Particulate Si Research Articles

The Role of Coastal Yedoma Deposits and Continental Shelf Sediments in the Arctic Ocean Silicon Cycle

AbstractThe availability of silicon (Si) in the ocean plays an important role in regulating biogeochemical and ecological processes. The Si budget of the Arctic Ocean appears balanced, with inputs equivalent to outputs, though it is unclear how a changing climate might aggravate this balance. In this study, we focus on Si cycling in Arctic coastal areas and continental shelf sediments to better constrain the Arctic Ocean Si budget. We provide the first estimate of amorphous Si (ASi) loading from erosion of coastal Yedoma deposits (30–90 Gmol yr−1), demonstrating comparable rates to particulate Si loading from rivers (10–90 Gmol yr−1). We found a positive relationship between surface sediment ASi and organic matter content on continental shelves. Combining these values with published Arctic shelf sediment properties and burial rates we estimate 70 Gmol Si yr−1 is buried on Arctic continental shelves, equivalent to 4.5% of all Si inputs to the Arctic Ocean. Sediment dissolved Si fluxes increased with distance from river mouths along cruise transects of shelf regions influenced by major rivers in the Laptev and East Siberian seas. On an annual basis, we estimate that Arctic shelf sediments recycle approximately up to twice as much DSi (680 Gmol Si) as is loaded from rivers (340–500 Gmol Si).

The importance of microbiota and terrestrial inflows in controlling seston C:N:P:O:Si:Ca:Mn:Mg:Fe:K:Na:Cl:S:Cu:Zn stoichiometry of a deep coastal fjord

Comprehensive fjord-systems represent major extensions of the coastline and are therefore important transfer zones of materials from land to ocean. Despite increased terrestrial inflows to fjords due to climate changes, we know little about the effects on the ecosystem, especially biogeochemical cycling. We present novel data on spatiotemporal variations of seston multielement stoichiometry in the Sognefjord, the second longest (204 km) and deepest (1308 m) fjord in the world, relative to environmental conditions and microbiota. Concentration of major elements was highest in the upper brackish layer whereas trace metals and minor elements were highest close to the bottom. Seasonally varying microbiota was an important part of the seston in surface waters. None of the seston C:N:P (molar) annual means at specific depths corresponded to the Redfield ratio (106:16:1). At 5 m, annual means of N/P and C/N were 8.4 and 6.5, respectively, while at depth (50–1220 m) N/P were on scale 3 times higher (21–31) and C/N 3 times lower (1.6–2.6), suggesting alternative N-sequestration mechanisms. Overall, correlations between C-Ca and C-S indicate a strong influence from calcite (CaCO3) and organosulfur producing microorganisms, while correlations between particulate Si and Mg–K–Ca–O at depth are consistent with clay and sinking diatom frustules. Mn concentrations increased strongly towards the bottom, likely from resuspension of MnO2 rich sediments and clay particles. Based on seston concentrations, we arrived at the following stoichiometric relationship: C55N16P1Si3.6Ca3.4O16Fe0.74Mn0.51Zn0.33S0.21Cu0.08Cl1.7Na0.68Mg0.71K0.37, although rarely measured, such information is a prerequisite for evaluating environmental impact on coastal ecosystems, biogeochemical cycling, pollution risk analysis and monitoring guidelines.

Diminished carbon and nitrate assimilation drive changes in diatom elemental stoichiometry independent of silicification in an iron-limited assemblage

In the California Current Ecosystem, upwelled water low in dissolved iron (Fe) can limit phytoplankton growth, altering the elemental stoichiometry of the particulate matter and dissolved macronutrients. Iron-limited diatoms can increase biogenic silica (bSi) content >2-fold relative to that of particulate organic carbon (C) and nitrogen (N), which has implications for carbon export efficiency given the ballasted nature of the silica-based diatom cell wall. Understanding the molecular and physiological drivers of this altered cellular stoichiometry would foster a predictive understanding of how low Fe affects diatom carbon export. In an artificial upwelling experiment, water from 96 m depth was incubated shipboard and left untreated or amended with dissolved Fe or the Fe-binding siderophore desferrioxamine-B (+DFB) to induce Fe-limitation. After 120 h, diatoms dominated the communities in all treatments and displayed hallmark signatures of Fe-limitation in the +DFB treatment, including elevated particulate Si:C and Si:N ratios. Single-cell, taxon-resolved measurements revealed no increase in bSi content during Fe-limitation despite higher transcript abundance of silicon transporters and silicanin-1. Based on these findings we posit that the observed increase in bSi relative to C and N was primarily due to reductions in C fixation and N assimilation, driven by lower transcript expression of key Fe-dependent genes.

