Silica Accumulation Research Articles

Estimating the contribution of the authigenic mineral component to the long-term reactive silica accumulation on the western shelf of the Mississippi River Delta

Previous studies have shown how biogenic silica particles undergo conversion to aluminosilicate phases in large tropical deltaic systems, thus affecting the world ocean budget of major seawater cations. This study tackles the important question of the silica budget in the coastal zone of the Mississippi River Delta, providing evidence for the role of biogenic silica diagenesis in this subtropical system from direct examination of individual diatom particles, sediment leachates and pore-water composition. The estimated reactive silica stored in the study area (5990 km 2) is based on operational leaches that account for altered biogenic silica particles and other authigenic aluminosilicate phases in addition to fresh biogenic silica. Early diagenesis of silica in the delta front occurs mainly where more siliceous material is deposited. An inner-shelf area, where hypoxic conditions are found, significantly contributes to the formation of authigenic products of Si alteration. Data suggest that the limiting factor of silica alteration processes is the availability of detrital phases such as Al and Fe. The estimated total reactive silica accumulation in the study area is 1.45×10 10 mol Si year −1, representing ∼2.2% of the long-term bulk sediment accumulation. On the basis of a conservative appraisal, the authigenic mineral components account for ∼40% of the long-term reactive silica storage. This study shows that non-tropical deltaic systems are significantly more important sinks of silica than previously thought and that, where conditions are favourable, a consistent portion of reactive silica not leaving the shelf is stored within the delta in the form of authigenic components.

Distribution and accumulation of biogenic silica in the intertidal sediments of the Yangtze Estuary

Abstract Sedimentary biogenic silica is known to be an important parameter to understand biogeochemical processes and paleoenviromental records in estuarine and coastal ecosystems. Consequently, it is of great significance to investigate accumulation and distribution of biogenic silica in sediments. The two-step mild acid-mild alkaline extraction procedure was used to leach biogenic silica and its early diagenetic products in intertidal sediments of the Yangtze Estuary. The results showed that total biogenic silica (t-BSi) in the intertidal sediments varied from 237.7-419.4 micromol Si/g, while the mild acid leachable silica (Si-HCl) and the mild alkaline leachable silica (Si-Alk) were in the range of 25.1-72.9 micromol Si/g and 208.1-350.4 micromol Si/g, respectively. Significant correlations were observed for the grain size distributions of sediments and different biogenic silica pools in intertidal sediments. This confirms that grain size distribution can significantly affect biogenic silica contents in sediments. Close relationships of biogenic silica with organic carbon and nitrogen were also found, reflecting that there is a strong coupling between biogenic silica and organic matter biogeochemical cycles in the intertidal system of the Yangtze Estuary. Additionally, the early diagenetic changes of biogenic silica in sediments are discussed in the present study.

Morphological adaptations of hot springs panic grass ( Dichanthelium lanigunosum var sericeum (Schmoll) to thermal stress

Although growth and productivity of most crops are significantly reduced by exposure to temperatures in excess of 35 °C, some plants thrive in geothermally heated soils characterized by chronic temperatures in excess of 40 °C. The morphological adaptations that enable these plants to colonize thermal environments are poorly understood. We characterized trichome development, silica accumulation, stomatal density, and waxy cuticle development in populations of Dichanthelium lanuginosum var sericeum (Schmoll) isolated from thermal and non-thermal environments to determine whether morphological changes existed in populations adapted to thermal environments. Plants isolated from thermal environments of Yellowstone National Park developed an extensive series of trichomes when exposed to chronic temperatures of 45/35 °C (day/night). In contrast, isolates from non-thermal environments of western Oregon showed minimal trichome development during exposure to the elevated temperatures. Leaves that developed during exposure to elevated temperatures had reduced thickness, though the reduction was less marked in plants from populations isolated from thermal environments. Plants isolated from thermal environments also had greater numbers of stomata on adaxial leaf surfaces relative to biotypes isolated from non-thermal environments. These results suggest phenotypic characteristics that may be useful in selection of new varieties of crop species with improved tolerance to supraoptimal temperatures.

Insights into the chemical composition of Equisetum hyemale by high resolution Raman imaging.

