Si Isotope Signatures Research Articles

Origin of silica in rice plants and contribution of diatom Earth fertilization: insights from isotopic Si mass balance in a paddy field

Background and aims: The benefits of Si for crops is well evidenced but the biogeochemical cycle of Si in agriculture remains poorly documented. This study aims at identifying and quantifying the Si sources (primary and secondary soil minerals, amorphous silica, irrigation, Si-fertilizer) to rice plants. Method: Field experiments were carried out with and without application of diatomaceous earth (DE) under rice and bare conditions to determine the water and dissolved mass balance in paddy fields (Karnataka, Southern India). The fate of the Si brought by irrigation (DSi) (uptake by rice, uptake by diatoms, adsorption) was assessed through a solute mass balance combined with silicon isotopic signatures. Results: Above the ground-surface, about one third of the DSi flux brought by borewell irrigation (545 mmol Si.m−2) to bare plots and half of DSi in rice plots were removed from solution within minutes or hours following irrigation. Such rate is consistent with the rate of DSi adsorption onto Fe-oxyhydroxides but not with diatom blooms. In rice and rice + DE experiments, the isotopic fractionation factor (30e) between bore well and stagnant water compositions is close to −1 ‰, i.e. the isotopic fractionation factor known for rice, indicating that above-ground DSi removal would be dominated by plant uptake upon adsorption. Within the soil layer, pore water DSi decreases much faster in rice experiments than in bare ones, demonstrating the efficiency of DSi rice uptake upon adsorption. Total irrigation-DSi to plant-Si would then represent 24 to 36% in rice experiments (over 1460 ± 270 mmol Si m−2 in biomass) and 15 to 23% in rice + DE ones (over 2250 ± 180 mmol Si m−2). The δ30Si signature of whole plants was significantly different in the rice + DE plot analyzed, 0.99 ± 0.07 ‰, than in the rice one, 1.29 ± 0.07 ‰. According to these δ30Si signatures, the main Si source from the soil would be the amorphous silica pool (ASi). A slight contribution of DE to the rice plant could be detected from the Si isotopic signature of rice. Conclusions: The δ30Si signatures of the various soil-plant compartments, when associated to Si mass balance at soil-plant scale, constitute a reliable proxy of the Si sources in paddy fields. The solute Si balance is controlled by rice uptake in rice plots and by adsorption in bare ones. The main Si sources for the rice plants were soil ASi, irrigation Si and to a lesser extent Si fertilizer when it was applied.

Silicon isotopic compositions of altered oceanic crust: Implications for Si isotope heterogeneity in the mantle

This study reports Si isotope compositions of the altered oceanic crust recovered from IODP Site 1256, East Pacific Rise to investigate the behavior of Si isotopes during low-temperature seawater and/or high-temperature hydrothermal alteration processes in the oceanic crust. These samples, including basalts and gabbros, were altered by seawater and hydrothermal fluids at various temperatures and water/rock ratios. The δ30Si of these samples range from −0.38‰ to −0.27‰, with an average of δ30Si=−0.32±0.06‰ (2SD, N=50), regardless of the depth and extent of alteration. The δ30Si values of the altered oceanic crust are in general agreement with the previous estimate of the global fresh MORBs (−0.27±0.06‰, 2SD; Savage et al., 2014), suggesting that either low-temperature seawater alteration or high-temperature hydrothermal alteration does not cause resolvable Si isotopic fractionation at the bulk-rock scale in the altered oceanic crust.Because the altered oceanic crust could be partially melted during subduction or in the convective mantle, we estimated the Si isotope fractionation between the melt and residue minerals of the altered oceanic crust. Our calculations show that Si isotopes can be fractionated between the melt and residues enriched in garnet and clinopyroxene. Although the bulk altered oceanic crust has a mantle-like Si isotopic signature, the residual solids of the recycled altered oceanic crust could be enriched in light Si isotopes and the melt could be enriched in heavy Si isotopes. This indicates that melting and recycling of the subducted oceanic crust can produce heterogeneous Si isotope compositions in the convective mantle, which could contribute to the source of ocean island basalts with low δ30Si.

Silica cycling and isotopic composition in northern Marguerite Bay on the rapidly-warming western Antarctic Peninsula

The Southern Ocean is a key region for silica (Si) cycling, and the isotopic signatures established here influence the rest of the world's oceans. The climate and ecosystem of the Southern Ocean are changing rapidly, with the potential to impact Si cycling and isotope dynamics. This study examines high-resolution time-series dataset of dissolved Si concentrations and isotopic signatures, particulate Si concentrations and diatom speciation at a coastal site on the western Antarctic Peninsula (WAP), in order to characterise changes in Si cycling with respect to changes occurring in productivity and diatom assemblages. Dissolved and particulate Si phases reflect the dominant control of biological uptake, and combined with isotopic fractionation were consistent with a season of low/intermediate productivity. Biogenic Si is tightly coupled to both chlorophyll and particulate organic carbon at the sampling site, consistent with diatom-dominated phytoplankton assemblages along the WAP. Variability in diatom speciation has a negligible impact on the isotopic signature of dissolved Si in surface waters, although this is unlikely to hold for sediments due to differential dissolution of diatom species. A continued decline in diatom productivity along the WAP would likely result in an increasing unused Si inventory, which can potentially feed back into Si-limited areas, promoting diatom growth and carbon drawdown further afield.

