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81.
Sulfate reduction in deep coastal marine sediments 总被引:1,自引:0,他引:1
Sulfate reduction rates in sediments of four deep stations in the Saguenay Fjord and the Laurentian Trough, Gulf of St. Lawrence, are among the lowest reported for the coastal environment. Maximum rates were 0.4–7.0 nmol cm−3 day−1. The low rates are due to relatively low sedimentation rates and continuously low temperatures. Regional differences in both integrated and maximum sulfate reduction rates in the sediment correlate with sediment trap measurements of sedimentation rate and organic carbon flux.Sulfate reduction accounts for the degradation of 5–26% of the estimated downward flux of organic matter to these sediments. Unlike the absolute rate of sulfate reduction, the relative proportion of the carbon flux that is degraded via sulfate reduction is not directly correlated with the sedimentation rate but is a function of organic matter composition, intensity of bioturbation, and the abundance of sub-oxic electron acceptors. Thus, the lowest proportion of carbon degradation via sulfate reduction occurred at a Gulf site, where a combination of low sedimentation and bioturbation rates allowed a long residence time for organic matter near the sediment surface and, in consequence, a low flux of labile carbon into the sulfate reduction zone. The highest proportion was observed at a station with a similar organic carbon flux but with higher rates of sedimentation and bioturbation. At a third site, with the highest rates of sulfate reduction as well as the highest rates of sedimentation and bioturbation, the contribution of sulfate reduction to organic matter degradation was only intermediate. This is attributable to the exhaustion of the supply of porewater sulfate. In deep coastal environments the proportion of organic matter degraded via sulfate reduction can be highly variable both spatially and temporally. 相似文献
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85.
Einar Svendsen Morten Skogen Paul Budgell Geir Huse Jan Erik Stiansen Bjrn dlandsvik Frode Vikeb Lars Asplin Svein Sundby 《Deep Sea Research Part II: Topical Studies in Oceanography》2007,54(23-26):2810
The Norwegian Ecological Model (NORWECOM) biophysical model system implemented with the ROMS ocean circulation model has been run to simulate conditions over the last 25 years for the North Atlantic. Modeled time series of water volume fluxes, primary production, and drift of cod larvae through their modeled ambient temperature fields have been analyzed in conjunction with VPA estimated time series of 3-year-old cod recruits in the Barents Sea. Individual time series account for less than 50% of the recruitment variability; however, a combination of simulated flow of Atlantic water into the Barents Sea and local primary production accounts for 70% of the variability with a 3-year lead. The associated regression predicts increased recruitment between 2007 and 2008 from about 450–700 million individuals with a standard error of nearly 150 million. 相似文献
86.
BjØrn Sundby Philippe Martinez Charles Gobeil 《Geochimica et cosmochimica acta》2004,68(11):2485-2493
The concentrations of authigenic phases of Cd, Re, U, and Mo increase with depth in four 45-cm-long sediment box cores collected along the axis of the Laurentian Trough, Gulf of St. Lawrence. Average authigenic accumulation rates, estimated from element inventories, are similar to rates in other continental margin environments. Strong regional variations in sediment accumulation rate and sulfide concentration have little influence on the accumulation rates of Cd and Re. This suggests that slow precipitation kinetics controls the accumulation of Cd and Re in these sediments. The accumulation rate of authigenic U is more variable; it may be tied to the kinetics of microbially mediated U reduction and be controlled by the availability of reactive organic matter. Authigenic Mo is distinguished by a sharp subsurface concentration minimum, above which Mo cycles with manganese. Mo released to pore water upon reduction of Mn oxides diffuses downward and enriches the subsurface sediment. Mo accumulates most rapidly in the sediment with the highest sulfide content. Slow conversion of molybdate to thiomolybdate may explain the much slower Mo accumulation rate in the less sulfidic sediments. A component of authigenic Mo accumulates with pyrite in an approximately constant Mo:Fe ratio. The accumulation rate of pyrite and associated Mo is insensitive to AVS abundance. Pyrite formation may be limited by the reactivity of iron oxide minerals. 相似文献
87.
Structural interaction between dissolved fluorine and silicate glass (25°C) and melt (to 1400°C) has been examined with 19F and 29Si MAS NMR and with Raman spectroscopy in the system Na2O-Al2O3-SiO2 as a function of Al2O3 content. Approximately 3 mol.% F calculated as NaF dissolved in these glasses and melts. From 19F NMR spectroscopy, four different fluoride complexes were identified. These are (1) Na-F complexes (NF), (2) Na-Al-F complexes with Al in 4-fold coordination (NAF), (3) Na-Al-F complexes with Al in 6-fold coordination with F (CF), and (4) Al-F complexes with Al in 6-fold, and possibly also 4-fold coordination (TF). The latter three types of complexes may be linked to the aluminosilicate network via Al-O-Si bridges.The abundance of sodium fluoride complexes (NF) decreases with increasing Al/(Al + Si) of the glasses and melts. The NF complexes were not detected in meta-aluminosilicate glasses and melts. The NAF, CF, and TF complexes coexist in peralkaline and meta-aluminosilicate glasses and melts.From 29Si-NMR spectra of glasses and Raman spectra of glasses and melts, the silicate structure of Al-free and Al-poor compositions becomes polymerized by dissolution of F because NF complexes scavenge network-modifying Na from the silicate. Solution of F in Al-rich peralkaline and meta-aluminous glasses and melts results in Al-F bonding and aluminosilicate depolymerization.Temperature (above that of the glass transition) affects the Qn-speciation reaction in the melts, 2Q3 ⇔ Q4 + Q2, in a manner similar to other alkali silicate and alkali aluminosilicate melts. Dissolved F at the concentration level used in this study does not affect the temperature-dependence of this speciation reaction. 相似文献
88.
