The stable isotopes 2H and 18O, combined with noble gases and radioisotopes (e.g., 3H, 14C, 36Cl), are used to infer groundwater age and climate during recharge. Flow regimes within low-velocity flowpaths and long residence times could allow an aquitard-aquifer diffusive flux to alter isotope abundance. Consequently, the diffusion of isotopes (e.g., 14C, 2H and 18O) between aquifers and confining layers needs to be considered in such conditions. In this study, COMSOL Multiphysics was used to determine if diffusion of 18O (and 2H by proxy) from a bounding aquitard could explain observed downgradient enrichment of 2H and 18O within a regional aquifer. Using the geologic and hydraulic properties of the lower Wilcox aquifer of the Mississippi Embayment aquifer system in Missouri and Arkansas, the advection-dispersion equation was solved along a 1-D groundwater flow domain, coupled with a cross-contact aquitard-aquifer diffusive flux. Although the observed signal within the lower Wilcox was not matched, a sensitivity analysis indicated the importance of the isotope composition gradient between the aquifer and aquitard. Furthermore, groundwater velocity was suggested as a controlling influence on aquitard-aquifer exchange that could alter aquifer isotope composition.

Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2007. / Excitation functions and production rates are presented for various Ce and Pr radionuclides formed in the bombardment of 141Pr, natLa and natCe with protons. Extensive measure- ments were performed for 141Pr + p up to 100 MeV and for natLa + p up to 20 MeV. The possibility is investigated to utilize tandem targetry for the production of no-carrier-added 139Ce of high radionuclidic purity, having a Pr target in the higher energy slot followed by a La target in a lower energy slot. 141Pr(p,3n)139mNd!139Pr is investigated as an al- ternative to the direct production route 140Ce(p,2n)139Pr for producing no-carrier-added 139Pr of high radionuclidic purity. The advantages and disadvantages of both production routes are discussed. The simultaneous production of 139Pr and 140Nd using Pr as target is also investigated. Experimental thick-target production rates are presented for Pr ra- dionuclides formed in the bombardment of natCe with protons at incident energies of 20, 26 and 32 MeV. All the experimental excitation functions obtained in this work are compared with theoretical predictions by means of the geometry-dependent hybrid (GDH) model as implemented in the code ALICE-IPPE. The results of this work are also compared with previous literature experimental data, if available.