Equisetaceae has been of research interest for decades, as it is one of the oldest living plant families, and also due to its high accumulation of silica up to 25% dry wt. Aspects of silica deposition, its association with other biomolecules, as well as the chemical composition of the outer strengthening tissue still remain unclear. These questions were addressed by using high resolution (<1 μm) Confocal Raman microscopy. Two-dimensional spectral maps were acquired on cross sections of Equisetum hyemale and Raman images calculated by integrating over the intensity of characteristic spectral regions. This enabled direct visualization of differences in chemical composition and extraction of average spectra from defined regions for detailed analyses, including principal component analysis (PCA) and basis analysis (partial least square fit based on model spectra). Accumulation of silica was imaged in the knobs and in a thin layer below the cuticula. In the spectrum extracted from the knob region as main contributions, a broad band below 500 cm−1 attributed to amorphous silica, and a band at 976 cm−1 assigned to silanol groups, were found. From this, we concluded that these protrusions were almost pure amorphous, hydrated silica. No silanol group vibration was detected in the silicified epidermal layer below and association with pectin and hemicelluloses indicated. Pectin and hemicelluloses (glucomannan) were found in high levels in the epidermal layer and in a clearly distinguished outer part of the hypodermal sterome fibers. The inner part of the two-layered cells revealed as almost pure cellulose, oriented parallel along the fiber.

Relationships Between Photosynthetic Activity and Silica Accumulation with Ages of Leaf in Sasa veitchii (Poaceae, Bambusoideae)

Bamboos have long-lived, evergreen leaves that continue to accumulate silica throughout their life. Silica accumulation has been suggested to suppress their photosynthetic activity. However, nitrogen content per unit leaf area (N(area)), an important determinant of maximum photosynthetic capacity per unit leaf area (P(max)), decreases as leaves age and senescence. In many species, P(max) decreases in parallel with the leaf nitrogen content. It is hypothesized that if silica accumulation affects photosynthesis, then P(max) would decrease faster than N(area), leading to a decrease in photosynthetic rate per unit leaf nitrogen (photosynthetic nitrogen use efficiency, PNUE) with increasing silica content in leaves. The hypothesis was tested in leaves of Sasa veitchii, which have a life span of 2 years and accumulate silica up to 41 % of dry mass. Seasonal changes in P(max), stomatal conductance, N(area) and silica content were measured for leaves of different ages. Although P(max) and PNUE were negatively related with silica content across leaves of different ages, the relationship between PNUE and silica differed depending on leaf age. In second-year leaves, PNUE was almost constant although there was a large increase in silica content, suggesting that leaf nitrogen was a primary factor determining the variation in P(max) and that silica accumulation did not affect photosynthesis. PNUE was strongly and negatively correlated with silica content in third-year leaves, suggesting that silica accumulation affected photosynthesis of older leaves. Silica accumulation in long-lived leaves of bamboo did not affect photosynthesis when the silica concentration of a leaf was less than 25 % of dry mass. Silica may be actively transported to epidermal cells rather than chlorenchyma cells, avoiding inhibition of CO2 diffusion from the intercellular space to chloroplasts. However, in older leaves with a larger silica content, silica was also deposited in chlorenchyma cells, which may relate to the decrease in PNUE.

Historical eutrophication in Lake Taihu: evidence from biogenic silica and total phosphorus accumulation in sediments from northern part of Lake Taihu

Sediment and water from the Meiliang Bay of Lake Taihu were analyzed to examine the historical relation between the accumulation of biogenic silica (BSi) and total phosphorus (TP). The results indicate that BSi accumulation in the northern part of Lake Taihu had been controlled by diatom production and phosphorus loading since the 1950s. BSi accumulation increased with the growing agricultural activity since the 1950s, up to a maximum level in the 1960s. After that, BSi accumulation decreased due to the diatom dissolution till the 1980s, and then the diatom biomass decreased with BSi accumulation increased. Lake Taihu came into an accelerated eutrophication periods since the 1990s, while BSi accumulation began to increase but the proportion of diatom decreased. Although the onset of silica depletion cannot be confirmed in the present work, it is clear that BSi accumulation was restrained by the input of TP.

