Si Isotope Research Articles

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.

Quantum effects in silicon isotopes at low temperatures under normal and high pressures

Precision studies of Raman scattering in isotopically pure 28Si, 29Si, and 30Si single crystals have been performed in the temperature range from 8 to 300 K. It was found that the quantum effects manifest themselves noticeably at low temperatures (T ≤ 100 K). This can be seen from the difference ∼(0.4 ± 0.1) cm−1 between the normalized frequencies of two extreme 28Si and 30Si isotopes, while at room temperature, quantum effects are not visible and are comparable with the measurement error of frequencies (0.1 cm−1). Raman scattering in silicon isotopes in the region of noticeable quantum effects has been studied under hydrostatic conditions at pressures to 12 GPa and at a temperature of Т = 80 K. Quantum effects are not changed under compression within the accuracy of measurements.

Structure of the 1d2s Shell from the Data on Single-Nucleon-Transfer Reactions

For the stable silicon isotopes 28Si and 30Si, the energies and occupancies of single-particle states of the 1d2s shell are determined by analyzing the combined data for the nucleon stripping and pickup reactions. From the results of analysis and the available experimental data, the variation of the 1d2s proton-shell structure with increasing neutron number for the silicon isotopes with A = 26–34 is discussed.

Whole‐Ocean Changes in Silica and Ge/Si Ratios During the Last Deglacial Deduced From Long‐Lived Giant Glass Sponges

AbstractSilicon is a keystone nutrient in the ocean for understanding climate change because of the importance of Southern Ocean diatoms in taking up CO2 from the surface ocean‐atmosphere system and sequestering carbon into the deep sea. Here we report on silicon isotopes and germanium‐to‐silicon ratios in giant glass spicules of deep‐sea sponge Monorhaphis chuni over the past 17,000 years. In situ measurements of Si isotopes and Ge concentrations show systematic variations from rim to center of the cross sections. When calibrated against seawater concentrations using data from modern spicule rims, sponge data indicate that dissolved silica concentrations in the deep Pacific were ~12% higher during the early deglacial. These deep Pacific Ocean data help to fill an important global gap in paleo‐nutrient records. Either continental sources supplied more silica to the deglacial ocean and/or biogenic silica burial was lower, both of which may have affected atmospheric CO2.

Temperature dependent 29Si incorporation during deposition of highly enriched 28Si films.

In this study, we examine the mechanisms leading to 29Si incorporation into highly enriched 28Si films deposited by hyperthermal ion beams at elevated temperatures in the dilute presence of natural abundance silane (SiH4) gas. Enriched 28Si is a critical material in the development of quantum information devices because 28Si is free of nuclear spins that cause decoherence in a quantum system. We deposit epitaxial thin films of 28Si enriched in situ beyond 99.99998 % 28Si onto Si(100) using an ion beam deposition system and seek to develop the ability to systematically vary the enrichment and measure the impact on quantum coherence. We use secondary ion mass spectrometry to measure the residual 29Si isotope fraction in enriched samples deposited from ≈ 250 °C up to 800 °C. The 29Si isotope fraction is found to increase from < 1 × 10-6 at the lower temperatures, up to > 4 × 10-6 at around 800 °C. From these data, we estimate the temperature dependence of the incorporation fraction, s, of SiH4, which increases sharply from about 2.9 × 10-4 at 500 °C to 2.3 × 10-2 at 800 °C. We determine an activation energy of 1.00(8) eV associated with the abrupt increase in incorporation and conclude that below 500 °C, a temperature independent mechanism such as activation from ion collisions with adsorbed SiH4 molecules is the primary incorporation mechanism. Direct incorporation from the adsorbed state is found to be minimal.

