Dissolved Silica Concentrations Research Articles

Geochemistry of urban waters and their evolution within the urban landscape

Urban populations and the sprawl of urban environments are increasing in the United States as well as globally. The local hydrologic cycle is directly impacted by urban development through greater generation of surface runoff and export of water through subterranean pipes networks to surface water bodies. These pipe networks carry waters that have potentially dramatic effects on the chemistry of groundwater and surface water bodies. In this work, we sampled waters from the Olentangy River and two subterranean outfalls that flow into the river in Columbus, Ohio United States. We measured the major ion, nutrient, and dissolved silica concentrations of each water source to identify how the urban landscape impacts the chemistry of a river that travels from an agricultural landscape to an urban environment. The outfalls had elevated concentrations of all major ions (Na+, K+, Mg2+, Ca2+, Cl−, SO42-) and H4SiO4. However, the Olentangy river typically had greater NO3− and soluble reactive phosphorus (SRP) concentrations. Sources of elevated ion export include road salts and combined storm runoff (Na+, Cl−), municipal water treatment practices (K+, Na+, SO42-), and concrete pipe weathering (Ca2+, Mg2+, K+, H4SiO4, SO42-). Utilizing stable isotopes of water, δ18O and δ2H, we identified that the water in the pipe networks is typically a mix of multiple precipitation events, but there is evidence of flushing following high-volume precipitation events. The contribution of high TDS waters from subterranean urban outfalls modified the ion abundance in the Olentangy river and produces a tendency towards freshwater salinization syndrome. This is particularly apparent when comparing the chemistry of the urban Olentangy to the agricultural corridor of the river and its other source waters. This research details the transformation of a river as it flows from an agricultural to urban landscape and provides data on the chemistry of source waters that facilitate the river’s chemical changes.

Climate, Hydrology, and Nutrients Control the Seasonality of Si Concentrations in Rivers

AbstractThe seasonal behavior of fluvial dissolved silica (DSi) concentrations, termed DSi regime, mediates the timing of DSi delivery to downstream waters and thus governs river biogeochemical function and aquatic community condition. Previous work identified five distinct DSi regimes across rivers spanning the Northern Hemisphere, with many rivers exhibiting multiple DSi regimes over time. Several potential drivers of DSi regime behavior have been identified at small scales, including climate, land cover, and lithology, and yet the large‐scale spatiotemporal controls on DSi regimes have not been identified. We evaluate the role of environmental variables on the behavior of DSi regimes in nearly 200 rivers across the Northern Hemisphere using random forest models. Our models aim to elucidate the controls that give rise to (a) average DSi regime behavior, (b) interannual variability in DSi regime behavior (i.e., Annual DSi regime), and (c) controls on DSi regime shape (i.e., minimum and maximum DSi concentrations). Average DSi regime behavior across the period of record was classified accurately 59% of the time, whereas Annual DSi regime behavior was classified accurately 80% of the time. Climate and primary productivity variables were important in predicting Average DSi regime behavior, whereas climate and hydrologic variables were important in predicting Annual DSi regime behavior. Median nitrogen and phosphorus concentrations were important drivers of minimum and maximum DSi concentrations, indicating that these macronutrients may be important for seasonal DSi drawdown and rebound. Our findings demonstrate that fluctuations in climate, hydrology, and nutrient availability of rivers shape the temporal availability of fluvial DSi.

Comparison of nutrients status in Liaodong Bay and Northern Yellow Sea, China: Controlling factors and nutrient budgets

Under the dual stress of global warming and human interaction, Liaodong Bay (LDB) and northern Yellow Sea (NYS) are undergoing significant ecological changes. Little is known about the driving nutrients characteristics supporting fishery resource output in these areas. We carried out three field observations in 2019 to investigate nutrient status. Results showed that dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved silica (DSi) concentrations changed seasonally, with lowest values in spring, and highest values in autumn. High DIN, DIP, and DSi concentrations were detected in LDB and NYS's estuary areas. The Yellow Sea Cold Water Mass plays a role in the distribution and seasonal variation of nutrients. Exchanges across the sediment-water interface, SFGD, atmospheric deposition, and the adjacent sea input dominated DIN dynamics of these areas. DIP primarily came from the adjacent sea input and DSi mainly originated from sediment release and the adjacent sea input. NYS seawater invasion accounted for 13.8% of DIN, 63.4% of DIP, and 35.1% of DSi in LDB. These results provide new insights to better facilitate the formulation of nitrogen and phosphorus reduction and control policies in these marginal seas.