Structural and analytical studies of silica accumulations in Equisetum hyemale

Horsetail (Equisetum spp.) is known as one of the strongest accumulators of silicon among higher terrestrial plants. We use the combination of position-resolved analytical techniques, namely microtomography, energy-dispersive X-Ray elemental mapping, Raman microscopy, as well as small-angle and wide-angle scattering of X-rays, to study the type, distribution and nanostructure of silica in the internodes of Equisetum hyemale. The predominant silicification pattern is a thin continuous layer on the entire outer epidermis with the highest density in particular knob regions of the long epidermal cells. The knob tips contain up to 33 wt% silicon in the form of pure hydrated amorphous silica, while the silica content is lower in the inner part of the knobs and on the continuous layer. In contrast to the knob tips, the silica in these regions lacks silanol groups and is proposed to be in close association with polysaccharides. No mentionable amount of crystalline silica is detected by wide-angle X-ray scattering. The small-angle X-ray scattering data are consistent with the presence of colloidal, sheet-like silica agglomerates with a thickness of about 2 nm. From these results we conclude that there are at least two distinct forms of silica in E. hyemale which may have different functions. The close association of silica with cell wall polymers suggests that they may act as a polymeric template that controls the shape and size of the colloidal silica particles similar to many other biominerals and mineralised tissues. We propose that owing to its specific distribution in E. hyemale, a protective role and possibly also an important biomechanical role are among the most likely functions of silica in these plants.

Modern accumulation rates of biogenic silica in Baikal bottom sediments: significance of diatom species composition

The modern distribution of biogenic silica (SiO 2biog) in Baikal surface (0–0.2 and 0–0.5 cm) sediments, its mass accumulation rates (MAR), and the processes responsible for this distribution were studied. The contents of SiO 2biog were determined in samples from 87 stations evenly distributed throughout the lake water area. These data were used to calculate the MAR of SiO 2biog and compile SiO 2biog and MAR distribution maps. The maps showed that the MAR varies significantly throughout the study area. Its highest values have been established in North Baikal. In Central and South Baikal the MAR values are much lower, and the minimum values are observed in the Selenga shoal. This MAR distribution pattern is consistent with the distribution of frustules of Aulacoseira baicalensis endemic diatoms throughout the surface sediments. Compared with other modern diatom species, this one has the most massive and heaviest frustules, which undergo the least dissolution when settling. This suggests that the above species plays a leading role in the accumulation of biogenic silica in modern Baikal bottom sediments, whereas the role of thin-walled species is minor because of their small mass and partial or complete dissolution in water on settling. Dilution of the sediments with terrigenous material is less significant for the SiO 2biog accumulation.

Les assemblages de diatomées du bassin messinien de Boudinar (Maroc nord-oriental)

The Messinian is one of the strongest biogenic silica accumulation periods in the world and more particularly in the Mediterranean, where it is marked by an important diatomitic sedimentation. In the Boudinar basin (Morocco north-Eastern, Western Mediterranean), the section of Sidi Haj Youssef, localised near the volcano of Ras Tarf, has approximately 100 m thickness of clayey-marly series in which 12 diatomitic levels of variable thickness are intercalated. The microfloristic study of diatoms on 86 samples, carried out in detail for the first time, made it possible to recognize 50 genus of diatoms (24 of centric and 26 of pennate) represented by 185 species (75 species of centric and 110 species of pennate). Four hundred individuals were taken from each sample to determine the relative frequency of each taxon within the diatoms assemblages. Thus, several assemblages were defined by the predominance of the following species: Coscinodiscus marginatus, Actinoptychus senarius, Thalassionema nitzschioides, Actinocyclus curvatulus, Thalassiothrix longissima, Rhizosolenia styliformis and Actinocyclus ehrenbergii. These diatoms assemblages display a littoral marine environment in communication with the opened sea. The abundance of the cold water species towards the base and the top of the section suggests broad exchanges of the basin with the Atlantic Ocean in Messinian. The predominance of the species Thalassionema nitzschioides and/or Thalassiothrix longissima indicates periods of strong productivity that can be associated to upwelling systems.