Silicon and chromium stable isotopic systematics during basalt weathering and lateritisation: A comparison of variably weathered basalt profiles in the Deccan Traps, India

Global biomass production is fundamentally affected by the hydrological cycling of elements at the Earth's surface. Continental weathering processes are the major source for most bio-essential elements in marine environments and therefore affect primary productivity. In addition, critical zone biomass depends on energy and chemical exchange reactions in weathering profiles. The latter reservoirs are in turn influenced by different climatic conditions that control weathering and pore water parameters like pH and Eh, which regulate mineral break down rates and dictate the mobility and mass flux of elements.Two Deccan Traps basalt weathering profiles of contrasting age and alteration intensity provide a natural laboratory for investigating the effects of rock alteration on Si and Cr and their isotopic systematics. The Quaternary Chhindwara profile has progressed to a moderate degree of alteration (saprolite), while the Paleogene Bidar example displays an extremely altered laterite. The Chhindwara saprolite profile shows a near uniform Cr and Si concentration and isotopic composition, whereas the Bidar laterite profile is characterised by an intense loss of Si, a large enrichment of Cr within the most altered uppermost levels, and a wide range of Cr stable isotope ratios (−0.85 to 0.36‰ δ53/52Cr). A co-variation between Si and Cr isotopes, as well as their co-variation with iron content, provides empirical evidence that iron redistribution within the profile has a large effect on Cr mobility and Si isotopic fractionation. Therefore, it is concluded that iron oxides exert a primary control over the isotopic composition of both Cr and Si in pore waters of laterites.Since laterite formation is promoted by tropical climates, the results of this study provide new evidence to suggest that the hydrological Cr and Si fluxes originating from continental weathering have changed in accordance with large-scale, deep time climate variation and continental plate configuration. An increased flux of Si and greater magnitude of Cr mobility and isotopic fractionation are possibly amplified under CO2-rich, greenhouse episodes and/or when large landmasses were tectonically arranged at near equatorial latitudes.

Effect of Weak Binding on the Apparent Spin-Orbit Splitting in Nuclei.

The apparent splitting between orbitals that are spin-orbit partners can be substantially influenced by the effects of weak binding. In particular, such effects can account for the observed decrease in separation of the neutron 1p_{3/2} and 1p_{1/2} orbitals between the ^{41}Ca and ^{35}Si isotopes. This behavior has been the subject of recent experimental and theoretical works and cited as evidence for a proton "bubble" in ^{34}Si causing an explicit weakening of the spin-orbit interaction. The results reported here suggest that the change in the separation between the 1p_{3/2} and 1p_{1/2} partners occurs dominantly because of the behavior of the energies of these 1p neutron states near zero binding.

Determination of the isotopic composition and molar mass of a new 'Avogadro' crystal: homogeneity and enrichment-related uncertainty reduction

The molar mass M and isotopic composition (expressed in amount-of-substance fractions x(iSi) of the silicon isotopes 28Si, 29Si, and 30Si) of a new silicon crystal (notation: Si28-23Pr11) highly enriched in the 28Si isotope have been determined independently at PTB and NMIJ by measuring exactly the same sample solutions using both a high resolution multicollector-inductively coupled plasma mass spectrometer (MC-ICP-MS). This crystal will be used for the complementary determination of the Avogadro constant NA and thus providing one of many key parameters in the planned redefinition of the SI units kilogram and mole, using fundamental constants. Samples from three different axial positions in the crystal ingot, each divided into several radial positions were measured in order to probe possible variations of the molar mass and isotopic composition. Results obtained at PTB and NMIJ agreed within the limits of uncertainty. The application of the latest improved measurement techniques as well as an improved determination of the calibration factors (K) required to correct for mass bias effects resulted in an averaged M = 27.976 942 666(40) g mol−1 with a relative combined uncertainty uc,rel(M) = 1.4 × 10−9. The course of M as a function of the origin of the measured samples suggests no significant inhomogeneity within the limits of the claimed uncertainty throughout the crystal supporting its applicability for the determination of a new NA. This extends to x(28Si) and x(29Si). Variations in x(30Si) as a function of the sample location were observed, but a systematic relation to physical origins cannot be claimed. Compared to the previous silicon crystal (‘AVO28’, notation: Si28-10Pr11) used for the latest determination of NA, the enrichment increases from x(28Si) = 0.999 957 52(12) mol mol−1 (‘AVO28’) to x(28Si) = 0.999 984 470(39) mol mol−1 (Si28-23Pr11, discussed in this paper) which is at least in part responsible for a reduction of the associated measurement uncertainty u(M).

