Total Silica Research Articles

Environmental factors that control the abundance of Cyclostephanos duhius (Bacillariophyceae) in Danish lakes, from seasonal to century scale

Studies of Cyclostephanos dubius were made using contemporary field data, lake surface-sediment samples and palaeolimnological records. Variations in the species' abundance (in time and across several lakes) in relation to environmental variables were explored using statistical methods. In five shallow, nutrient-rich, Danish lakes, the seasonal abundance of C. dubius was modelled using multiple linear regression techniques. Four significant variables (NO2 + NO and silica concentrations, Secchi depth and zooplankton abundance) explained 24% of the variation in the seasonal diatom data. The abundance of C. dubius was positively correlated with [NO2 + NO] and negatively correlated with [SiO2] , Secchi depth and zooplankton abundance. In a dataset of 29 surface-sediment samples from Danish lakes, significant species response curves could be modelled for C. dubius with lake depth, and concentrations of total phosphorus (TP), silica and chlorophyll a. Analyses of species data over different timescales demonstrate that multiple, interactive factors and dynamic processes influence the abundance of the diatom in the phytoplankton and in sediment assemblages.

Complex effects of winter warming on the physicochemical characteristics of a deep lake

Recent winter warming over Central Europe associated with a positive phase of the North Atlantic Oscillation (NAO) strongly influenced the thermal and water column stability properties of deep Lake Constance (zmean = 101 m). Volumetrically weighted average water temperatures have increased since the 1960s by an average of 0.017°C yr−1, and its interannual variability was strongly related to the variability in winter air temperature and the NAO winter index (NAOW). The influence of NAOW on water temperature was more persistent than its influence on air temperature. The seasonal persistence of the NAOW signal increased with water depth. Deep‐water temperatures were related to the NAOW from one spring mixing period to the next. This caused a time lag of 1 yr in the response of deep‐water winter temperatures to the NAOW. Reduced winter cooling during high‐NAOW years resulted in the persistence of small temperature gradients that possibly resisted complete mixing. This, in turn, resulted in less upward mixing of nutrients (total phosphorus and total silica), which accumulated in the hypolimnion during the previous stratification period. A second effect of incomplete mixing was the lack of the replenishment of deepwater oxygen during high‐NAOW years. Hence, besides its strong impact on the thermal regime, climate variability influenced both the causes (nutrient supply for phytoplankton growth) and symptoms (the degree of hypolimnetic oxygen deficiency) of trophic changes in Lake Constance.

Silica recovery from Sumikawa and Ohnuma geothermal brines (Japan) by addition of CaO and cationic precipitants in a newly developed seed circulation device

With the objective of preventing silica scaling, several types of chemical compounds such as silica gels, clay minerals, chlorides of Ca, Mg, Al and Fe, and calcium compounds were first examined to remove the excess silica in the Ohnuma geothermal brines. The total silica concentrations of silica in the brines after reaction by addition of these chemicals were observed to decrease linearly with an increase in the reagent concentration. Quicklime (CaO), added at dosages of 0.5 and 1.0 g/l, can remove excess and most silica at 90 °C, respectively. These results indicate that the silica concentration of Ohnuma brines (520 mg/l initial concentration) can be controlled by the amount of quicklime added, and that low-temperature resources can be used without scaling. Continuous flow tests were then performed at Ohnuma and Sumikawa fields by addition of CaO or cationic precipitants in a newly developed seed circulation device (flow rate of 10 l brine/min). At Ohnuma, the total silica concentrations of brines after reaction with CaO showed almost constant values: 10, 200 and 300 mg/l for additions of 1.0, 0.65 and 0.50 g CaO/l, respectively. At Sumikawa, 11 experiments were run, adding cationic precipitants. The total silica concentrations decrease with the increase in reagent concentrations (25–100 mg/l) and with an increase in the retaining time (15–120 min). With the addition of CaO (2 g/l) to Sumikawa geothermal brines with a retaining time of 120 min at 90 °C, the total silica concentration was almost non-detectable after 10 h from the start of the reaction. After reaction for 118 h, addition of CaO was suspended and only silica deposits were reused through the circulation pipe to desilicate brine. After this treatment, the total silica concentration quickly increased to 400 mg/l and remained constant for 210 h before increasing to 650 mg/l after 265 h. There are good correlations between the zeta potentials, a measure of colloidal stability, and the Si/Ca atomic ratios of the silica deposits and amounts of silica removed from brines at Ohnuma and Sumikawa. The CaO addition and circulation method (SACS method) is a very useful way of removing excess silica in brines for silica scale prevention.