Dissolved silica‐driven dolomite precipitation in the Great Salt Lake, Utah, and its implication for dolomite formation environments

AbstractSince the discovery of dolomite, numerous attempts have been made to understand its precipitation mechanism at Earth's surface conditions. One such mechanism relies on a relationship with microbial life, where laboratory synthesis experiments have shown that specific organic molecules, such as polysaccharides, exopolymeric substances and hydrogen sulphide can promote dolomite precipitation. Other mechanisms for precipitating dolomite focus on abiotic chemical environments, such as adding dissolved silica, which lower the dehydration energy barrier for the surface Mg2+‐water complex and promote disordered dolomite precipitation. Modern occurrences of dolomite in the Great Salt Lake, Utah, have been studied since the early 20th Century. The distribution of primary dolomite in the Great Salt Lake is spatially heterogeneous, with only the carbonate mud in the South Arm and ridge‐site between desiccation cracks in the North Arm being dominated by dolomite and calcite, while stromatolites in both Arms and ooidal sands in the North Arm are composed entirely of aragonite. It was proposed that dolomite precipitation in the Great Salt Lake was possibly induced by microbial activities such as organic degradation, bacteria sulphate reduction, or other microbial metabolic by‐products. However, these hypotheses could not explain the lack of dolomite in microbial mats, especially in the North Arm, which is constituted by mostly aragonite with no dolomite. Our results suggest that dissolved silica concentration is the primary control for dolomite and Mg‐clay formation in the Great Salt Lake. Even though the North Arm has a much more concentrated Mg and Ca water from lack of freshwater input, dissolved silica levels in the South Arm (>0.5 mm) and the Ridge‐site (ca 0.5 mm) are much higher than in the North Arm (<0.2 mm). Our finding could also provide a new proxy for reconstructing climate changes in the Great Salt Lake area based on dolomite content variation. Phanerozoic dolomite abundance variations may be linked to global CO2 level that facilitates global chemical weathering and dissolved silica input into palaeo‐ocean.

Spatio-temporal variation of major ion chemistry and nutrient stoichiometry in a tropical monsoonal estuary: insight into biogeochemical processes

Major ions and nutrient chemistry of surface water from the Devi river estuary, eastern India has been studied in order to understand the governing biogeochemical processes. Concentration of Na, K, Ca, Mg, Cl, SO4 and PO4 increases toward higher salinity; however, NO3 and dissolved silica (DSi) decreases from upper to lower estuary. All major ions, except NO3 and DSi, show the highest and lowest concentration in summer and monsoon season, respectively. Increased surface runoff has resulted in the highest concentration of NO3 and DSi in the monsoon period. The Na, Cl, SO4, NO3 and PO4 ions show conservative mixing behavior in the estuary throughout the year. Non-conservative behavior of DSi is attributed to the combined process of mineral weathering and biological removal by diatoms. Enrichment and depletion of Ca and Mg in the estuary is owed to carbonate mineral precipitation and dissolution processes. Decreasing carbonate saturation indices (SI) with reducing pH accompanied by lowering of DO% suggests the influence of organic matter degradation on the carbonate precipitation and/or dissolution potential. Nutrient stoichiometric ratios—NO3:PO4, DSi:PO4 and DSi:NO3—are found to vary spatially and seasonally as a result of differences in geochemical processes prevailing in the fresh- and marine-water environment. A prominent PO4 limitation is observed in the upper estuary during summer and winter. However, this limitation reduces toward the lower estuary owing to precipitation of Fe-sulfide under the influence of higher salinity. In contrast, the influx of PO4-limited freshwater into the estuary controls the nutrient stoichiometric ratios in the monsoon season. The spatial variation of chlorophyll concentration in the estuary is in accordance to the nutrient stoichiometric ratios. Thus, this study emphasizes on the role of unique biogeochemical processes in controlling the spatial and seasonal distribution of major ions and nutrient stoichiometry in a tropical estuarine system.

