Silicic Acid Concentrations Research Articles

Physical Mechanisms Sustaining Silica Production Following the Demise of the Diatom Phase of the North Atlantic Spring Phytoplankton Bloom During EXPORTS

AbstractEach spring, the North Atlantic experiences one of the largest open‐ocean phytoplankton blooms in the global ocean. Diatoms often dominate the initial phase of the bloom with succession driven by exhaustion of silicic acid. The North Atlantic was sampled over 3.5 weeks in spring 2021 following the demise of the main diatom bloom, allowing mechanisms that sustain continued diatom contributions to be examined. Diatom biomass was initially relatively high with biogenic silica concentrations up to 2.25 μmol Si L−1. A low initial silicic acid concentration of 0.1–0.3 μM imposed severe Si limitation of silica production and likely limited the diatom growth rate. Four storms over the next 3.5 weeks entrained silicic acid into the mixed layer, relieving growth limitation, but uptake limitation persisted. Silica production was modest and dominated by the >5.0 μm size fraction although specific rates were highest in the 0.6–5.0 μm size fraction over most of the cruise. Silica dissolution averaged 68% of silica production. The resupply of silicic acid via storm entrainment and silica dissolution supported a cumulative post‐bloom silica production that was 32% of that estimated during the main bloom event. Diatoms contributed significantly to new and to primary production after the initial bloom, possibly dominating both. Diatom contribution to organic‐carbon export was also significant at 40%–70%. Thus, diatoms can significantly contribute to regional biogeochemistry following initial silicic acid depletion, but that contribution relies on physical processes that resupply the nutrient to surface waters.

Collapse of a giant iceberg in a dynamic Southern Ocean marine ecosystem: In situ observations of A-68A at South Georgia

Large icebergs (>20 km long) are responsible for most of the freshwater discharged into the Southern Ocean. We report on in situ and satellite observations made during the break-up phase around South Georgia of the giant tabular iceberg A-68A. The in situ measurements were obtained during a 4-day visit by a research vessel in February 2021, where physical, chemical and biological measurements were made at a range of distances away from the main and subsidiary icebergs. These results were compared to a far-field station 133 km away. Upstream of the iceberg field, water column structure was similar to ambient water although there was evidence of iceberg-associated phytoplankton as a likely remnant of the passage of the icebergs. Nevertheless, enhancement of primary productivity along the path of the icebergs was not resolved in either in situ or monthly mean satellite observations. There was a considerable brash-ice field moving ahead of the icebergs which limited the number of downstream sampling stations. One downstream station within 2 km of iceberg A-68P showed several ice-melt influenced features that distinguished it from most other stations. Firstly, there was a strong stratified meltwater influenced layer that reached to around 120 m. This had the effect of deepening underlying water masses, with the core of the temperature minimum layer around 50 m deeper than elsewhere. Secondly, there was evidence of rapid downward displacement of both particulate material and certain phytoplankton taxa that may be a further result of this water mass deepening. Thirdly, macronutrient profiles were altered, with concentrations of nitrate, silicic acid and phosphate characteristic of deeper layers being found closer to the surface and a dilution of the ambient nutrient pool just above the iceberg draft that we ascribe to meltwater released from basal melting. Meanwhile, nutrient recycling processes associated with organic matter remineralisation were also modified by the physical restructuring of the water column and biotic components. Finally, the ice-associated phytoplankton taxa Pseudo-nitszchia/Nitszchia, found in both upstream and downstream locations, were abundant at this < 2 km-distant station through melting out from the iceberg and subsequent rapid growth. Overall, we resolved alterations to water column structure, nutrient profiles and phytoplankton community composition at fine to medium scales around the iceberg field. Nevertheless, although there may have been longer term and larger scale impacts, the dynamic oceanographic environment, including the presence of a strong oceanographic front and shelf-edge processes, dominated during the collapse of A-68A.

