Si Isotope Fractionation Factor Research Articles

Interpreting silicon isotopes in the Critical Zone

Metal and metalloid stable isotope ratios have emerged as potentially powerful proxies for weathering, element cycling and export in the Critical Zone. The simplest possible interpretative framework for these isotope ratios has three parameters: (i) the isotope ratio of the parent minerals undergoing weathering, (ii) the partitioning of the element between solute and the new secondary phases, and (iii) the fractionation factors associated with the formation of new secondary phases. Using the example of silicon, we show how all three of these parameters vary along a gradient of erosion rate and regolith residence time defined by three sites located on granitoid bedrock. These sites run from the kinetically limited Rhone Valley in the Central Swiss Alps to the tectonically inactive and supply-limited Sri Lankan highlands, with the Sierra Nevada mountains as a site of intermediate weathering intensity. At each site, primary mineral specific ³⁰Si/²⁸Si ratios span >0.4‰. These minerals weather differentially, such that the isotope ratio of silicon solubilised from rock differs at the three sites and is not necessarily equal to bulk bedrock composition. The partitioning of silicon between secondary clay and solute is reflected in the clay mineralogy and chemical composition: more intense weathering produces Si-poor clays. The clay composition thus comprises a first-order mass-balance control on the extent to which any fractionation factor can be expressed. Finally, the Si isotope fractionation factor associated with clay formation varies systematically with clay mineralogy: the formation of Si-deplete clay minerals is associated with larger fractionation factors. The magnitude of the fractionation may be mechanistically linked to relative aluminium availability. These findings provide the framework needed to use Si isotope ratios as a quantitative proxy to explore Si cycling and reconstruct weathering in the present and past.

Silicon isotopic fractionation during metamorphic fluid activities: constraints from eclogites and ultrahigh-pressure veins in the Dabie orogen, China

To understand Si isotope fractionation during metamorphic fluid activities in the subduction zone, this study presents the Si isotopic compositions of two high- to ultrahigh-pressure (HP–UHP) eclogite–vein systems from the Ganghe and Hualiangting areas in the Dabie orogen, eastern China. The results show that Si isotopes are significantly fractionated between the veins and their host eclogites in both areas. The δ30Si values of Ganghe eclogites range from −0.50‰ to −0.39‰, higher than that of the studied omphacite−epidote vein (−0.63 ± 0.07‰). The Hualiangting eclogites have δ30Si values of −0.36‰ to −0.29‰, whereas the δ30Si values of the Hualiangting multi-stage veins show greater variation, from −0.45‰ to +0.05‰, revealing significant Si isotope fractionation during metamorphic fluid evolution and vein formation.For the Hualiangting and Ganghe samples, the enrichment of heavy Si isotopes in minerals follows the order of δ30Siphengite (−0.01 to +0.13‰) ≈ δ30Siquartz (−0.14 to +0.10‰) > δ30Siomphacite (−0.63 to −0.33‰) ≈ δ30Siepidote (−0.60 to −0.30‰) ≈ δ30Sikyanite (−0.42 to −0.28‰) > δ30Sigarnet (−0.92 to −0.44‰). The Si isotope fractionation between metamorphic minerals was generally consistent with the equilibrium Si isotope fractionation factors calculated by the first-principles methods. These results suggest that the vein minerals are likely in Si isotopic equilibrium with each other.The δ30Si of the Hualiangting veins are linearly correlated to SiO2 contents with a steeper slope than that of the magma differentiation trend. By also considering the mineralogy of the veins, we conclude that this linear relationship reflects the sequential variation of mineral composition on the Si isotope signature of the veins. The SiO2 content and δ30Si of veins increased with continuing crystallization from the fluid, and the δ30Si of the fluid also increased. The results suggest that Si isotope data can be used to constrain element recycling and metamorphic fluids activities in the subduction zones.

