Analyse des rapports C/N/P du seston dans la partie orientale de l'atlantique equatorial

A study of the C/N ratios of particulate matter in the eastern part of the Gulf of Guinea demonstrated a statistically significant spatial variation in these ratios. The variation in the ratios ranged between 7.2 and 9.5 and were due in part to differences in the composition of the suspended organic matter and in part to its age and history. Young nutrient rich waters had low ratios (generally less than 7), whereas older waters often had ratios greater than 10. A relation between Chlorophyl a and the C/N ratio was also observed. The distribution of C/P ratios with depth showed a linear correlation with the vertical distribution of oxygen (r = 0.74). C/P values in the neighbourhood of 100 at the surface, increased with depth to around 300 at 1000 m.     

A budget for modern‐Holocene sediment on the South Otago continental shelf

A sediment budget for the South Otago continental shelf and coast, between Nugget Point and Otago Peninsula, reveals modern (post 6500 y) sediment input is dominated by the Clutha River (total 3.14 Mt y^‐1; Mt = 10⁶ tonnes). Contributions from the Taieri River (0.6 Mt y^‐1), the adjacent Southland shelf (0.4 Mt y^‐1), and the biogenic production of calcareous shell debris (0.25 Mt y^‐1) account for only 28% of the input. About half of the bedload (sand and gravel) reaching the Otago shelf is stored within a large nearshore sand wedge in the protected waters of Molyneux Bay, off the Clutha River. Bedload that escapes storage (1.1 Mt y^‐1) is transported north‐eastwards to be deposited on beach and inner shelf environments just north of Otago Peninsula. Suspended load (mud) accounts for over half of the sediment input (2.33 Mt y^‐1) and is nearly all transported from the study area to accumulate in north‐easterly shelf and slope depocentres.     

Capillary geochemistry in non-saturated zone of soils. Water content and geochemical signatures

The unsaturated zone (UZ) retains aqueous solutions against gravity by capillary forces. This suction state corresponds to a decreasing internal pressure of the water, which modifies its thermodynamic properties. Accordingly, the speciation of solutes and the solubility of solids and gases in such capillary solutions change. The volumetric capillary water content of the soil at high suction can be calculated extrapolating the water retention curves (WRC) with the Rossi–Nimmo model. Interestingly, several tens of liters per cubic meter of soil can be thus suctioned, a sufficiently large volume to support that: (1) capillary water is not restricted to nanosized pores, which means it disobeys the Young–Laplace law and is metastable with respect to vapor (superheating); and (2) the geochemistry of capillary solutions might significantly influence the subsurface mass transfer. Two field situations are here interpreted using the capillary thermodynamic properties: (1) the trapping of sand grains during the growth of desert roses (gypsum), and (2) the development of abnormal paragenetic sequences in some saprolites.The capillary approach is extended to the soil solids, so that the micro-mineralogy can be explicitly (though sketchily) integrated in the calculations. The key conclusion is that capillarity changes the saturation indexes (and so the reaction rates) at given solution composition, in a way consistent with the field observations. This perspective amounts to geochemically distinguishing the capillary and percolating solutions, which is interestingly analogous to the immobile and mobile water distinction already often integrated in UZ flow models.     

Embryo aberrations in sea ice amphipod Gammarus wilkitzkii exposed to water soluble fraction of oil

Offshore oil and gas activities have gained momentum in the European Arctic, raising concerns of the potential impact of oil-related chemicals on the polar marine ecosystem, notably on sea ice communities. Herein, malformations on embryos of the Arctic sea ice amphipod Gammaruswilkitzkii exposed to the water soluble fraction of oil were studied. The females ranged from development stage three to nine. No differences in reproductive stage were observed among the different treatments after 30 days of exposure. Frequency of embryo aberrations was significantly higher in the high-dose compared to controls, indicating that the embryos of G. wilkitzkiii were affected by oil.     