Iron and silicon isotope behaviour accompanying weathering in Icelandic soils, and the implications for iron export from peatlands

Incipient warming of peatlands at high latitudes is expected to modify soil drainage and hence the redox conditions, which has implications for Fe export from soils. This study uses Fe isotopes to assess the processes controlling Fe export in a range of Icelandic soils including peat soils derived from the same parent basalt, where Fe isotope variations principally reflect differences in weathering and drainage. In poorly weathered, well-drained soils (non-peat soils), the limited Fe isotope fractionation in soil solutions relative to the bulk soil (Δ57Fesolution-soil=−0.11±0.12‰) is attributed to proton-promoted mineral dissolution. In the more weathered poorly drained soils (peat soils), the soil solutions are usually lighter than the bulk soil (Δ57Fesolution-soil=−0.41±0.32‰), which indicates that Fe has been mobilised by reductive mineral dissolution and/or ligand-controlled dissolution. The results highlight the presence of Fe-organic complexes in solution in anoxic conditions. An additional constraint on soil weathering is provided by Si isotopes. The Si isotope composition of the soil solutions relative to the soil (Δ30Sisolution-soil=0.92±0.26‰) generally reflects the incorporation of light Si isotopes in secondary aluminosilicates. Under anoxic conditions in peat soils, the largest Si isotope fractionation in soil solutions relative to the bulk soil is observed (Δ30Sisolution-soil=1.63±0.40‰) and attributed to the cumulative contribution of secondary clay minerals and amorphous silica precipitation. Si supersaturation in solution with respect to amorphous silica is reached upon freezing when Al availability to form aluminosilicates is limited by the affinity of Al for metal–organic complexes. Therefore, the precipitation of amorphous silica in peat soils indirectly supports the formation of metal–organic complexes in poorly drained soils. These observations highlight that in a scenario of decreasing soil drainage with warming high latitude peatlands, Fe export from soils as Fe-organic complexes will increase, which in turn has implications for Fe transport in rivers, and ultimately the delivery of Fe to the oceans.

Single-particle properties of N = 12 to N = 20 silicon isotopes within the dispersive optical model

Experimental neutron and proton single-particle energies in N = 12 to N = 20 silicon isotopes and data on neutron and proton scattering by nuclei of the isotope 28Si are analyzed on the basis of the dispersive optical model. Good agreement with available experimental data was attained. The occupation probabilities calculated for the single-particle states in question suggest a parallel-type filling of the 1d and 2s 1/2 neutron states in the isotopes 26,28,30,32,34Si. The single-particle spectra being considered are indicative of the closure of the Z = 14 proton subshell in the isotopes 30,32,34Si and the N = 20 neutron shell.

