Agarase,amylase, cellulase,and chitinase activity at deep-sea pressures

An agarase produced by a marine bacterium, a bacterial amylase, a fungal cellulase, and an actinomycetal chitinase were examined for their stability and hydrolytic activity at hydrostatic pressures existing in the deep sea. All four enzymes were from barophobic organisms which fail to grow at pressures exceeding 400 to 500 atm, but some observations were made on the hydrolase activity of barophilic bacteria grown at 500 to 1,150 atm. Al-though differing in the rates at which they catalyzed the hydrolysis of agar, starch, cellulose, and chitin respectively, all four enzymes were found to be active at deep-sea pressures and temperatures. These glycoside hydrolases were found to be much more baroduric than most oxidoreductases, transferases, and ligases (synthetases) which have been investigated. The chitinase and-amylase were about as stable at 1,000 atm as at 1 atm. Cellulase was also baroduric and active at 1,000 atm at pH 4.2, its pH optimum at 1 atm. The pressure tolerance of cellulase was much less at higher pH values characteristic of normal sea water, i.e., near pH 8.2. The activity of agarase (synthesized at 1 atm) was retarded by increased pressure, due largely to the inactivation of this enzyme system. However, barophilic bacteria, isolated from the deep sea, synthesize agarases which are active at 1,000 atm as indicated by the hydrolysis of agar.Contribution from the Scripps Institution of Oceanography, University of California, San Diego, La Jolla, Calif. 92037, U.S.A.     

Impact of partial steps and momentum advection schemes in a global ocean circulation model at eddy-permitting resolution

Series of sensitivity tests were performed with a z-coordinate, global eddy-permitting (1/4°) ocean/sea-ice model (the ORCA-R025 model configuration developed for the DRAKKAR project) to carefully evaluate the impact of recent state-of-the-art numerical schemes on model solutions. The combination of an energy–enstrophy conserving (EEN) scheme for momentum advection with a partial step (PS) representation of the bottom topography yields significant improvements in the mean circulation. Well known biases in the representation of western boundary currents, such as in the Atlantic the detachment of the Gulf Stream, the path of the North Atlantic Current, the location of the Confluence, and the strength of the Zapiola Eddy in the south Atlantic, are partly corrected. Similar improvements are found in the Pacific, Indian, and Southern Oceans, and characteristics of the mean flow are generally much closer to observations. Comparisons with other state-of-the-art models show that the ORCA-R025 configuration generally performs better at similar resolution. In addition, the model solution is often comparable to solutions obtained at 1/6 or 1/10° resolution in some aspects concerning mean flow patterns and distribution of eddy kinetic energy. Although the reasons for these improvements are not analyzed in detail in this paper, evidence is shown that the combination of EEN with PS reduces numerical noise near the bottom, which is likely to affect current–topography interactions in a systematic way. We conclude that significant corrections of the mean biases presently seen in general circulation model solutions at eddy-permitting resolution can still be expected from the development of numerical methods, which represent an alternative to increasing resolution.     

From rifting to spreading in the eastern Gulf of Aden: a geophysical survey of a young oceanic basin from margin to margin

A geophysical survey in the eastern Gulf of Aden, between the Alula–Fartak (52°E) and the Socotra (55°E) transform faults, was carried out during the Encens–Sheba cruise. The conjugate margins of the Gulf are steep, narrow and asymmetric. Asymmetry of the rifting process is highlighted by the conjugate margins (horst and graben in the north and deep basin in the south). Two transfer fault zones separate the margins into three segments, whereas the present‐day Sheba Ridge is divided into two segments by a transform discontinuity. Therefore segmentation of the Sheba Ridge and that of the conjugate margins did coincide during the early stages of oceanic spreading. Extensive magma production is evidenced in the central part of the western segment. Anomaly 5d was identified in the northern and southern parts of the oceanic basin, thus confirming that seafloor spreading in this part of Gulf of Aden started at least 17.6 Ma ago.     