Understanding smectite to illite transformation at elevated (>100 °C) temperature: Effects of liquid/solid ratio, interlayer cation, solution chemistry and reaction time

Bentonite (mainly smectite) has been proposed as a buffer material in a nuclear waste repository, due to its low permeability, high swelling capability, and high ion exchange capacity. To evaluate the isolation performance of the material, it is important to understand a possible hydrothermal alteration of smectite to illite. We investigated the hydrothermal alteration of smectite under various physical and chemical conditions to identify key controlling factors of the process. The experiments were performed on <2 μm fractions of Na-rich or K-exchanged smectite in DI water or 1 M KCl solution at 200 °C for various liquid/solid ratios over several time periods up to 112 days. Multiple analytical techniques were used to characterize the solid and liquid products to determine the extent of the conversion and the related mineral structural changes. Our results suggest that the conversion could be relatively fast (i.e., weeks) relative to waste disposal isolation at the optimal conditions (high K+ concentration and high liquid/solid ratio), which is much faster than previously postulated for the temperature used here. A high K+ concentration in the solution is required to trigger the structural transition of a low charge smectite layer to a high charge illite layer. This transition appears to be continuous, thus contradicting the dissolution-precipitation mechanism for smectite to illite transformation. Based on the solution chemistry measurements, with respect to silica solubility, the equilibrium constant for the transformation of K-exchanged SWy-2 smectite to illite is estimated. In addition, the overall conversion is limited, to a large extent, by the dissolved silica concentration in the solution. Given a generally low potassium content, a very low liquid/solid ratio, and an extremely slow diffusion in an actual engineered barrier system, the extent of smectite conversion to illite in a repository environment, if happening at all, would be limited. The conversion can be further inhibited by introducing brucite and amorphous silica as chemical additives to the buffer material.

Role of Marginal Seas in Deep Ocean Regeneration of Dissolved Silica: A Case Study in the Marginal Seas of the Western Pacific

Deep ocean regeneration of dissolved silica (DSi) is an essential part of the ocean silica cycle and is driven by a complex series of biogeochemical processes. Here we compare the distributions of DSi and other environmental parameters in several western Pacific marginal seas to explore the role of marginal seas in deep ocean DSi regeneration. Results show that in oligotrophic marginal seas (such as the South China Sea), the DSi content in deep waters is similar to that of the adjacent Pacific waters. However, in productive marginal seas (such as the Bering Sea), the DSi content in deep waters is markedly higher than that in adjacent Pacific waters at the same depths. This is mainly due to deep ocean DSi regeneration in the marginal sea basin, which is fueled by the high biogenic particle flux from the productive surface waters. On a global scale, deep ocean DSi regeneration is accelerated in productive marginal seas, causing marginal seas such as the Bering Sea to have the highest DSi concentrations of all global waters.

In-situ Raman spectroscopic analysis of dissolved silica structures in Na2CO3 and NaOH solutions at high pressure and temperature

The dissolved silica structures in quartz-saturated 0.50 and 1.50 m [mol kg H2O–1] Na2CO3 and 0.47 m NaOH solutions at up to 750 °C and 1.5 GPa were investigated by in-situ Raman spectroscopy using a Bassett-type hydrothermal diamond anvil cell. The solubility of quartz in the solutions was determined by in-situ observations of the complete dissolution of the grain. The Raman spectra of the quartz-saturated Na2CO3 and NaOH solutions at high pressures and temperatures exhibited the tetrahedral symmetric stretching band of silica monomers. The lower frequency and broader width of the band than those in pure H2O indicated the presence of both neutral and deprotonated monomers. In addition, we newly confirmed the intense bridging oxygen band and the tetrahedral symmetric stretching band of Q1 (silicate center having a single bridging oxygen atom) in the spectra of the Na2CO3 solutions. The integrated intensity ratios of the bridging oxygen band to the monomer band increased with the addition of Na2CO3 and NaOH to fluids, corresponding to an elevation of the measured quartz solubilities. These observations indicate that the formation of silica oligomers in addition to neutral and deprotonated monomers explains the high dissolved silica concentrations in the solutions. The presence of deprotonated monomers under the experimental conditions suggests that deprotonated oligomers exist in the solutions, because the production of the latter more significantly reduces the Gibbs free energy. The anionic silica species and oligomers formed in alkaline silicate fluids may act as effective ligands for certain metal ions or complexes in deep subduction zones.