A quantitative assessment of diatom dissolution and late quaternary primary productivity in the Eastern Equatorial Pacific

The Eastern Equatorial Pacific (EEP) is an important area for ocean/atmosphere carbon exchange, and yields different opal accumulation records for sites influenced by equatorial divergence and Peru upwelling. The divergence zone is characterized by a lower opal accumulation rate during the last glacial, compared with the Holocene, but the reverse is true for the Peru upwelling zone. As a gauge of opal production versus preservation in the EEP, differential diatom preservation was investigated via a quantitative four-stage preservation index using the vellum of the ubiquitous tropical centric diatom Azpeitia nodulifer. We find similar dissolution records at both sites from the last glacial maximum (LGM) through the Holocene despite differing accumulation rates, implying that production, not preservation, controlled the accumulation rates. Thus, the increased silica accumulation in the Peru upwelling region is related to increased production, suggesting higher availability of silica in the photic zone, in accord with the ‘silica leakage hypothesis’ described by Matsumoto et al. [2002. Silicic acid leakage from the Southern Ocean: a possible explanation for glacial atmospheric CO. Global Biogeochemical Cycles. 5, doi:10.1029/2001GB001442] and discussed by Richaud et al. (2007). Furthermore, we suggest an ecological control on opal preservation due to silica recycling by diatoms in the water column, most likely in the photic zone. Increased production causes rapid silica recycling and thinner walled frustules, resulting in enhanced water-column dissolution. In this setting diatom dissolution is independent of the rate of accumulation of silica or sediment. High opal productivity does not necessarily lead to high preservation, as proposed for Southern Ocean sediments [Sayles, F., Martin, W., Chase, Z., Anderson, R., 2001. Benthic remineralization and burial of biogenic SiO2, CaCO3, organic carbon and detrital material in the Southern Ocean along a transect at 170 west. Deep-Sea Research II 48, 4323–4383]. Our observations are compatible with a production increase in the late deglacial and early Holocene as reported by Loubere [2001. Nutrient and oceanographic changes in the Eastern Equatorial Pacific from the last full glacial to the present. Global and Planetary Change 29, 77–98] and inferred by Spero, H., and Lea, D. [2002. The cause of carbon isotope maximum events on glacial terminations. Science 296, 522–525] and Rickaby, R., and Osborne, A. [2006. Oceanic redistribution of phosphate during the last deglaciation, in preparation].

Changes in opal flux and the rain ratio during the last 50,000 years in the equatorial Pacific

Abstract Changes in the orgC/CaCO3 ratio in particles sinking from the surface to the deep ocean have the potential to alter the atmospheric pCO2 over the span of a glacial/interglacial cycle. Recent paleoceanographic and modern observational studies suggest that silica is a key factor in the global carbon biogeochemical cycle that can influence the flux ratio, especially at low latitudes, through “silicic acid leakage” [Brzezinski, M., Pride, C., Franck, M., Sigman, D., Sarmiento, J., Matsumoto, K., Gruber, N., Rau, R., Coale, K., 2002. A switch from Si(OH)4 to NO 3 - depletion in the glacial Southern Ocean. Geophysical Research Letters 29, 5]. To test this hypothesis, we reconstruct biogenic fluxes of CaCO3, orgC and Si for three equatorial Pacific cores. We find evidence that a floral shift from a SiO2-based community to a CaCO3-based occurred, starting in mid-marine isotope stage (MIS) 3 (24–59 cal. ka) and declining toward MIS 2 (19–24 cal. ka). This could reflect the connection of the Peru upwelling system to the subantarctic region, and we postulate that excess silica was transported from the subantarctic via the deep Equatorial Undercurrent to the eastern equatorial Pacific. In the eastern equatorial Pacific only, we document a significant decrease in rain ratio starting mid-MIS 3 toward MIS 2. This decrease is concomitant with a significant decrease in silica accumulation rates at the seabed. This pattern is not observed in the Pacific influenced by equatorial divergence and shallow upwelling, where all reconstructed fluxes (CaCO3, orgC, and opal) increase during MIS 2. We conclude that the overall calcium carbonate pump weakened in the EEP under Peru upwelling influence.