The role of groundwater discharge fluxes on Si:P ratios in a major tributary to Lake Erie

Groundwater discharge can be a major source of nutrients to river systems. Although quantification of groundwater nitrate loading to streams is common, the dependence of surface water silicon (Si) and phosphorus (P) concentrations on groundwater sources has rarely been determined. Additionally, the ability of groundwater discharge to drive surface water Si:P ratios has not been contextualized relative to riverine inputs or in-stream transformations. In this study, we quantify the seasonal dynamics of Si and P cycles in the Grand River (GR) watershed, the largest Canadian watershed draining into Lake Erie, to test our hypothesis that regions of Si-rich groundwater discharge increase surface water Si:P ratios. Historically, both the GR and Lake Erie have been considered stoichiometrically P-limited, where the molar Si:P ratio is greater than the ~16:1 phytoplankton uptake ratio. However, recent trends suggest that eastern Lake Erie may be approaching Si-limitation. We sampled groundwater and surface water for dissolved and reactive particulate Si as well as total dissolved P for 12months within and downstream of a 50-km reach of high groundwater discharge. Our results indicate that groundwater Si:P ratios are lower than the corresponding surface water and that groundwater is a significant source of bioavailable P to surface water. Despite these observations, the watershed remains P-limited for the majority of the year, with localized periods of Si-limitation. We further find that groundwater Si:P ratios are a relatively minor driver of surface water Si:P, but that the magnitude of Si and P loads from groundwater represent a large proportion of the overall fluxes to Lake Erie.

Elemental Composition of Natural Nanoparticles and Fine Colloids in European Forest Stream Waters and Their Role as Phosphorus Carriers

AbstractBiogeochemical cycling of elements largely occurs in dissolved state, but many elements may also be bound to natural nanoparticles (NNP, 1–100 nm) and fine colloids (100–450 nm). We examined the hypothesis that the size and composition of stream water NNP and colloids vary systematically across Europe. To test this hypothesis, 96 stream water samples were simultaneously collected in 26 forested headwater catchments along two transects across Europe. Three size fractions (~1–20 nm, >20–60 nm, and >60 nm) of NNP and fine colloids were identified with Field Flow Fractionation coupled to inductively coupled plasma mass spectrometry and an organic carbon detector. The results showed that NNP and fine colloids constituted between 2 ± 5% (Si) and 53 ± 21% (Fe; mean ± SD) of total element concentrations, indicating a substantial contribution of particles to element transport in these European streams, especially for P and Fe. The particulate contents of Fe, Al, and organic C were correlated to their total element concentrations, but those of particulate Si, Mn, P, and Ca were not. The fine colloidal fractions >60 nm were dominated by clay minerals across all sites. The resulting element patterns of NNP <60 nm changed from North to South Europe from Fe‐ to Ca‐dominated particles, along with associated changes in acidity, forest type, and dominant lithology.

Silica cycling and isotopic composition in northern Marguerite Bay on the rapidly-warming western Antarctic Peninsula

The Southern Ocean is a key region for silica (Si) cycling, and the isotopic signatures established here influence the rest of the world's oceans. The climate and ecosystem of the Southern Ocean are changing rapidly, with the potential to impact Si cycling and isotope dynamics. This study examines high-resolution time-series dataset of dissolved Si concentrations and isotopic signatures, particulate Si concentrations and diatom speciation at a coastal site on the western Antarctic Peninsula (WAP), in order to characterise changes in Si cycling with respect to changes occurring in productivity and diatom assemblages. Dissolved and particulate Si phases reflect the dominant control of biological uptake, and combined with isotopic fractionation were consistent with a season of low/intermediate productivity. Biogenic Si is tightly coupled to both chlorophyll and particulate organic carbon at the sampling site, consistent with diatom-dominated phytoplankton assemblages along the WAP. Variability in diatom speciation has a negligible impact on the isotopic signature of dissolved Si in surface waters, although this is unlikely to hold for sediments due to differential dissolution of diatom species. A continued decline in diatom productivity along the WAP would likely result in an increasing unused Si inventory, which can potentially feed back into Si-limited areas, promoting diatom growth and carbon drawdown further afield.