Past and current geochemical conditions influence silicon isotope signatures of pedogenic clay minerals at the soil profile scale, Ethiopia

Soil processes strongly govern silicon (Si) mobility in terrestrial environments and its relation to other global biogeochemical cycles. The nature of inherited soil clay minerals can be highly diverse given the variability of their weathering environment. The influence of Si isotope fractionation factor in the initial geochemical conditions of clay precipitation can therefore be still expressed in inherited clay minerals in their new environment. We studied top- and subsoil of an Ethiopian Vertic Planosol derived from parent materials of similar sources of volcanism. The selected Planosol has an abrupt textural change at a depth of ~40 cm separating a bleached, silty (25 ± 1.8% clay) ash-derived soil horizon with a crumby structure from a heavy clayey (68 ± 3.4% clay) lacustrine-derived vertic soil horizon. The mineralogical assemblage of the clay fraction in top- and subsoil is characterized by similar proportions of 1:1 (kaolinite) and 2:1 (illite and smectite) layer-type clay minerals. This specific soil profile provides a unique opportunity to elucidate the influence of clay formation processes (inheritance versus neoformation) on the Si isotope signature of pedogenic clay minerals. Minerals of the clay fraction in the clayey vertic horizon are significantly enriched in light Si isotope (δ30Si = −1.41 ± 0.02‰) compared to the bleached, silty horizon (δ30Si = −0.69 ± 0.03‰). These results are corroborated by the preferential enrichment in Ge, relative to Si, in the clay fraction of the clayey subsoil compared to the silty topsoil (Ge/Si = 6.3 ± 0.14 and 4.0 ± 0.10 μmol mol−1, respectively). Our results demonstrate that geochemical conditions in lacustrine environment favor kinetically-driven Si isotope fractionation factor leading to lower Si isotope ratios in 2:1 clay minerals inherited in the new soil profile environment. The inherited soil textural conditions in the soil profile also contribute to on-going processes that result in larger Si isotope differences between the soil solution (CaCl2 extractable) and clay minerals. This implies that Si isotope signatures of clay minerals in the studied soil profile are influenced by a combination of inheritance processes in lacustrine environment and on-going neoformation processes in the soil profile. This finding has important implications for environmental studies using geochemical and Si stable isotope tracers to better understand current soil processes, to model elemental cycling in soil-plant systems and to quantify land-ocean element mass-balances.

The experimental determination of equilibrium Si isotope fractionation factors among H4SiO4o, H3SiO4− and amorphous silica (SiO2·0.32 H2O) at 25 and 75 °C using the three-isotope method

Abstract The accurate interpretation of Si isotope signatures in natural systems requires knowledge of the equilibrium isotope fractionation between Si-bearing solids and the dominant Si-bearing aqueous species. Aqueous silicon speciation is dominated by silicic acid (H4SiO4o) in most natural aqueous fluids at pH 9), equilibrium Si isotope fractionation factors between solid and aqueous solution are higher, at 1.63 ± 0.23‰ at 25 °C, and 1.06 ± 0.13‰ at 75 °C. Taking account of the distribution of the aqueous Si species, equilibrium Si isotope fractionation factors between H3SiO4− and H4SiO4o of −2.34 ± 0.13‰ and −2.21 ± 0.05‰ at 25 and 75 °C, respectively, were determined. The distinct equilibrium isotope fractionation factors of H3SiO4− and H4SiO4o, and its variation with temperature can be used to establish paleo-pH and temperature proxies. The application of the three-isotope method also provides insight into the rates of isotopic exchange. For the solid grain size used (∼20 nm), these rates match closely the measured bulk dissolution rates for amorphous silica for most of the isotope exchange process, suggesting the dominant and rate controlling isotope exchange mechanism in the experiments is detachment and reattachment of material at the amorphous silica surface.