A 700-year record of climate change based on geochemical and palynological data from varved sediments (Lac Pavin, France)

The Fe-flux budget of the meromictic crater Lac Pavin (French Massif Central) is re-evaluated in the light of new results from detailed sediment investigations (geochemistry, sedimentology, pollen, varve counting). Sedimentation of Lac Pavin during the last 700 years is characterised by high net accumulation of biogenic silica (100–500 g m − 2 yr − 1 SiO 2) and autochthonous Fe-precipitates ( F-Fe exc = 2–200 g m − 2 yr − 1 Fe) at overall high sediment accumulation (e.g. AD 1530–1890: 3.7 mm yr − 1 ). High net accumulation of biogenic silica and Fe exc is sustained by nutrient and Fe-influx via sub-lacustrine springs. The feeding of the groundwater inflow is closely correlated with the seepage of meteoric waters which declines in winter due to freezing of the ground. In the context of the derived local vegetation dynamics, balanced Fe exc-flux is interpreted as a proxy for rainfall during the growing season. Meteorological monitoring data document a significant inverse correlation between monthly temperature and rainfall between May and October which reflects enhanced cloudiness for precipitation above average. Beech, growing under sub-optimal conditions at the high altitude of the study site, represents a major tree component in the catchment of Lac Pavin. Minor temperature decrease in the growing season can explain reduced flowering of beech in the following year. Distinct short term variability of beech characterised by an inverse correlation with the balanced F-Fe exc supports the data interpretation. Two periods with prevailing of overall wetter and cooler conditions in the growing season were derived: ca. AD 1525–1600, and AD 1630–1660. Climate change to drier and warmer conditions is implied around AD 1450 and AD 1620. The Late Maunder Minimum (AD 1675–1715) is characterised by distinct short term variability of the palaeo-hydrological conditions at the study site.

Comparison of the desiccation effects of marine and freshwater diatomaceous earths on insects

The desiccating effects of two different types of diatomaceous earths (DEs), of freshwater and marine origin, applied by two different methods, were compared on four species of insects, Cryptolestes ferrugineus, Rhyzopertha dominica, Sitophilus oryzae and Sitophilus granarius. The purpose was to investigate the performance of DEs, including their abrasive and absorptive effects through the accumulation of silica dusts on the insect cuticle, resulting in death of the insects by desiccation. The data obtained showed that similar insects treated with the same type of diatom, but applied either by spraying or dusting, did not have the same initial mortality rate. After examining and comparing the data obtained, it appeared that weight loss at death of insects subjected to the action of sprayed DEs was slightly less than for insects subjected to dust application, while mortality was slightly more rapid during the observation period. The extra weight of dusted insects was ascribed to greater adherence of dust particles to the treated insects.

Biogenic silica accumulation in the sediments of Iron Gate I Reservoir on the Danube River

Damming of rivers can result in severe downstream effects such as changing sediment and nutrient fluxes that potentially affect coastal ecosystems. Closing of the Iron Gates Dams in the lower Danube River was linked to a decrease in dissolved silica flux to the Black Sea of 600,000 t yr−1. A recent study on the Iron Gate I, however, indicated a dissolved silica removal within the reservoir of only 16,000 t yr−1. Such an order of magnitude difference between actual budgets and earlier estimates is unlikely to be caused by changes in hydrological or biogeochemical conditions. In order to separate annual variations and downstream effects of damming, we analyzed the sedimentary records of biogenic silica using dated sediments. Results confirm the detailed budgets of dissolved silica. In 2001, a total biogenic silica accumulation in the sediments of the Iron Gate I Reservoir of 19,000 t Si yr−1 was determined and represents the highest retention over the past 20 years. The accumulation of biogenic silica in the Iron Gate I Reservoir was compared with data from the coastal Black Sea. Biogenic silica in the sediments of the coastal Black Sea start decreasing before Iron Gate I Dam was completed in 1971. In conclusion, construction of the largest impoundment on the Danube River, the Iron Gate I Reservoir, was not solely responsible for decreasing the silica loads downstream at the coastal Black Sea.