(Invited) Atom Probe Tomography of 1D, 2D and 3D Semiconductors

Atom probe tomography (APT) is a nanoscale materials characterization technique that combines field ion microscopy with time-of-flight mass spectrometry in order to yield a three-dimension imaging of sub-micrometer structure with atomic scale spatial resolution and isotope level sensitivity. Although it was originally developed for metals, significant instrumentation advances in the past decade, including the implementation of ultrafast lasers, have made this technique also attractive for characterization of semiconductors. Today, atom probe tomography of semiconductors is increasingly being used in correlation with other techniques (e.g. electron microscopy) to investigate doping distribution and segregation, phase separation, and diffusion across heterointerfaces. Experimentally, using a local electrode atom probe (LEAP), either a precisely prepared or an already cone tip-shaped specimen is cryogenically cooled in vacuum (25~150 K), electrically biased (2~10 kV) and excited by ultrafast laser pulses (0.02~2000 pJ pulse energies) to induce field evaporation of ions from its surface. In this talk, we review and present our recent results on the atom probe tomography of 1D, 3D and 2D compound semiconductors and heterostructures. We will also address specimen preparation challenges as well as challenges in properly interpreting collected data. We investigated the laser-assisted APT of 1D semiconductors, including ZnO nanowires and GaAs/AlGaAs core-shell nanowires. Columnar in shape, averaging ~100 nm in diameter, nanowires did not require extensive specimen preparation. Our early APT study of ZnO nanowire addressed the challenge of properly differentiate Zn and O in the mass spectrum, as the main isotopes of Zn and O (64Zn and 16O) exhibited the same mass-to-charge ratio of 32 under ionic species 64Zn++ and 16O2 +. A comparison of ZnO nanowires grown by chemical vapor deposition under Ar and N2 ambients revealed the presence of nitrogen dopant incorporated in the ZnO, which was confirmed by a lower electrical resistivity. Our APT work on GaAs/Al0.33Ga0.67As core-shell nanowires yielded a quantitative measure of the radial composition transition change from the GaAs core to AlGaAs shell, revealing a nearly linear-graded heterojunction over a distance of 6.5 nm. A comparative study of the voltage-mode and the laser-assisted APT of GaN (3D semiconductors) showed that the latter yielded sharper mass spectra. The APT of III-V semiconductors such as GaN often suffers from the loss of nitrogen as neutral N2 species that are not detected, which we showed can be alleviated by using suitable laser pulse energies. Doing so enabled us to obtain overall near-stoichiometric field evaporation of Ga and N elements from the material. Undoped GaN, Mg-doped GaN ([Mg]~1×1019 cm-3) and Si-doped GaN ([Si]~5×1018 cm-3) were investigated and compared. Mg dopants were successfully detected by APT in GaN:Mg as evidenced by the detection of the three natural isotopes of Mg at masses 12, 12.5 and 13, corresponding to 24Mg++, 25Mg++, and 26Mg++. The detection of Si was slightly more challenging because the main isotope 28Si led to a mass spectrum peak of 28 that can be either 28Si+ or 14N2 +. However, a direct comparison between undoped and Si-doped GaN allowed us to confidently identify the Si detected and perform 3D image reconstruction of the dopant distribution. Other 3D semiconductors investigated included AlxGa1-xN and Al1-yInyN ternary compounds. In terms of device heterostructures, we employed APT to probe the buried heterointerface in strained AlGaN/GaN and lattice-matched AlInN/GaN high electron mobility transistor structures. We compared the measured composition interface roughness and interface diffuseness with the measured electrical transport characteristics of the two-dimensional electron gas, as well as before and after gamma ray and proton irradiation under various doses. Finally, we will also present our recent success in using APT to characterize a single monolayer transition metal dichalcogenide materials (2D semiconductors) grown by chemical vapor deposition.

Resolving mass spectral overlaps in atom probe tomography by isotopic substitutions – case of TiSi15N

Mass spectral overlaps in atom probe tomography (APT) analyses of complex compounds typically limit the identification of elements and microstructural analysis of a material. This study concerns the TiSiN system, chosen because of severe mass-to-charge-state ratio overlaps of the 14N+ and 28Si2+ peaks as well as the 14N2+ and 28Si+ peaks. By substituting 14N with 15N, mass spectrum peaks generated by ions composed of one or more N atoms will be shifted toward higher mass-to-charge-state ratios, thereby enabling the separation of N from the predominant Si isotope. We thus resolve thermodynamically driven Si segregation on the nanometer scale in cubic phase Ti1-xSix15N thin films for Si contents 0.08 ≤ x ≤ 0.19 by APT, as corroborated by transmission electron microscopy. The APT analysis yields a composition determination that is in good agreement with energy dispersive X-ray spectroscopy and elastic recoil detection analyses. Additionally, a method for determining good voxel sizes for visualizing small-scale fluctuations is presented and demonstrated for the TiSiN system.

Iron Availability Influences Silicon Isotope Fractionation in Two Southern Ocean Diatoms (Proboscia inermis and Eucampia antarctica) and a Coastal Diatom (Thalassiosira pseudonana)

The fractionation of silicon (Si) isotopes was measured in two Southern Ocean diatoms (Proboscia inermis and Eucampia Antarctica) and a coastal diatom (Thalassiosira pseudonana) that were grown under varying iron (Fe) concentrations. Varying Fe concentrations had no effect on the Si isotope enrichment factor (e) in T. pseudonana, whilst E. antarctica and P. inermis exhibited significant variations in the value of e between Fe-replete and Fe-limited conditions. Mean e values in P. inermis and E. antarctica decreased from (± 1SD) -1.11 ± 0.15‰ and -1.42 ± 0.41‰ (respectively) under Fe-replete conditions, to -1.38 ± 0.27‰ and -1.57 ± 0.5‰ (respectively) under Fe-limiting conditions. These variations likely arise from adaptations in diatoms arising from the nutrient status of their environment. T. pseudonana is a coastal clone typically accustomed to low Si but high Fe conditions whereas E. antarctica and P. inermis are typically accustomed to High Si, High nitrate low Fe conditions. Growth induced variations in silicic acid (Si(OH)4) uptake arising from Fe-limitation is the likely mechanism leading to Si-isotope variability in E. antarctica and P. inermis. The multiplicative effects of species diversity and resource limitation (e.g. Fe) on Si-isotope fractionation in diatoms can potentially alter the Si-isotope composition of diatom opal in diatamaceous sediments and sea surface Si(OH)4. This work highlights the need for further in-vitro studies into intracellular mechanisms involved in Si(OH)4 uptake, and the associated pathways for Si-isotope fractionation in diatoms.