Thesis (PhD)--Stellenbosch University, 2002. / ENGLISH ABSTRACT: The equilibrium distribution coefficients of 32 metal ions [Al(llI), As(V), CdïIl), Ce(III), Ce(IV), Co(n), Cr(III), Cu(II), Fe(III), Ga(III), Ge(IV), !n(III), La(III), Mn(II), Mo(VI), Nb(V), Ni(II), Pr(III), Sb(V), Sc(III), Se(IV), Sn(IV), Ta(V), Tb(III), Te(IV), Ti(IV), Vev), W(VI), Y(III), Yb(III), Zn(lI) and Zr(IV)] on a cation exchanger (Bio- Rad® AG50W-X8) and an anion exchanger (Bio-Rad® AG1-X8) in varying oxalic acid - sulphuric acid mixtures were successfully determined. The equilibrium distribution coefficients of these selected metal ions were determined in both 0.05 M and 0.25 M oxalic acid at various concentrations of sulphuric acid (0.005 M, 0.05 M, 0.10 M, 0.25 M, 0.50 M, 1.00 M, 1.50 M and 2.00 M). Attempts to explain the sorption behaviour of the selected metal ions were made by using MINEQL+, a speciation modelling program, and the speciation systems for AI(III), Cd(II), Co(II) and Zn(lI) in varying oxalic acid - sulphuric acid mixtures were determined. Two component [Zr(IV)-La(III); AI(III)-La(III); Ga(IlI)-Zn(II); As(V)-Zn(II); Cu(II)- Ce(IV); Ga(III)-Ce(IV); Ge(IV)-Ce(III); Mo(VI)- Y(III); Nb(V)- Y(III); Ga(III)-Co(II); As(V)-Co(lI) and Fe(III)-Mn(II)] and three component [Fe(III)-Ga(III)-Zn(lI) and Zr(IV)-Ta(V)-Yb(III)] mixtures on a 10 ml or 13 ml cation exchange resin in a variety of oxalic acid - sulphuric acid mixtures were successfully separated. Two component [As(V)-Zr(IV); Co(II)-Fe(III); Ni(II)-Co(lI) and Ni(II)-Fe(III)] and three component [Ni(II)-As(V)-Se(IV); AI(III)-Zn(II)-Ge(IV) and As(V)-Cu(II)-Ge(IV)] mixtures on a 10 ml or 13 ml anion exchange resin in a variety of oxalic acid - sulphuric acid mixtures were also successfully separated and studied. It was also shown how some of the elution curves developed above could easily be adapted for radiochemical separations. Usin~ the relevant data from the above study, a separation for 68Gefrom a Ga20 target was developed according to a method based on acid dissolution of the target and chromatography on an anion exchange resin (Bio-Rad® AG1-X8). The separated 68Geshowed high radionuclidic purity and an acceptable chemical purity. / AFRIKAANSE OPSOMMING: Die ewewigsverdelingskoëffisiënte van 32 metaalione [Al(III), As(V), Cd(II), Ce(III), Ce(IV), Co(II), Cr(III), Cu(II), Fe(III), Ga(III), Ge(IV), In(III), La(III), Mn(ll), Mo(VI), Nb(V), Ni(ll), Pr(Ill), Sb(V), Sc(III), Se(IV), Sn(IV), Ta(V), Th(Ill), Te(IV), Ti(IV), vrv; W(VI), Y(III), Yb(III), Zn(lI) en Zr(IV)] op 'n katioonruiler (Bio-Rad® AG50W-X8) en 'n anioonruiler (Bio-Rad® AG1-X8) in veranderende oksaalsuurswaelsuurmengsels is met welslae bepaal. Die ewewigsverdelingskoëffisiënte van hierdie geselekteerde elemente is in beide 0.05 Men 0.25 M oksaalsuur by verskeie konsentrasies swaelsuur (0.005 M, 0.05 M, 0.10 M, 0.25 M, 0.50 M, 1.00 M, 1.50 M en 2.00 M) bepaal. Daar is gepoog om die sorpsiegedrag van die geselekteerde metaalione te verklaar deur die gebruik van MINEQL+, 'n spesiëringmodelleringsprogram, en die spesiëringsisteme vir AI(III), Cd(II), Co(II) en Zn(lI) in veranderende oksaalsuur-swaelsuurmengsels is bepaal. Tweekomponent [Zr(IV)- La(III); AI(III)-La(II!); Ga(III)-Zn(Il); As(V)-Zn(II); Cu(II)-Ce(IV); Ga(III)-Ce(IV); Ge(IV)-Ce(Il!); Mo(VI)- Y(III); Nb(V)- Y(ll!); Ga(Ill)-Co(Il); As(V)-Co(lI) en Fe(III)- Mn(II)] en driekomponent [Fe(III)-Ga(III)-Zn(ll) en Zr(IV)- Ta(V)- Yb(III)] mengsels op 'n 10 ml of 13 ml katioonruilhars in 'n verskeidenheid oksaalsuurswaelsuurmengsels is met welslae geskei. Tweekomponent [As(V)-Zr(IV); Co(II)- Fe(III); Ni(Il)-Co(lI) en Ni(II)-Fe(III)] en driekomponent [Ni(II)-As(V)-Se(IV); AI(III)-Zn(Il)-Ge(IV) en As(V)-Cu(II)-Ge(IV)] mengsels op 'n 10 ml of 13 ml anioonruilhars in 'n verskeidenheid oksaalsuur-swaelsuurmengsels is ook met welslae geskei en bestudeer. Daar is ook aangetoon hoe sommige van die elueringskrommes wat hierbo ontwikkel is, maklik vir radiochemiese skeidings aangepas sou kon word. Deur gebruik te maak van die relevante data uit die studie hierbo, is 'n skeiding vir 68Geuit 'n Ga20-teiken ontwikkel volgens 'n metode gebaseer op suurdissolusie van die teiken en chromatografie op 'n anioonruilhars (Bio-Rad® AG1-X8). Die geskeide 68Gehet hoë radionukliedsuiwerheid en 'n aanvaarbare chemiese suiwerheid getoon.

This dissertation describes a proof-of-principle experiment demonstrating a technique for stable isotope enrichment called Magnetically Activated and Guided Isotope Separation (MAGIS) (1). Over the past century a large number of enriched isotopes have become available, thanks largely to electromagnetic separators called calutrons that were developed during World War II. These isotopes have found applications across an array of fields including medicine, basic science, and energy. Due to substantial maintenance and operating costs, the United States decommissioned the last of its calutrons in 1998, leading to demand for alternative methods of isotope separation. Our experiment suggests the promise for MAGIS as a viable alternative for replenishing stockpiles previously provided by calutrons. Our apparatus combines optical pumping with a scalable magnetic field gradient to enrich lithium-7 (Li-7) by suppressing lithium-6 (Li-6) throughput in a lithium atomic beam. We first evaporate lithium metal in a crucible in order to generate thermal, high flux beam. We then perform optical pumping on Li-6 atoms, magnetically polarizing a substantial fraction of Li-6 atoms into the entirely high-field seeking 2²S₁/₂, F = 1/2 ground state. The resultant beam then samples a magnetic field gradient produced by a 1.5 m long array of rare-earth permanent magnets bent over its length by 20 mrad. This geometry prevents high-field seeking lithium atoms from reaching the plane beyond the magnets, while efficiently deflecting low-field seeking atoms. We measured Li-6 suppression – using independent techniques – along the plane after the magnets beyond a factor of 200, corresponding to Li-7 enrichment to better than 99.95%. As apparatus-specific hindrances appeared to limit this suppression, we believe that we should achieve better enrichment on a commercial apparatus. We also measured both the absolute flux beyond the single, 1.5 in tall magnet array and the efficiency for guiding feedstock material to the collection plane. Given the planar configuration for the field gradient, the flux that we measured should scale linearly with both magnet height and the number of arrays surrounding the source. Our measurements therefore indicate that – at source temperatures that we actually investigated – a commercial apparatus fitting within a volume of just several cubic meters should yield hundreds of grams of enriched (to beyond 99.95%) Li-7 per year. In addition, we observed a competitive ratio between collected material and feedstock with greater than 20% of lithium incident upon the magnet array reaching beyond the magnets. Benchmarking our work against the calutron, we demonstrated comparable enrichment in a manner that should scale to the production of similar quantities. In contrast, however, MAGIS should require vastly less energy input. While calutrons required massive currents for maintaining a static magnetic field over a substantial area, the only non-shared energy expense for MAGIS is the cost for running the low power lasers for optical pumping. Via additional analysis, we have supplemented this proof-of-principle experiment with schemes for applying MAGIS to over half of the stable isotopes in the periodic table. Due to the success of this demonstration and the broad applicability of the principles, we believe that MAGIS will play an important role in the future of stable isotope enrichment. / text