Reactive silicon dynamics in a large prairie reservoir (Lake Diefenbaker, Saskatchewan)

Abstract There is an up-coming global surge in dam construction. River damming impacts nutrient cycling in watersheds through transformation and retention in the reservoirs. The bioavailability of silicon (Si) relative to nitrogen (N) and phosphorus (P) concentrations, in combination with light environment, controls diatom growth and therefore influences phytoplankton community compositions in most freshwater aquatic ecosystems. In this study, we quantified reactive Si cycling and annual Si retention in Lake Diefenbaker, a 394 km 2 prairie reservoir in Saskatchewan, Canada. Retention estimates were derived from 7 sediment cores combined with high-resolution spatiotemporal sampling of water column dissolved Si (DSi). Current annual DSi retention in the reservoir is approximately 28% of the influx, or 2.5 × 10 8 mol yr − 1 . The relative retention of DSi is higher than that of bioavailable N, but lower than that of bioavailable P, indicating a decoupling of the P, N, and Si cycles in the reservoir. The largest accumulation of reactive particulate Si (PRSi) is found in sediments deposited in the mid-reservoir region, which corresponds to the region of highest epilimnetic DSi decrease during the 2013 open water season. This region corresponds to the fluvial-to-lacustrine transition zone, and represents a hot spot for Si retention. Differences in retention efficiencies of macronutrients may not only affect the in-reservoir ecology and water quality, but also modifies the N:P:Si ratios exported downstream, which has the potential to alter ecosystem processes in receiving water bodies.

Electrochemical Performance of Si Modified TiO2 Nanotube Arrays Investigated in Li-Ion Containing Ionic Liquid Electrolyte

Widely researched in the recent years, Si offers one of the possibilities for significant increase of the capacity of Li-ion battery anode. During its lithiation Si undergoes series of phase transitions related to formation of a number of Li-Si alloy phases with a total capacity of about 4200 mAhg-1[1-3]. Consequently, the electrochemical lithiation of Si is associated with a drastic volume increase, usually resulting in rapid capacity fade and mechanical decomposition of the structure [2]. A number of different approaches, including nanostructuring [3], application of amorphous Si [1,2,4-8], limitation of state of charge of Si anodes by means of capacity or voltage control [8-10] composites with carbon [2] and more recently with TiO2[11-12], has been undertaken in order to improve the cycling stability of Si based electrodes.This paper will discuss recent results from the author's labs on the application of TiO2nanotube arrays as templates for magnetron sputtering of Si. Tuning of the deposition parameters (power and time) allowed obtaining a specific particulate Si morphology with sufficient free space, suitable for application as anode material in Li ion batteries (Fig.1).Fig1. Scanning electron microscopy (SEM) image of the Si-modified TiO2 nanotube layer.The electrochemical performance of the Si containing structures elucidates the importance of the Si surface morphology for the cycling stability of the anodes. The structures deposited at 50 W are favorable for Si stabilization during electrochemical cycling in Li ion electrolyte.The nanostructured Ti/TiO2layers were electrochemically tested for Li-ion exchange in 1-butyl-1-methylpyrrolidinium bis (trifluoromethyl) sulfonylimide ([BMP][TFSI]) containing 1M Li[TFSI]. The results show that the ionic liquid [BMP][TFSI] is a promising electrolyte for batteries with silicon anodes.The substrate type significantly influences the long term galvanostatic cycling of the samples. It was found that the structures deposited at 50 W on amorphous TiO2 exhibit a superior constant current cycling, finishing the 200th galvanostatic cycle with discharge capacity value of 1150 mAh g-1, with a tendency for further stabilization of the cycling. The good electrochemical performance of this sample type was attributed to the specific morphology of the Si deposit and structural stability of the amorphous TiO2 nanotubes.