Synthesis of 2-cyanoethyl-modified MCM-48 stable to surfactant removal by solvent extraction: Influence of organic modifier, base and surfactant

A 2-cyanoethyl grafted MCM-48 mesoporous silicate, which was stable to surfactant removal by solvent extraction, was synthesised by co-condensation of 2-cyanoethyltriethoxysilane and tetraethyl orthosilicate in basic conditions using cetyltrimethylammonium bromide as surfactant. The presence of the organic group was verified by infrared analysis. Additional studies confirmed that the concentration of sodium hydroxide used for this synthesis influenced the mesopore properties. MCM-41 and MCM-48 were formed using a narrow range of base concentrations while preparations outside this range gave disordered materials with reduced porosity. Materials synthesised using various quantities of cetyltrimethylammonium bromide showed that MCM-48 was formed over a range of surfactant concentrations although the order varied considerably. The mesopore phase was further adjusted by altering the concentration of 2-cyanoethyltriethoxysilane modifier. Preparations using 0–10 and 20 mol% organic modifier (of total silica) produced MCM-48 and MCM-41, respectively, while greater modifier concentrations gave disordered materials with significantly diminished porosity characteristics.

Phototrophic activity and redox condition in Lake Hamana, Japan indicated by sedimentary photosynthetic pigments and molybdenum over the last 250 years

We analyzed photosynthetic pigments, total organic carbon (TOC), biogenic silica, and Mo, a redox-sensitive element, in 210Pb-dated sediment cores to reconstruct the historical changes in primary productivity and anoxia in the central basin of Lake Hamana, a brackish lake in Shizuoka Prefecture, Japan, over the last ∼250 years. The algal photosynthetic pigments we analyzed included chlorophyll a (and its derivatives), chlorophyll b (and its derivatives), and carotenoids such as β-carotene, lutein, zeaxanthin, diatoxanthin, fucoxanthin, alloxanthin, and α-carotene. Marker pigments for phototrophic sulfur bacteria were also recorded, including okenone and bacteriopheophytin a, originating from Chromatium (a genus of purple sulfur bacteria), and isorenieratene and bacteriochlorophylls e1, e2 and e3 (and corresponding bacteriopheophytins) from brown Chlorobium (a brown-colored group of green sulfur bacteria). The occurrence of these pigments throughout the length of all cores indicates that the anoxia in Lake Hamana has existed over at least the last ∼250 years. The indicators related to primary productivity – TOC and pigments of aerobic and anaerobic phototrophs – and an indicator of anoxia, Mo, increased after ∼1860, indicating that productivity had increased in both the oxic and anoxic (sulfidic) zones. The depth profiles of the indicators in the sediment cores showed that among phototrophic sulfur bacteria, Chromatium preferentially increased relative to brown Chlorobium when the lake productivity was high, and hence high anoxia existed in the lake. This can be explained by a shallowing of the oxic/anoxic boundary zone due to changes in temporal and/or spatial extents of seasonal anoxia, which made the light intensity in the upper anoxic zone high enough for Chromatium to grow. The upper Chromatium layer may absorb the wavelengths of light that favor the growth of brown Chlorobium in the water column, resulting in a relative decrease in brown Chlorobium. During the 1950s, the trends among the indicators changed significantly. This change is attributed to the construction of training walls, built to direct tidal currents into the lake, on the Imagire-guchi Channel, the sole inlet of seawater to the lake, during 1954–1956, and the resultant increases of seawater intrusion and lake salinity. A decrease in okenone and bacteriopheophytin a, or in okenone/isorenieratene ratio, after ∼1960 accompanying a decrease in Mo, is attributed to a deepening of the anoxic zone, where the light intensity became too low for growth of Chromatium, more light-demanding than the brown Chlorobium. A decrease of zeaxanthin (cyanobacteria) after ∼1960 relative to lutein (green algae) and diatoxanthin (diatoms and dinoflagellates) indicated a change in algal assemblage, presumably due to the increased salinity. Principal component analysis with a data set of total algal carotenoids, okenone, isorenieratene, and Mo also suggested that a major change occurred around the 1950s.

