Stoichiometries of remineralisation and denitrification in global biogeochemical ocean models

Since the seminal paper of Redfield (1934), constant stoichiometric elemental ratios linking biotic carbon and nutrient fluxes are often assumed in marine biogeochemistry, and especially in coupled biogeochemical circulation models, to couple the global oxygen, carbon and nutrient cycles. However, when looking in more detail, some deviations from the classical Redfield stoichiometry have been reported, in particular with respect to remineralization of organic matter changing with depth or with ambient oxygen levels. We here compare the assumptions about the stoichiometry of organic matter and its remineralization that are used explicitly and implicitly in common biogeochemical ocean models. We find that the implicit assumptions made about the hydrogen content of organic matter can lead to inconsistencies in the modeled remineralization and denitrification stoichiometries. It is suggested that future marine biogeochemical models explicitly state the chemical composition assumed for the organic matter, including its oxygen and hydrogen content

Inorganic fillers influence on the radiation-induced ageing of a space-used silicone elastomer

A space-used filled silicone rubber (silica and iron oxide fillers) and its polysiloxane isolated matrix were exposed to high energy electrons in order to determine their ageing mechanisms from a structural point of view. Physicochemical analysis evidenced that both filled and unfilled materials predominantly crosslink under such irradiation. Solid-state 29Si NMR spectroscopy allowed the identification of T-type SiO3 units as the main new crosslinks formed in the polymer network. It also revealed an increase in Qtype SiO4 units in the irradiated filled sample. Thanks to the combination of NMR spectroscopy and ammonia-modified swelling tests, these Q-type units were associated with new crosslinks formed at the silica fillers-matrix interface. While the main interaction between the polysiloxane network and the fillers was shown to proceed mainly through hydrogen bonding in the pristine filled samples, it was suggested that the hydrogen bonds were progressively replaced with SiO4 chemical bonds. These additional chemical crosslinks induced evolutions of the shear modulus on the rubber plateau and crosslink density that were significantly more pronounced in the filled material than in the neat one

Electrical conductivity of a silicone network upon electron irradiation: influence of formulation

In this study, the electrical conductivity of a silicone elastomer filled with inorganic fillers was investigated upon electron irradiation. Neat samples consisting of the isolated polysiloxane matrix (with no fillers) were studied in parallel to identify the filler contribution to this evolution. It was shown that exposure to 400 keV electron doses induced a decrease in electrical conductivity for both the filled and neat materials. This decrease was much more pronounced with the filled samples than with the neat ones. Moreover, the activation energy of electrical conductivity (Arrhenius behaviour) doubled in the filled case, while it varied only weakly for the neat case. In light of these results, structure–property relationships were proposed on the basis of the radiation-induced crosslink processes to which this material is subject. In the framework of electronic percolation theory, it is suggested that the radiation-induced formation of SiO3 crosslinks in the polysiloxane network and SiO4 crosslinks at filler–matrix interfaces affects the percolation path of the material, which can be simply modelled by a network of resistors in series. On one hand, their densification increases the overall resistance of the percolation path, which results in the observed decrease of effective electrical conductivity. On the other hand, the steep increase in activation energy in the filled material attributes to the SiO4 crosslinks becoming the most restrictive barrier along the percolation path. In spite of the misleading likeness of electrical conductivities in the pristine state, this study presented evidence that silicone formulation can affect the evolution of electrical properties in radiative environments. To illustrate this conclusion, the use of this material in space applications, especially when directly exposed to the radiative space environment, was discussed. The decrease in electrical conductivity was associated with a progressively increasing risk for the occurrence of electrostatic discharge and consequent spacecraft failures

Electrical behaviour of a silicone elastomer under simulated space environment

The electrical behavior of a space-used silicone elastomer was characterized using surface potential decay and dynamic dielectric spectroscopy techniques. In both cases, the dielectric manifestation of the glass transition (dipole orientation) and a charge transport phenomenon were observed. An unexpected linear increase of the surface potential with temperature was observed around Tg in thermally-stimulated potential decay experiments, due to molecular mobility limiting dipolar orientation in one hand, and 3D thermal expansion reducing the materials capacitance in the other hand. At higher temperatures, the charge transport process, believed to be thermally activated electron hopping with an activation energy of about 0.4 eV, was studied with and without the silica and iron oxide fillers present in the commercial material. These fillers were found to play a preponderant role in the low-frequency electrical conductivity of this silicone elastomer, probably through a Maxwell–Wagner–Sillars relaxation phenomenon

Aging Effect and Induced Electric Phenomena on Dielectric Materials Irradiated With High Energy Electrons

The high radiation dose received by space used polymers may greatly alter their electric properties. This effect could, for instance, reduce significantly radiation-induced conductivity (RIC) leading to high charging risks that were not predicted from pristine sample. For an optimized qualification and prediction, it is therefore highly important to characterize the charging properties of polymers and their evolution as a function of the received dose. This paper aimed at studying aging of electric properties of four different polymers (Teflon FEP, Kapton, polyepoxy DP 490 adhesive, and silicon QS1123 adhesive) at high dose level (105 and 106 Gy). We have been able to demonstrate that aging could lead to the reduction of RIC on some polymers (for polyepoxy and silicone adhesives, and FEP) or inversely to the increase of RIC on others (e.g., Kapton). Ionization effect must, however, be considered in the analysis of the results. Relaxation processes could drastically affect the charging profile and macroscopic electric properties

