Tectonic shortening and gravitational spreading in the Gulf of Cadiz accretionary wedge: Observations from multi-beam bathymetry and seismic profiling

The Gulf of Cadiz lies astride the complex plate boundary between Africa and Eurasia west of the Betic-Rif mountain belt. We report on the results of recent bathymetric swathmapping and multi-channel seismic surveys carried out here. The seafloor is marked by contrasting morphological provinces, spanning the SW Iberian and NW Moroccan continental margins, abyssal plains and an elongate, arcuate, accretionary wedge. A wide variety of tectonic and gravitational processes appear to have shaped these structures. Active compressional deformation of the wedge is suggested by folding and thrusting of the frontal sedimentary layers as well as basal duplexing in deeper internal units. There is evidence for simultaneous gravitational spreading occurring upslope. The very shallow mean surface and basal slopes of the accretionary wedge (1° each) indicate a very weak decollement layer, geometrically similar to the Mediterranean Ridge accretionary complex. Locally steep slopes (up to 10°) indicate strongly focused, active deformation and potential gravitational instabilities. The unusual surface morphology of the upper accretionary wedge includes “raft-tectonics” type fissures and abundant sub-circular depressions. Dissolution and/or diapiric processes are proposed to be involved in the formation of these depressions.     

Holocene seasonal sea‐surface temperature variations in the southern Adriatic Sea inferred from a multiproxy approach

Holocene cooling events have been reconstructed for the southern Adriatic Sea (central Mediterranean) by means of analyses of organic walled dinoflagellate cysts, planktonic foraminifera, oxygen isotopes, calcareous nanoplankton, alkenones and pollen from a sediment core. Two cooling events have been detected, during which sea‐surface temperatures (SSTs) were ca. 2°C lower. Unravelling the SST signal into dominant seasonal components suggests maximum winter cooling of 2°C at around 6.0 ka, whereas the cooling at ca. 3.0 ka might be the result of a spring temperature cooling of 2–3°C. The events, lasting several hundred years, are apparently synchronous with those in the Aegean Sea, where they have been related to known cooling events from the Greenland ice‐core record. A distinct interruption in Adriatic Sea sapropel S1 is not clearly accompanied by a local drop in winter temperatures, but seems to be forced by ventilation, which probably occurred earlier in the Aegean Sea and was subsequently transmitted to the Adriatic Sea. Copyright © 2003 John Wiley & Sons, Ltd.     

The planktonic food web of the Bizerte lagoon (south-western Mediterranean) during summer: I. Spatial distribution under different anthropogenic pressures

The structure and the trophic interactions of the planktonic food web were investigated during summer 2004 in a coastal lagoon of south-western Mediterranean Sea. Biomasses of planktonic components as well as bacterial and phytoplankton production and grazing by microzooplankton were quantified at four stations (MA, MB, MJ and R) inside the lagoon. Station MA was impacted by urban discharge, station MB was influenced by industrial activity, station MJ was located in a shellfish farming sector, while station R represented the lagoon central area. Biomasses and production rates of bacteria (7–33 mg C m⁻³; 17.5–35 mg C m⁻³ d⁻¹) and phytoplankton (80–299 mg C m⁻³; 34–210 mg C m⁻³ d⁻¹) showed high values at station MJ, where substantial concentrations of nutrients (NO₃⁻ and Si(OH)₄) were found. Microphytoplankton, which dominated the total algal biomass and production (>82%), were characterized by the proliferation of several chain-forming diatoms. Microzooplankton was mainly composed of dinoflagellates (Torodinium, Protoperidinium and Dinophysis) and aloricate (Lohmaniellea and Strombidium) and tintinnid (Tintinnopsis, Tintinnus, Favella and Eutintinnus) ciliates. Higher biomass of these protozoa (359 mg C m⁻³) was observed at station MB, where large tintinnids were encountered. Mesozooplankton mainly represented by Calanoida (Acartia, Temora, Calanus, Eucalanus, Paracalanus and Centropages) and Cyclopoida (Oithona) copepods, exhibited higher and lower biomasses at stations MA/MJ and MB, respectively. Bacterivory represented only 35% of bacterial production at stations MB and R, but higher fractions (65–70%) were observed at stations MA and MJ. Small heterotrophic flagellates and aloricate ciliates seemed to be the main controllers of bacteria. Pico- and nanophytoplankton represented a significant alternative carbon pool for micrograzers, which grazing represented 67–90% of pico- and nano-algal production in all stations. Microzooplankton has, however, a relatively low impact on microphytoplankton, as ≤45% of microalgal production was consumed in all stations. This implies that an important fraction of diatom production would be channelled by herbivorous meso-grazers to higher consumers at stations MA and MJ where copepods were numerous. Most of the microalgal production would, however, sink particularly at station MB where copepods were scare. These different trophic interactions suggest different food web structures between stations. A multivorous food web seemed to prevail in stations MJ and MA, whereas microbial web was dominant in the other stations.     