STELLAR ORIGINS OF EXTREMELY 13C- AND 15N-ENRICHED PRESOLAR SIC GRAINS: NOVAE OR SUPERNOVAE?

ABSTRACT Extreme excesses of 13C (12C/13C < 10) and 15N (14N/15N < 20) in rare presolar SiC grains have been considered diagnostic of an origin in classical novae, though an origin in core collapse supernovae (CCSNe) has also been proposed. We report C, N, and Si isotope data for 14 submicron- to micron-sized 13C- and 15N-enriched presolar SiC grains (12C/13C < 16 and 14N/15N < ∼100) from Murchison, and their correlated Mg–Al, S, and Ca–Ti isotope data when available. These grains are enriched in 13C and 15N, but with quite diverse Si isotopic signatures. Four grains with 29,30Si excesses similar to those of type C SiC grains likely came from CCSNe, which experienced explosive H burning occurred during explosions. The independent coexistence of proton- and neutron-capture isotopic signatures in these grains strongly supports heterogeneous H ingestion into the He shell in pre-supernovae. Two of the seven putative nova grains with 30Si excesses and 29Si depletions show lower-than-solar 34S/32S ratios that cannot be explained by classical nova nucleosynthetic models. We discuss these signatures within the CCSN scenario. For the remaining five putative nova grains, both nova and supernova origins are viable because explosive H burning in the two stellar sites could result in quite similar proton-capture isotopic signatures. Three of the grains are sub-type AB grains that are also 13C enriched, but have a range of higher 14N/15N. We found that 15N-enriched AB grains (∼50 < 14N/15N < ∼100) have distinctive isotopic signatures compared to putative nova grains, such as higher 14N/15N, lower 26Al/27Al, and lack of 30Si excess, indicating weaker proton-capture nucleosynthetic environments.

Silicon isotope systematics of acidic weathering of fresh basalts, Kilauea Volcano, Hawai’i

Silicon stable isotopes are fractionated by a host of low-temperature aqueous processes, making them potentially useful as a weathering proxy. Here we characterize the silicon isotope signature of surficial chemical weathering of glassy basaltic lava flows at Kilauea Volcano, Hawaii. Fresh basalt flow surfaces (<40years old) frequently feature opaque amorphous silica surface coatings up to 80μm thick. These silica coatings and associated silica cements are enriched in the heavier isotopes of Si (δ30SiNBS-28=+0.92‰ to +1.36‰) relative to their basaltic substrate (δ30SiNBS-28=−0.3‰ to −0.2‰). Secondary clays and opals are typically depleted in 30Si relative to the dissolved reservoirs from which they precipitated, so this sense of isotopic fractionation is unusual. Mechanisms capable of producing isotopically heavy secondary minerals were explored by conducting batch alteration experiments on fresh basaltic glass. Batch acidic alteration of basalt glass in HCl, H2SO4, and HF produced silica-rich surface layers resembling the Hawaiian surface coatings. Differences in fluid chemical composition affected the direction and magnitude of Si isotope fractionation. Basalt leaching in HCl or H2SO4 produced 30Si-enriched fluids (1000 lnαprecip-Si(aq)≅−0.8‰) and 30Si-depleted secondary silica layers. In contrast, HF-bearing experiments produced highly 30Si-depleted fluid compositions (1000 lnαprecip-Si(aq) up to +8‰). Larger isotopic fractionations were observed in experiments with lower fluid–rock ratios. In Hawaii, where altering fluids contain H2SO4 and HCl but minimal HF, high δ30Si values for the silica coatings were likely achieved by Rayleigh fractionation. Aqueous 30Si-enriched silica was released during incongruent basalt dissolution then subsequently transported and deposited from an evaporating solution at the flow surface. Our results indicate that (1) altering fluid chemistry and fluid–rock ratio impact the Si isotope signature of chemical weathering and (2) δ30Si of solids produced by low temperature aqueous alteration may diverge sharply from watershed- or landscape-scale weathering trends.