The dynamics of Si cycling during weathering in two small catchments in the Black Forest (Germany) traced by Si isotopes

Silicon stable isotopes have emerged as a powerful proxy to investigate weathering because Si uptake from solution by secondary minerals or by the vegetation causes significant shifts in the isotope composition. In this study, we determined the Si isotope compositions of the principle Si pools within two small catchments located on sandstone and paragneiss, respectively, in the temperate Black Forest (Germany). At both settings, clay formation is dominated by mineral transformation preserving largely the signature of parental minerals with δ30Si values of around −0.7‰. Bulk soils rich in primary minerals are similar to bulk parental material with δ30Si values close to −0.4‰. Topsoils are partly different because organic matter degradation has promoted intense weathering leading to δ30Si values as low as −1.0‰. Water samples expose highly dynamic weathering processes in the soil zone: 1) after spring snowmelt, increased release of DOC and high water fluxes trigger clay mineral dissolution which leads to δ30Si values down to −0.7‰ and 2) in course of the summer, Si uptake by the vegetation and secondary mineral formation drives dissolved Si to typical positive δ30Si values up to 1.1‰. Groundwater with δ30Si values of around 0.4‰ records steady processes in bedrock reflecting plagioclase weathering together with kaolinite precipitation. An isotope mass balance approach reveals incongruent weathering conditions where denudation of Si is largely driven by physical erosion. Erosion of phytoliths contributes 3 to 21% to the total Si export flux, which is in the same order as the dissolved Si flux. These results elucidate the Si dynamics during weathering on catchments underlain of sedimentary origin, prevailing on the Earth surface and provide therefore valuable information to interpret the isotope signature of large river systems.

The isotope composition of inorganic germanium in seawater and deep sea sponges

Although dissolved concentrations of germanium (Ge) and silicon (Si) in modern seawater are tightly correlated, uncertainties still exist in the modern marine Ge cycle. Germanium stable isotope systematics in marine systems should provide additional constraints on marine Ge sources and sinks, however the low concentration of Ge in seawater presents an analytical challenge for isotopic measurement. Here, we present a new method of pre-concentration of inorganic Ge from seawater which was applied to measure three Ge isotope profiles in the Southern Ocean and deep seawater from the Atlantic and Pacific Oceans. Germanium isotopic measurements were performed on Ge amounts as low as 2.6ng using a double-spike approach and a hydride generation system coupled to a MC-ICP-MS. Germanium was co-precipitated with iron hydroxide and then purified through anion-exchange chromatography. Results for the deep (i.e. >1000m depth) Pacific Ocean off Hawaii (nearby Loihi Seamount) and the deep Atlantic off Bermuda (BATS station) showed nearly identical δ74/70Ge values at 3.19±0.31‰ (2SD, n=9) and 2.93±0.10‰ (2SD, n=2), respectively. Vertical distributions of Ge concentration and isotope composition in the deep Southern Ocean for water depth>1300m yielded an average δ74/70Ge=3.13±0.25‰ (2SD, n=14) and Ge/Si=0.80±0.09μmol/mol (2SD, n=12). Significant variations in δ74/70Ge, from 2.62 to 3.71‰, were measured in the first 1000m in one station of the Southern Ocean near Sars Seamount in the Drake Passage, with the heaviest values measured in surface waters. Isotope fractionation by diatoms during opal biomineralization may explain the enrichment in heavy isotopes for both Ge and Si in surface seawater. However, examination of both oceanographic parameters and δ74/70Ge values suggest also that water mass mixing and potential contribution of shelf-derived Ge also could contribute to the variations. Combining these results with new Ge isotope data for deep-sea sponges sampled nearby allowed us to determine a Ge isotope fractionation factor of −0.87±0.37‰ (2SD, n=12) during Ge uptake by sponges. Although Ge has long been considered as a geochemical twin of Si, this work underpins fundamental differences in their isotopic behaviors both during biomineralization processes and in their oceanic distributions. This suggests that combined with Si isotopes, Ge isotopes hold significant promise as a complementary proxy for delineating biological versus source effects in the evolution of the marine silicon cycle through time.

