Geophysical characterisation of karstic networks – Application to the Ouysse system (Poumeyssen,France)

In the framework of the management of karstic aquifers, geophysical reconnaissance can be used to locate conduits and caves, and to characterise the surrounding limestone matrix. Suitable characterisation of heterogeneities in the karstic environment is, however, challenging for ground-based geophysical methods. The present article describes the results, and evaluates the response and accuracy of combined geophysical measurements carried out at the Poumeyssen test site in France, involving electrical resistivity imaging (ERI), magnetic resonance sounding (MRS), “mise-à-la-masse” electrical mapping, and seismic tomography. This site provides the opportunity to study a relatively wide, shallow, water-filled conduit whose location and shape are known from topographic work carried out by cave divers. Seismic and MRS provided the exact location and width of the conduit, to within a few meters. The seismic and electrical data suggest that the limestone medium surrounding the conduit is not homogeneous.     

Bentho-planktonic evidence from the Austrian Alps for a decline in sea-surface carbonate production at the end of the Triassic

A high-resolution micropalaeontological study, combined with geochemical and sedimentological analyses was performed on the Tiefengraben, Schlossgraben and Eiberg sections (Austrian Alps) in order to characterize sea-surface carbonate production during the end-Triassic crisis. At the end-Rhaetian, the dominant calcareous nannofossil Prinsiosphaera triassica shows a decrease in abundance and size and this is correlated with a increase in δ¹⁸O and a gradual decline in δ¹³C_carb values. Simultaneously, benthic foraminiferal assemblages show a decrease in diversity and abundance of calcareous taxa and a dominance of infaunal agglutinated taxa. The smaller size of calcareous nannofossils disturbed the vertical export balance of the biological carbon pump towards the sea-bottom, resulting in changes in feeding strategies within the benthic foraminiferal assemblages from deposit feeders to detritus feeders and bacterial scavengers. These micropalaeontological data combined with geochemical proxies suggest that changes in seawater chemistry and/or cooling episodes might have occurred in the latest Triassic, leading to a marked decrease of carbonate production. This in turn culminated in the quasi-absence of calcareous nannofossils and benthic foraminifers in the latest Triassic. The aftermath (latest Triassic earliest Jurassic) was characterised by abundance peaks of “disaster” epifaunal agglutinated foraminifera Trochammina on the sea-floor. Central Atlantic Magmatic Province (CAMP) paroxysmal activity, superimposed on a major worldwide regressive phase, is assumed to be responsible for a deterioration in marine palaeoenvironments. CAMP sulfuric emissions might have been the trigger for cooling episodes and seawater acidification leading to disturbance of the surface carbonate production at the very end-Triassic.     

Understanding Design with VGI using an Information Relevance Framework

Volunteered Geographic Information (VGI) has the potential to offer benefits to end‐users over and above those of Professional Geographic Information (PGI). A multi‐methods approach, consisting of participatory observation, focus groups and diary studies, was used to study how VGI and PGI were actually used by a target user group. A theoretical framework of information relevance was used to understand the attributes of information that were most important in relation to the characteristics of the users' activity and their community. The key finding was that the discussion amongst GIS designers should not be whether to choose VGI or PGI as the information data set, but to consider which combination of VGI and PGI relating to different geographic features and task characteristics would best meet the users' needs. VGI is likely to be most relevant to the user when a geographic feature is dynamic rather than static in nature, and can also provide a level of detail that is unlikely to arise through PGI. These findings have implications for how different forms of information are most effective for different usage situations. Above all, a case is presented for the implementation of User Centred Design (UCD) principles when integrating VGI and PGI together in a single mashup‐based product.     

