The Experts below are selected from a list of 201 Experts worldwide ranked by ideXlab platform
Jean-jacques Ehrhardt - One of the best experts on this subject based on the ideXlab platform.
-
Competition between selenium (IV) and Silicic Acid on the hematite surface
Chemosphere, 2008Co-Authors: Norbert Jordan, Nicolas Marmier, Claire Lomenech, Eric Giffaut, Jean-jacques EhrhardtAbstract:Abstract Competition between selenium (IV) and Silicic Acid for the hematite (α-Fe2O3) surface has been studied during this work. Single batch experiments have been performed to study separately the sorption of selenium (IV) and Silicic Acid as a function of the pH. With the help of the 2-pK surface complexation model, experimental data have been fitted using the FITEQL 4.0 program. Two monodentate inner-sphere surface complexes have been used to fit selenite ions retention, FeSeO 3 - and FeHSeO3. In order to fit sorption of Silicic Acid, the two following surface complexes, namely FeH3SiO4, and FeH 2 SiO 4 - , have been used. Using the surface complexation constants coming from these two binary systems, prediction curves of the effect of Silicic Acid on the retention of selenium (IV) onto hematite have been obtained. Finally, performed experiments showed a competition between selenium (IV) and Silicic Acid for the surface sites of hematite. Experimental data matched DDLM predictions, confirming the ability of the surface complexation model to predict quantitatively and qualitatively the ternary system selenium (IV)/H4SiO4/hematite.
-
Sorption of selenium(IV) onto magnetite in the presence of Silicic Acid
Journal of colloid and interface science, 2008Co-Authors: Norbert Jordan, Nicolas Marmier, Claire Lomenech, Eric Giffaut, Jean-jacques EhrhardtAbstract:Sorption of selenium(IV) and Silicic Acid onto magnetite (Fe(3)O(4)) was investigated in binary systems, with concentrations of Silicic Acid under the solubility limit of amorphous silica. Using the double diffuse layer model (DDLM), surface complexation constants of selenium(IV) and H(4)SiO(4) onto magnetite were extracted using Fiteql 4.0. Then, prediction curves of the sorption of selenium(IV) in the presence of Silicic Acid onto magnetite were obtained, using the calculated surface complexation constants. Finally, laboratory experiments were performed and showed a competition between selenium(IV) and Silicic Acid for the surface sites of magnetite. Experimental results matched the model predictions, confirming its ability to model qualitatively and quantitatively the ternary system.
Jorge L. Sarmiento - One of the best experts on this subject based on the ideXlab platform.
-
A corollary to the Silicic Acid leakage hypothesis
Paleoceanography, 2008Co-Authors: Katsumi Matsumoto, Jorge L. SarmientoAbstract:[1] The Silicic Acid leakage hypothesis (SALH) attempts to explain part of the large and regular atmospheric CO2 changes over the last glacial-interglacial cycles. It calls for a reduction in the carbonate pump through a growth in diatoms at the expense of coccolithophorids in low-latitude surface waters, driven by a “leakage” of high-Si:N waters from the Southern Ocean. Recent studies that present low opal accumulation rates from the glacial eastern equatorial Pacific have challenged SALH. In a corollary to SALH, we argue that the key to SALH is the dominance of diatoms over coccolithophorids, and this does not depend on the magnitude of diatom production per se. In support of our claim, we show in a numerical model that atmospheric CO2 can be lowered with even a reduced absolute flux of Silicic Acid leakage, provided that Si:N in the leakage is elevated and that the excess Si can be used by diatoms to shift the floral composition in their favor.
-
Deep ocean biogeochemistry of Silicic Acid and nitrate
Global Biogeochemical Cycles, 2007Co-Authors: Jorge L. Sarmiento, J. Simeon, Anand Gnanadesikan, Nicolas Gruber, Robert M. Key, Reiner SchlitzerAbstract:[1] Observations of Silicic Acid and nitrate along the lower branch of the global conveyor belt circulation show that Silicic Acid accumulation by diatom opal dissolution occurs at 6.4 times the rate of nitrate addition by organic matter remineralization. The export of opal and organic matter from the surface ocean occurs at a Si:N mole ratio that is much smaller than this almost everywhere (cf. Sarmiento et al., 2004). The preferential increase of Silicic Acid over nitrate as the deep circulation progresses from the North Atlantic to the North Pacific is generally interpreted as requiring deep dissolution of opal together with shallow remineralization of organic matter (Broecker, 1991). However, Sarmiento et al. (2004) showed that the primary reason for the low Silicic Acid concentration of the upper ocean is that the waters feeding the main thermocline from the surface Southern Ocean are depleted in Silicic Acid relative to nitrate. By implication, the same Southern Ocean processes that deplete the Silicic Acid in the surface Southern Ocean must also be responsible for the enhanced Silicic Acid concentration of the deep ocean. We use observations and results from an updated version of the adjoint model of Schlitzer (2000) to confirm that this indeed the case.