Worldwide retention of nutrient silicon by river damming: From sparse data set to global estimate

AbstractDamming of rivers represents a major anthropogenic perturbation of the hydrological cycle, with the potential to profoundly modify the availability of nutrient silicon (Si) in streams, lakes, and coastal areas. A global assessment of the impact of dams on river Si fluxes, however, is limited by the sparse data set on Si budgets for reservoirs. To alleviate this limitation, we use existing data on dissolved Si (DSi) retention by dams to calibrate a mechanistic model for the biogeochemical cycling of DSi and reactive particulate Si (PSi) in reservoir systems. The model calibration yields a relationship between the annual in‐reservoir siliceous primary productivity and the external DSi supply. With this relationship and an estimate of catchment Si loading, the model calculates the total reactive Si (RSi = DSi + PSi) retention for any given reservoir. A Monte Carlo analysis accounts for the effects of variations in reservoir characteristics and generates a global relationship that predicts the average reactive Si retention in reservoirs as a function of the water residence time. This relationship is applied to the Global Reservoirs and Dams database to estimate Si retention by damming worldwide. According to the results, dams retain 163 Gmol yr−1 (9.8 Tg SiO2 yr−1) of DSi and 372 Gmol yr−1 (22.3 Tg SiO2 yr−1) of RSi, or 5.3% of the global RSi loading to rivers.

Characterization of materials released into water from paint containing nano-SiO2

In order to assess the possible risks of applications containing engineered nanomaterials, it is essential to generate more data about their release and exposure, so far largely overlooked areas of research. The aim of this work was to study the characterization of the materials released from paint containing nano-SiO2 during weathering and exposure to water. Panels coated with nano-SiO2 containing paint and a nano-free reference paint were exposed to accelerated weathering cycles in a climate chamber. The total release of 89 six-hour cycles of UV-illumination and precipitation was 2.3% of the total SiO2 contained in the paint. Additional tests with powdered and aged paint showed that the majority of the released Si was present in dissolved form and that only a small percentage was present in particulate and nano-particulate form. TEM imaging of the leachates indicated that the majority of the particulate Si was contained in composites together with Ca, representing the paint matrix, and only few single dispersed SiO2-NPs were detected. The results suggest that toxicological and ecotoxicological studies need to consider that the released particles may have been transformed or are embedded in a matrix.

Particulate silica and Si recycling in the surface waters of the Eastern Equatorial Pacific

The distributions of biogenic and lithogenic silica concentrations and net silica production rates in the upper 120 m of the Eastern Equatorial Pacific (EEP) were examined in December 2004, on two transects situated at 110°W (4°N to 3°S) and along the equator (110°W to 140°W). Lithogenic silica (lSiO 2) was generally <10 nmol Si l −1 with maximum concentrations reaching 25 nmol l −1 in surface waters. These low concentrations confirm low atmospheric inputs of particulate Si, consistent with reported low inputs of wind-borne material in the EEP. In spite of active upwelling of silicic acid-rich waters the biogenic silica (bSiO 2) concentrations were generally low, falling between 100 and 180 nmol Si l −1 in the upper 50 m and decreasing to less than 50 nmol Si l −1 below ∼90 m. Estimates of net bSiO 2 production rates revealed that the rate of production exceeded that of dissolution in the upper euphotic layer (0-40 m) along 110°W with net production extending somewhat deeper (60-100 m) to the west along the equator. Net production rates in the surface layer were low, ranging between 5 and 40 nmol Si l −1 d −1, consistent with previous observations that diatoms are small contributors to autotrophic biomass in the EEP. Net silica dissolution predominated in the lower euphotic layer (40-120 m), indicating active Si recycling which diminished the strength of the silica pump in this region.