Oxygen Isotope Composition Of Human Teeth And The Record Of Climate Changes In France (Lorraine) During The Last 1700 Years

In order to study climate variations during the last 1700 years in eastern France, fifty-eight oxygen isotope compositions of phosphate were measured in human tooth enamel. The individuals, who lived in Lorraine, are assumed to have drunk local water derived directly from rainfall. According to previous work, drinking water is the main source of oxygen that sets the isotopic composition of phosphatic tissues in humans. The empirical fractionation equation determined from our data combined with those of Longinelli’s one [Geochim. Cosmochim. Acta 48 (1984) 385] was used to calculate the oxygen isotope composition of meteoric waters. The mean air temperature was inferred from these isotope ratios and the Von Grafenstein et al.’s [Geochim. Cosmochim. Acta 60 (1996) 4025] relationship between δ¹⁸O and air temperature. Oxygen isotope composition of present-day individuals yields a mean air temperature of 9.9± 1.7 ^∘C which is consistent with meteorological data. Application of this method to historical individuals results in mean air temperatures estimates 0 to 3 ^∘C higher than present-day air temperature. These warm air temperatures are not realistic during the so-called Little Ice Age for which an air-cooling of about 0.5 to 2 ^∘C has been documented. We propose that these relatively high δ¹⁸O values of human tooth enamel reflect higher mean δ¹⁸O values of meteoric water which can be attributed to an increased proportion of summer rainfall during the “Little Ice Age” time frame in Lorraine.     

Enregistrements sismologiques de l'explosion sur le site de l'usine AZF (Toulouse,France)Seismological records of an explosion at the AZF chemical complex in Toulouse,France

This study presents a detailed analysis of the seismic records of a strong explosion that occurred on 21 September 2001 at a chemical complex located south of Toulouse, France, and provoked important damages. The explosion, which is equivalent to a 3.4 magnitude earthquake, has been recorded at most of the stations of the National Seismological Network, as well as at a station under test at the ‘Observatoire Midi-Pyrénées’, 4.2 km away from the epicentre. The main seismic phases are interpreted using the known crustal structures, and a modelling with synthetic seismograms is performed. To cite this article: A. Souriau et al., C. R. Geoscience 334 (2002) 155–161.     

The detection of new methanol masers in the 5−1–40 E line

Forty-eight objects were detected in the 5_?1–4₀ E methanol line at 84.5 GHz during a survey of Class I maser sources. Narrow maser features were found in 14 of these. Broad quasi-thermal lines were detected toward other sources. One of the objects with narrow features at 84.5 GHz, the young bipolar outflow L1157, was also observed in the 8₀–7₁ A ⁺ line at 95.2 GHz; a narrow line was detected at this frequency. Analysis showed that the broad lines are usually inverted. The quasi-thermal profiles imply that there are no more than a few line opacities. These results confirm the plausibility of models in which compact Class I masers appear in extended sources as a result of a preferential velocity field.     

Love-Rayleigh wave incompatibility and possible deep upper mantle anisotropy in the Iberian peninsula

Love and Rayleigh wave phase velocities are analyzed with the goal of retrieving information about the anisotropic structure of the Iberian lithosphere. The cross-correlation method is used to measure the interstation phase velocities between diverse stations of the ILIHA network at periods between 20 and 120 s. Despite the 2-D structure of the network, the Love wave data are too few to enable an analysis of phase velocity azimuthal variations. Azimuthal averages of Love and Rayleigh wave phase velocities are calculated and inverted both in terms of isotropic and anisotropic structures. Realistic isotropic models explain the Rayleigh wave and short-period Love wave phase velocities. Therefore no significant anisotropy needs to be introduced in the crust and down to 100 km depth in the upper mantle to explain our data. A discrepancy is observed only at long periods, where the data are less reliable. Love wave data at periods between 80 and 120 s remain 0.15 km/s faster than predicted by isotropic models explaining the long-period Rayleigh wave data. Possibilities of biases in the measurements due to interferences with higher modes are examined but seem unlikely. A transversely isotropic model with 8% of S-wave velocity anisotropy in the upper mantle at depths larger than 100 km can explain the whole set of data. In terms of a classical model of mantle anisotropy, this corresponds to 100% of the crystals perfectly oriented in the horizontal plane in a pyrolitic mantle. This is a rather extreme model, which predicts at time delay between 0 and 2 seconds for split SKS.     