Reservoirs change pCO2 and water quality of downstream rivers: Evidence from three reservoirs in the Seine Basin

The global increase in the construction of reservoirs has drawn attention given its documented hydrological and biogeochemical impacts on downstream rivers; however, the impact of reservoirs on downstream pCO2 (partial pressure of carbon dioxide) is still poorly understood. To evaluate these impacts, the interactions between reservoirs and their corresponding upstream and downstream rivers were analyzed for three reservoirs in the Seine Basin based on monthly measurement during two hydrological years. The seasonal variations of water quality in the reservoirs were mainly driven by the entering water and the biogeochemical processes occurring in the reservoirs. Our results unravel the crucial role of reservoir in downstream water quality, which significantly increased DOC (dissolved organic carbon) and BDOC (biodegradable DOC) concentrations, while lowered DSi (dissolved silica) concentrations during emptying period (p < 0.01). Furthermore, the impacts of reservoirs on the annual fluxes of DOC, BDOC, and DSi were quantified and suggested that the three reservoirs respectively increased 20% and 23% of annual fluxes of DOC and BDOC, while decreased 33% of annual DSi fluxes in their downstream rivers. Additionally, the reservoirs significantly decreased downstream riverine pCO2 (p < 0.01), and enhanced the gas transfer coefficient of CO2 in downstream rivers by 1.3 times during the emptying period, which highlights the necessity to consider the potential impact of reservoirs on riverine CO2 emissions. Overall, our results highlight the importance of combining biogeochemical and hydrological characteristics to understand the impacts of reservoirs on downstream rivers, and emphasize the need of similar studies under the current context of increasing reservoir constructions.

Nature of Mineralizing Fluid in Paleoproterozoic Dolomite Hosted Talc Deposits, Cuddapah Basin, India

Abstract Dolomite hosted talc deposits of the Vempalle Formation (VF), Cuddapah basin, Andhra Pradesh produces nearly 6% of total talc demand of India. The strata bound talc zones extending over kilometers along strike have tabular geometry and are concordant with the stratification and primary depositional contacts, though sometimes the talc bodies locally cross-cut the stratification. Field and petrographic evidences suggest that the strata-bound talc deposit, located at the flank of the basin, was formed by replacement of platformal carbonate sequences of the passive margin (stromatolitic dolomite), preserving the primary texture and structure of the host. Lack of anomalous concentrations of V, Fe, Ni, Cr and Co; depletion of Ta, Hf, Zr and Th and Ni, Cu, Pb, Zn in the talc and talc bearing dolomites may be indicative of the absence of magmatic and/or metamorphic source for the mineralization fluid. The igneous intrusions within the Vempalle Formation (VF) – earlier thought to be the source of mineralization fluid – thus are neither coeval with talc mineralization nor they acted as the heat source. Furthermore, depletion of REE, Al3+, K+, Na and Sr in the talc and talc bearing dolomites suggest that the mineralizing fluid was probably evolved from Paleoproterozoic ocean water that was super-saturated with respect to talc and had higher dissolved silica concentration. This evolved fluid source can also explain the supply and sustenance of large volume of water needed for the talcification of the host dolomite.

Unveiling the elusive role of tetraethyl orthosilicate hydrolysis in ionic-liquid-templated zeolite synthesis

Despite previous reports showing that crystallization kinetics affects zeolite phase selectivity because zeolites are metastable species in their synthesis solution rather than thermodynamic end points, the critical kinetics-controlling parameter is yet to be determined. This work elucidated the effect of tetraethyl orthosilicate (TEOS) hydrolysis before hydrothermal treatment on the final zeolite phase selectivity in the ionic liquid-templated synthesis of 10-membered ring zeolites (MFI- or TON-type zeolites). The results showed that the dissolved silica concentration in the synthesis solution, which is controlled by varying the TEOS hydrolysis temperature and addition rate, induced heterogeneous nuclear growth. Specifically, in 1-butyl-3-methylimidazolium ([BMIM]Br)-directed syntheses, the high and low concentrations of dissolved silica species led to MFI and TON zeolite formation, respectively. The experimental results are supported by silica polymerization modeling using the Reaction Ensemble Monte Carlo method and theoretical calculations on composite building unit formation. The results are valuable for understanding the nucleation mechanism in zeolite crystallization.