Planetary and meteoritic Mg/Si and [formula omitted] variations inherited from solar nebula chemistry

The bulk chemical compositions of planets are uncertain, even for major elements such as Mg and Si. This is due to the fact that the samples available for study all originate from relatively shallow depths. Comparison of the stable isotope compositions of planets and meteorites can help overcome this limitation. Specifically, the non-chondritic Si isotope composition of the Earth's mantle was interpreted to reflect the presence of Si in the core, which can also explain its low density relative to pure Fe–Ni alloy. However, we have found that angrite meteorites display a heavy Si isotope composition similar to the lunar and terrestrial mantles. Because core formation in the angrite parent-body (APB) occurred under oxidizing conditions at relatively low pressure and temperature, significant incorporation of Si in the core is ruled out as an explanation for this heavy Si isotope signature. Instead, we show that equilibrium isotopic fractionation between gaseous SiO and solid forsterite at ∼1370 K in the solar nebula could have produced the observed Si isotope variations. Nebular fractionation of forsterite should be accompanied by correlated variations between the Si isotopic composition and Mg/Si ratio following a slope of ∼1, which is observed in meteorites. Consideration of this nebular process leads to a revised Si concentration in the Earth's core of 3.6 (+6.0/−3.6) wt% and provides estimates of Mg/Si ratios of bulk planetary bodies.

Landscape cultivation alters δ³⁰Si signature in terrestrial ecosystems.

Despite increasing recognition of the relevance of biological cycling for Si cycling in ecosystems and for Si export from soils to fluvial systems, effects of human cultivation on the Si cycle are still relatively understudied. Here we examined stable Si isotope (δ30Si) signatures in soil water samples across a temperate land use gradient. We show that – independent of geological and climatological variation – there is a depletion in light isotopes in soil water of intensive croplands and managed grasslands relative to native forests. Furthermore, our data suggest a divergence in δ30Si signatures along the land use change gradient, highlighting the imprint of vegetation cover, human cultivation and intensity of disturbance on δ30Si patterns, on top of more conventionally acknowledged drivers (i.e. mineralogy and climate).

Silicon isotopes record dissolution and re-precipitation of pedogenic clay minerals in a podzolic soil chronosequence

By providing the largest part of the reactive surface area of soils, secondary minerals play a major role in terrestrial biogeochemical processes. The understanding of the mechanisms governing neo(trans-)formation of pedogenic clay minerals in soils is therefore of the utmost importance to learn how soils evolve and impact the chemistry of elements in terrestrial environments. Soil-forming processes governing the evolution of secondary aluminosilicates in Podzols are however still not fully understood. The evolution of silicon (Si) isotope signature in the clay fraction of a podzolic soil chronosequence can provide new insight into these processes, enabling to trace the source of Si in secondary aluminosilicates during podzol-forming processes characterized by the mobilization, transport and precipitation of carbon, metals and Si. The Si isotope compositions in the clay fraction (comprised of primary and secondary minerals) document an increasing light 28Si enrichment and depletion with soil age, respectively in illuvial B horizons and eluvial E horizon. The mass balance approach demonstrates that secondary minerals in the topsoil eluvial E horizons are isotopically heavier with δ30Si values increasing from −0.39 to +0.64‰ in c.a. 200years, while secondary minerals in the illuvial Bhs horizon are isotopically lighter (δ30Si=−2.31‰), compared to the original “unweathered” secondary minerals in BC horizon (δ30Si=−1.40‰). The evolution of Si isotope signatures is explained by the dissolution of pedogenic clay minerals in the topsoil, which is a source of light 28Si for the re-precipitation of new clay minerals in the subsoil. This provides consistent evidence that in strong weathering environment such as encountered in Podzols, Si released from secondary minerals is partially used to form “tertiary clay minerals” over very short time scales (ca. 300years). Our dataset demonstrates the usefulness to measure Si isotope signatures in the clay fraction to discern clay mineral changes (e.g., neoformation versus solid state transformation) during soil evolution. This offers new opportunity to better understand clay mineral genesis under environmental changes, and the short-term impact of the dissolution and re-precipitation of pedogenic clay minerals on soil fertility, soil carbon budget and elemental cycles in soil–plant systems.

Testing the limits of micro-scale analyses of Si stable isotopes by femtosecond laser ablation multicollector inductively coupled plasma mass spectrometry with application to rock weathering