Magma vesiculation and pyroclastic volcanism on Venus

Theoretical consideration of the magma vesiculation process under observed and inferred venusian surface conditions suggests that vesicles should form in basaltic melts, especially if CO₂ is the primary magmatic volatile. However, the high surface atmospheric pressure ((～90 bars) and density on Venus retard bubble coalescence and disruption sufficiently to make explosive volcanism unlikely. The products of explosive volcanism (fire fountains, convecting eruption clouds, pyroclastic flows, and topography-mantling deposits of ash, spatter, and scoria) should be rare on Venus, and effusive eruptions should dominate. The volume fraction of vesicles in basaltic rocks on Venus are predicted to be less than in chemically similar rocks on Earth. Detection of pyroclastic landforms or eruption products on Venus would indicate either abnormally high volatile contents of Venus magmas (2.5–4 wt%) or different environmental conditions (e.g., lower atmospheric pressure) in previous geologic history.     

The influence of fine particle tropospheric aerosol on local air and soil temperature in the central U.S.A.

The optical properties of two haze conditions prevalent during well-observed occasions in the central U.S.A. were estimated by Ball and Robinson (1982). A comprehensive, interactive, time-dependent model of the coupled atmospheric planetary boundary-upper soil (water) layers, specifically formulated to include such radiative parameters, is described in Brown et al. (1982) and earlier papers. A version of that model, including the Robinson and Ball radiative parameters, is applied with detailed micrometeorological measurements tabulated in Lettau and Davidson (1957), to obtain some quantitative estimates of the effects of haze on near-surface air and soil temperatures in agriculturally important regions of the central U.S.A.A calibration simulation of an observed two-day period in late summer over a dry-prairie surface shows the model's qualitative verisimilitude in the calculation of observed solar irradiance and near-surface air temperature. This is followed by a second model simulation experiment for late spring clear-sky conditions over a hypothetical moist bare-soil (sown cropland) surface, to estimate the effects of the two hazes on temperature and on the accompanying surface heat budgets. Significant alterations of near-surface temperatures, in the range 0.5 °C to 2.5 °C are found over a five-day simulation period. The alterations are such as to increase air-soil temperature difference, with relative warming of the air and relative cooling of the soil with both aerosol characterizations.     

Impact of the Phaeocystis globosa spring bloom on the intertidal benthic compartment in the eastern English Channel: a synthesis

From 1999 to 2005, studies carried out in the frame of regional and national French programs aimed to determine whether the Phaeocystisglobosa bloom affected the intertidal benthic communities of the French coast of the eastern English Channel in terms of composition and/or functioning. Study sites were chosen to cover most of the typical shore types encountered on this coast (a rocky shore, an exposed sandy beach and a small estuary). Both the presence of active Phaeocystis cells and their degradation product (foam) did have a significant impact on the studied shores. The primary production and growth rates of the kelp Saccharina latissima decreased during the bloom because of a shortage of light and nutrient for the macroalgae. On sandy sediments, the benthic metabolism (community respiration and community primary production), as well as the nitrification rate, were enhanced during foam deposits, in relation with the presence of bacteria and active pelagic cells within the decaying colonies. In estuarine sediments, the most impressive impact was the formation of a crust at the sediment surface due to drying foam. This led to anoxic conditions in the surface sediment and resulted in a high mortality among the benthic community. Some organisms also tended to migrate upward and were then directly accessible to the higher trophic level represented by birds. Phaeocystis then created a shortcut in the estuarine trophic network. Most of these modifications lasted shortly and all the systems considered came back to their regular properties and activities a few weeks after the end of the bloom, except for the most impacted estuarine area.     

Robust diving control of an AUV

Mobile systems traveling through a complex environment present major difficulties in determining accurate dynamic models. Autonomous underwater vehicle motion in ocean conditions requires investigation of new control solutions that guarantee robustness against external parameter uncertainty. A diving-control design, based on Lyapunov theory and back-stepping techniques, is proposed and verified. Using adaptive and switching schemes, the control system is able to meet the required robustness. The results of the control system are theoretically proven and simulations are developed to demonstrate the performance of the solutions proposed.