Constraining silicon isotope exchange kinetics and fractionation between aqueous and amorphous Si at room temperature

Abstract Silicon (Si) isotopes are useful tracers for the modern and ancient Si cycle, but their interpretation is limited by inadequate understanding of Si isotope exchange kinetics and fractionation factors at low temperature. This study investigated Si isotope exchange and fractionation between aqueous and amorphous Si at circumneutral pH and room temperature through a series of 29Si-spiked isotope-exchange experiments. Four different amorphous Si solids with varied surface areas were reacted with aqueous Si solutions of high ionic strength similar to seawater, or low ionic strength typical of freshwater, under conditions close to chemical equilibrium with respect to amorphous Si solubility. In contrast to the common perception of negligible Si isotope exchange at low temperature, ∼50–85% isotope exchange was achieved between aqueous and amorphous Si within ∼60 days. Larger solid surface areas and higher aqueous ionic strength generally promoted Si isotope exchange. Drying/aging of Si gel, however, impedes Si isotope exchange between amorphous and aqueous Si relative to freshly prepared Si gels. Excluding the experiments that used the aged Si gel, temporal trajectories of Si isotope evolution of the two phases from all other experiments showed significant curvature in three-isotope space (29Si/28Si and 30Si/28Si). These results can be best explained by a model that comprises two Si isotope exchange processes with different exchange rates and fractionation factors during the interactions between aqueous and amorphous Si towards isotope equilibrium. The faster exchange is associated with surface sites, and slower exchange occurs between exterior and interior Si atoms of the solid. Exchange with surface sites tends to partition heavy Si isotopes in the aqueous phase relative to the solid surface, whereas exchange between surface and interior sites in the solid tends to enrich heavy Si isotopes in the interior. Two experiments that achieved >80% isotope exchange provided the best estimates of equilibrium Si isotope fractionation factors between bulk amorphous Si solid and aqueous monomeric silicic acid H4SiO4 (Δ30Siamorphous–aqueous) at 23 °C: +0.52‰ (±0.15‰, 1sd) at seawater ionic strength, and −0.98‰ (±0.12‰) at freshwater ionic strength. The observed “salt effect” on Si isotope exchange kinetics and fractionation factor is interpreted to reflect an influence of cations on Si speciation of solid surfaces. This work highlights the value of three-isotope method in studying both reaction kinetics and isotope fractionation mechanisms. The observed Si isotope exchange between amorphous and aqueous Si at low temperature implies that Si isotope re-equilibration, a previously neglected process, may be important in controlling Si isotope compositions of natural samples.

Silicon isotope and silicic acid uptake in surface waters of Marguerite Bay, West Antarctic Peninsula

The silicon isotope composition (δ30Si) of dissolved silicon (DSi) and biogenic silica (BSi) provides information about the silicon cycle and its role in oceanic carbon uptake in the modern ocean and in the past. However, there are still questions outstanding regarding the impact of processes such as oceanic mixing, export and dissolution on the isotopic signature of seawater, and the impacts on sedimentary BSi. This study reports the δ30Si of DSi from surface waters at the Rothera Time Series (RaTS) site, Ryder Bay, in a coastal region of the West Antarctic Peninsula (WAP). The samples were collected at the end of austral spring through the end of austral summer/beginning of autumn over two field seasons, 2004/5 and 2005/6. Broadly, for both field seasons, DSi diminished and δ30Si of DSi increased through the summer, but this was accomplished during only a few short periods of net nutrient drawdown. During these periods, the δ30Si of DSi was negatively correlated with DSi concentrations. The Si isotope fractionation factor determined for the net nutrient drawdown periods, ɛuptake, was in the range of −2.26 to −1.80‰ when calculated using an open system model and −1.93 to −1.33‰ when using a closed system model. These estimates of ɛ are somewhat higher than previous studies that relied on snapshots in time rather than following changes in δ30Si and DSi over time, which therefore were more likely to include the effects of mixing of dissolved silicon up into the mixed layer. Results highlight also that, even at the same station and within a single growing season, the apparent fractionation factor may exhibit significant temporal variability because of changes in the extent of biological removal of DSi, nutrient source, siliceous species, and mixing events.Paleoceanographic studies using silicon isotopes need careful consideration in the light of our new results.

