Log-normal distributions of suspended particles in the open ocean

A scanning electron microscope-electron microprobe technique was used to chemically distinguish and size particles as fine as 0.2/µm on GEOSECS suspended matter filters from the open ocean…

Sources and transport of suspended calcites in Pacific Deep Water

International audienceSignificantly large concentrations of suspended matter, especially calcites, are found at some locations in the Deep Pacific, particularly at the depth of the Pacific Benthic Thermocline. These particle-rich layers cannot be explained by simple classical settling processes nor by resuspension of the underlying sediment. A theoretical approach based on these data, on the hydrodynamic characteristics of the regional circulation and on some known physical and chemical parameters, leads us to postulate two possible mechanisms: the settling of large particles from surface waters and their preferential disaggregation at some specific depth, particularly at the Pacific Benthic Thermocline depth; or alternatively, the resuspension atthose depths of old calcareous sediments from the flanks of East Pacific Rise associated with a horizontal advective diffusive transport mechanism. New chemical and electron-microscopeanalyses of GEOSECS samples give rise to the conclusion that the resuspension mechanism is the most probable. A simple model based on this mechanism, using our calcite concentrationsat the PBT depth implies that the calcium carbonate flux arising from this source ranges between 5 x 10^6 and 5 x 10$^8$ tons per year. The erosion intensity and the quantity of calciumcarbonate contained by the rises or the seamounts, must be large enough to maintain this flux. Those two constraints have been successfully tested

Towards a parametrization of river discharges into ocean general circulation models: a closure through energy conservation

Diagnostic methods are defined in order to compare two numerical simulations of ocean dynamics in a region of freshwater influence. The first one is a river plume simulation based on a high resolution numerical configuration of the POM coastal ocean model in which mixing parametrizations have been previously defined. The second one is a simulation based on the NEMO Global Ocean Model used for climate simulations in its half-a-degree configuration in which a river inflow is represented as precipitation on two coastal grid cells. Both simulations are forced with the same freshwater inflows and wind stresses. The divergence of volumetric fluxes above and below the halocline are compared. Results show that when an upwelling wind blows, the two models display similar behavior although the impact of lack of precision can be observed in the NEMO configuration. When a downwelling wind blows, the NEMO Global Ocean configuration can not reproduce the coastally trapped baroclinic dynamics because its grid resolution is too coarse. To find a parametrization to help represent these dynamics in ocean general circulation models, a method based on energy conservation is investigated. This method shows that it is possible to link the energy fluxes provided by river inflows to the divergence of energy fluxes integrated over the grid cells of ocean general circulation models. A parametrization of the dynamics created by freshwater inflows is deduced from this method. This enabled creation of a box model that proved to have the same behavior as the fluxes previously computed from the high resolution configuration

Tidal bore: eulerian velocities and suspended sediment concentration measurements

International audienc

Evaluation of Particle Size Distribution Metrics to Estimate the Relative Contributions of Different Size Fractions Based on Measurements in Arctic Waters

The size distribution of suspended particles influences several processes in aquatic ecosystems, including light propagation, trophic interactions, and biogeochemical cycling. The shape of the particle size distribution (PSD) is commonly modeled as a single-slope power law in oceanographic studies, which can be used to further estimate the relative contributions of different particle size classes to particle number, area, and volume concentration. We use a data set of 168 high size-resolution PSD measurements in Arctic oceanic waters to examine variability in the shape of the PSD over the particle diameter range 0.8 to 120 μm. An average value of -3.6 ± 0.33 was obtained for the slope of a power law fitted over this size range, consistent with other studies. Our analysis indicates, however, that this model has significant limitations in adequately parameterizing the complexity of the PSD, and thus performs poorly in predicting the relative contributions of different size intervals such as those based on picoplankton, nanoplankton, and microplankton size classes. Similarly, median particle size was also generally a poor indicator of these size class contributions. Our results suggest that alternative percentile diameters derived from the cumulative distribution functions of particle number, cross-sectional area, and volume concentration may provide better metrics to capture the overall shape of the PSD and to quantify the contributions of different particle size classes

Regional modelling of the biogeochemical cycles in the Western Mediterranean (EROS 2000)

peer reviewe

Centers of coordination : a case and some themes.

Abstract This chapter identifies a class of worksites characterizable in terms of participants ’ ongoing orientation to problems of space and time, involving the deployment of people and equipment across distances according either to a timetable or to the emergent requirements of a time-critical situation. To meet simultaneous requirements of mobility and control, centers of coordination must function as centers to which participants distributed in space can orient, and which at any given moment they know how to find. At the same time, to coordinate activities distributed in space and time personnel within the site must somehow have access to the situation of co-workers in other locations. One job of technologies in such settings is to meet these requirements through the reconfiguration of relevant spatial and temporal relations. This general characterization is explored through ethnographic materials from an investigation of the work of airline ground operations at a metropolitan airport on the west coast of the United States