Incorporation of Al into diatom frustule as a proxy of seawater Al/Si ratios in the Antarctic Zone of the Southern ocean: Implications for surface silicic acid concentrations during the last glacial period

Knowledge of silicic acid concentrations at the surface Southern Ocean helps better understand the role of the biological pump in regulating atmospheric CO2 concentrations in the past. While the silicon isotope compositions of diatom provide a measure of Si utilization efficiency, proxies on surface silicic acid concentration are not yet available. Here we determined the diatom frustule Al/Si ratio and dust flux to constrain the surface silicic acid concentrations in the Atlantic sector of the Southern Ocean. Our results based on multiple methods, including inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscope (SEM) and energy-dispersive spectroscopy (EDS), and nuclear magnetic resonance spectroscopy (NMR), suggest the isomorphic substitution of Al for Si by frustule biosynthesis. This enables us to transfer the diatom Al/Si ratio into seawater Al/Si ratio using the partition coefficient of Al between diatom silica and seawater. By further constraining dust flux using the sediment 230Th-normalized 232Th flux, we found that the markedly higher frustule Al/Si ratios cannot be solely explained by increased Al dissolution from dust and call for much lower surface silicic acid concentrations in the Antarctic Zone during the Last Glacial Period. The lower silicic acid concentration coupled with lower Si utilization efficiency inferred previously from silicon isotopes in the Antarctic Zone is likely due to the combined effects of reduced upwelling and iron-fertilization-regulated uptake stoichiometry, contributing to a lower atmospheric CO2 during the Last Glacial Period.

Importance of microzooplankton for sustaining high mesozooplankton biomass during post-bloom period in the Oyashio region of the western subarctic Pacific

We investigated the plankton community structure and biomass during the post-bloom season in the Oyashio region of the western subarctic Pacific, including pico-, nano-, microplankton and mesozooplankton. We found that the nitrate, phosphate and silicic acid concentrations remained high at >4.2 μM, >0.77 μM and >7.1 μM, respectively, in the euphotic layer at almost all sampling stations, but that the chlorophyll a concentrations were low (<3 µg Chl. a l−1). These findings indicate high nutrient and low chlorophyll (HNLC)-like conditions. In the phytoplankton community, pennate diatoms, the larger subpopulation of pico-sized eukaryotic phytoplankton, and nano-flagellates substantially contributed to the low biomass of the chain-forming centric diatoms that mainly comprised the spring phytoplankton bloom. The microzooplankton biomass was 2.7–4.4 fold greater than the phytoplankton biomass in the surface layer. Naked ciliates substantially contributed to the microzooplankton community (40–87 %). The naked ciliate growth rates during our in situ bottle incubation experiments were significantly greater than the maximum growth rates as calculated from cell volume and water temperature. The mesozooplankton biomass was mainly composed of krill and copepods and was 5.9–9.3 fold higher than the microzooplankton biomass. This inverted biomass pyramid with relatively low microzooplankton and high mesozooplankton biomass may be explained by the high production and growth rates of the microzooplankton. The ratio of phytoplankton growth (µ, d−1) to grazing mortality (m, d−1) by microzooplankton were relatively low at 0.26–0.44 m/µ in our dilution experiments. These low values indicate that microzooplankton grazing does not regulate phytoplankton growth and suggests that microzooplankton feed on an alternative nutritional source, such as heterotrophic prey items, or mixotrophy to fulfill their growth needs. Additional research is needed during the post-bloom period to further evaluate the mechanisms that sustain microzooplankton dominance and production in the Oyashio region under the HNLC-like conditions, especially for naked ciliates.

