Tectonosedimentary subdivisions in the neoproterozoic to Early Cambrian cover of the taoudenni Basin (Algeria-Mauritania-Mali)

During two mapping campaigns in Algeria, in the eastern part of the northern edge of the Taoudenni Basin, the imprints of tectonic events in the sediments of the Neoproterozoic to Early Cambrian cover was observed. These were obviously linked to tectonic activity in the nearby domain of the Pan-African Orogenic Belt towards the east. This allows the definition of new subdivisions in the cover and the proposal of correlations with the Mauritanian part of the basin to the west, which can be extended to the entire northern Taoudenni Basin cover. A new geodynamic interpretation of the sedimentation history within the cratonic basin is proposed.     

Interaction Between Macrophytobenthic Groups on the Sublittoral Belt of a Greek Island

Abstract. The interrelationships between the main phytobenthic groups of Chlorophyceae, Phaeophyceae, and Rhodophyceae around Milos Island, Central Aegean Sea, were studied using standard ecological and mathematical methods. The analysis was based on seasonal sampling of 114 different macrophytobenthic species.
The employed ecological indices revealed that the sampling areas located on the north, south, and east side of Milos Island show similar annual fluctuations in abundance and species diversity. The resemblance was particularly high between all stations in October-February, between south and east stations during May, and between north and east stations during July.
Additionally, an annual cycle of group interaction with regard to species diversity and abundance exists between the Chlorophyceae, Rhodophyceae, and Phaeophyceae. The Rhodophyceae are the most abundant and dominant group throughout the year, showing percentages in the samples higher than 70% in all sampling areas.     

MODIS卫星遥感北京地区气溶胶光学厚度及与地面光度计遥感的对比

介绍MODIS卫星遥感气溶胶的方法,利用北京大学地面多波段太阳光度计的观测进行了对比,二者的相关性比较好.给出了描述北京地区气溶胶光学厚度分布的几幅图片.卫星遥感对于更好地研究空气污染提供了一种新手段,卫星遥感的气溶胶光学厚度弥补了地面观测空间覆盖不足的缺陷.卫星遥感的气溶胶资料不仅对全球和区域气候研究而且对城市污染分析提供了丰富的资料.     

Near-bottom particle concentration and flux: Temporal variations observed with sediment traps and nepholometer on the Meriadzek Terrace, Bay of Biscay

The Meriadzek Terrace (a 2100m deep plateau on the North-East Atlantic continental slope) was chosen as the experimental site for a multidisciplinary programme to observe the parameters needed for a better understanding of biological processes in the benthic environment.Two approaches were used to study the input of particulate matter to the bathyal seabed: sediment traps and indirect particle concentration measurements with nephelometry. These two technologies do not measure particles of the same size range, but as we are interested in the fluctuations of the particle supply, their results are complementary.Vertical profiles of nephelometry show that over the Meriadzek Terrace there is 125m thick nepheloid layer immediately above the bottom.The dynamics in the deep layer has been determined by measurements made with a Module Autonome Pluridisciplinaire (MAP), an in situ monitoring device developed at IFREMER which measures currents, nephelometry, temperature vertical profile near the bottom.Throughout six months of measurements in 1984, the currents at 0.5m and 120m above the bottom were subject to semi-diurnal tidal oscillations. The intensity of light scattering recorded with the nephelometer on the MAP was highly correlated with current velocities especially with semidiurnal tidal oscillations which seem to induce local resuspension. There are also longer term fluctuations, notably a very strong event which lasted several days during August. This event lagged behind a period of high intensity of internal waves correlated with a reversal in current direction. The sediment trap (Pièges à Particules “PAP”) observations showed that the particle fluxes on the Meriadzek Terrace have a cycle of variation similar to primary production which is characterized by a maximum in May during the phytoplankton bloom and a minimum during January. There was also interannual fluctuation.These two kinds of results point out the different time scales (from some hours to several months) of the large temporal fluctuations which affect the near-bottom particle behaviour.     

Early Cretaceous (late Berriasian to early Aptian) palaeoceanographic change along the northwestern Tethyan margin (Vocontian Trough,southeastern France): δ13C, δ18O and Sr-isotope belemnite and whole-rock records

Stable carbon, oxygen, and strontium isotope records were obtained from uppermost Hauterivian to lowermost Aptian belemnite rostra, which were collected in well-dated sections from the Vocontian Trough (southeastern France). This data set complements previously published belemnite-isotope records from the uppermost Berriasian-Hauterivian interval from the same basin. The belemnite carbon and oxygen isotope record is compared to the carbonate bulk-rock isotope record from the same sections, and from additional Italian sections. With regards to their long-term trends, both belemnite and whole-rock δ¹⁸O records are well correlated, except for the uppermost Hauterivian-lower Barremian interval, within which they deviate. This discrepancy is interpreted to be linked to the latest Hauterivian Faraoni oceanic anoxic event and its early Barremian aftermath. The Faraoni level is characterized by enhanced sea-water stratification, probably induced by the onset of a warmer and more humid climate along the northern Tethyan margin. The early Barremian was characterized by stronger vertical sea-water mixing reflected by a decrease in density contrast between sea-surface and deeper waters. The belemnite oxygen isotope record shows a more stable evolution with smaller fluctuations than its bulk-rock counterpart, which indicates that deeper water masses were not as much subjected to density fluctuations as sea-surface water. The comparison of belemnite and bulk-rock carbon isotope records allows observing the impact of regional influence exerted by platform carbonate ooze shedding on the carbon cycle. Discrepancies in the two records are observed during time of photozoan carbonate platform growth. The strontium isotopic record shows a gradual increase from the uppermost Berriasian to the uppermost lower Barremian followed by a rapid decrease until the uppermost Barremian and a renewed small increase within the lowermost Aptian. The major inflection point in the uppermost lower Barremian appears to predate the onset in the formation of the Ontong-Java volcanic plateau.     