First-principles calculations of equilibrium fractionation of O and Si isotopes in quartz, albite, anorthite, and zircon

In this study, we used first-principles calculations based on density functional theory to investigate silicon and oxygen isotope fractionation factors among the most abundant major silicate minerals in granites, i.e., quartz and plagioclase (including albite and anorthite), and an important accessory mineral zircon. Combined with previous results of minerals commonly occurring in the crust and upper mantle (orthoenstatite, clinoenstatite, garnet, and olivine), our study reveals that the Si isotope fractionations in minerals are strongly correlated with SiO4 tetrahedron volume (or average Si–O bond length). The 30Si enrichment order follows the sequence of quartz > albite > anorthite > olivine ≈ zircon > enstatite > diopside, and the 18O enrichment follows the order of quartz > albite > anorthite > enstatite > zircon > olivine. Our calculation predicts that measurable fractionation of Si isotopes can occur among crustal silicate minerals during high-temperature geochemical processes. This work also allows us to evaluate Si isotope fractionation between minerals and silicate melts with variable compositions. Trajectory for δ30Si variation during fractional crystallization of silicate minerals was simulated with our calculated Si isotope fractionation factors between minerals and melts, suggesting the important roles of fractional crystallization to cause Si isotopic variations during magmatic differentiation. Our study also predicts that δ30Si data of ferroan anorthosites of the Moon can be explained by crystallization and aggregation of anorthite during lunar magma ocean processes. Finally, O and Si isotope fractionation factors between zircon and melts were estimated based on our calculation, which can be used to quantitatively account for O and Si isotope composition of zircons crystallized during magma differentiation.

Heavy silicon isotopic composition of silicic acid and biogenic silica in Arctic waters over the Beaufort shelf and the Canada Basin

AbstractThe silicon isotopic composition of silicic acid (δ30Si(OH)4) and biogenic silica (δ30Si‐bSiO2) were measured for the first time in marine Arctic waters from the Mackenzie River delta to the deep Canada Basin in the late summer of 2009. In the upper 100 m of the water column, δ30Si(OH)4 signals (+1.82‰ to +3.08‰) were negatively correlated with the relative contribution of Mackenzie River water. The biogenic Si isotope fractionation factor estimated using an open system model, 30ε = −0.97 ± 0.17‰, agrees well with laboratory and global‐ocean estimates. Nevertheless, the δ30Si dynamics of this region may be better represented by closed system isotope models that yield lower values of 30ε, between −0.33‰ and −0.41‰, depending on how the contribution of sea‐ice diatoms is incorporated. In the upper 400 m, δ30Si‐bSiO2 values were among the heaviest ever measured in marine suspended bSiO2 (+2.03‰ to +3.51‰). A positive correlation between δ30Si‐bSiO2 and sea‐ice cover implies that heavy signals can result from isotopically heavy sea‐ice diatoms introduced to pelagic assemblages. Below the surface bSiO2 production zone, the δ30Si(OH)4 distribution followed that of major water masses. Vertical δ30Si(OH)4 profiles showed a minimum (average of +1.84 ± 0.10‰) in the upper halocline (125–200 m) composed of modified Pacific water and heavier average values (+2.04 ± 0.11‰) in Atlantic water (300–500 m deep). In the Canada Basin Deep Water (below 2000 m), δ30Si(OH)4 averaged +1.88 ± 0.12‰, which represents the most positive value ever measured anywhere in the deep ocean. Since most Si(OH)4 enters the Arctic from shallow depths in the Atlantic Ocean, heavy deep Arctic δ30Si(OH)4 signals likely reflect the influx of relatively heavy intermediate Atlantic waters. A box model simulation of the global marine δ30Si(OH)4 distribution successfully reproduced the observed patterns, with the δ30Si(OH)4 of the simulated deep Arctic Ocean being the heaviest of all deep‐ocean basins.

Abiologic silicon isotope fractionation between aqueous Si and Fe(III)–Si gel in simulated Archean seawater: Implications for Si isotope records in Precambrian sedimentary rocks