Silica polymerization and nanocolloid nucleation and growth kinetics in aqueous solutions

Amorphous silica precipitation from aqueous solutions at low temperatures (<100 °C) involves the initial nucleation and growth of silica polymers followed by aggregation, cementation and recrystallization to form opaline silica. In this study, silica polymerization was studied at temperature 23–80 °C, pH 2.48–9.65, and ionic strength 0.01–0.53 mol kg−1. Measured changes in the concentration of monomeric silica (SiO2(mono)) were fit assuming the polymerization reaction is fourth-order with respect to the difference between the concentration of silicic acid (Si(OH)4(aq)) and its concentration at equilibrium (Si(OH)4(aq)eq) with respect to amorphous silica:r=-k4AsmSi(OH)4(aq)-mSi(OH)4(aq)eq4where r is the overall rate of the reaction, As is the specific surface area of the formed polymers, calculated assuming particle-size-dependent solubility and a spherical polymer geometry, and k4 is the rate constant. The rate constant at zero ionic strength is given by:k0=Aexp-Ea/RTaOH-nwhere A is a pre-exponential constant (0.3822 mmol−3 cm−2 s−1), Ea is the activation energy (29.87 kJ mol−1), and n is equal to −0.80. The overall rate constant as a function of ionic strength (I) is given by:k4=k0Imwhere m is equal to 0.65. The rate constant increases by approximately six orders of magnitude from pH of 2 to 10 and one order of magnitude between temperature ∼20 and 80 °C. However, hydrolysis of silicic acid (Si(OH)4(aq)) at pH above ∼8 decreases the relative abundance of Si(OH)4(aq) while increasing the solubility of amorphous silica, thereby decreasing the rate of silica polymerization. The inferred reaction order is consistent with a reaction mechanism rate-limited by the formation of the cyclic tetramer across a wide range of pH. The time-dependence of the specific surface area during polymerization reflects the formation of critical nuclei towards the beginning of the polymerization process and Ostwald ripening of larger polymers towards the end of the polymerization process. The developed rate equation can predict rates of silica polymerization across a wide range of aqueous solution compositions.

Scaling and cleaning of silica scales on reverse osmosis membrane: Effective removal and degradation mechanisms utilizing gallic acid

Silica scaling on membranes represents one of the most important issues in industrial water systems because of its complex composition and difficulty in removal. However, there is a lack of understanding of the mechanisms for cleaning silica scales from reverse osmosis (RO) membranes. To address this research gap, this study investigated the scaling and cleaning behavior of silica on RO membrane processes, with a specific focus on the silica scale cleaning mechanism using gallic acid (GA). The membrane flux continuously decreased with operation time, even at the lowest initial silicic acid concentration, owing to silica scale blockage. The GA solution exhibited a strong efficacy in cleaning silica-scaling RO membranes. The membrane flux returned to 89.7% of the initial value by removing 81.87% of the silica scale within the first 30 min of the study period. The cleaning mechanism of GA involved its adsorption onto the surface of silica scale particles to form a surface complex and subsequently transition into a water-soluble 1:3 complex within the solution. This complex interaction facilitated the gradual decomposition of the silica scales that adhered to the membrane surface. This study has valuable implications for the development of efficient and effective silica scale cleaning solutions, providing insights into the complex interplay between GA and silica scaling mechanisms.

Biomimetic Sol-Gel Chemistry to Tailor Structure, Properties, and Functionality of Bionanocomposites by Biopolymers and Cells.

Biosilica, synthesized annually only by diatoms, is almost 1000 times more abundant than industrial silica. Biosilicification occurs at a high rate, although the concentration of silicic acid in natural waters is ~100 μM. It occurs in neutral aqueous solutions, at ambient temperature, and under the control of proteins that determine the formation of hierarchically organized structures. Using diatoms as an example, the fundamental differences between biosilicification and traditional sol-gel technology, which is performed with the addition of acid/alkali, organic solvents and heating, have been identified. The conditions are harsh for the biomaterial, as they cause protein denaturation and cell death. Numerous attempts are being made to bring sol-gel technology closer to biomineralization processes. Biomimetic synthesis must be conducted at physiological pH, room temperature, and without the addition of organic solvents. To date, significant progress has been made in approaching these requirements. The review presents a critical analysis of the approaches proposed to date for the silicification of biomacromolecules and cells, the formation of bionanocomposites with controlled structure, porosity, and functionality determined by the biomaterial. They demonstrated the broad capabilities and prospects of biomimetic methods for creating optical and photonic materials, adsorbents, catalysts and biocatalysts, sensors and biosensors, and biomaterials for biomedicine.