Silica removal from Mokai, New Zealand, geothermal brine by treatment with lime and a cationic precipitant

Preliminary experiments using two chemicals (CaO, a quicklime, and a cationic nitrogen-bearing precipitant, EC-004) to remove silica from geothermal brine were undertaken at the Mokai geothermal plant, New Zealand. The brine was mixed with the reagent (CaO or EC-004). The reaction was studied from the start of the experiment (NRT, 0 min, no retaining time) and after 15 min (15RT) at 90 °C. The concentration of silica in the brine was initially 954 mg/l, and decreased linearly with increasing reagent concentration. When CaO is added, the silica concentration at 15RT was 200 mg/l lower than at NRT and became almost zero on addition of 1.5 g/l. In contrast, when EC-004 is added, the total silica concentration nearly reaches the solubility of amorphous silica at 90 °C. In order to prevent silica scaling in Mokai brines cooled to 90 °C, the CaO and EC-004 added should be individually adjusted to 0.5 g/l and 80 mg/l, respectively.

The geometry and anatomy of the Los Pijiguaos bauxite deposit, Venezuela

The Los Pijiguaos bauxite mine in the western part of the State of Bolivar, Venezuela, represents the only currently exploited bauxite deposit in the country. Since mining commenced in 1987, total crude bauxite production has amounted to more than 43 million tonnes (Mt), with a present annual production close to 5 Mt. Proven and probable reserves are around 570 Mt, at an economic cut-off grade of about 44 wt.% alumina (Al 2O 3) and a total silica (SiO 2) content of less than 20 wt.%. The orebody has an irregular, dismembered geometry as a result of strong and prolonged fluvial erosion during and after uplift of the region. The isolated minable blocks are located on plateau remnants separated by deep valleys. As a result, only half of the entire lease area is accessible for mining. Los Pijiguaos represents a typical laterite bauxite deposit that formed on a flat topped plateau from weathering of the underlying mid-Proterozoic Rapakivi-type Parguaza granite. The complete laterite profile is characterized by a vertical zonation comprising a concretionary zone (the bauxite horizon), and a mottled zone (the saprolite horizon) on granitic bed rock. The economic bauxite zone attains an average thickness of 7.6 m. On a mine scale, best ore grades and greatest thickness of the ore horizon are at topographic highs. Laterally, grade distribution is also not isotropic but follows distinct WNW–SSE trends. The spatial control on the mineralization is a result of the westerly inclination of the planation surface and the formation of major NW-striking fault systems in the region. Gibbsite is the most abundant mineral in the economic-grade bauxite horizon. The occurrence of minor nordstrandite is also restricted to that zone. Kaolinite is not present in the bauxite zone, but in the lower saprolite zone kaolinite can be more than 5 wt.%. An inverse correlation exists between gibbsite and quartz. Abundance ratios for the two minerals decrease from around 45 in the bauxite horizon to around 2 in the lower saprolite zone. Hematite is commonly more abundant than goethite. In the bauxite horizon, however, the goethite/hematite ratio is greater than 1. Anatase occurs as an accessory phase mainly in the bauxite horizon. Mass balance considerations demonstrate that bauxitization resulted in progressive desilication, hydration, and Al 2O 3 and Fe 2O 3 concentration in the residual weathering blanket. Silica as well as alkali and alkaline earth metals were almost completely leached. Mass and volume losses in the order of 61% and 77%, respectively, attest to severe mineralogical, textural, and chemical changes. From an economic point of view, the Los Pijiguaos bauxite represents a high-quality almost pure gibbsitic ore that allows processing to alumina by the low-temperature Bayer process around 140 °C. In terms of material consumption, 2.3 tonnes of dry bauxite are used for every tonne of alumina produced. The accompanying mass of red mud amounts to 550 kg per tonne alumina. The relatively high abundance of hematite in the bauxite has a positive effect on the red mud settling rate and, thus, favors a high extraction yield of the alumina plant. Los Pijiguaos is based on a huge reserve that will provide bauxite for more than 100 years at the current rate of mining. The entire region may contain as much as 6 billion tonnes of probable bauxite resources.