Precambrian Si-rich sedimentary rocks, including cherts and banded iron formations (BIFs), record a >7‰ spread in 30Si/28Si ratios (δ30Si values), yet interpretation of this large variability has been hindered by the paucity of data on Si isotope exchange kinetics and equilibrium fractionation factors in systems that are pertinent to Precambrian marine conditions. Using the three-isotope method and an enriched 29Si tracer, a series of experiments were conducted to constrain Si isotope exchange kinetics and fractionation factors between amorphous Fe(III)–Si gel, a likely precursor to Precambrian jaspers and BIFs, and aqueous Si in artificial Archean seawater under anoxic conditions. Experiments were conducted at room temperature, and in the presence and absence of aqueous Fe(II) (Fe(II)aq).Results of this study demonstrate that Si solubility is significantly lower for Fe–Si gel than that of amorphous Si, indicating that seawater Si concentrations in the Precambrian may have been lower than previous estimates. The experiments reached ∼70–90% Si isotope exchange after a period of 53–126days, and the highest extents of exchange were obtained where Fe(II)aq was present, suggesting that Fe(II)–Fe(III) electron-transfer and atom-exchange reactions catalyze Si isotope exchange through breakage of Fe–Si bonds. All experiments except one showed little change in the instantaneous solid–aqueous Si isotope fractionation factor with time, allowing extraction of equilibrium Si isotope fractionation factors through extrapolation to 100% isotope exchange. The equilibrium 30Si/28Si fractionation between Fe(III)–Si gel and aqueous Si (Δ30Sigel–aqueous) is −2.30±0.25‰ (2σ) in the absence of Fe(II)aq. In the case where Fe(II)aq was present, which resulted in addition of ∼10% Fe(II) in the final solid, creating a mixed Fe(II)–Fe(III) Si gel, the equilibrium fractionation between Fe(II)–Fe(III)–Si gel and aqueous Si (Δ30Sigel–aqueous) is −3.23±0.37‰ (2σ). Equilibrium Si isotope fractionation for Fe–Si gel systems is significantly larger in magnitude than estimates of a near-zero solid–aqueous fractionation factor between pure Si gel and aqueous Si, indicating a major influence of Fe atoms on Si–O bonds, and hence the isotopic properties, of Fe–Si gel. Larger Si isotope fractionation in the Fe(II)-bearing systems may be caused by incorporation of Fe(II) into the solid structure, which may further weaken Fe–Si bonds and thus change the Si isotope fractionation factor. The relatively large Si isotope fractionation for Fe–Si gel, relative to pure Si gel, provides a new explanation for the observed contrast in δ30Si values in the Precambrian BIFs and cherts, as well as an explanation for the relatively negative δ30Si values in BIFs, in contrast to previous proposals that the more negative δ30Si values in BIFs reflect hydrothermal sources of Si or sorption to Fe oxides/hydroxides.

Equilibrium and kinetic Si isotope fractionation factors and their implications for Si isotope distributions in the Earth’s surface environments

Abstract Several important equilibrium Si isotope fractionation factors among minerals, organic molecules and the H4SiO4 solution are complemented to facilitate the explanation of the distributions of Si isotopes in Earth’s surface environments. The results reveal that, in comparison to aqueous H4SiO4, heavy Si isotopes will be significantly enriched in secondary silicate minerals. On the contrary, quadra-coordinated organosilicon complexes are enriched in light silicon isotope relative to the solution. The extent of 28Si-enrichment in hyper-coordinated organosilicon complexes was found to be the largest. In addition, the large kinetic isotope effect associated with the polymerization of monosilicic acid and dimer was calculated, and the results support the previous statement that highly 28Si-enrichment in the formation of amorphous quartz precursor contributes to the discrepancy between theoretical calculations and field observations. With the equilibrium Si isotope fractionation factors provided here, Si isotope distributions in many of Earth’s surface systems can be explained. For example, the change of bulk soil δ30Si can be predicted as a concave pattern with respect to the weathering degree, with the minimum value where allophane completely dissolves and the total amount of sesqui-oxides and poorly crystalline minerals reaches their maximum. When, under equilibrium conditions, the well-crystallized clays start to precipitate from the pore solutions, the bulk soil δ30Si will increase again and reach a constant value. Similarly, the precipitation of crystalline smectite and the dissolution of poorly crystalline kaolinite may explain the δ30Si variations in the ground water profile. The equilibrium Si isotope fractionations among the quadra-coordinated organosilicon complexes and the H4SiO4 solution may also shed light on the Si isotope distributions in the Si-accumulating plants.