Benthic contribution to seasonal silica budgets in two macrotidal estuaries in North-Western France

The paper aims to build seasonal silica budgets in two macrotidal estuaries, the Elorn and Aulne estuaries of the Bay of Brest (North-Western France), based on modeling and measurements, in order to increase our understanding of the silica (Si) cycle at land-sea interfaces. A diagenetic model was developed to quantify benthic Si fluxes, e.g. aSiO2 deposition fluxes that are difficult to assess through direct measurements. Sediment cores were also seasonally sampled at six stations to provide data essential to parametrize and validate the model. Vertical profiles of porosity, burrowing depth, biodiffusive coefficients, concentrations of amorphous silica (aSiO2) and silicic acid (Si(OH)4 and the proportion of reactive aSiO2 were measured. The results show that sites sampled along the Elorn and Aulne estuaries constitute significant net Si deposition areas (1-4.5 mmol Si m-2 d-1), particularly in the upstream during winter and in midstream and downstream during summer. Year round, reprecipitation is negligible (&amp;lt; 3%) while burial accounts for the retention of ~ 30-80% of deposited aSiO2. In winter, burial dominates the benthic Si budget. As surface-integrated benthic Si fluxes are low compared to riverine aSiO2 fluxes, the Si export to coastal waters is high (93%) during winter. In contrast, in summer, burial accounts for 38% of river Si fluxes, and Si(OH)4 flux from the sediment is high as a result of enhanced benthic recycling and bioirrigation. Internal estuarine processes, e.g., benthic and pelagic primary production, dissolution and benthic Si fluxes, surpass river fluxes in magnitude during summer. Overall, we conclude that the Elorn and Aulne macrotidal estuaries are efficient filters of Si, retaining about 4-38% of river Si fluxes, and even 6-67% when accounting for retention in intertidal marshes, but with massive exports occurring during winter floods.

Scenarios of temporal environmental alterations and phytoplankton diversity in a changing bay in the East China Sea

In the context of global change, the stressors of warming and eutrophication have significant ecological implications in coastal waters. In order to examine the diversity of phytoplankton and its relationship with water quality, we conducted a survey of phytoplankton community compositions and their correlation with environmental changes over four seasons in a eutrophic bay located in the East China Sea. Through a systematic analysis, we identified diatoms and dinoflagellates as the primary dominant groups, with the species Skeletonema costatum, Skeletonema marinoi, Biddulphia sinensis, Thalassiosira eccentrica, Leptocylindrus danicus, Coscinodiscus oculus-iridis, Coscinodiscus jonesianus, and Chaetoceros knipowitschi as the most abundant species in all seasons. Significant seasonal alterations were observed in both environmental settings and phytoplankton species richness, dominance, and abundance. The phytoplankton community varied in its response to diverse aquatic environments and was principally affected by temperature, silicic acid concentrations, and suspended solids. Elevated temperatures were found to promote an increase in phytoplankton abundance. However, no clear evidence of diatom and dinoflagellate succession in relation to N:P ratio was observed across seasons. Water quality analysis illustrated that the majority of the study area exhibited a mid-eutrophic with severe organic pollution. The abundance of phytoplankton was significantly influenced by eutrophication and organic pollution. The accelerated warming process related to coastal nuclear power plants and nutrient regime alterations significantly affect the temporal shift of the phytoplankton community. These findings contribute valuable insights into the effects of eutrophic environments on the structure of phytoplankton communities in coastal aquatic systems.