Limnological changes in Lake Victoria since the mid‐20th century

Abstract Lake Victoria experienced a strong degradation of water quality between the 1960s and the 1990s and, as a consequence of eutrophication, the dominant phytoplankton group changed from diatoms to N2‐fixing cyanobacteria and there was a 2‐ to 10‐fold increase in chlorophyll‐a. The goal of this study is to determine whether the 2018–2019 physical (light, stratification) and ecological (nutrient, chlorophyll‐a, phytoplankton composition) conditions in Lake Victoria changed from the 1990s. Samples were collected in 2018–2019 in nearshore and offshore waters (Uganda), during three contrasting seasons: heavy rains (March), low rains (October), and dry (June), which corresponded to distinct water column mixing regimes, respectively, late‐stratified, early‐stratified, and mixed regimes. At each station (48 nearshore and 25 offshore), we measured vertical profiles of temperature, oxygen, phytoplankton biomass and composition, inorganic nutrients, and particulate organic carbon, particulate nitrogen (N), and phosphorus (P). Chlorophyll‐a concentrations in 2018–2019 were 10.3 ± 7.1 and 2.8 ± 1.1 µg/L in the nearshore and offshore surface waters, respectively, close to those measured in the 1960s before eutrophication, but distinctly lower than those measured in the 1990s (71 ± 100 and 14 ± 6 µg/L). The phytoplankton of Lake Victoria in 2018–2019 still appears dominated by diatoms and cyanobacteria. However, we observed more non‐heterocystous filamentous and coccal/colonial cyanobacteria taxa that are better adapted to mixing conditions than gas‐vacuolated heterocystous taxa, which were dominant in the 1990s. Particulate N was significantly lower in 2018–2019 than in the 1990s, indicative of less efficient N fixation. The dissolved silica concentrations in 2018–2019 were significantly higher with the concomitant reappearance of Aulacoseira spp., which was not observed in the 1990s, presumably due to low dissolved silica concentrations. As data from long‐term monitoring are absent, the reasons for the lower chlorophyll‐a concentrations in 2018–2019 compared to the 1990s are unclear. However, climatic controls (El Niño/La Niña conditions) may be an important factor influencing the historical trend in chlorophyll‐a. Higher wind in 2018–2019 promoted vertical mixing, resulting in a deeper thermocline and surface mixed layers, which eventually lowered phytoplankton production in comparison to the 1990s. In contrast, the thermocline and surface mixed layers in the 1990s were shallower, enabling phytoplankton to stay suspended in the upper well illuminated water, allowing greater productivity. The lake in 2018–2019 is still P saturated, suggesting that another episode of high chlorophyll‐a concentrations could develop if less windy conditions occur in future, or if continued warming of surface waters eventually overcomes the mixing from present windy conditions. This study gives insights about the present ecological functioning of Lake Victoria and emphasises the impacts of variations in climate on lake physics that changes the light environment for phytoplankton. A possible less windy period in the future resulting from a new El Niño phase or from climate change, will probably lead to another episode of eutrophication in Lake Victoria. As in 2018–2019 the lake was still saturated by nutrients, there is need to reduce the nutrient concentrations (especially P) to prevent future destructive eutrophic periods caused by reduced mixing.

Isotopic analyses of Ordovician–Silurian siliceous skeletons indicate silica‐depleted Paleozoic oceans