19th Century Eutrophication of a Remote Boreal Lake: A Consequence of Climate Warming?

To investigate the response of a remote boreal lake to recent climate warming, a ∼200-year varved sediment record from Rainbow Lake A (RLA), located in the northern boreal forest of Wood Buffalo National Park, straddling northern Alberta and the Northwest Territories (Canada), was investigated using diatom assemblages and biogenic silica concentrations. Diatom community composition, trends in diatom-inferred total phosphorus (TP) and biogenic silica levels all showed significant changes beginning between circa 1830 and 1840, coincident with the onset of increasingly warm June/July temperatures in northern Canada. We evaluated several hypotheses which may have caused these nutrient changes, including local anthropogenic disturbances, forest fires, increased atmospheric deposition of nutrients or pollen, and internal sources of nutrient regeneration. We concluded that TP is likely increasing as a result of enhanced internal cycling of phosphorus due to either increased thermal stratification in response to warmer summer temperatures and/or decreased meromictic stability. The results presented here, in combination with other recent paleolimnological research in northern latitude regions, suggest widespread aquatic response to increasing temperatures beginning in the 19th century.

Silica in Several Rattan Species

Summary The overall silica content in 10 rattan species was analyzed. Its distribution within the stem of Calamus merrillii Becc. was also tested. Anatomical observations were performed to explain the silica content variation. Ash and silica content varied among species. Regardless of diameter class, silica accounts for 44% to 48% of the overall ash content of the stem. Silica in the medium and small diameter stem was twice as much as in the large diameter stem. Within Calamus merrillii Becc., silica was abundant in the epidermal layer. It was responsible for 74% to 85% of the total silica content of the stem, which explains the variation in silica content between diameter classes. The proportion of epidermal layer contained in the sample deviated among the species causing a variation in silica content. Epidermal silica did not vary along the length of the stem while silica in the stem itself varied significantly from base to top. Differences in silica body frequency per vascular bundle caused such deviation.

A new structure-directing agent for the synthesis of pure-phase ZSM-11

A new synthetic procedure for the preparation of pure-phase ZSM-11 (MEL) is reported. Pure-phase ZSM-11 is synthesized at 135 °C using 2,2-diethoxyethyltrimethylammonium as a structure-directing agent (SDA). The effects of the incorporation of boron in the reaction chemistry are also reported. A very high SDA–ZSM-11 specificity is observed since the inclusion of ZSM-5 (MFI) into the synthesis mixture at levels as high as 24 wt.% of the total silica content still leads to the crystallization of ZSM-11.

Surface-imprinted silica particles: the effects of added organosilanes on catalytic activity

Silicate surfaces were molecularly imprinted with a surfactant derivative of an α-chymotrypsin transition state analog (TSA) during particle formation in a water-in-oil microemulsion. The organosilanes N-(3-aminoethylaminomethyl)-phenyltrimethoxysilane (PEDA), carboxyethylsilanetriol (CTES), and N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole (IPTES) were added along with tetraethoxysilane (TEOS) to imprint the chemical functionality of the imprint molecule. Surfaces imprinted with N-α-decyl- d-phenylalanine-2-aminopyridine enantioselectively catalyzed the hydrolysis of benzoyl- l-arginine- p-nitroanilide ( l-BAPNA). No hydrolysis of d-BAPNA was observed. The catalytic activity of surface-imprinted particles formed with TEOS and PEDA was significantly greater than that of surface-imprinted particles formed with TEOS and IPTES. Particles formed with added 10 mol% (total silica) of PEDA hydrolyzed dl-BAPNA at a rate equivalent to that of surface-imprinted particles formed with the mixture of 5 wt% (total silica) of PEDA, CTES, and IPTES suggesting a cooperative effect with the organosilane mixture that promoted catalytic activity.