An analytical protocol for accurate in-situ Si stable isotope analysis has been established on a new second-generation custom-built femtosecond laser ablation system. The laser was coupled to a multicollector inductively coupled plasma mass spectrometer (fsLA-MC-ICP-MS). We investigated the influence of laser parameters such as spot size, laser focussing, energy density and repetition rate, and ICP-MS operating conditions such as ICP mass load, spectral and non-spectral matrix effects, signal intensities, and data processing on precision and accuracy of Si isotope ratios. We found that stable and reproducible ICP conditions were obtained by using He as aerosol carrier gas mixed with Ar/H2O before entering the plasma. Precise δ29Si and δ30Si values (better than ±0.23‰, 2SD) can be obtained if the area ablated is at least 50×50μm; or, alternatively, for the analysis of geometric features down to the width of the laser spot (about 20μm) if an equivalent area is covered. Larger areas can be analysed by rastering the laser beam, whereas small single spot analyses reduce the attainable precision of δ30Si to ca. ±0.6‰, 2SD, for <30μm diameter spots. It was found that focussing the laser beam beneath the sample surface with energy densities between 1 and 3.8J/cm2 yields optimal analytical conditions for all materials investigated here. Using pure quartz (NIST 8546 aka. NBS-28) as measurement standard for calibration (standard-sample-bracketing) did result in accurate and precise data of international reference materials and samples covering a wide range in chemical compositions (Si single crystal IRMM-017, basaltic glasses KL2-G, BHVO-2G and BHVO-2, andesitic glass ML3B-G, rhyolitic glass ATHO-G, diopside glass JER, soda-lime glasses NIST SRM 612 and 610, San Carlos olivine). No composition-dependent matrix effect was discernible within uncertainties of the method.The method was applied to investigate the Si isotope signature of rock weathering at the micro-scale in a corestone sampled from a highly weathered roadcut profile in the tropical Highlands of Sri Lanka. The results show that secondary weathering products accumulated in cracks and grain boundaries are isotopically lighter than their unweathered plagioclase host, consistent with isotopically heavy dissolved Si found in rivers.

Si stable isotope fractionation during adsorption and the competition between kinetic and equilibrium isotope fractionation: Implications for weathering systems

The adsorption of Si onto amorphous Al-hydroxides is the cause for the light Si isotope signature that secondary crystalline clay minerals in weathering systems carry. We propose this hypothesis from a series of adsorption experiments in which the light isotopes are being favored during Si adsorption onto crystalline gibbsite and in which the associated fractionation factor depends on the solution's initial Si concentration.Three adsorption experiments were carried out at pH7 with different initial Si concentrations of 0.36, 0.71 and 1.42mmol/l Si start concentrations. As Al-hydroxide adsorbent, 30g/l crystalline gibbsite were used to provide equal surface area in all experiments. Adsorption rates are higher with higher initial Si concentration. At the same time, calculated apparent isotope fractionation factors 103lnαadsorbed/solution decrease from −1.8 to −3‰ with increasing initial Si concentration. As care was taken to avoid isotope fractionation during transport of dissolved Si to the gibbsite surface, the mass dependence of the activation energy barrier at the interface is causing the kinetic isotope fractionation. Within the mass balance framework of DePaolo (2011) the shift in Si isotope fractionation with initial Si concentration is interpreted to be induced by different kinetic isotope fractionation factors associated with the forward reaction. Only after ca. two months do the isotope ratios begin to adjust to an equilibrium isotope fractionation factor that is close to 0‰. With such slow re-equilibration Si adsorption differs fundamentally from transition metals that re-equilibrate isotopically within hours after adsorption onto Fe and Mn oxide surfaces.These observations may provide an explanation for the light Si isotope signature clay minerals formed during weathering carry: the light Si isotope composition is being inherited early on during Si adsorption onto amorphous Al-hydroxides and is potentially carried over during all further stages of transformation.

The influence of water mass mixing on the dissolved Si isotope composition in the Eastern Equatorial Pacific

Silicon isotopes are a powerful tool to investigate the cycling of dissolved silicon (Si). In this study the distribution of the Si isotope composition of dissolved silicic acid (δ30Si(OH)4) was analyzed in the water column of the Eastern Equatorial Pacific (EEP) where one of the globally largest Oxygen Minimum Zones (OMZs) is located. Samples were collected at 7 stations along two meridional transects from the equator to 14°S at 85°50′W and 82°00′W off the Ecuadorian and Peruvian coast. Surface waters show a large range in isotope compositions δ30Si(OH)4 (+2.2‰ to +4.4‰) with the highest values found at the southernmost station at 14°S. This station also revealed the most depleted silicic acid concentrations (0.2 μmol/kg), which is a function of the high degree of Si utilization by diatoms and admixture with waters from highly productive areas. Samples within the upper water column and the OMZ at oxygen concentrations below 10 μmol/kg are characterized by a large range in δ30Si(OH)4, which mainly reflects advection and mixing of different water masses, even though the highly dynamic hydrographic system of the upwelling area off Peru does not allow the identification of clear Si isotope signals for distinct water masses. Therefore we cannot rule out that also dissolution processes have an influence on the δ30Si(OH)4 signature in the subsurface water column. Deep water masses (>2000 m) in the study area show a mean δ30Si(OH)4 of +1.2±0.2‰, which is in agreement with previous studies from the eastern and central Pacific. Comparison of the new deep water data of this study and previously published data from the central Pacific and Southern Ocean reveal substantially higher δ30Si(OH)4 values than deep water signatures from the North Pacific. As there is no clear correlation between δ30Si(OH)4 and silicic acid concentrations in the entire data set the distribution of δ30Si(OH)4 signatures in deep waters of the Pacific is considered to be mainly a consequence of the mixing of several end member water masses with distinct Si isotope signatures including Lower Circumpolar Deep Water (LCDW) and North Pacific Deep Water (NPDW).