ESR Study of Y2SiO5:Nd143 Isotopically Pure Impurity Crystals for Quantum Memory

Two Y2SiO5 single crystals doped with 0.001 at.% of the 143Nd3+ ion (sample I containing the only 28Si isotope) and (sample II with the natural abundance of silicon isotopes) were studied using magnetic resonance methods. The spin–spin and spin–lattice relaxation times were measured at 9.7 GHz between 4 and 10 K. It is established that three relaxation processes describe temperature dependences of the spin–lattice relaxation for both crystals. They are one-phonon, two-phonon Raman and two-phonon Orbach–Aminov relaxation processes. It is established that temperature dependence of spin–spin relaxation time differs for different hyperfine components of the electron paramagnetic resonance spectrum of neodymium ions and depends on the kind of the neodymium isotope (143Nd or 145Nd).

Uniform Silicon Isotope Ratios Across the Milky Way Galaxy

Abstract We report the relative abundances of the three stable isotopes of silicon, 28Si, 29Si, and 30Si, across the Galaxy using the transition of silicon monoxide. The chosen sources represent a range in Galactocentric radii ( ) from 0 to 9.8 kpc. The high spectral resolution and sensitivity afforded by the Green Bank Telescope permit isotope ratios to be corrected for optical depths. The optical-depth-corrected data indicate that the secondary-to-primary silicon isotope ratios and vary much less than predicted on the basis of other stable isotope ratio gradients across the Galaxy. Indeed, there is no detectable variation in Si isotope ratios with . This lack of an isotope ratio gradient stands in stark contrast to the monotonically decreasing trend with exhibited by published secondary-to-primary oxygen isotope ratios. These results, when considered in the context of the expectations for chemical evolution, suggest that the reported oxygen isotope ratio trends, and perhaps those for carbon as well, require further investigation. The methods developed in this study for SiO isotopologue ratio measurements are equally applicable to Galactic oxygen, carbon, and nitrogen isotope ratio measurements, and should prove useful for future observations of these isotope systems.

Impact of low denudation rates on soil chemical weathering intensity: A multiproxy approach

Quantifying the influence of denudation, i.e., physical erosion and chemical weathering, on soil weathering intensity is an important component for a comprehensive understanding of element biogeochemical cycles. The relation between the weathering intensity and the denudation rate is not clear and requires further investigation in a variety of climatic and erosional settings. Here, in the Betic Cordillera (southern Spain), we assess the soil chemical weathering intensity with a multiproxy approach combining different indicators of chemical weathering of the soil: the Total Reserve in Bases (TRB), the content in Fe-oxides, the quartz and clay content, the soil cation exchange capacity (CEC), and the silicon (Si) isotope composition of the clay-sized fraction. Our multiproxy approach demonstrates that in this semi-arid environment at low denudation rates, an increasing denudation rate decreases the soil weathering intensity, whereas Si mobility remains limited. Our results converge with previous conclusions based on chemical mass balance methods in the same geological setting. Mass balance methods, and particularly Chemical Depletion Fractions (CDF), are based on the immobility of a refractory element (commonly zirconium, Zr) relative to major cations in soils. Interestingly, our study suggests that a weathering index such as the TRB may provide a useful complement to assess soil chemical weathering intensity in eroding landscapes where the application of chemical mass balances may be hampered by potential Zr mobility in the soil or by heterogeneity of Zr concentrations in the bedrock.