The Phanerozoic Eon marked a major transition from marine silica deposition exclusively via abiotic pathways to a system dominated by biogenic silica sedimentation. For decades, prevailing ideas predicted this abiotic-to-biogenic transition were marked by a significant decrease in the concentration of dissolved silica in seawater; however, due to the lower perceived abundance and uptake affinity of sponges and radiolarians relative to diatoms, marine dissolved silica is thought to have remained elevated above modern values until the Cenozoic radiation of diatoms. Studies of modern marine silica biomineralizers demonstrated that the Si isotope ratios (δ30 Si) of sponge spicules and planktonic silica biominerals produced by diatoms or radiolarians can be applied as quantitative proxies for past seawater dissolved silica concentrations due to differences in Si isotope fractionations among these organisms. We undertook 446 ion microprobe analyses of δ30 Si and δ18 O of sponge spicules and radiolarians from Ordovician-Silurian chert deposits of the Mount Hare Formation in Yukon, Canada. These isotopic data showed that sponges living in marine slope and basinal environments displayed small Si isotope fractionations relative to coeval radiolarians. By constructing a mathematical model of the major fluxes and reservoirs in the marine silica cycle and the physiology of silica biomineralization, we found that the concentration of dissolved silica in seawater was less than ~150μM during early Paleozoic time-a value that is significantly lower than previous estimates. We posit that the topology of the early Paleozoic marine silica cycle resembled that of modern oceans much more closely than previously assumed.

Spatiotemporal dynamics of coupled dissolved silica and bicarbonate in a dam-induced urban lake system in the Three Gorges Reservoir Area

Dissolved silica (DSi) and bicarbonate (HCO3–) stoichiometric coupling is collectively regulated by chemical weathering and algal activity, and potentially indicates the aquatic silicon retention and carbon emission in aquatic systems. Damming markedly remodels the hydrological conditions and aquatic biological activity, however, its effects on the spatiotemporal dynamics of coupled DSi and HCO3– remain poorly understood. Here we deciphered a 2-year hydrological and water quality dataset collected at multiple sites within an urban lake, namely Hanfeng Lake, the largest pre-dam of the Three Gorges Reservoir, to unravel the spatiotemporal dynamics of DSi and HCO3– and selected hydrological and water quality variables using cluster analysis, and to explore the best predictors for DSi:HCO3– using partial least squares regression. The changes of DSi, HCO3– and selected water variables could be characterized by spatial east and west zones, and further by temporal periods 1, 2 and 3 corresponding respectively to fluctuation stage, fluvial stage and lacustrine stage in each zone based on the hydrographic features of Hanfeng Lake. Wet season presented higher flow velocity, permanganate index, concentrations of DSi, HCO3–, nitrate-nitrogen and total suspended solids, and lower water level, secchi depth and chlorophyll-a, but equal DSi:HCO3– relative to dry season. DSi was overall negatively correlated with HCO3–, highlighting the dominant role of chemical weathering rather algal activity. DSi:HCO3– exhibited significant relationships with various variables, but was the best predictable using water level, permanganate index and nitrogen-based variables, suggesting the strong effects of water level fluctuation and pollution discharge on the coupling of silicon and carbon. Our results have great significance for understanding and modeling the biogeochemical cycles of silicon and carbon in the dam-induced lake ecosystems worldwide.

Interpreting Groundwater Chemistry and Estimating Residence Time of Groundwater in the Mt. Fuji Area Based on a Dissolution Kinetics-Fluid Flow Model and Chlorofluorocarbons (CFCs) Concentration

Relationships of concentrations of elements (Ca, Mg, Si) in groundwater from the Mt. Fuji area are obtained using analytical data on groundwater. Relationships of Ca, Mg, and H4SiO4 concentrations can be explained in terms of the dissolution of basaltic glass and precipitations of secondary minerals (halloysite, allophane, montmorillonite, and α-cristobalite). The relationship between residence time and H4SiO4 concentration of groundwater is obtained based on dissolution kinetics-piston fluid flow and a perfectly mixing fluid flow model. Using this relationship, the dissolution rate constant experimentally determined in an open system (not in a closed system) (k = 10−10.7 molSi/m2 s) and analytical data on H4SiO4 in groundwater, the residence time of groundwater (Kakitagawa site, southeastern foot and Fuji Yoshida site, northern foot) is estimated to be ca. 20 years for A/M (A: surface area of rock (m2), M: mass of water (kg)) = 20 based on a dissolution kinetics-fluid flow model. This estimated residence time is roughly consistent with other methods (He isotope tritium, chlorofluorocarbons (CFCs) ages (10-40 years), indicating that the dissolution kinetics-fluid flow model using dissolved silica concentration is a potentially useful method for estimating the residence time of groundwater in a young volcanic region. It is also reported that CFCs ages are ca. 30-40 years, which are the first data obtained for the Mt. Fuji area. This suggests that CFCs data are very useful for providing the residence time of groundwater in an area at a high elevation.