Katsumi Matsumoto - One of the best experts on this subject based on the ideXlab platform.
-
Different mechanisms of Silicic Acid leakage and their biogeochemical consequences
Paleoceanography, 2014Co-Authors: Katsumi Matsumoto, Zanna Chase, Karen E. KohfeldAbstract:In the modern ocean, Silicic Acid is effectively trapped in the Southern Ocean. According to the Silicic Acid leakage hypothesis, a loosening of this trapping may have contributed to low atmospheric CO2 during glacial times. Using a model with dynamical feedbacks in ocean physics and biology, we explore three distinct mechanisms to loosen the trapping and trigger Silicic Acid leakage from the Southern Ocean: sea ice expansion, weaker winds, and iron addition. The basic idea of the iron addition mechanism was previously explored using a simple box model. Here we confirm the main results of the earlier work and demonstrate further that sea ice expansion and weaker southern westerlies can also trigger Silicic Acid leakage. The three mechanisms are not mutually exclusive, and their biogeochemical consequences are dissimilar in terms of the spatial patterns of Si:N uptake and the relative changes in opal and particulate organic carbon fluxes both in and outside the Southern Ocean. While it is not entirely clear how sea ice, winds, and iron deposition were different during the last glacial period compared to today, we examine the synergistic effects of these three triggers in one simulation. In this simulation, the combination of sea ice and iron reproduces the north-south dipole of productivity recorded in export production proxies, but effects of the iron perturbation seem to dominate the overall biogeochemical response.
-
A corollary to the Silicic Acid leakage hypothesis
Paleoceanography, 2008Co-Authors: Katsumi Matsumoto, Jorge L. SarmientoAbstract:[1] The Silicic Acid leakage hypothesis (SALH) attempts to explain part of the large and regular atmospheric CO2 changes over the last glacial-interglacial cycles. It calls for a reduction in the carbonate pump through a growth in diatoms at the expense of coccolithophorids in low-latitude surface waters, driven by a “leakage” of high-Si:N waters from the Southern Ocean. Recent studies that present low opal accumulation rates from the glacial eastern equatorial Pacific have challenged SALH. In a corollary to SALH, we argue that the key to SALH is the dominance of diatoms over coccolithophorids, and this does not depend on the magnitude of diatom production per se. In support of our claim, we show in a numerical model that atmospheric CO2 can be lowered with even a reduced absolute flux of Silicic Acid leakage, provided that Si:N in the leakage is elevated and that the excess Si can be used by diatoms to shift the floral composition in their favor.
Norbert Jordan - One of the best experts on this subject based on the ideXlab platform.
-
Competition between selenium (IV) and Silicic Acid on the hematite surface
Chemosphere, 2008Co-Authors: Norbert Jordan, Nicolas Marmier, Claire Lomenech, Eric Giffaut, Jean-jacques EhrhardtAbstract:Abstract Competition between selenium (IV) and Silicic Acid for the hematite (α-Fe2O3) surface has been studied during this work. Single batch experiments have been performed to study separately the sorption of selenium (IV) and Silicic Acid as a function of the pH. With the help of the 2-pK surface complexation model, experimental data have been fitted using the FITEQL 4.0 program. Two monodentate inner-sphere surface complexes have been used to fit selenite ions retention, FeSeO 3 - and FeHSeO3. In order to fit sorption of Silicic Acid, the two following surface complexes, namely FeH3SiO4, and FeH 2 SiO 4 - , have been used. Using the surface complexation constants coming from these two binary systems, prediction curves of the effect of Silicic Acid on the retention of selenium (IV) onto hematite have been obtained. Finally, performed experiments showed a competition between selenium (IV) and Silicic Acid for the surface sites of hematite. Experimental data matched DDLM predictions, confirming the ability of the surface complexation model to predict quantitatively and qualitatively the ternary system selenium (IV)/H4SiO4/hematite.
-
Sorption of selenium(IV) onto magnetite in the presence of Silicic Acid
Journal of colloid and interface science, 2008Co-Authors: Norbert Jordan, Nicolas Marmier, Claire Lomenech, Eric Giffaut, Jean-jacques EhrhardtAbstract:Sorption of selenium(IV) and Silicic Acid onto magnetite (Fe(3)O(4)) was investigated in binary systems, with concentrations of Silicic Acid under the solubility limit of amorphous silica. Using the double diffuse layer model (DDLM), surface complexation constants of selenium(IV) and H(4)SiO(4) onto magnetite were extracted using Fiteql 4.0. Then, prediction curves of the sorption of selenium(IV) in the presence of Silicic Acid onto magnetite were obtained, using the calculated surface complexation constants. Finally, laboratory experiments were performed and showed a competition between selenium(IV) and Silicic Acid for the surface sites of magnetite. Experimental results matched the model predictions, confirming its ability to model qualitatively and quantitatively the ternary system.