Silicon isotope fractionation during the precipitation of quartz and the adsorption of H4SiO4(aq) on Fe(III)-oxyhydroxide surfaces

Equilibrium Si isotope fractionation factors among orthosilicic acid (i.e.,H4SiO4(aq)), quartz and the adsorption complexes of H4SiO4(aq) on Fe(III)-oxyhydroxide surface were calculated using the full-electron wave-function quantum chemistry methods [i.e., B3LYP/6-311G(2df,p)] with a new cluster-model-based treatment. Solvation effects were carefully included in our calculations via water-droplet method combined with implicit solvent models (e.g., PCM). The results revealed that, if it is under equilibrium conditions, heavy Si isotopes would be significantly enriched in quartz in comparison to H4SiO4(aq). However, most of the field observations suggested that quartz would have identical or even depleted δ30Si values compared to that of H4SiO4(aq). To explain this discrepancy between the equilibrium calculation results and the field observations, the kinetic isotope effect (KIE) associated with the formation of amorphous silica, which usually is the precursor of crystalline quartz, was investigated using quantum chemistry methods. The KIE results showed that amorphous silica would be significantly enriched in light Si isotopes during its formation. Our equilibrium fractionation results, however, matched a special type of quartz (i.e., Herkimer “diamond”) very well, due to its nearly equilibrated precipitation condition. Opposite to the case of precipitated quartz, a large equilibrium Si isotope fractionation (i.e., −3.0 ‰) was found between the absorbed bidentate Si surface complexes (i.e., 2C > Fe2O2Si(OH)2) and H4SiO4(aq). This calculated equilibrium Si isotope fractionation factor largely differed from a previous experimental result (ca. −1.08 ‰). We found that the formation of transient or temporary surface complexes [e.g., 1V > Fe2OSi(OH)3] may have accounted for the smaller net fractionation observed. With the equilibrium and kinetic Si isotope fractionation factors provided here, the distributions and changes of Si isotope compositions in the Earth’s surface systems can be better understood.

Effects of growth and dissolution on the fractionation of silicon isotopes by estuarine diatoms

Studies of silicon (Si) isotope fractionation during diatom growth in open ocean systems have documented lower Si isotopic values (δ30Si) in the biogenic silica of diatom frustules compared to dissolved silicon. Recent findings also indicate that Si isotope fractionation occurs during dissolution of diatom frustules, producing higher δ30Si values in the remaining biogenic silica. This study focuses on diatoms from high production areas in estuarine and coastal areas that represent approximately 30–50% of the global marine primary production. Two species of diatoms, Thalassiosira baltica and Skeletonema marinoi, were isolated from the brackish Baltic Sea, one of the largest estuarine systems in the world. These species were used for laboratory investigations of Si isotope fractionation during diatom growth and the subsequent dissolution of the diatom frustules. Both species of diatoms give an identical Si isotope fractionation factor during growth of −1.50±0.36‰ (2σ) for 30Si, which falls in the range of −2.09‰ to −0.55‰ of published data. Our results also suggest a dissolution-induced Si isotope fractionation factor of −0.86‰ at early stage of dissolution, but this effect was observed only in DSi and no significant Si isotope change was observed for BSi. The growth and dissolution results are applied to a Baltic Sea sediment core to reconstruct DSi utilization by diatoms, and found to be in agreement with the observed DSi uptake rates in the overlying water column during diatom growth.

Experimental determination of the Si isotope fractionation factor between liquid metal and liquid silicate