Controls on riverine δ30Si signatures in a temperate watershed under high anthropogenic pressure (Scheldt — Belgium)

We analysed Si isotopic signatures (δ30Si signature) of dissolved silicon (DSi) in the Scheldt (Belgium) tidal river (four stations) and in its main tributaries (Zenne, Dender, Grote Nete and Kleine Nete). For each station, we analysed four samples representative of the four seasons. These are the first δ30Si data at the basin and seasonal scales measured in a watershed under high anthropogenic pressure. In the Scheldt main stem, we show that the role of diatoms on the silicon biogeochemical cycle and δ30Si signatures is dominant in summer and sometimes in fall. Out of the diatoms' growing season, the Scheldt main stem exhibits unexpected large δ30Si signatures variations, especially in winter. Although these isotopic variations are not reflected in DSi concentrations, they are consistent with δ30Si signatures recorded in the main tributaries. This indicates that except for summer diatom blooms, δ30Si signatures in the tidal river result from a conservative mixing with tributaries. Overall we suggest that the variations are mainly set by land use and lithology at the sub-basin level. Forest cover (inducing clay dissolution favoured by the presence of humic acids) and wetland cover (yielding to dissolution of biogenic silica) are highly and significantly correlated with δ30Si (R2>0.85). Both environments tend to render the tributaries δ30Si signatures lighter while agriculture cover has the reverse effect on δ30Si values (R2=0.70, p-value<0.08). In contrast, urban zone cover does not have any noticeable effect on Si isotopic signatures of the Scheldt tributaries. Despite this effect of land use, we estimate a mean annual δ30Si signature output from the Scheldt tidal river to the saline estuary at 0.95‰, well comparable to other rivers. This results from the mixing of tributaries within the tidal river which levels off at the outlet the Si isotopic differences of the sub-basins. In the Scheldt watershed, it thus seems that Si isotopes could trace anthropogenic activities at the sub-basin scale. In contrast, at the outlet, the variety of land-use and lithology found over the whole watershed are merged into a mean isotopic signature similar to the world river average. As it is now recognised that change in land use alter the silicon biogeochemical continental cycle, our data show that use of Si isotopic signatures might constitute a useful proxy to better trace these modifications on contrasted (sub-)basins.

Silicon Isotope Evidence Against an Enstatite Chondrite Earth

The compositions of Earth materials are strikingly similar to those of enstatite chondrite meteorites in many isotope systems. Although this suggests that Earth largely accreted from enstatite chondrites, definitive proof of this model has been lacking. By comparing the silicon (Si) isotope signatures of several extraterrestrial materials with terrestrial samples, we show that they cannot be explained by core-formation scenarios involving a bulk Earth of enstatite chondrite composition. Si isotope similarities between the bulk silicate Earth and the Moon preclude the existence of a hidden reservoir in the lower mantle, a necessary condition of the enstatite chondrite model, and require an equilibrium process after the Moon-forming impact. A three-end-member chondritic mixing model for Earth reconciles the Si isotope similarities between enstatite chondrites and Earth.

Climate dependent diatom production is preserved in biogenic Si isotope signatures

Abstract. The aim of this study was to reconstruct diatom production in the subarctic northern tip of the Baltic Sea, Bothnian Bay, based on down-core analysis of Si isotopes in biogenic silica (BSi). Dating of the sediment showed that the samples covered the period 1820 to 2000. The sediment core record can be divided into two periods, an unperturbed period from 1820 to 1950 and a second period affected by human activities (from 1950 to 2000). This has been observed elsewhere in the Baltic Sea. The shift in the sediment core record after 1950 is likely caused by large scale damming of rivers. Diatom production was inferred from the Si isotope composition which ranged between δ30Si −0.18‰ and +0.58‰ in BSi, and assuming fractionation patterns due to the Raleigh distillation, the production was shown to be correlated with air and water temperature, which in turn were correlated with the mixed layer (ML) depth. The sedimentary record showed that the deeper ML depth observed in colder years resulted in less production of diatoms. Pelagic investigations in the 1990's have clearly shown that diatom production in the Baltic Sea is controlled by the ML depth. Especially after cold winters and deep water mixing, diatom production was limited and dissolved silicate (DSi) concentrations were not depleted in the water column after the spring bloom. Our method corroborates these findings and offers a new method to estimate diatom production over much longer periods of time in diatom dominated aquatic systems, i.e. a large part of the world's ocean and coastal seas.