Major, Trace, and Rare Earth Elements in the Sediments of the Central Indian Ocean Basin: Their Source and Distribution

A large number of surface sediments as well as short sediment cores collected in the Central Indian Ocean Basin have been subjected to various geochemical investigations during the last one and half decade. The studies varied, covering different aspects of sediments and resulting in a number of publications. In the present article, we have put together the data from 82 surface sediments and 14 short sediment cores, including 25 new analyses, to study the trend of their distribution and source at large. The distribution maps of elements show that highest concentrations of Mn, Cu, Ni, Zn, Co, and biogenic opal in the surface sediment occurs between 10°S and 16°S latitude, where diagenetic ferromanganese nodules rich in Mn, Cu, Ni, and Zn are present. The studies highlight that the excess element concentration (detrital unsupported) such as Mn, Cu, Ba, Ni, Co, Pb, and Zn have contributed >80% of their respective bulk composition. These excess elements exhibit strong positive correlation with each other suggesting their association with a single authigenic phase such as Mn oxide. Biogenic opal contributes 30–50% of the total silica in the siliceous sediment. Aluminum, Fe, and K have contributed >60% from terrigenous detrital source compared to their bulk composition. In calcareous ooze, Ca, and Sr excess contribute >95% while, in siliceous ooze it is only 50% of their bulk composition. Nearly 35% of structurally unsupported Al in the sediment raises doubt of using Al as a terrigenous index element to normalize the trace and minor elements. Biogenic apatite is evident by the positive correlation between Ca (<1%) and P. Calcium, Sr, and P depict a common source such as biogenic. Bulk element concentration such as Li, V, Cr, Sc, and Zr are positively correlated with Ti indicating their terrigenous detrital source. Rare earth element (REE) concentration increases from calcareous ooze to siliceous ooze and reaches a maximum in the red clay. Presence of positive Eu-anomaly in these sediments has been attributed to aeolian input. REE in these sediments are mostly carried by authigenic phases such as manganese oxide and biogenic apatite. Based on the distribution of transition elements in the sediment cores, three distinct zones—oxic at top, suboxic at intermediate depth, and a subsurface maxima—have been identified. Oxic and suboxic zones are incidentally associated with high and low micronodule abundance in the coarse fraction (>63 μm) respectively. Ash layers encountered at intermediate depth between 10 to 35 cm are correlative with the Youngest Toba eruption of ∼74ka from Northern Sumatra. This ash is mainly responsible for the high bulk Al/Ti ratio up to 48.5 (three times higher than Post Archean Australian Shale), other than scavenging of dissolved Al by biogenic components.

Deposition of amorphous silica in porous packed beds — predicting the lifetime of reinjection aquifers

Predicting deposition rates of dissolved silica in geothermal reinjection aquifers is difficult due to a lack of reliable scaling rates and the complexity of modelling fluid transport simultaneously with deposition. In order to develop techniques, understand the problems and improve our predictive capabilities, we have undertaken field experiments at Wairakei geothermal field, New Zealand, to determine amorphous silica deposition rates in 25 mm diameter pipes packed with 2 mm diameter zirconia beads. These pipes served as model aquifers. Five experiments using flashed fluid containing 530 ppm total silica were completed at temperatures between 71 and 129°C and at flowrates between 0.002 and 0.02 kg s −1. The residence times in the pipes were shorter than the induction period required for silica polymerisation from solution. The scaling rates in the beds, measured over a month, were about 12 mg cm −2 year −1 and independent of flowrate between 80 and 129°C. Scaling at 129°C was unexpected, because the dissolved silica was expected to be undersaturated with respect to amorphous silica. At 71°C the rates were higher (up to 23 mg cm −2 year −1) and were proportional to flowrate. At Wairakei the 130°C fluid used in these experiments is disposed of by injection into a reservoir at 80°C. Using our field deposition rates, we estimate that 2.6×10 5 kg of amorphous silica would precipitate in 10 years around the injection well, assuming an injection rate of 50 kg s −1 into a 100 m thick reservoir of radius 500 m with permeability 100 mdarcy and a porosity of 0.2.

