Alteration of bottom roughness by benthic organisms in a sandy coastal environment

This work deals with the potential influence of benthic communities on the sediment dynamics of the coastal zone, and specifically with the modifications to bottom roughness caused by communities and their effects on wave propagation across the coastal profile. Time-series of video observations of the sea bottom on the Ebro delta coast were analysed in order to estimate the bottom roughness associated with physical and biological morphological components and the sediment reworking rates caused by epifaunal organisms. Biological roughness was mainly caused by ophiuroids and tanatocenosis of bivalve and gastropod shells, which changed their abundance during the study period. The total biological roughness (K_bio) ranged between 0.27 and 0.81 cm and represented a significant part (<20%) of the total form drag roughness. Flattening of ripples caused by bioturbation was observed under low-energy conditions. Surface sediment perturbation and bioturbation rates were also estimated. Based on these observations a wave propagation model was applied in order to carry out a sensitivity analysis of the significance of biological roughness on wave dissipation in the study area under different wave conditions.     

Rb-Sr whole-rock dating of late diagenetic to anchimetamorphic,palaeozoic sediments in southern France (Montagne Noire)

Rb-Sr whole-rock and mineral analyses are reported from Early Paleozoic sediments of the southern rim of the French Central Massif (Montagne Noire). The sediments—mainly pelites, silt- and sandstones—can be separated into three different zones (A, B, and C) according to their varying degrees of deformation and transformation, which took place during the Hercynian (= Variscan) orogeny (Carboniferous). For the least deformed, folded but not schistose, fossiliferous Lower Ordovician sediments (zone A), a scatter of the data points resulted, at least partly due to the varying amount of detrital muscovites, which did not equilibrate their Sr isotopes with those of the clay minerals. Fossiliferous Middle Cambrian sediments (zone B), however, are colinear in an isochron plot. The rocks are also folded, and show a beginning schistosity in the pelitic layers. The age of 333±16 m.y. therefore does not correspond to the time of sedimentation, as one would expect from most of the interpretations given in the literature. It is, however, in close agreement with the stratigraphically known time of folding and anchimetamorphism. The same result, within analytical error, of 326±17 m.y. was obtained for Lower Ordovician sediments (zone C), which show a schistosity not only in the pelites but also in some psammitic layers. As for the sediments of zone B, this result indicates open whole-rock systems for Rb and/or Sr during folding and anchimetamorphism. Minerals extracted from psammites of zones B and C gave the following results: albite coincides with the isochrons; detrital muscovites do not coincide and yield ages of about 650 m.y. (Cadomian orogeny) for both zone B and C sediments; detrital chlorites from zone B sediments seem to be only partly reset by anchimetamorphism and therefore plot closer to the whole-rock isochron. From this it must be concluded that an increasing amount of these detrital micas could significantly shift the position of the total-rock points, as might be the case for zone A sediments. From the whole-rock results of zone B and C sediments and the mineralogical and petrological changes during diagenesis and anchimetamorphism we conclude that many of the isochrons obtained from stratigraphically uncontrollable Precambrian sediments should be reinterpreted in favour of metamorphic and not sedimentary ages, especially if the metamorphism reached greenschist facies conditions. Devonian and Cambrian limestones intercalated between the pelites, silstones and sandstones contain Sr of a variable and slightly radiogenic isotopic composition in contrast to the expected ancient sea water Sr. This fact is interpreted again in favour of open whole-rock systems during the postdepositional history of the rock, rather than primary scatter of ancient, radiogenic sea water Sr.     