Silicon pool dynamics and biogenic silica export in the Southern Ocean inferred from Si-isotopes

Abstract. Silicon isotopic signatures (δ30Si) of water column silicic acid (Si(OH)4) were measured in the Southern Ocean, along a meridional transect from South Africa (Subtropical Zone) down to 57° S (northern Weddell Gyre). This provides the first reported data of a summer transect across the whole Antarctic Circumpolar Current (ACC). δ30Si variations are large in the upper 1000 m, reflecting the effect of the silica pump superimposed upon meridional water transfer across the ACC: the transport of Antarctic surface waters northward by a net Ekman drift and their convergence and mixing with warmer upper-ocean Si-depleted waters to the north. Using Si isotopic signatures, we determine different mixing interfaces: the Antarctic Surface Water (AASW), the Antarctic Intermediate Water (AAIW), and thermoclines in the low latitude areas. The residual silicic acid concentrations of end-members control the δ30Si alteration of the mixing products and with the exception of AASW, all mixing interfaces have a highly Si-depleted mixed layer end-member. These processes deplete the silicic acid AASW concentration northward, across the different interfaces, without significantly changing the AASW δ30Si composition. By comparing our new results with a previous study in the Australian sector we show that during the circumpolar transport of the ACC eastward, the δ30Si composition of the silicic acid pools is getting slightly, but significantly lighter from the Atlantic to the Australian sectors. This results either from the dissolution of biogenic silica in the deeper layers and/or from an isopycnal mixing with the deep water masses in the different oceanic basins: North Atlantic Deep Water in the Atlantic, and Indian Ocean deep water in the Indo-Australian sector. This isotopic trend is further transmitted to the subsurface waters, representing mixing interfaces between the surface and deeper layers. Through the use of δ30Si constraints, net biogenic silica production (representative of annual export), at the Greenwich Meridian is estimated to be 5.2 &amp;amp;pm; 1.3 and 1.1 &amp;amp;pm; 0.3 mol Si m−2 for the Antarctic Zone and Polar Front Zone, respectively. This is in good agreement with previous estimations. Furthermore, summertime Si-supply into the mixed layer of both zones, via vertical mixing, is estimated to be 1.6 &amp;amp;pm; 0.4 and 0.1 &amp;amp;pm; 0.5 mol Si m−2, respectively.

Micrometer silicon isotope diagnostics of soils by UV femtosecond laser ablation

This study presents the first Si isotope data of the principle Si pools in soils determined by a UV femtosecond laser ablation system coupled to a multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS). This method reveals accurate and precise Si isotope data on bulk materials, and at high spatial resolution, on the mineral scale. The following Si pools have been investigated: a) the Si source to soils on all major silicate minerals on thin sections from bedrock fragments in the soil profiles; b) bulk soils (particle size < 2 mm) after fusion to glass beads with an iridium-strip heater or pressed into powder pellets; c) separated clay fractions as pressed powder pellets and e) separated phytoliths as pressed powder pellets. Multiple analyses of three rock standards, BHVO-2, AGV-1 and RGM-1 as fused glass beads and as pressed powder pellets, reveal δ 30Si values within the expected range of igneous rocks. The MPI-DING reference glass KL2-G exhibits the same Si isotope composition after remelting by an iridium-strip heater showing that this technique does not alter the isotope composition of the glass. We used this approach to investigated two immature Cambisols developed on sandstone and paragneiss in the Black Forest (Germany), respectively. Bulk soils show a largely uniform Si isotope signature for different horizons and locations, which is close to those of primary quartz and feldspar with δ 30Si values around − 0.4‰. Soil clay formation is associated with limited Si mobility, which preserves initial Si isotope signatures of parental minerals. An exception is the organic horizon of the paragneiss catchment where intense weathering leads to a high mobility of Si and significant negative isotope signatures as low as to − 1.00‰ in bulk soils. Biogenic opal in the form of phytoliths, exhibits negative Si isotope signatures of about − 0.4‰. These results demonstrate that UV femtosecond laser ablation MC-ICP-MS provides a tool to characterize the Si isotope signature of the principle Si pools left behind after weathering and Si transport have altered soils. These results can now serve as a fingerprint of the residual solids that can be used to explain the isotope composition of dissolved Si in soil solutions and river water, which is mostly enriched in the heavy isotopes.

Isotopic constraints on the Si-biogeochemical cycle of the Antarctic Zone in the Kerguelen area (KEOPS)