The conditions of core formation and the abundances of the light elements in Earthʼs core remain debated. Silicon isotope fractionation provides a tool contributing to this subject. We present experimentally determined Si isotope fractionation factors between liquid metal and liquid silicate at 1450 °C and 1750 °C, which allow calibrating the temperature dependence of Si isotope fractionation. Experiments were performed in a centrifuging piston cylinder at 1 GPa, employing both graphite and MgO capsules. Tin was used to lower the melting temperature of the metal alloys for experiments performed at 1450 °C. Tests reveal that neither Sn nor C significantly affects Si isotope fractionation. An alkaline fusion technique was employed to dissolve silicate as well as metal phases prior to ion exchange chemistry and mass spectrometric analysis. The results show that metal is consistently enriched in light isotopes relative to the silicate, yielding average metal–silicate fractionation factors of −1.48±0.08‰ and −1.11±0.14‰ at 1450 °C and 1750 °C, respectively. The temperature dependence of equilibrium Si isotope fractionation between metal and silicate can thus be described as Δ30SiMetal-Silicate=−4.42(±0.05)×106/T2. The Si isotope equilibrium fractionation is thus about 1.7 times smaller than previously proposed on the basis of experiments. A consequence of this smaller fractionation is that the calculated difference between the Si isotope composition of the bulk Earth and that of the bulk silicate Earth generated by core formation is smaller than previously thought. It is therefore increasingly difficult to match the Si isotope composition of the bulk silicate Earth with that of chondrites for metal–silicate equilibration temperatures above ∼2500 K. This suggests that Si isotopes were more sensitive to the early stages of core formation when low oxygen fugacities allowed significant incorporation of Si into metal.

Silicon isotope enrichment in diatoms during nutrient-limited blooms in a eutrophied river system

We examined the Si isotope fractionation by following a massive nutrient limited diatom bloom in a eutrophied natural system. The Oder River, which is a eutrophied river draining the western half of Poland and entering the southern Baltic Sea, exhibits diatom blooms that cause extreme Si isotope fractionation. The rapid nutrient depletion and fast BSi increase observed during the spring bloom suggest a closed system Rayleigh behavior for DSi and BSi in the river at certain time scales. A Si isotope fractionation factor (30εDSi–BSi) of −1.6±0.31‰ (2σ) is found based on observations between April and June, 2004. A very high δ30Si value of up to +3.05‰ is measured in BSi derived from diatoms. This is about 2 times higher than previously recorded δ30Si in freshwater diatoms. The Rayleigh model used to predict the δ30Si values of DSi suggests that the initial value before the start of the diatom bloom is close to +2‰, which is relatively higher than the previously reported values in other river water. This indicates that there is a biological control of the Si isotope compositions entering the river, probably caused by Si isotope fractionation during uptake of Si in phytoliths. Clearly, eutrophied rivers with enhanced diatom blooms deliver 30Si-enriched DSi and BSi to the coastal ocean, which can be used to trace the biogeochemistry of DSi/BSi in estuaries.

Species-dependent silicon isotope fractionation by marine diatoms

Fractionation of silicon (Si) isotopes was measured in seven species (nine strains) of polar and sub-polar marine diatoms grown in semi-continuous unialgal cultures under optimal irradiance and temperature for each diatom strain. Results from this work provide the first evidence that Si isotope fractionation by diatoms is species-dependent. The greatest difference in the Si isotope fractionation factor (ε) was observed between two Southern Ocean diatoms, Fragilariopsis kerguelensis (−0.54‰, average for two strains) and Chaetoceros brevis (−2.09‰). The ε for the other species, both polar and sub-polar, ranged from −0.72‰ to −1.21‰. The two remaining polar diatoms had ε values of −0.74±0.05‰ for Thalassiosira antarctica, and −1.21±0.04‰ for Thalassiosira nordenskioeldii, while the sub-polar species had ε values of −0.72±0.04‰ for Thalassiosira weissflogii, −0.88±0.06‰ for Thalassiosira pseudonana (CCCM58), −0.97±0.14‰ for Thalassiosira pseudonana (CCMP1014), and −1.15±0.03‰ for Porosira glacialis. The range in ε for the diatoms evaluated in this study may be large enough to significantly impact the Si isotope composition measured in diatom opal (δ30Si-bSiO2) from marine sediments and its subsequent interpretation. To test the influence of diatom taxonomic composition on δ30Si-bSiO2, we developed a model that considered the relative abundance of diatom species and the ε values (from this study) for each species present within the sediment core (i.e. weighted-average ε). The model was applied to records from a Southern Ocean sediment core (TN057-13) where both diatom abundance and δ30Si-bSiO2 data were available. The analysis indicated that 67% of the variation in δ30Si-bSiO2 could be explained by species-dependent Si isotope fractionation. We suggest that future work should assess phytoplankton taxonomic composition when using δ30Si-bSiO2 as a proxy for Si utilization.