Macronutrient biogeochemistry in Antarctic land-fast sea ice: Insights from a circumpolar data compilation

Antarctic sea ice plays an important role in Southern Ocean biogeochemistry and mediating Earth's climate system. Yet our understanding of biogeochemical cycling in sea ice is limited by the availability of relevant data over sufficient temporal and spatial scales. Here we present a new publicly available compilation of macronutrient concentration data from Antarctic land-fast sea ice, covering the full seasonal cycle using datasets from around Antarctica, as well as a smaller dataset of macronutrient concentrations in adjacent seawater. We show a strong seasonal cycle whereby nutrient concentrations are high during autumn and winter, due to supply from underlying surface waters, and then are utilised in spring and summer by mixed ice algal communities consisting of diatoms and non-siliceous species. Our data indicate some degree of nutrient limitation of ice algal primary production, with silicon limitation likely being most prevalent, although uncertainties remain around the affinities of sea-ice algae for each nutrient. Remineralisation of organic matter and nutrient recycling drive substantial accumulations of inorganic nitrogen, phosphate and to a lesser extent silicic acid in some ice cores to concentrations far in excess of those in surface waters. Nutrient supply to fast ice is enhanced by brine convection, platelet ice accumulation and incorporation into the ice matrix, and under-ice tidal currents, whilst nutrient adsorption to sea-ice surfaces, formation of biofilms, and abiotic mineral precipitation and dissolution can also influence fast-ice nutrient cycling. Concentrations of nitrate, ammonium and silicic acid were generally higher in fast ice than reported for Antarctic pack ice, and this may support the typically observed higher algal biomass in fast-ice environments.

Seasonality of phytoplankton growth limitation by iron and manganese in subantarctic waters

Phytoplankton indirectly influence climate through their role in the ocean biological carbon pump. In the Southern Ocean, the subantarctic zone represents an important carbon sink, yet variables limiting phytoplankton growth are not fully constrained. Using three shipboard bioassay experiments on three separate voyages, we evaluated the seasonality of iron (Fe) and manganese (Mn) co-limitation of subantarctic phytoplankton growth south of Tasmania, Australia. We observed a strong seasonal variation in a phytoplankton Fe limitation signal, with a summer experiment showing the greatest response to Fe additions. An autumn experiment suggested that other factors co-limited phytoplankton growth, likely low silicic acid concentrations. The phytoplankton responses to Mn additions were subtle and readily masked by the responses to Fe. Using flow cytometry, we observed that Mn may influence the growth of some small phytoplankton taxa in late summer/autumn, when they represent an important part of the phytoplankton community. In addition, Mn induced changes in the bulk photophysiology signal of the spring community. These results suggest that the importance of Mn may vary seasonally, and that its control on phytoplankton growth may be associated with specific taxa.

Synergic role of frontal migration and silicic acid concentration in driving diatom productivity in the Indian sector of the Southern Ocean over the past 350 ka

The glacial-interglacial siliceous productivity in the Sub-Antarctic Zone (SAZ) of the Southern Ocean (SO) has been proposed to respond to higher dust-bearing iron fluxes and/or basin-wide increased nutrient supply to surface waters. However, long records of diatom productivity are mainly obtained from the Atlantic and Pacific sectors of the SO. We present a new diatom productivity record from the SAZ of the western Indian sector of the SO, where the Antarctic Circumpolar Current strongly interacts with bottom topography to create a productivity hotspot, during the last four glacial-interglacial cycles. Our results show that regional changes in diatom productivity did not follow a glacial-interglacial pattern. It was highest during the Marine Isotope stage (MIS) 6 and MIS4, lowest during MIS10-MIS8 and MIS3-MIS1, whereas intermediate diatom productivity was found during MIS7 and MIS5. Multi-millennial events of high diatom productivity were scattered throughout both the glacial and interglacial periods. Both long-term and rapid diatom productivity changes in the region were disconnected from dust flux changes, but might relate to frontal migrations and SO upwelling intensity changes which have both mediated the silica and iron availability for diatoms. Importantly, our data suggest that front migrations were not homogenous in the SO, especially where these fronts interact with bottom topography. The peculiarity of these productivity hotspots must be considered when drawing SO-wide carbon balance in the past.