Aude Leynaert - One of the best experts on this subject based on the ideXlab platform.
-
No limit? : the multiphasic uptake of Silicic Acid by benthic diatoms
Limnology and Oceanography, 2009Co-Authors: Aude Leynaert, Sorcha Ni Longphuirt, Pascal Claquin, Laurent Chauvaud, Olivier RagueneauAbstract:Silicic Acid uptake kinetics for a field population of benthic diatoms were examined using a radioisotope tracer over a concentration range of 0-300 μmol L-1. The microphytobenthos half saturation constant (54 μmol L-1) and specific uptake rates (0.096 h-1) for Silicic Acid were well above those usually found in the pelagic environment. Silicic Acid kinetics were hyperbolic only at low concentrations (< 60 μmol L-1). At higher concentrations, a second mechanism that did not suggest saturation was activated. Many benthic diatoms are motile and migrate vertically in the upper few centimeters of sediment where there are strong gradients of nutrient concentrations. The multiphasic uptake may allow them to take maximum advantage of the high Silicic Acid concentrations found at depth.
-
Diurnal heterogeneity in Silicic Acid fluxes in shallow coastal sites: Causes and implications
Estuarine Coastal and Shelf Science, 2009Co-Authors: Sorcha Ni Longphuirt, Laurent Chauvaud, Olivier Ragueneau, Sophie Martin, Fred Jean, Gérard Thouzeau, Aude LeynaertAbstract:In shallow coastal areas the amplitude and range of benthic Silicic Acid fluxes can have a significant influence on benthic–pelagic coupling and the functioning of the pelagic system. To explore the oscillation in fluxes over the diurnal cycle and in particular the influence of microphytobenthos (MPB), an experiment was carried out in a shallow subtidal site in the Bay of Brest (France). Benthic chambers were employed over a 48 h period to measure the variability in Silicic Acid and oxygen fluxes; MPB migration was investigated using a diving Pulse Amplitude Modulated (PAM) fluorometer and uptake rhythms of Silicic Acid by natural MPB populations were measured using the 32Si isotope. It was discovered that Silicic Acid fluxes fluctuated greatly throughout the diurnal period resulting in an oscillation in the availability of this nutrient for phytoplankton communities. The uptake of Silicic Acid by the MPB was quantified for the first time and highlighted a 2-fold increase in the demand from night to afternoon periods. The combined Silicic Acid uptake and the concentration of cells at the sediment–water interface, forming a dense biofilm of MPB, were postulated to be the main processes reducing effluxes at midday. Our work highlighted the many processes which influence Silicic Acid effluxes in shallow coastal areas and the possible interaction between uptake and dissolution processes. The variations in benthic fluxes over the diurnal period were comparable to observations reported at the seasonal scale. Therefore, up-scaling hourly flux observations to daily and annual estimates should be undertaken with caution. Further we suggest that the main processes influencing flux oscillations over the diurnal period should be considered when planning sampling strategies and extrapolating to larger time scales.
-
Effect of iron deficiency on diatom cell size and Silicic Acid uptake kinetics
Limnology and Oceanography, 2004Co-Authors: Aude Leynaert, Pascal Claquin, Eva Bucciarelli, Richard Dugdale, V. Martin-jézéquel, Philippe Pondaven, Olivier RagueneauAbstract:We studied the Silicic Acid uptake kinetics of the pennate diatom Cylindrotheca fusiformis grown under a wide range of iron concentrations (from Fe-limiting to Fe-sufficient conditions) to assess the effect of iron availability on diatom cell size, silicon content, and Silicic Acid uptake kinetic parameters. As the iron stress increased, the growth rate slowed, cell size decreased, and silicification increased. A series of Si kinetic uptake experiments (from Si-limiting to Si-sufficient conditions) performed at different iron concentrations demonstrated the extent of the colimitation domain, where the specific Si uptake rate (VSi) varied as a function of both Silicic Acid and Fe availability. A decrease in maximal specific uptake rate of Silicic Acid (VSi-max) under iron limitation was observed along with a decrease in half-saturation constant for Silicic Acid uptake (KSi). Because VSi-max and KSi vary in the same direction, the specific affinity for Silicic Acid does not change under iron stress. The variation in cell size is an acclimation to low nutrient concentrations. Our study shows that Si uptake kinetics parameters, especially VSi-max, are strongly related to cell size, which is itself constrained by the degree of iron limitation. Thus, the surface to volume ratio should be related to Silicic Acid flows in size-based biogeochemical models of planktonic ecosystems. Another way to describe the observed variations of Si uptake over the full range of Si and Fe concentrations tested in the present study would be to implicitly take into account the variations in cell size through a scaling of the maximal specific uptake rate (Vmax) by the iron-limitation term in a multiplicative manner.