Solubility and solution mechanism of H2O in alkali silicate melts and glasses at high pressure and temperature 总被引:2,自引:0,他引:2
The solubility behavior of H2O in melts in the system Na2O-SiO2-H2O was determined by locating the univariant phase boundary, melt = melt + vapor in the 0.8-2 GPa and 1000°-1300°C pressure and temperature range, respectively. The NBO/Si-range of the melts (0.25-1) was chosen to cover that of most natural magmatic liquids. The H2O solubility in melts in the system Na2O-SiO2-H2O (XH2O) ranges between 18 and 45 mol% (O = 1) with (∂XH2O/∂P)T∼14-18 mol% H2O/GPa. The (∂XH2O/∂P)T is negatively correlated with NBO/Si (= Na/Si) of the melt. The (∂XH2O/∂T)P is in the −0.03 to +0.05 mol% H2O/°C range, and is negatively correlated with NBO/Si. The [∂XH2O/∂(NBO/Si)]P,T is in the −3 to −8 mol% H2O/(NBO/Si) range. Melts with NBO/Si similar to basaltic liquids (∼0.6-∼1.0) show (∂XH2O/∂T)P<0, whereas more polymerized melts exhibit (∂XH2O/∂T)P>0. Complete miscibility between hydrous melt and aqueous fluid occurs in the 0.8-2 GPa pressure range for melts with NBO/Si ≤0.5 at T >1100°C. Miscibility occurs at lower pressure the more polymerized the melt. 相似文献
89.
From experimental data in the systems Na2O-Al2O3-SiO2-H2O, K2O-Al2O3-SiO2-H2O at 1100°C, and CaO-Al2O3-SiO2-H2O at 1200°C in the 1-2 GPa pressure range, the solution behavior of the individual oxides in coexisting H2O-saturated silicate melts and silicate-saturated aqueous fluids appears to be incongruent. Recalculated on an anhydrous basis, in the CaO-Al2O3-SiO2-H2O system, CaOfluid/CaOmelt < 1, whereas in the Na2O-Al2O3-SiO2-H2O and K2O-Al2O3-SiO2-H2O systems, K2Ofluid/K2Omelt and Na2Ofluid/Na2Omelt both are greater than 1. The aqueous fluids are depleted in alumina relative to silicate melt.In the Na2O-Al2O3-SiO2-H2O, K2O-Al2O3-SiO2-H2O, and CaO-Al2O3-SiO2-H2O systems, fluid/melt partition coefficients for the individual oxides range between ∼0.005 and 0.35 depending on oxide, bulk composition and pressure. The alkali partition coefficients are about an order of magnitude higher than that of CaO. Alumina and silica partition coefficient values in the CaO-Al2O3-SiO2-H2O system are 10-20% of the values for the same oxides in the Na2O-Al2O3-SiO2-H2O and K2O-Al2O3-SiO2-H2O systems.Positive correlations among individual partition coefficients and oxide concentrations in the aqueous fluids are consistent with complexing in the fluid that involves silicate polymers associated with alkalis and alkaline earths and aluminosilicate complexes where alkalis and alkaline earths may serve to charge-balance Al3+, which is, perhaps, in tetrahedral coordination. Alkali aluminosilicate complexes in aqueous fluid appear more stable than Ca-aluminosilicate complexes. 相似文献
90.
Diopside (CaMgSi2O6) and pseudowollastonite (CaSiO3) have been studied by X-ray powder diffraction and Raman spectroscopy up to their respective melting points. In agreement
with previous unit-cell parameters determinations below 1100 K, thermal expansion of diopside along the a and c axis is much smaller than along the b axis. For pseudowollastonite, the axis expansivity increases slightly in the order b>a>c. For both minerals, the change in unit-cell angles is very small and there are no anomalous variations of the other unit-cell
parameters near the melting point. With increasing temperatures, the main changes observed in the Raman spectra are strong
increases of the linewidths for those bands which mainly represent Si−O−Si bending (near 600 cm−1) or involve Ca−O or Mg−O stretching, in the range 270–500 cm−1 for diopside, and 240–450 cm−1 for pseudowollastonite. At temperatures near the onset of calorimetric premelting effects, this extensive band widening results
in a broad Raman feature that can no longer be deconvoluted into its individual components. No significant changes affect
the Si−O streching modes. For both diopside and pseudowollastonite, premelting appears to be associated with enhanced dynamics
of the alkaline-earth elements. This conclusion contrasts markedly with that drawn for sodium metasilicate in which weaker
bonding of sodium allows the silicate framework to distort and deform in such a way as to prefigure the silicate entities
present in the melt.
Received 16 July 1997 / Revised, accepted: 6 March 1998 相似文献