Sedimentology and geochemistry of basinal lithofacies in the Mesoarchean (2.93 Ga) Red Lake carbonate platform, northwest Ontario, Canada

The > 200 m thick Red Lake carbonate platform of the 2.940-2.925 Ga Ball Assemblage (Red Lake Greenstone Belt, Superior Province, Canada) is the oldest known carbonate platform preserved on Earth. This study examined surface outcrops and multiple industry cored drill holes comprising a diverse assemblage of chemical and siliciclastic sedimentary rocks of the deeper water lithofacies associated with the shallow water carbonates. The offshore lithologies, underlying, interlayered with, and overlying the mainly stromatolitic carbonates, consist of alternating packages of sandstone, siltstone, carbonaceous slate, pyritic carbonaceous slate, chert, and oxide facies iron formation (OFIF). These interlayer with carbonate in transitional slope environments, which consist in places of carbonate with magnetite laminae above slumped carbonate debris in iron formation and/or carbonaceous slate. Chert and OFIF were deposited further offshore in a suboxic environment, with lithology dependent on concentrations of seawater Fe(II) and silicic acid. The iron sulfides were deposited in a reducing, anoxic, and organic-rich bottom water and muddy pore water environment not conducive to Fe hydroxide or chert precipitation. Carbonaceous mud was the background sediment, accumulating during intervals when the precipitation of chert, Fe hydroxides, or massive layers of Fe sulfides was inhibited. The interlayering of differing assemblages of chemical and siliciclastic sedimentary rocks indicates temporal and spatial fluctuations in the chemistry of local paleo-seawater and variable sedimentation rates on the margins of the Red Lake carbonate platform. Post-Archean Australian Shale (PAAS)- normalized rare earth element (REE) systematics of carbonate and OFIF show positive La and Gd anomalies, super-chondritic Y/Ho ratios, and depleted light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), similar to modern seawater, while the positive Eu anomalies in the carbonate and OFIF samples reflect hydrothermally influenced Archean seawater. The chemical sedimentary rocks exhibit enrichment of redox-sensitive elements (i.e., Cr, Mo, V, and U) compared to their siliciclastic associates. This, combined with concentrations of MnO up to 6.3 wt% and positive Ce anomalies in the iron formation directly below the carbonate platform, all indicate the likely presence of some O2 ca 2.93 Ga.

Effect of silicic acid concentration on green mesoporous silica synthesis

Tetraethoxysilane is the most commonly used precursor for obtaining mesoporous silica, it is economically expensive, so the use of inexpensive precursors such as sodium silicate or silicic acid are of interest for the economic production of said materials; silicic acid is generally obtained from sodium silicate by adjusting the pH of the system to 3-4 or through the use of ion exchange resins, this second methodology allows to eliminate the use of acid or basic catalysts in the synthesis of materials, which potentiates these materials for bioparticle encapsulation applications. The effect of the poly-condensation of silicic acid in obtaining mesoporous silica has been little evaluated, so this work shows the study of the effect of the degree of polycondensation of silicic acid on the textural properties of the materials, observing that at 0 and After 48 h of Si(OH)4 aging, the synthesized materials show a bimodal distribution of their pores at 4 and 8 nm, observing the majority presence of pores at 4 nm at 6, 24 and 72 h of aging.

Hydroxyapatite Modified Silica Aerogel Nanoparticles: In Vitro Cell Migration Analysis

The ability of silica aerogel nanoparticles (SA-Np) to improve the stability of hydroxyapatite (HA) was investigated. Using the sol-gel method, the HA was incorporated into SA-Np at a weight ratio of 0.5 of HA to SiO2 (HA-SA-Np). The efficacy of HA-SA-Np, SA-Np, and HA on the in vitro migration of normal human dermal fibroblast cells (HSF1184) was compared. The cell migration was measured at 0, 6, and 24 hours after scratching using ImageJ and an inverted optical microscope. To ascertain the resorbability of HA-SA-Np, the phosphate and silicic acid concentrations in media treated for 2, 5, and 7 days were examined. The high dissolution of HA could be reduced by incorporating the HA into the silica nanosphere. The HA-SA-Np significantly stimulated cell migration and increased closure with increasing treatment time. It was caused by the release of silicic acid, which aided in healing cells. It also demonstrates the ability of HA-SA-Np to be resorbed and eventually increase the adhesion and migration of normal human fibroblast cells. As a result, the potential application of HA-SA-Np as an alternative biomaterial was confirmed.