Near IR spectroscopy to quantify the silica content and difference between silicified microcrystalline cellulose and physical mixtures of microcrystalline cellulose and silica

Silicified microcrystalline cellulose (SMCC) has been shown to have advantages over conventional microcrystalline cellulose (MCC). These advantages are (i) improved tablet strength compared to that achieved with MCC, (ii) the retention of compressibility after wet granulation, whereas MCC produces weaker tablets after wet granulation, and (iii) superior flow properties than MCC. In this study near IR spectroscopy has been used to study MCC, SMCC (with different loadings of colloidal silicon dioxide, CSD) and physical mixtures of MCC and CSD. It was found that even though SMCC and MCC were very similar, there was a region of the near IR spectra (second derivative peak at 2194 nm) where a distinctive response was seen for SMCC. The size of the peak was proportional to the CSD content for the co-processed SMCC samples. The peak was not present to the same extent for physical mixtures. A combination of near IR and a test for total silica content would make it possible to discern whether microcrystalline cellulose samples were SMCC material or simple physical mixtures.

Zeolite Growth by Addition of Subcolloidal Particles: Modeling and Experimental Validation

Seeded growth of tetrapropylammonium (TPA)-silicalite-1 is studied using simulations and dynamic light scattering, atomic force microscopy, and transmission electron microscopy experiments. The effects of the total silica concentration, temperature, and total seed concentration are examined. When the composition of silica in solution is above a critical value, growth is observed. In such a case, the size of the seeds increases linearly with time, with a growth rate that is not significantly affected by the total silica concentration. Growth appears to be activated with an activation energy of 90 kJ/mol for a range of seed concentrations. Transmission electron microscopy and dynamic light scattering indicate the presence of subcolloidal particles. Simulations of the growth of a static particle in a suspension of subcolloidal particles are carried out. Good agreement with experimental results regarding the growth rate and the apparent activation energy is possible by considering DerjaguinLandau-Verwey-Overbeek (DLVO) interactions with electrostatic repulsion described with a constant surface charge model. The type of interaction was also verified with atomic force microscopy force measurements between a silicalite surface and a glass sphere. By use of these types of interactions, it is also possible to explain the stability of the seeded suspension and show that only a relatively narrow size distribution of growth precursors participate in the growth of the seeds. Our results support the possibility that under the conditions studied, growth of silicalite seeds proceeds by a mechanism with the rate-limiting step being the addition of subcolloidal particles.

The concentration, apparent molecular weight and chemical reactivity of silica from groundwater in Southern Nevada

Sorption of radionuclides, metals and organic compounds to colloidal particles has been suggested to increase the mobility of these pollutants in groundwater. Because silicates and alumino–silicates can be important components of groundwater colloids, we have conducted a study to characterize the nature of silica in various springs and wells in Southern Nevada and to determine the extent that silica may be associated with colloidal particles that can participate in pollutant transport. The total silica content was measured using inductively coupled plasma emission spectroscopy (ICP). In addition, reactive silica was measured using the silica molybdate colorimetric technique. The apparent molecular weight of the silica was investigated using split-flow-lateral-transport-thin-cell-fractionation (SPLITT) which can readily distinguish between colloidal and low molecular weight associations. This study indicates that silica does not tend to form stable inorganic colloids in Southern Nevada groundwaters but exists as low molecular weight species. However, water from one of the test facilities on the Nevada Test Site (NTS) did contain stable siliceous colloids that could have important implications for the modeling the transport of radionuclides at this site.

Evaluation of lignite combustion residues as cement additives

In this study the physical and chemical properties of lignite fly ashes obtained from electrostatic precipitator and cyclone, lignite bottom ash, cement + lignite ash mixtures, and their effects on mechanical properties of concrete were investigated. The ashes were classified into two general types based on total silica, alumina, and iron-III oxide: class A and class B. When 25% of the cement was replaced by class A lignite bottom ash (the combined three oxide contents were 30.2%), the 28-day compressive strength increased by 18.9% compared to the control mix, and when 25% of the cement was replaced by class B lignite ash (the combined three oxide contents were 78.1%), the compressive strength decreased by 3.5% compared to the control mix. The results obtained were compared with the Turkish Standards and, in general, were found to be within the limits. As a result, the lignite fly ash and bottom ash samples may be used as cementitious materials.