Using low-density discrete Airborne Laser Scanning data to assess the potential carbon dioxide emission in case of a fire event in a Mediterranean pine forest

The aim of study is to map the carbon dioxide (CO₂) emission of the aboveground tree biomass (AGB) in case of a fire event. The suitability of low point density, discrete, multiple-return, Airborne Laser Scanning (ALS) data and the influence of several characteristics of these data and the study area on the results obtained have been evaluated. A sample of 45 circular plots representative of Pinus halepensis Miller stands were used to fit and validate the model of AGB. The ALS point clouds were processed to obtain the independent variables and a multivariate linear regression analysis between field data and ALS-derived variables allowed estimation of AGB. Then, the influence of several characteristics on the residuals of the model was analyzed. Finally, conversion factors were applied to obtain the CO₂ values. The AGB model presented a R² value of 0.84 with a relative root-mean-square error of 27.35%. This model included ALS variables related to vegetation height variability and to canopy density. Terrain slope, aspect, canopy cover, scan angle and the number of laser returns did not influence AGB estimations at plot level.     

High-resolution seismic studies of gas hydrates west of Svalbard

 A strong bottom-simulating reflection (BSR) with high-amplitude variations is detectable in high- resolution reflection seismic profiles west of Svalbard. Above the BSR, anomalously high velocities up to 1840 m/s, calculated from high-frequency ocean-bottom hydrophone (HF-OBH) data, indicate the existence of gas-hydrated sediments. Below the BSR, a low-velocity layer, interpreted as gas-bearing sediments, shows thickness variations from 12 to 25 m. In addition, two other low-velocity layers clearly containing free gas are detected within the classic hydrate stability zone (HSZ) where, a theoretical viewpoint, free gas cannot exist. Received: 6 August 1997 / Revision received: 26 January 1998     

Determination par activation neutronique des rapports Cl/Br des inclusions fluides de divers gypses. Correlation avec les donnees de la microcryoscopie et interpretations genetiques

Gypsum contains liquid inclusions which fill mainly primary cavities. Analysis of inclusions may give some information about the mineral formation process. In places, diagenetic solutions have filled secondary cavities along cleavage planes or replaced initial liquid inclusions in primary cavities; the presence of such secondary inclusions must, therefore, be taken into consideration. This study presents our results on the Cl/Br ratio of liquid inclusions in gypsum. The Cl/Br ratio is practically constant in present sea water, and has been measured in many different types of waters. The Br content of halite has also been extensively studied to trace the compositional changes of a salt-depositing brine. In gypsum Br is present only in microscopic fluid inclusions. For this reason we used the neutron-activation method. We adopted the technique in order to prevent any contamination and obtain directly Cl/Br ratio within 10% accuracy. In order to ascertain that liquid inclusions reside mainly in primary cavities of gypsum, we selected our samples only after microscopic observation. Fifty-eight samples from different types of gypsum were analysed. The salinities of the inclusions, obtained through the data on freezing, and the Cl/Br ratio, obtained through activation-analysis, give us clues as to the nature of the fluids depositing the gypsum. Recent gypsum deposits from French salt-pans of Mediterranean and Atlantic coasts were sampled. The latter has apparently been diagenetically altered by meteoric water; the salinity of many inclusions having been changed while the Cl/Br ratio (315 and 350) in both remains close to the value of present sea water (#300). Liquid inclusions of Recent crystals from New Caledonia, deposited in mangrove swamps are enriched in Br (Cl/Br = 150 and 200). The enrichment might be related to the presence of abundant organic materials. Inclusions in crystals from Sebkha el Melah (Tunisia) also show high Br content (Cl/Br = 194). In this case, the brine was highly concentrated and was saturated with NaCl. The Upper Miocene gypsum from Sicily was studied to test the various models for the genesis of this important evaporite formation of the Mediterranean. Freezing data show some decrease of salinities, probably by groundwater diagenesis, but the Br content is high. The Cl/Br ratio is 175, and this value is similar to that of Sebkha el Melah. Samples from three thick Eocene and Oligocene sequences near Paris and in the South of France were studied. Freezing data and Cl/Br ratio of their inclusions indicate that those gypsum deposits have crystallized in marine environments receiving considerable influx of river-waters.     