Active Silica Diagenesis in the Deepest Hadal Trench Sediments

AbstractPorewater dissolved silicic acid (DSi) concentrations and stable Si isotope compositions (δ30Si) together with biogenic silica (bSiO2) contents of sediments in five sediment cores collected from the southern Mariana Trench are presented. These data suggest the occurrence of bSiO2 dissolution and concomitant authigenic clay formation in three bSiO2‐bearing cores. A reaction‐transport model constrained by the measured geochemical data was applied to quantify the rates of Si turnover. Model results predicted the greatest rates of both bSiO2 dissolution and authigenic clay formation at the trench axis core that displayed low bSiO2 contents and abundant detrital materials, suggesting that detrital materials may be a limiting factor for bSiO2 diagenesis. Model results further predicted that ∼40%–70% of DSi generated by bSiO2 dissolution is consumed by authigenic clay formation. This is the first study that demonstrates active silica diagenesis in the hadal realm and has implications for understanding benthic Si cycling in deep‐sea settings.

Stable silicon isotopes uncover a mineralogical control on the benthic silicon cycle in the Arctic Barents Sea

Biogeochemical cycling of silicon (Si) in the Barents Sea is under considerable pressure from physical and chemical changes, including dramatic warming and sea ice retreat, together with a decline in dissolved silicic acid (DSi) concentrations of Atlantic inflow waters since 1990. Associated changes in the community composition of phytoplankton blooms will alter the material comprising the depositional flux, which will subsequently influence recycling processes at and within the seafloor. In this study we assess the predominant controls on the early diagenetic cycling of Si, a key nutrient in marine ecosystems, by combining stable isotopic analysis (δ30Si) of pore water DSi and of operationally defined reactive pools of the solid phase. We show that low biogenic silica (BSi) contents (0.26–0.52 wt% or 92–185 μmol g dry wt−1) drive correspondingly low asymptotic concentrations of pore water DSi of ∼100 μM, relative to biosiliceous sediments (>20 wt% BSi) wherein DSi can reach ∼900 μM. While Barents Sea surface sediments appear almost devoid of BSi, we present evidence for the rapid recycling of bloom derived BSi that generates striking transient peaks in sediment pore water [DSi] of up to 300 μM, which is a feature that is subject to future shifts in phytoplankton community compositions. Using a simple isotopic mass balance calculation we show that at two of three stations the pore water DSi pool at 0.5 cm below the seafloor (+0.96 to +1.36 ‰) is sourced from the mixing of core top waters (+1.46 to +1.69 ‰) with the dissolution of BSi (+0.82 to +1.50 ‰), supplemented with a lithogenic Si source (LSi) (−0.89 ±0.16‰). Further, our sediment pore water δ30Si profiles uncover a coupling of the Si cycle with the redox cycling of metal oxides associated with isotopically light Si (−2.88 ±0.17‰). We suggest that a high LSi:BSi ratio and apparent metal oxide influence could lead to a degree of stability in the annual background benthic flux of DSi, despite current pressures on pelagic phytoplankton communities. Coupled with supporting isotopic evidence for the precipitation of authigenic clays in Barents Sea sediment cores, our observations have implications for the regional Si budget.