A long pollen record from Lac du Bouchet, Massif Central, France: for the period ca. 325 to 100 ka BP (OIS 9c to OIS 5e)

Pollen analysis was carried out on sediments older than the Würm pleniglacial (OIS 4), in two new sequences (H and I) derived from the centre of Lac du Bouchet, Massif Central. The inferred vegetation history enables, for the first time in France, five temperate episodes to be defined which pre-date the last interglacial. These temperate episodes alternate with episodes during which the changes in vegetation are indicative of glacial climates. Comparison of these climatic episodes with the oceanic isotope record shows that the pollen record of sequences H and I from Lac du Bouchet spans the time interval from OIS 9c (Ussel interstadial) to OIS 5e (Ribains/Eemian interglacial). In the organic sediments from the Amargiers interstadial (OIS 9a), a trachytic layer, Ar/Ar dated to ca 275 ka, enables a correlation to be established with the upper part of a sequence derived from the nearby Praclaux crater, the lower part of this sequence being of Holsteinian age (OIS 11c). The cross-dating of the pollen sequences from Lac du Bouchet (cores H, I and D) and from Praclaux provides a complete record from the Massif Central, southern France, of successive glacial and interglacial episodes that span the last ca. 400 ka, that is the interval from the Holsteinian to the Holocene.     

Rôle de l'halocinèse dans l'évolution du bassin d'Essaouira (Sud-Ouest marocain)

The seismic reflection studies carried in the Essaouira Basin, intermediate zone between the High Atlas and the Atlantic Margin, show the presence of halokinetic structures that played a significant role in the evolution of the basin. Salt dynamics was controlled by the Atlantic rifting and the Atlasic orogen. These tectonic controls and the margin segmentation are responsible for the diachronism of salt movement. Halokinesis varied in time and space in the basin and was more active offshore, where autochtonous and allochtonous salt layers are present. To cite this article: K. Mehdi et al., C. R. Geoscience 336 (2004).     

Structure et caractérisation des matériaux utilisés dans la construction d'une mosaı̈que romaine de la cité de Volubilis (Maroc)

In Volubilis, Roman mosaics are very beautiful and reveal, from the bottom to the surface, three layers: (i) ‘hedgehog’ layer, (ii) coarse grain mortar layer (rudus + nucleus) and (iii) tesselatum. Mineralogical analysis of coarse grain mortar sampled in Flavius Germanus mosaic shows that it consisted of quartz and calcite, with some feldspar and probably mica and dolomite. Fine-grained mortar in tesselatum is made from a mixture of calcite and quartz only. Limestone tesserae (white, pink and brown) show petrographic facies that change from micritic to oolithic limestone. Conversely, black and brick red tesserae are respectively made of marble, red sandstone and from fire clay. Other colours as yellow, blue, green and grey are obtained from artificial glass with different chemical compositions. To cite this article: A. Dekayir et al., C. R. Geoscience 336 (2004).     

L'hétérogénéité du manteau supérieur à l'aplomb du volcan de Nyos (Cameroun) révélée par les enclaves ultrabasiques

Volcanic deposits from the lake Nyos (Cameroon) contain ultrabasic xenoliths: lherzolites, harzburgites and websterites, sometimes containing amphibole. Major and trace element compositions of clinopyroxene and amphibole allow the recognition of an old depletion event followed by two episodes of incompatible-element enrichment. One is high in light rare-earth elements, while the other one is low in these elements, but high in U and Th. Their relative chronology is not yet established. To cite this article: R. Temdjim et al., C. R. Geoscience 336 (2004).     

MELIMEX,an experimental heavy metal pollution study: Oligochaetes and chironomid larvae in heavy metal loaded and control limno-corrals

In two limno-corrals loaded with heavy metals the density of tubificid worms and chironomid larvae was higher than in the control. The better oxygen conditions observed in the heavy metal loaded corrals might explain this discrepancy. The higher oxygen concentration at the sediment-water interface was due to lower organic sedimentation, consequence of the primary production reduced by heavy metals. In all limno-corrals, the tubificid populations consisted ofTubifex tubifex. The size structure of tubificid populations in the heavy metal loaded corrals indicated a delayed reproduction compared to the control.