Origins and genesis of loess deposits in central Spain,as indicated by heavy mineral compositions and grain‐size variability

The formation of loess deposits strongly depends on the availability of deflatable dust‐sized material. Identification of source areas and mechanisms of dust production is essential for an appropriate characterization of related palaeoenvironmental conditions. So far, little research has been done on loess deposits in the Mediterranean region and information about mechanisms of dust production is very rare. In this context, it is not clear from where the loess deposits from the upper Tagus Basin in central Spain originated. The main objective of this study was to find out whether these loess deposits were formed primarily by mountain processes and fluvial comminution, or whether they have their origin in the weathering of Tertiary marls in the centre of the Madrid Basin. A further concern was to link the determined source areas with loess formation mechanisms and concomitant environmental contexts. Following a comprehensive approach using heavy minerals and grain‐size data, together with information on stratigraphic features and geomorphic positions, different local loess sources could be identified. This study shows that during the last 35 kyr a major proportion of the Tagus loess deposits was deflated from river floodplains, while the mountain region of the Iberian Range was identified as a significant source of respective floodplain sediments. Based on heavy mineral compositions and dating results, it was found that during Heinrich Stadial 3, sediment supply from the Iberian Range strongly increased, suggesting an environmental shift from initially warmer towards colder temperatures in late Marine Isotope Stage 3. Furthermore, it was found that grain‐size patterns clearly indicate maximum wind strengths during Heinrich Stadial 3, followed by Heinrich Stadial 2 and Heinrich Stadial 1. These results demonstrate that the formation of dust‐sized particles in this part of the Mediterranean operated similar to many other places in temperate and continental regions, and that weathering processes linked to hot and dry environments were only of minor importance.     

North polar region of Mars: Advances in stratigraphy, structure, and erosional modification

We have remapped the geology of the north polar plateau on Mars, Planum Boreum, and the surrounding plains of Vastitas Borealis using altimetry and image data along with thematic maps resulting from observations made by the Mars Global Surveyor, Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter spacecraft. New and revised geographic and geologic terminologies assist with effectively discussing the various features of this region. We identify 7 geologic units making up Planum Boreum and at least 3 for the circumpolar plains, which collectively span the entire Amazonian Period. The Planum Boreum units resolve at least 6 distinct depositional and 5 erosional episodes. The first major stage of activity includes the Early Amazonian (∼3 to 1 Ga) deposition (and subsequent erosion) of the thick (locally exceeding 1000 m) and evenly-layered Rupes Tenuis unit (Abrt), which ultimately formed approximately half of the base of Planum Boreum. As previously suggested, this unit may be sourced by materials derived from the nearby Scandia region, and we interpret that it may correlate with the deposits that regionally underlie pedestal craters in the surrounding lowland plains. The second major episode of activity during the Middle to Late Amazonian (1 Ga) began with a section of dark, sand-rich and light-toned ice-rich irregularly-bedded sequences (Planum Boreum cavi unit, Abb_c) along with deposition of evenly-bedded light-toned ice- and moderate-toned dust-rich layers (Planum Boreum 1 unit, Abb₁). These units have transgressive and gradational stratigraphic relationships. Materials in Olympia Planum underlying the dunes of Olympia Undae are interpreted to consist mostly of the Planum Boreum cavi unit (Abb_c). Planum Boreum materials were then deeply eroded to form spiral troughs, Chasma Boreale, and marginal scarps that define the major aspects of the polar plateau's current regional topography. Locally- to regionally-extensive (though vertically minor) episodes of deposition of evenly-bedded, light- and dark-toned layered materials and subsequent erosion of these materials persisted throughout the Late Amazonian. Sand saltation, including dune migration, is likely to account for much of the erosion of Planum Boreum, particularly at its margin, alluding to the lengthy sedimentological history of the circum-polar dune fields. Such erosion has been controlled largely by topographic effects on wind patterns and the variable resistance to erosion of materials (fresh and altered) and physiographic features. Some present-day dune fields may be hundreds of kilometers removed from possible sources along the margins of Planum Boreum, and dark materials, comprised of sand sheets, extend even farther downwind. These deposits also attest to the lengthy period of erosion following emplacement of the Planum Boreum 1 unit. We find no evidence for extensive glacial flow, topographic relaxation, or basal melting of Planum Boreum materials. However, minor development of normal faults and wrinkle ridges may suggest differential compaction of materials across buried scarps. Timing relations are poorly-defined mostly because resurfacing and other uncertainties prohibit precise determinations of surface impact crater densities. The majority of the stratigraphic record may predate the recent (<20 Ma) part of the orbitally-driven climate record that can be reliably calculated. Given the strong stratigraphic but loose temporal constraints of the north polar geologic record, a comparison of north and south polar stratigraphy permits a speculative scenario in which major Amazonian depositional and erosional episodes driven by global climate activity is plausible.     

Mouvements actuels des blocs tectoniques dans l’arc Bético-Rifain à partir des mesures GPS entre 1999 et 2005

GPS velocities and seismicity across the Betic–Rif Arc structural domains (Morocco and Iberia) provide a basis to evaluate present-day seismotectonic processes between different deformation belts. The results show asymmetric movements in the complex Alboran system accommodating the convergence between the African (Nubian) and Eurasian plates. While the Betic Mountains are attached to Iberia, moving toward the southeast with respect to Africa, the Rif is divided into three blocks with distinct displacements relative to Nubia: (1) the Tangier block moving southeastward, (2) the Central Rif block moving SSW, and (3) the Oriental Rif block undergoing clockwise rotation. GPS-derived motions decrease in rate from the Rif nappes complex to the foreland.