Three Secondary Reference Materials for Lithium Isotope Measurements: Li7-N, Li6-N and LiCl-N Solutions

The CRPG (Nancy, France) has prepared secondary reference materials for Li isotope measurements by mixing ⁷Li or ⁶Li spikes and either L-SVEC or IRMM-016 certified reference materials to produce solutions having a known Li concentration and isotopic composition. The Li7-N and Li6-N solution samples (1.5 mol l⁻¹ HNO₃) have nominal δ⁷Li isotopic compositions of 30.1‰ and -9.7‰ respectively relative to L-SVEC and concentrations of 100 mg l⁻¹. Repeated measurement of these samples using the QUAD-ICP-MS at the CRPG yielded δ⁷Li of 30.4 ± 1.1‰ (n = 13) and -8.9 ± 0.9‰ (n = 9) at the 2s level of confidence. An additional LiCl-N solution was measured and yielded a delta value of 9.5 ± 0.6‰ (n = 3). Identical results were obtained at the BRGM (Orléans, France) from determinations performed with a Neptune MC-ICP-MS (30.2 ± 0.3‰, n = 89 for the Li7-N, -8.0 ± 0.3‰, n = 38 for the Li6-N and 10.1 ± 0.2‰, n = 46 for LiCl-N at the 2s level of confidence). The deviation of measured composition relative to the nominal value for the Li6-N solution might be explained by either contamination during preparation or an error during sample weighing. These secondary reference materials, previously passed through ion exchange resin or directly analysed, may be used for checking the accuracy of Li isotopic measurements over a range of almost 40‰ and will be available to the scientific community upon request to J. Carignan or N. Vigier, CRPG.     

The relative influence of soil moisture and SST in climate predictability explored within ensembles of AMIP type experiments

Three ensembles of AMIP-type simulations using the Arpege-climat coupled land–atmosphere model have been designed to assess the relative influence of SST and soil moisture (SM) on climate variability and predictability. The study takes advantage of the GSWP2 land surface reanalysis covering the 1986–1995 period. The GSWP2 forcings have been used to derive a global SM climatology that is fully consistent with the model used in this study. One ensemble of ten simulations has been forced by climatological SST and the simulated SM is relaxed toward the GSWP2 reanalysis. Another ensemble has been forced by observed SST and SM is evolving freely. The last ensemble combines the observed SST forcing and the relaxation toward GSWP2. Two complementary aspects of the predictability have been explored, the potential predictability (analysis of variance) and the effective predictability (skill score). An analysis of variance has revealed the effects of the SST and SM boundary forcings on the variability and potential predictability of near-surface temperature, precipitation and surface evaporation. While in the tropics SST anomalies clearly maintain a potentially predictable variability throughout the annual cycle, in the mid-latitudes the SST forced variability is only dominant in winter and SM plays a leading role in summer. In a similar fashion, the annual cycle of the hindcast skill (evaluated as the anomalous correlation coefficient of the three ensemble means with respect to the “observations”) indicates that the SST forcing is the dominant contributor over the tropical continents and in the winter mid-latitudes but that SM is supporting a significant part of the skill in the summer mid-latitudes. Focusing on boreal summer, we have then investigated different aspects of the SM and SST contribution to climate variations in terms of spatial distribution and time-evolution. Our experiments suggest that SM is potentially an additional source of climate predictability. A realistic initialization of SM and a proper representation of the land–atmosphere feedbacks seem necessary to improve state-of-the-art dynamical seasonal predictions, but will be actually efficient only in the areas where SM anomalies are themselves predictable at the monthly to seasonal timescale (since remote effects of SM are probably much more limited than SST teleconnections).     

Overcoming problems of density and thickness measurements in FTIR volatile determinations: a spectroscopic approach

The Beer–Lambert law is traditionally used to determine water and carbon concentrations in glasses from their infrared (IR) spectra. In practice, this method requires estimation of the thickness and density of the glass as well as the calibration of the molecular absorptivities of the species concerned. All of these parameters can be sources of practical difficulties and analytical uncertainty. These weaknesses in the application of the Beer–Lambert law have been overcome by an empirical analysis of the infrared spectra. Using a set of 292 spectra obtained on 113 natural and experimental tholeiitic glasses (SiO₂ 48.5–51 wt%; water contents 0–4000 ppm H₂O), it can be shown that the thickness–density (ρ d) product of a glass sample can be directly and reliably inferred from its IR spectrum. This allows the Beer–Lambert law to be rewritten. The new form no longer requires thickness or density estimations to determine volatile contents. Moreover, if needed, the thickness of the glass slab can also be accurately determined from the IR spectra. This new method is developed for quantitative determination of water concentrations in MORB glasses but can also be applied to any minor species (carbon, sulfur, etc.) provided it is active in the IR domain and that a suitable independent frequency of IR absorption can be identified. Precision is about 60 ppm H₂O on O–H⁻ contents. This method, tested on natural and experimental MORB-type glasses, can be applied to any chemical composition provided a set of reference spectra is available. Received: 16 September 1999 / Accepted: 18 February 2000     

Cooling of Accretion-Heated Neutron Stars

We present a brief, observational review about the study of the cooling behaviour of accretion-heated neutron stars and the inferences about the neutron-star crust and core that have been obtained from these studies. Accretion of matter during outbursts can heat the crust out of thermal equilibrium with the core and after the accretion episodes are over, the crust will cool down until crust-core equilibrium is restored. We discuss the observed properties of the crust cooling sources and what has been learned about the physics of neutron-star crusts. We also briefly discuss those systems that have been observed long after their outbursts were over, i.e, during times when the crust and core are expected to be in thermal equilibrium. The surface temperature is then a direct probe for the core temperature. By comparing the expected temperatures based on estimates of the accretion history of the targets with the observed ones, the physics of neutron-star cores can be investigated. Finally, we discuss similar studies performed for strongly magnetized neutron stars in which the magnetic field might play an important role in the heating and cooling of the neutron stars.