Si/TiSi2 Heteronanostructures as High-Capacity Anode Material for Li Ion Batteries

We synthesized a unique heteronanostructure consisting of two-dimensional TiSi(2) nanonets and particulate Si coating. The high conductivity and the structural integrity of the TiSi(2) nanonet core were proven as great merits to permit reproducible Li(+) insertion and extraction into and from the Si coating. This heteronanostructure was tested as the anode material for Li(+) storage. At a charge/discharge rate of 8400 mA/g, we measured specific capacities >1000 mAh/g. Only an average of 0.1% capacity fade per cycle was observed between the 20th and the 100th cycles. The combined high capacity, long capacity life, and fast charge/discharge rate represent one of the best anode materials that have been reported. The remarkable performance was enabled by the capability to preserve the crystalline TiSi2 core during the charge/discharge process. This achievement demonstrates the potency of this novel heteronanostructure design as an electrode material for energy storage.

Si cycle in the Cariaco Basin, Venezuela: Seasonal variability in silicate availability and the Si:C:N composition of sinking particles

A 9‐year time series of water column and sediment trap measurements was used to examine silicon cycling within the anoxic Cariaco Basin. The dynamic hydrographic regime within Cariaco Basin results in strong seasonal changes in nutrient availability and the Si:C:N of sinking particles. Upwelling in early winter injects silicic acid (Si(OH)4= ) and nitrate (NO3−) rich waters into the photic zone which stimulates primary production and results in opal fluxes in excess of 4 mmol Si m−2 d−1. However, even during upwelling, surface waters tend to be depleted (&lt;1 μM) in both Si(OH)4= and NO3− as a result of rapid utilization. In most years, the upper water column during winter and spring is marked by Si(OH)4= :NO3− and Si* values of less than 1. This indicates that silicate limitation in Cariaco Basin is most severe during upwelling and may restrict diatom production. Conversely, during the summer and fall when upwelling is reduced, Si(OH)4= :NO3− ratios in the upper 50 m of the water column exceed 10, implying that nitrate rather than silicate is acting to limit production during this time of year. On average, sinking particles collected at 150‐m depth in the Cariaco Basin have Si:C and Si:N values of 0.17 ± 0.01 and 1.14 ± 0.10, respectively. These ratios increase with depth to 400 m and then remain relatively constant, suggesting minimal selective removal of elements with remineralization in the anoxic portion of the water column. Similar depth‐dependent changes in these ratios are seen in surface sediments from the basin. Seasonally, particulate Si:C and Si:N are highest during the early part of the year when upwelling is most intense, while both ratios decrease to their lowest values during summer and fall. The observed seasonal variability in these ratios is due to changes in both nutrient utilization by diatoms and the contribution of diatoms to the total phytoplankton. The high ratios during upwelling suggest enhanced export of Si relative to C and N during this time of year.

The uptake and export of silicon and nitrogen in HNLC waters of the NE Pacific Ocean

The high-nitrate, low-chlorophyll (HNLC) waters of the Gulf of Alaska tend towards silicate rather than nitrate depletion as phytoplankton utilize nutrients during summer. This tendency is enhanced when iron supply is elevated through natural inputs such as from coastally generated mesoscale eddies or through artificial enrichment as was carried out in an in situ experiment in July 2002. However, ship-board incubations with iron enrichment demonstrate nitrate rather than silicate depletion for these waters. The difference between in situ and in vitro experiments occurs at least in part because deck incubations do not allow export of particulate Si and N. Due to the more efficient recycling of nitrogen and carbon, export favours the removal of silicon from the upper ocean (the Si pump). Previous measurements at Ocean Station Papa (50°N, 145°W) show that ∼25% of the Si, but only ∼7% of the C and ∼4% of the N utilized during spring growth, is exported to a depth of 200 m. These results in the Gulf of Alaska agree with the present understanding of phytoplankton controls in other HNLC regions and show that any estimates of carbon export from iron enrichment should be based on Si- rather than N-limitation.