Biogeochemical cycling in an organic rich coastal marine basin—II. Nutrient sediment-water exchange processes

The release of remineralized N and P from the organic-rich anoxic sediments of Cape Lookout Bight is controlled by processes occurring within the sediment column and at the sediment-water interface. The relatively rapid rates of temperature dependent microbial degradation of organic matter support seasonally varying nutrient fluxes ranging from 20 to 1200 μmol·m^?2·hr^?1 for dissolved ammonium and from ? 20 to 120 μmol·m^?2·hr^?1 for total dissolved phosphate (measured in situ over the period October, 1976 to October, 1978). Molecular diffusion along steep vertical pore water concentration gradients measured simultaneously cannot explain the high fluxes observed during warmer months. Gradients for ammonium and phosphate ranged from 0.33 to 1.10 and from 0 to 0.29 μmol·cm^?3_pw·cm^?1_s respectively. These high summertime fluxes appear to result from increased sediment-water transport associated with bubble tubes created and maintained by low-tide methane gas bubble ebullition. When these tubes are present, apparent bulk sediment diffusivities calculated from concurrent studies of methane and radon-222 sediment-water exchange are 1.0–3.1 times greater than molecular diffusivities. Nutrient fluxes calculated via Fick's first law taking into account this enhanced transport and the differential diffusive mobilities of dissolved ammonium, phosphate and phosphate ion pairs indicate that removal by aerobic adsorption and/or biological uptake at the sediment-water interface plays an important role in controlling nutrient exchange in these sediments.     

Major and trace element geochemistry in Zeekoevlei, South Africa: A lacustrine record of present and past processes

This study reports a multi-parameter geochemical investigation in water and sediments of a shallow hyper-eutrophic urban freshwater coastal lake, Zeekoevlei, in South Africa. Zeekoevlei receives a greater fraction of dissolved major and trace elements from natural sources (e.g., chemical weathering and sea salt). Fertilizers, agricultural wastes, raw sewage effluents and road runoff in contrast, constitute the predominant anthropogenic sources, which supply As, Cd, Cu, Pb and Zn in this lake. The overall low dissolved metal load results from negligible industrial pollution, high pH and elevated metal uptake by phytoplankton. However, the surface sediments are highly polluted with Pb, Cd and Zn. Wind-induced sediment resuspension results in increased particulate and dissolved element concentrations in bottom waters. Low C/N ratio (10) indicates primarily an algal source for the sedimentary organic matter. Variation in sedimentary organic C content with depth indicates a change in primary productivity in response to historical events (e.g., seepage from wastewater treatment plant, dredging and urbanization). Primary productivity controls the enrichment of most of the metals in sediments, and elevated productivity with higher accumulation of planktonic debris (and siltation) results in increased element concentration in surface and deeper sediments. Aluminium, Fe and/or Mn oxy-hydroxides, clay minerals and calcareous sediments also play an important role in adsorbing metals in Zeekoevlei sediments.     

Quantitative assessment of the reliability of chironomid remains in paleoecology: effects of count density and sample size

Random distributions for a wide range (1–100,000) of chironomid head capsules (HC) were simulated on a 1-m² surface. The number of HC found in circular surfaces equivalent to standard core diameters (90 and 63 mm) was estimated 1000 times, over the range of tested densities. For each number of HC found in the samples, the range of simulated densities was estimated using a threshold probability (p > 0.95). This enabled us to develop equations to infer HC density from sample counts. Because of the threshold probability for comparable sample counts, the equations yield higher estimated densities under a random distribution than for a regular distribution. The probability of sampling at least one HC was >0.95 for densities of 900 HC m^?2 for the 90-mm core and 1400 HC m^?2 for the 63-mm core. For a specific sample count, the range of actual densities was ~10 times higher for the 63-mm core than the 90-mm core. Comparison with field larval densities revealed that most densities were too low to be suitable for annually resolved reconstruction of a quantitative signal, using current corer sizes, although a large number of populations can support sub-decadal analyses. Nonetheless, some lakes exhibit population sizes large enough to reconstruct robust quantitative estimates of past chironomid abundances. This work provides guidance to reconstruct species dynamics and fine-scale time series analyses in paleoecology.