Middle Eocene radiolarian and diatom accumulation in the equatorial Atlantic (Demerara Rise, ODP Leg 207): Possible links with climatic and palaeoceanographic changes

Middle Eocene siliceous bioaccumulation on Demerara Rise is explored using taphonomic and palaeoecological studies of radiolaria and diatoms, respectively. Enrichment and improved preservation of silica began just above the Lower to Middle Eocene transition (ca. 49 Ma) and was mainly due to radiolarian productivity. It reflects a profound change in the zooplankton communities of the western Equatorial Atlantic, dominated previously by planktic foraminifera. Diatoms are a minor siliceous component of the Middle Eocene chalk. This microflora is dominated by robust planktic species, typical of neritic environments, and includes some upwelling indicators. Benthic species are absent. No continental taxa were recorded. Onset of biogenic silica accumulation was probably due to the establishment of a vigorous bottom current circulation and higher levels of productivity. It coincides with a major palaeoceanographic shift from a Lower Eocene stagnant to a more vigorous Middle Eocene ocean, following some as yet poorly understood global climatic change.

Ultrastructure and microanalysis of silica bodies in Dactylis Glomerata L.

Silica bodies of orchard grass (Dactylis glomerata L.) were analysed by transmission electron microscopy and X-ray microanalysis. Silica bodies are inserted into the epidermis of the leaf and consist of a rather compact amorphous silica and low carbon. In particular regions of the interface between silica body and cell wall, silica is crossed by electron opaque strands, which are similar to filaments of the cell wall and most probably represent cellulose fibrils. The presence of a higher carbon content in this region than in the compact silica supports this notion. Cells with silicified walls or lumen were found apart from typical silica bodies. They most likely represent early or intermediate stages of silica body formation. Some epidermal cells showed silica deposits between protoplast and cell wall with small but regularly detected nitrogen and phosphorus peaks indicating a particular extracellular chemical composition probably involved in silica accumulation.

Heat and fire resistance of polyurethane‐polydimethylsiloxane hybrid material

AbstractThis work examines the synthesis of polyurethane (PU)—polydimethylsiloxane (PDMS) hybrid in order to provide low flammability to PU. Chemical characterization of PU/PDMS hybrid by solid state NMR of carbon and FT‐IR shows that some PDMS segments are incorporated in the macromolecular chains of PU. Heat stability of PU (thermo‐oxidative decomposition) is not enhanced by the incorporation of PDMS except that the amount of the high temperature residue is improved and depends on the amount of PDMS incorporated in the PU chains. It is explained by the formation and the accumulation of silica. On the contrary, thermal stabilization (charring) occurs in the pyrolytic conditions when incorporating PDMS. The accumulation of silica as well as charring enhancement enables the flammability properties of the PU/PDMS hybrid to form a protective layer limiting heat transfer between the flame and the substrate. The heat release rate measured by cone calorimetry of PU/PDMS hybrid is significantly decreased compared to virgin PU and temperature of the back of the sample is comparatively low compared to virgin PU. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Diatom Paleolimnological Record of Holocene Climatic and Environmental Change in the Altai Mountains, Siberia

The sedimentary diatom records of three shallow lakes in the Altai Mountains, southern Siberia, were examined to assess the nature and timing of Holocene environmental changes. Few paleoenvironmental records, especially reconstructions not based on pollen, have been reported from this region. The lakes differ in elevation, annual precipitation, and catchment vegetation. Diatom assemblages in all lakes were dominated for the entire period of record by small benthic species of Pseudostaurosira Williams & Round, Staurosira Ehrenberg, and Staurosirella Williams & Round. Planktonic taxa only occur in very low abundances (<5%). The most diverse diatom flora was found in Dzhangyskol, which is situated at the lowest elevation within a forested catchment. A lack of detailed information on the ecological preferences of the dominant taxa and the complexity of environmental drivers make direct interpretation of the diatom record difficult. However, other proxies suggest that dramatic shifts in dominance between Staurosira elliptica and Staurosirella pinnata in Grusha Ozero reflect millennial-scale variability in climate. Together, chironomids and diatoms provide evidence of a cooling possibly correlative to the Younger Dryas Stade and subsequent early-Holocene warming consistent with pollen evidence of afforestation, which also is likely linked to increased humidity. By ~6000 cal year BP, the transition to a cooler, more continental climate had begun. The diatom record of Akkol shows significantly less variation in diatom community composition, but biogenic silica accumulation rates, a proxy for diatom productivity, appear to reflect climatic variability driven by insolation trends over the past 8000 years. Long-term variability in Dzhangyskol is not clearly linked to climate.