Titanium isotopic compositions of rare presolar SiC grain types from the Murchison meteorite

We report the Ti isotopic compositions of 8 mainstream, 22 Y, 9 Z, and 26 AB presolar SiC grains from two SiC-rich residues of the Murchison CM2 meteorite together with Si, C and some Mg-Al isotopic data for the same grains. Mainstream, Y and Z grains are believed to originate in asymptotic giant branch (AGB) stars of varying metallicities, but the stellar sources of AB grains are poorly understood. We find that the 46,47,49Ti/48Ti ratios are correlated with 29Si/28Si for all of the grain types, indicating that these ratios are mainly dominated by Galactic chemical evolution (GCE). The mainstream, Y and Z grains all show enrichments in 50Ti from neutron capture nucleosynthesis. However, AGB models predict smaller excesses in 50Ti (and 49Ti) than are observed in these grains. For Z grains and especially for Y grains, the enhancement of 50Ti is greater than the enhancement in 30Si, indicating that the 13C neutron source produced a greater total fluence of neutrons than the 22Ne source in the low metallicity parent AGB stars. The Z grains plot below the mainstream correlation lines at more 48Ti- and 28Si-rich compositions in plots of 46,47,49Ti/48Ti vs. 29Si/28Si. On the other hand, the Y grains plot close to the mainstream correlation line. This could imply that the Ti isotopes evolved non-linearly at metallicities below ∼1/3 solar. The AB grains in this study have Ti isotopic compositions that fall along correlation lines defined by the mainstream grains, suggesting origins in close to solar metallicity stars. However, these grains fall below the mainstream correlation lines in plots of 46,49,50Ti/48Ti vs. 29Si/28Si and do not show enhancements in 50Ti, indicating that their parent stars did not undergo significant s-process nucleosynthesis. These data support origins of AB grains in J-type C stars rather than born-again AGB stars that undergo s-process nucleosynthesis. AB grains that do not have 50Ti excesses may provide the best measure of Si and Ti isotope GCE since their parent stars were less affected by s-process nucleosynthesis than the mainstream grains.

New Possibilities for Magnetic Control of Chemical and Biochemical Reactions.

Chemistry is controlled by Coulomb energy; magnetic energy is lower by many orders of magnitude and may be confidently ignored in the energy balance of chemical reactions. The situation becomes less clear, however, when reaction rates are considered. In this case, magnetic perturbations of nearly degenerate energy surface crossings may produce observable, and sometimes even dramatic, effects on reactions rates, product yields, and spectroscopic transitions. A case in point that has been studied for nearly five decades is electron spin-selective chemistry via the intermediacy of radical pairs. Magnetic fields, external (permanent or oscillating) and the internal magnetic fields of magnetic nuclei, have been shown to overcome electron spin selection rules for pairs of reactive paramagnetic intermediates, catalyzing or inhibiting chemical reaction pathways. The accelerating effects of magnetic stimulation may therefore be considered to be magnetic catalysis. This type of catalysis is most commonly observed for reactions of a relatively long-lived radical pair containing two weakly interacting electron spins formed by dissociation of molecules or by electron transfer. The pair may exist in singlet (total electron spin is zero) or triplet (total spin is unity) spin states. In virtually all cases, only the singlet state yields stable reaction products. Magnetic interactions with nuclear spins or applied fields may therefore affect the reactivity of radical pairs by changing the angular momentum of the pairs. Magnetic catalysis, first detected via its effect on spin state populations in nuclear and electron spin resonance, has been shown to function in a great variety of well-characterized reactions of organic free radicals. Considerably less well studied are examples suggesting that the basic mechanism may also explain magnetic effects that stimulate ATP synthesis, eliminating ATP deficiency in cardiac diseases, control cell proliferation, killing cancer cells, and control transcranial magnetic stimulation against cognitive deceases. Magnetic control has also been observed for some processes of importance in materials science and earth and environmental science and may play a role in animal navigation. In this Account, the radical pair mechanism is applied as a consistent explanation for several intriguing new magnetic phenomena. Specific examples include acceleration of solid state reactions of silicon by the magnetic isotope 29Si, enrichment of 17O during thermal decomposition of metal carbonates and magnetic effects on crystal plasticity. In each of these cases, the results are consistent with an initial one-electron transfer to generate a radical pair. Similar processes can account for mass-independent fractionation of isotopes of mercury, sulfur, germanium, tin, iron, and uranium in both naturally occurring samples and laboratory experiments. In the area of biochemistry, catalysis by magnetic isotopes has now been reported in several reactions of DNA and high energy phosphate. Possible medical applications of these observations are pointed out.