Estimation of the silicon (Si) mass balance in the ocean from direct measurements (Si uptake-dissolution rates …) is plagued by the strong temporal and spatial variability of the surface ocean as well as methodological artifacts. Tracers with different sensitivities toward physical and biological processes would be of great complementary use. Silicon isotopic composition is a promising proxy to improve constraints on the Si-biogeochemical cycle, since it integrates over longer timescales in comparison with direct measurements and since the isotopic balance allows to resolve the processes involved, i.e. uptake, dissolution, mixing. Si-isotopic signatures of seawater Si(OH) 4 and biogenic silica (bSiO 2) were investigated in late summer 2005 during the KEOPS experiment, focusing on two contrasting biogeochemical areas in the Antarctic Zone: a natural iron-fertilized area above the Kerguelen Plateau (< 500 m water depth) and the High Nutrient Low Chlorophyll area (HNLC) east of the plateau (> 1000 m water depth). For the HNLC area the Si-isotopic constraint identified Upper Circumpolar Deep Water as being the ultimate Si-source. The latter supplies summer mixed layer with 4.0 ± 0.7 mol Si m − 2 yr − 1 . This supply must be equivalent to the net annual bSiO 2 production and exceeds the seasonal depletion as estimated from a simple mixed layer mass balance (2.5 ± 0.2 mol Si m − 2 yr − 1 ). This discrepancy reveals that some 1.5 ± 0.7 mol Si m − 2 yr − 1 must be supplied to the mixed layer during the stratification period. For the fertilized plateau bloom area, a low apparent mixed layer isotopic fractionation value (∆ 30Si) probably reflects (1) a significant impact of bSiO 2 dissolution, enriching the bSiO 2 pool in heavy isotope; and/or (2) a high Si uptake over supply ratio in mixed layer at the beginning of the bloom, following an initial closed system operating mode, which, however, becomes supplied toward the end of the bloom (low Si uptake over supply ratio) with isotopically light Si(OH) 4 from below when the surface Si(OH) 4 pool is significantly depleted. We estimated a net integrated bSiO 2 production of 10.5 ± 1.4 mol Si m − 2 yr − 1 in the AASW above the plateau, which includes a significant contribution of bSiO 2 production below the euphotic layer. However, advection which could be significant for this area has not been taken into account in the latter estimation based on a 1D approach of the plateau system. Finally, combining the KEOPS Si-isotopic data with those from previous studies, we refined the average Si-isotopic fractionation factor to − 1.2 ± 0.2‰ for the Antarctic Circumpolar Current.

Deciphering formation processes of banded iron formations from the Transvaal and the Hamersley successions by combined Si and Fe isotope analysis using UV femtosecond laser ablation

To investigate the genesis of BIFs, we have determined the Fe and Si isotope composition of coexisting mineral phases in samples from the ∼2.5 billion year old Kuruman Iron Formation (Transvaal Supergroup, South Africa) and Dales Gorges Member of the Brockman Iron Formation (Hamersley Group, Australia) by UV femtosecond laser ablation coupled to a MC-ICP-MS. Chert yields a total range of δ 30Si between −1.3‰ and −0.8‰, but the Si isotope compositions are uniform in each core section examined. This uniformity suggests that Si precipitated from well-mixed seawater far removed from its sources such as hydrothermal vents or continental drainage. The Fe isotope composition of Fe-bearing mineral phases is much more heterogeneous compared to Si with δ 56Fe values of −2.2‰ to 0‰. This heterogeneity is likely due to variable degrees of partial Fe(II) oxidation in surface waters, precipitation of different mineral phases and post-depositional Fe redistribution. Magnetite exhibits negative δ 56Fe values, which can be attributed to a variety of diagenetic pathways: the light Fe isotope composition was inherited from the Fe(III) precursor, heavy Fe(II) was lost by abiotic reduction of the Fe(III) precursor or light Fe(II) was gained from external fluids. Micrometer-scale heterogeneities of δ 56Fe in Fe oxides are attributed to variable degrees of Fe(II) oxidation or to isotope exchange upon Fe(II) adsorption within the water column and to Fe redistribution during diagenesis. Diagenetic Fe(III) reduction caused by oxidation of organic matter and Fe redistribution is supported by the C isotope composition of a carbonate-rich sample containing primary siderite. These carbonates yield δ 13C values of ∼−10‰, which hints at a mixed carbon source in the seawater of both organic and inorganic carbon. The ancient seawater composition is estimated to have a minimum range in δ 56Fe of −0.8‰ to 0‰, assuming that hematite and siderite have preserved their primary Fe isotope signature. The long-term near-zero Fe isotope composition of the Hamersley and Transvaal BIFs is in balance with the assumed composition of the Fe sources. The negative Fe isotope composition of the investigated BIF samples, however, indicates either a perturbation of the steady state, or they have to be balanced spatially by deposition of isotopically heavy Fe. In the case of Si, the negative Si isotope signature of these BIFs stands in marked contrast to the assumed source composition. The deviation from potential source composition requires a complementary sink of isotopically heavy Si in order to maintain steady state in the basin. Perturbing the steady state by extraordinary hydrothermal activity or continental weathering in contrast would have led to precipitation of light Si isotopes from seawater. Combining an explanation for both elements, a likely scenario is a steady state ocean basin with two sinks. When all published Fe isotope records including BIFs, microbial carbonates, shales and sedimentary pyrites, are considered, a complementary sink for heavy Fe isotopes must have existed in Precambrian ocean basins. This Fe sink could have been pelagic sediments, which however are not preserved. For Si, such a complementary sink for heavy Si isotopes might have been provided by other chert deposits within the basin.