Historic low‐level phosphorus enrichment in the Great Lakes inferred from biogenic silica accumulation in sediments

Sedimentary biogenic silica (BSi) accumulation was used in conjunction with a hypothetical model of BSi accumulation to show that BSi is a sensitive proxy for low‐level phosphorus enrichment in the Great Lakes. We hypothesize that historic nutrient‐driven changes in diatom production altered silica biogeochemistry and induced biologically mediated silica depletion (BMSD) and that a record of the underlying mechanism, enhanced diatom production and BSi sedimentation stimulated by anthropogenic phosphorus enrichment, is preserved in the sediment record. Paleolimnological results support three hypotheses based on this model. First, BSi accumulation increased in Lake Superior and Lake Huron at total phosphorus (TP) concentrations (4 and 5 µg TP L−1 or 0.13 and 0.16 µmol L−1, respectively) too small to induce BMSD and with changes in TP concentration too small to be detected by routine water‐column sampling. Second, a peak in BSi accumulation in Lake Michigan resulted from epilimnetic silica depletion that developed rapidly in the 1950s and 1960s when TP averaged 8 mg L−1 (0.26 µmol L−1). In addition, epilimnetic silica depletion in the late 1800s was inferred from BSi accumulation in Lake Erie and Lake Ontario when the TP concentration was &lt;10 µg L−1 (0.32 µmol L−1). Third, a secondary peak in BSi accumulation in the 1950s and 1960s signaled water‐column silica depletion in Lake Ontario and the eastern basin of Lake Erie that developed as TP concentration increased to 27 µg L−1 (0.87 µmol L−1). Ratios of NAIP :TP, BSi :TP, and BSi : NAIP also provide sensitive proxies for phosphorus enrichment. BSi accumulation is a sensitive proxy for phosphorus enrichment because BSi production by diatoms integrates silica utilization over an annual cycle, silica is recycled slowly (on annual time scales) compared with phosphorus, and sedimented BSi is focused into depositional zones.

Influence of mechanical properties and mineral salts in wood species on tool wear of high-speed steels and stellite-tipped tools – Consideration of tool wear of the newly developed tip-inserted band saw

This paper is the third in a series of work done on a novel technique for bandsawing, which uses a tip-inserted saw, and is gaining increasing popularity in Japan. It builds up on previous reports on the novel technique for bandsawing, which considered cutting tool hardness, tool wear, accuracy of kerf width, washboarding, and surface profiles of the workpiece. In this paper, ash content analysis and three-point static bending tests were conducted to clarify the influence of mineral salts and mechanical properties of wood samples of Elais guineensis (Oil palm), Strombosia glaucescens (Afina), and Cryptomeria japonica (Sugi) on cutting tool wear. The cutting tools were made from Stellite and High Speed Steels (HSS) of designations SUS420J2 and TiN coated SKH 51 according to the Japan Industrial Standards (JIS). Among the tested wood samples, the highest cutting tool edge recession when machining with SUS420J2 and TiN coated SKH 51 tools were recorded in Oil palm in spite of Afina possessing the highest mechanical strength properties. On the contrary, Stellite in spite of possessing the smallest hardness (HV580) among all the tested cutting tools recorded the lowest cutting tool edge recession when machining wood samples of Oil palm. However, Stellite recorded the highest edge recession when machining Afina, a high density species. Ash content analysis and a scanning electron micrograph of wood samples of the tested wood species show the presence of high proportions of mineral salts in the transverse, radial and tangential sections of Oil palm. This could perhaps account for the high cutting tool edge recessions recorded in wood samples of the Oil palm. The studies have demonstrated that silica accumulation species could have significant effect on tool wear of high speed steels.