A silicon isotopic perspective on the contribution of diagenesis to the sedimentary silicon budget in the Southern Ocean

Diatoms are known to fractionate silicon isotopes during the formation of their frustules causing the silicon isotopic composition of biogenic silica to track the degree of silicic acid consumption in surface waters. Despite a growing body of work that uses this proxy to reconstruct past changes in silicic acid utilization, the understanding of the benthic silicon cycle, particularly the identification and quantification of the processes that potentially alter the silicon isotopic composition of biogenic silica during early diagenesis is still lacking. We investigated these processes by comparing the silicon isotopic composition of pore water silicic acid, biogenic silica and, for the first time, lithogenic silica from five sediment cores collected in the deep basin of the Southern Ocean representing a diversity of sedimentation regimes. Silicic acid concentrations and the isotopic composition of Southern Ocean pore waters were the result of a dynamic balance between the dissolution of biogenic silica, reactive lithogenic silica phases and Si re-precipitation with the relative importance of each processes differing significantly between regions. The results are consistent with the formation of authigenic alumino-silicates derived from dissolved biogenic silica in the Sub-Antarctic Zone and in the Antarctic Zone (on average 12 ± 5% and 17 ± 13%, respectively). Since this latter process can fractionate silicon isotopes, this implies that, even if the silicon isotopic composition of diatoms preserved in the sediments is a reliable proxy for silicic acid utilization in the past ocean, care must be taken to extract a clean biogenic silica phase free of authigenic clays and lithogenic phases from sediments to eliminate this potential bias when interpreting isotopic records.

Effects of ground bamboo application on weed suppression and rice production: a 3-year paddy field experiment

BackgroundIn light of the dramatic expansion of Japan’s bamboo forests, it is necessary to develop a strategy for the effective use of bamboo biomass resources. In this study, we tested the effects of ground bamboo as an organic mulching material or soil conditioner during a 3-year, agrochemical-free rice cultivation period.MethodsWe performed field experiments in 16 experimental paddy fields and established five treatments with three or four replicates each: control, manual weeding, low-volume ground bamboo application (0.5 kg m−2), medium-volume ground bamboo application (1.0 kg m−2), and high-volume ground bamboo application (2.0 kg m−2).ResultsWe observed no suppression of aquatic weeds with the ground bamboo treatments. Nevertheless, in the first year, rice yields were 1.7–1.8 times greater in the medium- and high-volume ground bamboo treatments than in the controls. In the second and third years, rice yields did not differ among treatments. During the 3-year period, mean rice yields dropped dramatically to around 20%. Simple linear regression analyses indicated that rice yields were positively associated with available phosphate, and negatively associated with the silicic acid content of post-experiment paddy soils after the second and third years of cultivation. Multiple linear regression analyses indicated that available phosphate and silicic acid were important variables explaining rice yields. Application of ground bamboo did not appear to reduce external rice grain quality.ConclusionsApplication of ground bamboo may enhance the production of high-quality rice, particularly when soil phosphorus is not deficient.

Effects of Silicic Acid on Leaching Behavior of Arsenic from Spent Magnesium-Based Adsorbents Containing Arsenite

The spent adsorbents left after treating arsenic-contaminated water contain large amounts of arsenic. These spent adsorbents may come into contact with silicic acid leached from soil or cementitious solidification materials in the disposal environment. Thus, it is important to evaluate the effects of silicic acid on spent adsorbents containing arsenic. In this study, the effects of silicic acid on spent Mg-based adsorbents (magnesium oxide (MgO) and magnesium hydroxide (Mg(OH)2)) containing arsenite were investigated. The arsenic leaching ratios of both spent adsorbents decreased slightly with an increase in the initial silicic acid concentration of the eluent. The arsenic leaching ratio decreased from 1.24% to 0.69% for MgO and from 5.97% to 4.71% for Mg(OH)2 at an initial Si-normalized concentration of 100 mg/L. The primary mechanism behind the inhibition of arsenic leaching by silicic acid was determined to be the difficulty of arsenic desorption due to the coating effect following the adsorption of silicic acid species. The results indicate that the arsenic leaching related to the ion exchange reaction with silicic acid hardly occurred for the spent Mg-based adsorbents. Compared with various spent Mg-based and Ca-based adsorbents, the spent MgO adsorbent exhibited the highest environmental stability and best performance.