Experimental evidence supports early silica cementation of the Ediacara Biota

Abstract Casts and molds of soft-bodied organisms in Ediacaran sandstones (“Ediacara-style” fossilization) have played an important role in reconstruction of the emergence and radiation of early complex macroscopic life. However, the preservational processes responsible for the Ediacara fossil record are still vigorously debated. Whereas classic studies proposed fossilization via rapid sulfide mineralization of carcass and matground surfaces, a more recent view posits silica as the key mineral involved in their preservation. We performed experiments in which a variety of soft-bodied organisms were exposed to silica-rich solutions at concentrations considered characteristic of Ediacaran seawater (2 mM). Our results document continuous precipitation of amorphous silica onto the surfaces of these organic tissues under constant and normal marine pH values (7.8). Mineral formation was accompanied by a progressive decrease in the dissolved silica (DSi) concentration of the experimental solution to levels well below amorphous silica saturation. Additionally, we find that the magnitude of silica precipitation is correlated to each organism’s functional-group chemistry, as measured by potentiometric acid-base titrations. We suggest that a wide range of soft-bodied organisms were prone to silicification in Ediacaran marine environments characterized by anactualistically high DSi concentrations. This provides further support for the model that the extraordinary moldic preservation of the Ediacara Biota was promoted by early silica cementation and that this mode of preservation can offer an accurate glimpse into the composition of those early animal ecosystems.

Spatial and temporal variations of dissolved silicon isotope compositions in a large dammed river system

The dissolved silicon isotope composition (δ30Si) of river water serves as a key parameter in tracing the terrestrial silicon cycle. However, the lack of long-term observations of rivers often makes it difficult to constrain the isotopic features of riverine dissolved silica (DSi) well. In this study, samples were collected during four basin-scale surveys and monthly monitoring at the mouth of Changjiang over the course of two years, during the full operation period of the Three Gorges Dam (TGD), the largest hydropower project in the world, and the DSi concentration and δ30Si were measured. The DSi concentration and δ30Si ranged from 91.3 μM to 124.7 μM and 1.00‰ to 1.86‰ for the main stream of Changjiang, and from 58.4 μM to 194.0 μM and 0.83‰ to 2.27‰ for the tributaries/lakes, respectively. The mid-lower tributaries/lakes possessed mean DSi concentrations comparable to those of the upstream tributaries but higher δ30Si. δ30Si increased by ~0.7‰ in the main stream, accompanied by a decrease in DSi concentration and elemental ratio of dissolved aluminum to Si (Al/Si) downstream from the TGD when the mid-lower tributaries/lakes were in the dry period, suggesting co-influence of enhanced clay formation, productivity of freshwater diatoms as well as influx of 30Si-enriched tributaries/lakes in controlling the evolution of δ30Si in the mainstream. In contrast, little longitudinal variation was observed when the mid-lower stream was in the flooding period, and δ30Si discharged by Changjiang to the estuarine region was mainly controlled by the mixing between the main stream and the mid-lower tributaries/lakes. Groundwater could be relevant source of DSi in the main stream of Changjiang, as indicated by water mass balance calculation, while its impact on δ30Si in the main stream was insignificant. Based on the long-term monitoring at the river mouth, the DSi-flux weighted δ30Si value of Changjiang was re-estimated to be 1.4‰, which represented the best estimation of the riverine δ30Si discharged into the coastal waters by Changjiang at this stage. The temporal variation in δ30Si observed at the river mouth was affected by the annual flow cycles of the upper and mid-lower streams of Changjiang superimposed by seasonal aquatic diatom productivity. Combination with the historical DSi concentrations in the Changjiang end-member suggested that the Three Gorges Reservoir does not play a significant role in direct retaining DSi at this stage, while the severe sediment trapping within the reservoir may cause potential alteration in the silicate weathering and freshwater diatoms productivity in the mid-lower reaches.