Variations of δ 30Si and Ge/Si with weathering and biogenic input in tropical basaltic ash soils under monoculture

In soils, silicon released by mineral weathering can be retrieved from soil solution through clay formation, Si adsorption onto secondary oxides and plant uptake, thereby impacting the Si-isotopic signature and Ge/Si ratio of dissolved Si (DSi) exported to rivers. Here we use these proxies to study the contribution of biogenic Si (BSi) in a soil–plant system involving basaltic ash soils differing in weathering degree under intensive banana cropping. δ 30Si and Ge/Si ratios were determined in bulk soils (<2 mm), sand (50–2000 μm), silt (2–50 μm), amorphous Si (ASi, 2–50 μm) and clay (<2 μm) fractions: δ 30Si by MC–ICP–MS Nu Plasma in medium resolution, operating in dry plasma with Mg doping (δ 30Si vs. NBS28 ± 0.12‰ ± 2 σ SD), Ge/Si computed after determination of Ge and Si concentrations by HR–ICP–MS and ICP–AES, respectively. Components of the ASi fraction were quantified by microscopic counting (phytoliths, diatoms, ashes). Compared to fresh ash (δ 30Si = −0.38‰; Ge/Si = 2.21 μmol mol −1), soil clay fractions (<2 μm) were enriched in light Si isotopes and Ge: with increasing weathering degree, δ 30Si decreased from −1.19 to −2.37‰ and Ge/Si increased from 4.10 to 5.25 μmol mol −1. Sand and silt fractions displayed δ 30Si values close to fresh ash (−0.33‰) or higher due to saharian dust quartz deposition, whose contribution was evaluated by isotopic mass balance calculation. Si-isotopic signatures of bulk soils (<2 mm) were strongly governed by the relative proportions of primary and secondary minerals: the bulk soil Si-isotopic budget could be closed indicating that all the phases involved were identified. Microscopic counting highlighted a surface accumulation of banana phytoliths and a stable phytolith pool from previous forested vegetation. δ 30Si and Ge/Si values of clay fractions in poorly developed volcanic soils, isotopically heavier and Ge-depleted in surface horizons, support the occurrence of a DSi source from banana phytolith dissolution, available for Si sequestration in clay-sized secondary minerals (clay minerals formation and Si adsorption onto Fe-oxide). In the soil–plant system, δ 30Si and Ge/Si are thus highly relevant to trace weathering and input of DSi from phytoliths in secondary minerals, although not quantifying the net input of BSi to DSi.

Impact of soil weathering degree on silicon isotopic fractionation during adsorption onto iron oxides in basaltic ash soils, Cameroon

The sequestration of silicon in soil clay-sized iron oxides may affect the terrestrial cycle of Si. Iron oxides indeed specifically adsorb aqueous monosilicic acid (H 4SiO 4 0), thereby influencing Si concentration in soil solution. Here we study the impact of H 4SiO 4 0 adsorption on the fractionation of Si isotopes in basaltic ash soils differing in weathering degree (from two weathering sequences, Cameroon), hence in clay and Fe-oxide contents, and evaluate the potential isotopic impact on dissolved Si in surrounding Cameroon rivers. Adsorption was measured in batch experiment series designed as function of time (0–72 h) and initial concentration (ic) of Si in solution (0.61–1.18 mM) at 20 °C, constant pH (5.5) and ionic strength (1 mM). After various soil-solution contact times, the δ 30Si vs. NBS28 compositions were determined in selected solutions by MC-ICP-MS (Nu Plasma) in medium resolution, operating in dry plasma with Mg doping with an average precision of ±0.15‰ (±2σ SEM). The quantitative adsorption of H 4SiO 4 0 by soil Fe-oxides left a solution depleted in light Si isotopes, which confirms previous study on synthetic Fe-oxides. Measured against its initial composition (δ 30Si = +0.02 ± 0.07‰ (±2σ SD)), the solutions were systematically enriched in 30Si reaching maximum δ 30Si values ranging between +0.16‰ and +0.95‰ after 72 h contact time. The enrichment of the solution in heavy isotopes increased with increasing values of three parameters: soil weathering degree, iron oxide content, and proportion of short-range ordered Fe-oxide. The Si-isotopic signature of the solution was partly influenced by Si release, possibly through mineral dissolution and Si desorption from oxide surfaces, depending on soil type, highlighting the complex pattern of natural soils. Surrounding Cameroon rivers displayed a mean Si-isotopic signature of +1.19‰. Our data imply that in natural environments, H 4SiO 4 0 adsorption by soil clay-sized Fe-oxides at least partly impacts the Si-isotopic signature of the soil solution exported to water streams.