Coupled physical/biogeochemical modeling including O2-dependent processes in the Eastern Boundary Upwelling Systems: application in the Benguela

International audienceThe Eastern Boundary Upwelling Systems (EBUS) contribute to one fifth of the global catches in the ocean. Often associated with Oxygen Minimum Zones (OMZs), EBUS represent key regions for the oceanic nitrogen (N) cycle. Important bioavailable N loss due to denitrification and anammox processes as well as greenhouse gas emissions (e.g, N2O) occur also in these EBUS. However, their dynamics are currently crudely represented in global models. In the climate change context, improving our capability to properly represent these areas is crucial due to anticipated changes in the winds, productivity, and oxygen content. We developed a biogeochemical model (BioEBUS) taking into account the main processes linked with EBUS and associated OMZs. We implemented this model in a 3-D realistic coupled physical/biogeochemical configuration in the Namibian upwelling system (northern Benguela) using the high-resolution hydrodynamic ROMS model. We present here a validation using in situ and satellite data as well as diagnostic metrics and sensitivity analyses of key parameters and N2O parameterizations. The impact of parameter values on the OMZ off Namibia, on N loss, and on N2O concentrations and emissions is detailed. The model realistically reproduces the vertical distribution and seasonal cycle of observed oxygen, nitrate, and chlorophyll a concentrations, and the rates of microbial processes (e.g, NH4+ and NO2− oxidation, NO3− reduction, and anammox) as well. Based on our sensitivity analyses, biogeochemical parameter values associated with organic matter decomposition, vertical sinking, and nitrification play a key role for the low-oxygen water content, N loss, and N2O concentrations in the OMZ. Moreover, the explicit parameterization of both steps of nitrification, ammonium oxidation to nitrate with nitrite as an explicit intermediate, is necessary to improve the representation of microbial activity linked with the OMZ. The simulated minimum oxygen concentrations are driven by the poleward meridional advection of oxygen-depleted waters offshore of a 300 m isobath and by the biogeochemical activity inshore of this isobath, highlighting a spatial shift of dominant processes maintaining the minimum oxygen concentrations off Namibia. In the OMZ off Namibia, the magnitude of N2O outgassing and of N loss is comparable. Anammox contributes to about 20% of total N loss, an estimate lower than currently assumed (up to 50%) for the global ocean

Climatically-Active Gases in the Eastern Boundary Upwelling and Oxygen Minimum Zone (OMZ) Systems

International audienceThe EBUS (Eastern Boundary Upwelling Systems) and OMZs (Oxygen Minimum Zone) contribute very significantly to the gas exchange between the ocean and the atmosphere, notably with respect to the greenhouse gases (hereafter GHG). From in-situ ocean measurements, the uncertainty of the net global ocean-atmosphere CO2 fluxes is between 20 and 30%, and could be much higher in the EBUS-OMZ. Off Peru, very few in-situ data are available presently, which justifies alternative approaches for assessing these fluxes. GHG air-sea fluxes determination can be inferred from inverse modeling applied to Vertical Column Densities (VCDs) from GOSAT, using state of the art modeling, at low spatial resolution. For accurately linking sources of GHGs to EBUS and OMZs, the resolution of the source regions needs to be increased. This task develops on new non-linear and multiscale processing methods for complex signals to infer a higher spatial resolution mapping of the fluxes and the associated sinks and sources between the atmosphere and the ocean. The use of coupled satellite data (e.g. SST and/or Ocean colour) that carry turbulence information associated to ocean dynamics is taken into account at unprecedented detail level to incorporate turbulence effects in the evaluation of the air-sea fluxes. We will present a framework as described above for determining sources and sinks of GHG from satellite remote sensing with the Peru OMZ as a test bed

Eddies and an extreme water mass anomaly observed in the eastern south Pacific at the Stratus mooring

In the tropical eastern South Pacific the Stratus Ocean Reference Station (ORS) (∼20°S, 85.5°W) is located in the transition zone between the oxygen minimum zone (OMZ) and the well-oxygenated subtropical gyre. In February/March 2012, extremely anomalous water mass properties were observed in the thermocline at the Stratus ORS. The available eddy oxygen anomaly was −10.5 × 1016 µmol. This anomalous water was contained in an anticyclonic mode-water eddy crossing the mooring site. This eddy was absorbed at that time by an anticyclonic feature located south of the Stratus mooring. This was the largest water property anomaly observed at the mooring during the 13.5 month deployment period. The sea surface height anomaly (SSHA) of the strong mode-water eddy in February/March 2012 was weak, and while the lowest and highest SSHA were related to weak eddies, SSHA is found not to be sufficient to specify the eddy strength for subsurface-intensified eddies. Still, the anticyclonic eddy, and its related water mass characteristics, could be tracked backward in time in SSHA satellite data to a formation region in April 2011 off the Chilean coast. The resulting mean westward propagation velocity was 5.5 cm s−1. This extremely long-lived eddy carried the water characteristics from the near-coastal Chilean water to the open ocean. The water mass stayed isolated during the 11 month travel time due to high rotational speed of about 20 cm s−1 leading to almost zero oxygen in the subsurface layer of the anticyclonic mode-water eddy with indications of high primary production just below the mixed layer