A coupled carbon-silicon cycle model over Earth history: Reverse weathering as a possible explanation of a warm mid-Proterozoic climate

The balance between carbon outgassing and carbon burial controls Earth's climate on geological timescales. Carbon removal in carbonates consumes both atmospheric carbon and ocean carbonate alkalinity sourced from silicate weathering on the land or seafloor. Reverse weathering (RW) refers to clay-forming reactions that consume alkalinity but not carbon. If the cations (of alkalinity) end up in clay minerals rather than in the carbonates, carbon remains as atmospheric CO2, warming the climate. Higher silicate weathering fluxes and warmer temperatures are then required to balance the carbon cycle. It has been proposed that high dissolved silica concentrations resulting from the absence of ecologically significant biogenic silica precipitation in the Precambrian drove larger RW fluxes than today, affecting the climate. Here, we present the first fully coupled carbon-silica cycle model for post-Hadean Earth history that models climate evolution self-consistently (available as open source code). RW fluxes and biogenic silica deposition fluxes are represented using a sediment diagenesis model that can reproduce modern conditions. We show that a broad range of climate evolutions are possible but most plausible scenarios produce Proterozoic warming (+5 K relative to without RW), which could help explain the sustained warmth of the Proterozoic despite lower insolation. RW in the Archean is potentially more muted due to a lower land fraction and sedimentation rate. Key model uncertainties are the modern reverse weathering flux, the rate coefficient for RW reactions, and the solubility of authigenic clays. Consequently, within the large uncertainties, other self-consistent scenarios where Proterozoic RW was unimportant cannot be excluded. Progress requires better constraints on parameters governing RW reaction rates including explicit consideration of cation-limitations to Precambrian RW, and perhaps new inferences from Si or Li isotopes systems.

Technical note: The silicon isotopic composition of choanoflagellates: implications for a mechanistic understanding of isotopic fractionation during biosilicification

Abstract. The marine silicon cycle is intrinsically linked with carbon cycling in the oceans via biological production of silica by a wide range of organisms. The stable silicon isotopic composition (denoted by δ30Si) of siliceous microfossils extracted from sediment cores can be used as an archive of past oceanic silicon cycling. However, the silicon isotopic composition of biogenic silica has only been measured in diatoms, sponges and radiolarians, and isotopic fractionation relative to seawater is entirely unknown for many other silicifiers. Furthermore, the biochemical pathways and mechanisms that determine isotopic fractionation during biosilicification remain poorly understood. Here, we present the first measurements of the silicon isotopic fractionation during biosilicification by loricate choanoflagellates, a group of protists closely related to animals. We cultured two species of choanoflagellates, Diaphanoeca grandis and Stephanoeca diplocostata, which showed consistently greater isotopic fractionation (approximately −5 ‰ to −7 ‰) than cultured diatoms (−0.5 ‰ to −2.1 ‰). Instead, choanoflagellate silicon isotopic fractionation appears to be more similar to sponges grown under similar dissolved silica concentrations. Our results highlight that there is a taxonomic component to silicon isotope fractionation during biosilicification, possibly via a shared or related biochemical transport pathway. These findings have implications for the use of biogenic silica δ30Si produced by different silicifiers as proxies for past oceanic change.

Silica fouling during groundwater RO treatment: The effect of colloids’ radius of curvature on dissolution and polymerisation

Silica fouling during groundwater reverse osmosis (RO) treatment can have a significant impact on filtration performance. To better understand this phenomenon, the equilibrium kinetics of amorphous colloidal silica were studied at conditions relevant to RO of silica-rich alkaline groundwater. The impact of particle size was investigated using synthetic monodisperse silica nanoparticles. Bench scale experiments were conducted by monitoring dissolved silica concentration of aqueous suspensions of colloids of 100 and 300 nm diameter and pH 8.5 to 9.5. The equilibrium data was determined from existing established rate law equations. This study concluded that surface energy has a major impact on silica dissolution rate constant, particularly for colloidal silica. Observations of Ostwald ripening in bidisperse silica dispersions further confirmed these results, which indicate that dissolution and redeposition is responsible for the problematic silica fouling behaviour during RO treatment. 2D modelling based on inferred equilibrium data allows visualization of scale layer growth in agreement with cross-sectional scanning electron micrographs of autopsied membranes.