Evidence of ameliorated Middle Weichselian climate and sub‐arctic environment in the western Baltic region: coring lake sediments at Klintholm,Møn,Denmark

Coring through glaciotectonically stacked Quaternary sediments situated below sea level on the island of Møn, Denmark, recovered a succession of interstadial sediments of Middle Weichselian age. Plant and animal remains including insects found in laminated sand and mud indicate deposition in a lake surrounded by dwarf shrubs, herbs, mosses and rare trees. The insect fauna indicates a mean July temperature of 8–12 °C, suggesting an arctic to sub‐arctic environment, while winter temperatures around ?8 to ?22 °C suggest periglacial conditions with permafrost. Luminescence dating of sediment samples gave ages from 48–29 ka, and radiocarbon dating indicates deposition of plant fragments between 45 and 36 ka BP. The fossil assemblage from Møn shows close resemblance to those from other sites with similar ages found in the vicinity of the western Baltic Basin.     

The impact of extreme turbidity events on the nursery function of a temperate European estuary with regulated freshwater inflow

Estuaries are used as nursery grounds by numerous marine species despite being usually subject to strong anthropogenic disturbances. Abundances of marine recruits (fish and crustacean decapods) and their main prey (mysids) were monitored by monthly sampling, from June 1997 to February 2009, in the lower reaches of the Guadalquivir estuary (SW Spain). During that period, unusually high and persistent turbidity events (HPTEs) were observed twice. Both HPTEs started with strong and sudden freshwater discharges after relatively long periods of very low freshwater inflow. Data from this time-series were used to test the hypothesis that HPTEs may negatively impact the nursery function of estuaries either by decreasing prey availability or by decreasing survival/arrival of marine recruits. During HPTEs, the commonest mysid (Mesopodopsis slabberi), a key species in the estuarine food web, showed a significant decrease in abundance. Likewise, some marine recruits that prey on M. slabberi and whose peaks of abundance within the estuary occur in summer–autumn (Engraulis encrasicolus and Pomadasys incisus) were less abundant during HPTEs. It is also suggested that HPTEs might have triggered a shift in the distribution of the most euryhaline prey (Neomysis integer) and predator (Dicentrarchus punctatus and Crangon crangon) species, towards more saline waters. This could have contributed to an increase in the inter-specific competition (for food/habitat) within the estuarine nursery area. The results discussed in this study call attention to the need to reduce as much as possible the anthropogenic pressures that may stimulate the occurrence of high and persistent turbidity events (HPTEs) in order to preserve the nursery function of temperate estuaries.     

Probabilistic compressive strength of sound dry granite

Summary The different functions of cumulative probability of fracture that can be used in the Probabilistic Strength of Materials in the case of constant uniaxial compression are described. Sound fine-grained granite was used to study volume influence by fracturing rectangular prisms, and then no noticeable influence was observed. Since this is showing that all the fracture stresses are belonging to a single set they were included in a single group that exhibited two functions of specific risk of fracture. The population with the lesser fracture stress has no critical zone while the other population does have it and a critical zone in the order of 10^–6 m³ is the minimum size exhibiting a complete fracture of the specimen when the same collapses. All the statistical functions were found to be acceptable according to theX ² criterion.     

Finite-element modelling of Tertiary paleostress fields in the eastern part of the Tajo Basin (central Spain)

Three subsequent Tertiary paleostress fields that are deduced from fault-slip data for the eastern part of the Tajo Basin are analyzed by finite-element studies. The modelling results show that maximum horizontal stresses (S_Hmax) are mainly controlled by the geometry of the model limits and the boundary conditions applied. The models are used to test two hypotheses on the origin of the Altomira Range. A local stress field responsible for its formation (‘Altomira') can be modelled successfully by superposition in time and place of two major paleostress fields (‘Iberian' and ‘Guadarrama'). Stress trajectories have been modelled with respect to a homogeneous cover and heterogeneous basement to investigate the role of rheological contrasts between different basement blocks on the orientation of the stress field. Results of this kind of modelling suggest a mechanical decoupling between the cover and the basement, especially for the ‘Altomira' paleostress field.