Spatial and temporal variability in phytoplankton iron limitation along the California coast and consequences for Si, N, and C biogeochemistry

Iron limitation was investigated along the Northern California coast in the summer of 1999. Small‐volume (1 l) shipboard iron addition bottle experiments were performed at 44 stations to gain the greatest possible temporal and spatial coverage of the area. Parameters measured in these 4‐day incubations included size‐fractionated chlorophyll a and particulate nutrients (C, N, and Si). Degrees of community iron limitation were quantified and compared using various iron limitation indexes, calculated as the ratio of chlorophyll a, particulate organic nitrogen, or particulate organic carbon produced in Fe‐amended bottles to the amounts produced in controls. Iron limitation occurred most frequently and was most severe on stations off the continental shelf, away from sedimentary sources of iron, as well as during relaxation events in aged upwelled water. Size‐fractionated chlorophyll a data did not suggest large Fe‐mediated changes in phytoplankton community composition. Fe limitation reduced phytoplankton production of particulate organic nitrogen and carbon, but had much less effect on biogenic silica production. The result is an increase in particulate Si:N and Si:C ratios of control samples, which were frequently double those of Fe‐amended samples. Particulate C:N ratios also decreased under Fe limitation, indicating that iron availability exerts a strong control on C, N, and Si elemental composition of phytoplankton, and thus on the biogeochemical cycling of these nutrients in the California upwelling region.

Characterization of suspended solids in Lake Biwa by measuring their elemental composition of Al, Si, P, S, K, Ca, Ti, Mn, and Fe

We investigated the concentrations of particulate Al, Si, P, S, K, Ca, Ti, Mn, and Fe at various areas of Lake Biwa, such as the limnetic zone in the northern basin, the offshore zone, the dredged area and Akanoi Bay in the southern basin, and reed community areas in the shore. In the wide and deep northern basin, the density of suspended solids was low, and substantial fractions of particulate Si, P, S, and Ca were biogenic matter produced by phytoplankton. In the offshore zone in the narrow and shallow southern basin, the concentration ratios of particulate Si, K, Ca, Ti, and Fe against particulate Al were almost constant throughout the year and were similar to those found in crusts. Therefore, these elements are most likely derived from terrigenous clay matter. In the dredged area where the hypolimnion became anoxic during the stratification period, the concentrations of particulate Fe, Mn, P, S, and Ca increased because of the formation of hydrous Fe and Mn oxides and Fe sulfide. In Akanoi Bay, the ratios of Mn/Al and Fe/Al were high because of the stirring of sediments enriched with Mn and Fe oxides. In the reed community areas, the concentration of suspended solids correlated well with particulate Fe and P, but not with Al or chlorophyll a; therefore these areas seem to be rich in humic organic matter enriched with P and Fe due to microbial activity. Thus, the elemental composition of suspended solids reflects the chemical, biological, and physical processes in the lake and gives us useful indices of the characteristics of the water.

EFFECT OF NUTRIENT AVAILABILITY ON THE BIOCHEMICAL AND ELEMENTAL STOICHIOMETRY IN THE FRESHWATER DIATOM STEPHANODISCUS MINUTULUS (BACILLARIOPHYCEAE).

The objective of this study was to examine the differences in the biochemical and elemental stoichiometry of a freshwater centric diatom, Stephanodiscus minutulus (Grun.), under various nutrient regimes. Stephanodiscus minutulus was grown at μmax or 22% of μmax under limitation by silicon, nitrogen, or phosphorus. Cell sizes for nutrient-limited cultures were significantly smaller than the non-limited cell sizes, with N-limited cells being significantly smaller than all other treatments. Compared with the nutrient-replete treatment, both carbohydrates and lipids increased in Si- and P-limited cells, whereas carbohydrates increased but proteins decreased in N-limited cells. All of the growth-limited cells showed an increase of carbohydrate and triglyceride, and a decrease of cell size and polar lipids as a percentage of total lipids. The non-limited cells also had a significantly higher chl a concentration and galactolipids as a percentage of total lipids than any of the limited treatments, and the low-Si and low-P cells had significantly higher values than the low-N cells. The particulate C concentrations showed significant differences between treatments, with the Si- and P-limited treatments being significantly higher than the N- and non-limited treatments. Particulate Si did not show a strong relationship with any of the parameters measured, and it was the only parameter with no differences between treatments. The low-Si cells had a significantly higher P content (about two times more) than any other treatment, presumably owing to the luxury consumption of P, and a correspondingly high phospholipid concentration. The elemental data showed that S. minutulus had a high P demand with low optimum N:P (4) and Si:P (10) ratios and a C:N:P ratio of 109:16:2.3. The particulate C showed a positive relationship with POM (r = 0.93), dry weight (r = 0.88), lipid (r = 0.87) and protein (r = 0.84, all P < 0.0001). Particulate N showed a positive relationship with galactolipids (r = 0.95), protein (r = 0.90), dry weight (r = 0.78), lipid (r = 0.75), and cell volume (r = 0.64, all P < 0.0001). It is evident that nutrient limitation in the freshwater diatom S. minutulus has pronounced effects on its biochemical and elemental stoichiometry.