Silicon isotopes and the tracing of desilication in volcanic soil weathering sequences, Guadeloupe

Silicon (Si) stable isotopes have the potential to become a useful weathering proxy, given that light Si isotopes are preferentially incorporated into secondary clay minerals. Here we investigate how Si depletion in soils and associated clay mineralogy influence the Si isotope fractionation associated with clay mineral formation. We report δ30Si compositions in bulk soils and clay fractions relative to their parent andesite in three soil weathering sequences from Guadeloupe that were formed under contrasting climatic conditions. Strongly desilicated soils containing kaolinite that formed in wet areas (high precipitation) are compared with less desilicated soils containing smectite formed in drier conditions (low precipitation). Clay fractions are isotopically lighter than the parent andesite (δ30Si-0.23‰), and increasingly lighter with Si depletion in soils, which supports the view that the Si isotope composition in secondary clay fractions is controlled by the degree of soil desilication. It is shown that the Si isotope fractionation factor between the parent silicate material and the secondary clay minerals is smaller for Si-rich secondary clay minerals such as smectite and larger for Si-poor secondary clay minerals such as kaolinite. This study provides new insights to better define Si isotopes as a proxy for environmental conditions for clay neoformation.

Silicon isotope fractionation in bamboo and its significance to the biogeochemical cycle of silicon

A systematic investigation on silica contents and silicon isotope compositions of bamboos was undertaken. Seven bamboo plants and related soils were collected from seven locations in China. The roots, stem, branch and leaves for each plant were sampled and their silica contents and silicon isotope compositions were determined. The silica contents and silicon isotope compositions of bulk and water-soluble fraction of soils were also measured. The silica contents of studied bamboo organs vary from 0.30% to 9.95%. Within bamboo plant the silica contents show an increasing trend from stem, through branch, to leaves. In bamboo roots the silica is exclusively in the endodermis cells, but in stem, branch and leaves, the silica is accumulated mainly in epidermal cells. The silicon isotope compositions of bamboos exhibit significant variation, from −2.3‰ to 1.8‰, and large and systematic silicon isotope fractionation was observed within each bamboo. The δ 30Si values decrease from roots to stem, but then increase from stem, through branch, to leaves. The ranges of δ 30Si values within each bamboo vary from 1.0‰ to 3.3‰. Considering the total range of silicon isotope composition in terrestrial samples is only 7‰, the observed silicon isotope variation in single bamboo is significant and remarkable. This kind of silicon isotope variation might be caused by isotope fractionation in a Rayleigh process when SiO 2 precipitated in stem, branches and leaves gradually from plant fluid. In this process the Si isotope fractionation factor between dissolved Si and precipitated Si in bamboo ( α pre-sol) is estimated to be 0.9981. However, other factors should be considered to explain the decrease of δ 30Si value from roots to stem, including larger ratio of dissolved H 4SiO 4 to precipitated SiO 2 in roots than in stem. There is a positive correlation between the δ 30Si values of water-soluble fractions in soils and those of bulk bamboos, indicating that the dissolved silicon in pore water and phytoliths in soil is the direct sources of silicon taken up by bamboo roots. A biochemical silicon isotope fractionation exists in process of silicon uptake by bamboo roots. Its silicon isotope fractionation factor ( α bam-wa) is estimated to be 0.9988. Considering the distribution patterns of SiO 2 contents and δ 30Si values among different bamboo organs, evapotranspiration may be the driving force for an upward flow of a silicon-bearing fluid and silica precipitation. Passive silicon uptake and transportation may be important for bamboo, although the role of active uptake of silicic acid by roots may not be neglected. The samples with relatively high δ 30Si values all grew in soils showing high content of organic materials. In contrast, the samples with relatively low δ 30Si values all grew in soil showing low content of organic materials. The silicon isotope composition of bamboo may reflect the local soil type and growth conditions. Our study suggests that bamboos may play an important role in global silicon cycle.