Rhizosolenia mats: An overlooked source of silica production in the open sea

The contribution of Rhizosolenia mats to silica cycling in the central North Pacific and the coupling of mat silicon metabolism and their vertical migration was examined in areas to the west of the Hawaiian Islands (23–28°N and 159–175°W) in 1995 and to the east of Hawaii along 31°N (160–127°W) in 1996. The biogenic silica content of Rhizosolenia mats sampled in 1995 averaged 1.82 ± 1.87 (SD) µmol Si mat−1. Larger mats that averaged 4.56 ± 3.54 (SD) µmol Si mat−1 were observed in 1996. Kinetic experiments indicated that substrate limitation of mat silica production was widespread across the study region, with ambient [Si(OH)4] restricting silica production to 33% of maximum potential rates. Three lines of evidence indicate that silicon metabolism is not tightly coupled to the migration of mats to and from the nutricline. In 1996, mats in surface waters could double their Si content in 0.55 d on average without migrating to the nutricline to obtain Si. However, average doubling times (9.8 d) in 1996 were of the same order as a migration cycle, necessitating significant Si uptake at depth. Si uptake rates did not differ significantly between ascending and descending mats, suggesting that mats ascending from the nutricline had not fulfilled their Si uptake requirements. Finally, small internal pools of Si in ascending mats indicated that if significant amounts of Si were taken up at depth, they were not stored for use in the surface waters. The biomass and silica production rates of mats collected using SCUBA in the upper 20 m were extrapolated to 150 m by using abundances determined using a video plankton recorder (VPR). The results suggest that mats account for about 3% of the standing stock of biogenic silica and 26% of silica production in the upper 150 m. The daily silica production by Rhizosolenia mats (317 µmol Si m−2 d−1) is 50–76% of the total silica production in the Sargasso Sea. This high rate of silica production combined with the wide geographic distribution of mats throughout several mid‐ocean gyres suggests that mats may contribute significantly to global silica production.

Airborne quartz concentration in an urban site

The International Agency for Research on Cancer, IARC, has recently classified crystalline silica as carcinogenic to man. This new assessment calls for the determination of the concentration levels of crystalline silica to which the general population is exposed to. This paper reports the results of the determinations of airborne quartz – the most common form of crystalline silica – in a site of the metropolitan area of Rome, Italy. We studied the respirable fraction of the airborne particulate (PM10) sampled in spring 1996 with a cascade impactor, by means of a scanning electron microscope equipped with an X-ray microprobe. Ten components were identified of which 3.7% on the average was total silica (crystalline and amorphous). On the basis of these data we started a methodical study of quartz concentrations in the historical series of the PM10 daily sampled with an Airborne Particles Monitor (APM) apparatus at the above-mentioned site. PM10 was collected on cellulose filters which were then incinerated in oxygen plasma at low temperature to eliminate the organic component. The residue was deposited on silver membrane filters that were then read with a conventional X-ray diffractometer to determine quartz amounts. Twenty eight filters were analysed which corresponded to four weeks representative of the four seasons of 1994, the year when samples started to be systematically collected. Results show that the mean weekly concentration of airborne quartz ranged between 0.6 and 1.5 μg/Nm 3, which corresponded to 1.7% and 3.4% of total PM10, respectively. These data are substantially in agreement with our previous data obtained with electron microscopy, and show that the quartz levels present in the urban atmosphere under study were approximately 100-fold lower than the threshold limit value (TLV), currently suggested by the American Conference of Governmental Industrial Hygienists (ACGIH) for the workplaces, namely 100 μg/Nm 3 – threshold that is currently applied in Italy. ©

Geochemical evidence for Holocene paleoclimatic changes in Maxwell Bay of South Shetland Islands, West Antarctica

Total organic carbon (TOC), biogenic silica (Bsi), and total sulfur (TS) contents in core sediments of Maxwell Bay are examined in order to investigate paleoclimatic changes during the Holocene. Both TOC and Bsi contents generally show a downcore decrease, which reflects that primary productivity in surface waters has increased during the Holocene. In contrast, TS contents show a downcore increase, which is probably due to a decrease in terrigenous input of hydrothermal-origin pyrite. In Maxwell Bay, primary productivity was low prior to 4700 yr BP because sea ice permanently covered the surface. As the climate became relatively warm after 4700 yr BP, sea ice in surface waters sporadically melted, causing an increase in productivity. The climate probably became warm between about 4700 and 3000 yr BP, evidenced by the rapid increase in TOC and Bsi. Grounding glaciers were located near the core sites of Maxwell Bay prior to about 4700 yr BP. As the climate became warm, grounding glaciers have retreated and been located at the present site since about 3000 yr BP.