The performance of aluminium-matrix composites with nanometric particulate Si–N–C reinforcement

Aluminium-matrix composites reinforced with nanomatric Si–N–C particles (Si–N–C/Al) were fabricated by a powder-metallurgy process. X-ray diffraction, metallography, tensile, dynamic compression and high-temperature creep tests were used to characterize the microstructure and mechanical properties of these composites. The results showed that the tensile strength of the 1 vol. % Si–N–C/Al composite is equivalent to that of the 15 vol. % SiC p(3.5 μm)/Al composite. However, the tensile properties of the Si–N–C/Al composite deteriorated considerably with increasing particle content owing to the clustering of particles. The creep resistance of the 1 vol. % Si–N–C/Al composite was at least two orders of magnitude higher than that of the 15 vol. % SiC p(3.5 μm)/Al composite. Furthermore, the 1 vol. % Si–N–C/Al composite exhibited an apparent stress exponent ranging from 15.7 to 23.0 and an apparent activation energy of 248 kJ/mol. The creep data of the composite was rationalized by using the substructure-invariant model with a stress exponent of 8 together with a threshold stress. The dynamic compression tests indicated that the strength and strain hardening rate of the 1 vol. % Si–N–C/Al composite increase significantly with increasing the strain rate from 10 −3 s −1 to 10 3 s −1.

An iron limitation mosaic in the California upwelling regime

Although Fe limitation is well documented in open ocean high ‐nutrient, low ‐chlorophyll (HNLC) areas, little is known about the potential for Fe limitation in coastal environments. We present a series of four Fe addition experiments that demonstrate varying degrees of Fe limitation in the central California coastal upwelling area. Fe concentrations vary widely here (&lt;0.1 to &gt;8.0 nM) because inputs from rivers and resuspended shelf sediments are unevenly distributed. The biological response to Fe availability is also extremely variable. Fe‐replete areas experience extensive blooms of large diatoms and almost complete depletion of nutrients. In slightly Fe‐stressed areas, Fe limits the growth of large diatoms but does not control nutrient biogeochemistry or growth of other planktonic organisms. In severely Fe‐limited waters, Fe exerts a fundamental control on nitrate and silicic acid drawdown, particulate organic carbon production, and the growth of phytoplankton, zooplankton, and bacteria. We propose a four ‐level classification scheme for Fe limitation in coastal California waters. Each level is characterized by a set of specific biological and biogeochemical responses to Fe. Parameters that show a characteristic response to Fe addition and thus define a region's Fe limitation status include particulate Si:N and Si:C production ratios, NO3− and H2SiO3 drawdown, C fixation, large diatom and picoplankton growth, bacterial production, and zooplankton grazing and biomass. Fe limitation of coastal upwelling regions needs to be recognized as an important biogeochemical process that could profoundly affect global new production and carbon cycling. The physical, chemical, and biological complexity of coastal upwelling regimes requires that Fe limitation effects be addressed with a more sophisticated approach than has generally been used to describe oceanic HNLC regimes.

Chemical Composition Determination of Suspended Mineral Particles and Colloidal Fractions of River Samples

Abstract In this paper the technique and procedure were developed to determine the chemical compositions of suspended mineral particles in the different fractions of river samples by gravimetry, aqua rega-HF-H3BO3 digestion and ICP-AES analysis. Particulate Si, Al, Ca, Fe, Mg and Mn in the different fractions can be determined simutaneously, precisely and accurately with a simple procedure at very low concentrations. The loss of Si in the digestion was not observed. The presence of boric acid did not cause any interference in the analysis. The comparisons between digested and non-digested particles in the fractions of diameters 0.8 μm were made. A negative error occurred in non digested particles.