a quasi-closed system evolution of this element in the continental crust. The Sr, Nd data fall in the enriched part of the¹⁴³Nd/¹⁴⁴Nd vs.⁸⁷Sr/⁸⁶Sr diagram and define a smooth hyperbolic mixing curve. Over a wide area, straddling different orogens, most granites may be accounted for by a binary mixture between a recycled crustal component and a depleted mantle-like component. No correlation is observed between either Pb and Sr or Nd isotopic ratios, or any isotopic ratio and major element contents. Quantitative modelling suggests that two cases fit the Sr and Nd characteristics of these granites: they both require anatexis of the crust on a scale large enough to average the isotopic properties of heterogeneous terranes. In the first case, the mantle-derived component may be represented by differentiated Island Arc-type magmas, and the granites result from mixing these magmas with anatectic melts. In the second case, mantle-derived igneous rocks, such as obducted ophiolites, are part of the crustal source and their variable involvement in the anatectic process causes isotopic variations.CRPG Contribution n 630.     

Chemical mass balance of calcrete genesis on the Toledo granite (Spain)

The chemical mass balance of calcrete genesis is studied on a typical sequence developed in granite, in the Toledo mountains, Central Spain.

Field evidence and petrographic observations indicate that the texture and the bulk volume of the parent rock are strictly preserved all along the studied calcrete profile.

Microscopic observations indicate that the calcitization process starts within the saprolite, superimposed on the usual mechanisms of granite weathering: the fresh rock is first weathered to secondary clays, mainly smectites, which are then pseudomorphically replaced by calcite. Based on this evidence, chemical mass transfers are calculated, assuming iso-volume transformation from the parent rock to the calcrete.

The mass balance results show the increasing loss of matter due to weathering of the primary phases, from the saprolite towards the calcrete layers higher in the sequence. Zr, Ti or Th, which are classically considered as immobile during weathering, are also depleted along the profile, especially in the calcrete layer. This results from the prevailing highly alkaline conditions, which could account for the simultaneous precipitation of CaCO₃ and silicate dissolution.

The calculated budget suggests that the elements exported from the weathering profile are provided dominantly by the weathering of plagioclase and biotite. We calculate that 8–42% of the original Ca remains in granitic relics, while only 15% of the authigenic Ca released by weathering is reincorporated in the calcite. This suggests that 373 kg/m² of calcium (i.e., three times the original amount) is imported into the calcrete from allochtonous sources, probably due to aeolian transport from distant limestone formations.