No description available.

The source of potassium and argon in ordinary chondrites is determined by comparing the argon activation energies of feldspar, pyroxene, and olivine with that of the L chondrites Chico and North West Africa (NWA) 091. In addition, shock pressures of 29 to 55.8 GPa are shown to lower the activation energy of feldspar. Comparable shock pressures lowers the activation energy of pyroxene outside of error, but the variability of this value, even among unshocked samples, makes a clear distinction difficult. The effect of shock on olivine has not been investigated, by myself or others. Like many ordinary chondrites, Chico and NWA 091 have two major releases of argon, one at low temperature, and the other at high temperature. The low temperature release of Chico contains two releases, which match the activation energies of shocked and unshocked feldspar. The low temperature release of NWA 091 only contains a single release, which matches shocked feldspar. The high temperature release of both Chico and NWA 091 has an activation energy that is similar to pyroxene, but not olivine. A potassium mass balance of Chico shows that all the potassium in the meteorite is contained in feldspar, and Raman spectroscopy shows this feldspar has not been converted into a high pressure phase, indicating the high temperature release is inclusions in a high temperature mineral. This mineral is probably pyroxene based upon the activation energy, though thin sections provide evidence that feldspar is more closely associated with olivine. NWA 091 exhibits multiple isochrons, showing the presence of two nonprimordial and (probably) non-terrestrial trapped components of argon. The removal of these trapped components reveals a thermal event produced by a collisional impact on the L chondrite parent body at 475 ± 6 Ma (which supports a link between L chondrites and Ordovician fossil meteorites), as well as a similar event at ~800 Ma (which, combined with similar ages on other Solar System objects, suggests an increased impact flux at that time). Chico did not exhibit an isochron, and the age data for Chico is not reported.

Recent years have seen increasing use of atmospheric radionuclides for dating and tracing hydrogeologic processes. Hydrologists often assume that meteoric sources of these nuclides are dominant in ground water and that age-dating methods are limited primarily by analytical detection capability. However, in some cases, subsurface production may also limit the usefulness of these nuclides for dating. Equilibrium radionuclide concentrations are calculated as a function of depth for a variety of rock types. Production mechanisms include fissioning of heavy radionuclides; spallation by cosmic-ray nucleons; capture of neutrons, a-particles, muons and protons; and photonuclear reactions. Calculations indicate that deep subsurface production of ³H, ¹⁴c, ⁸⁵Kr and ⁹⁹Tc is generally below detection but that deep production of ³⁶C1, ³⁹Ar, ⁸¹Kr and ¹²⁹I establishes limits to age-dating of water in most rocks. Parameters for estimating production of ¹⁰Be, ²²Na, ²⁶Al, ³⁷Ar, ³²Si, ⁴¹Ca and ⁷⁹Se are included in appendices. Evidence for in-situ production of ³⁶C1 and ¹²⁹I is presented for two field studies. Concentrations in ground water from the Stripa granite, Sweden, were determined by accelerator mass spectrometry. ¹²⁹I values range from 1,000 to 200,000 atoms/ml, compared to an estimated background concentration in pre-1945 water of 20 atoms/ml. The high levels are attributed to production by spontaneous fission of ²³⁸U in the granite (44 ppm U). ³⁶C1/C1 ratios range from 50-200 x 10 -15 compared to about 40 x 10⁻¹⁵ in meteoric recharge. An increase in ratios with depth has been attributed to production of ³⁶C1 by neutron- capture on ³⁵C1 and is used to set upper limits on the residence time of water in the granite. The validity of using ³⁶C1/C1 ratios as a monitor of deep lithospheric neutron fluxes was tested by measuring the ratios in Cl extracted from Stripa granite. The average ratio, 190 x 10⁻¹⁵, agrees with ratios calculated based on rock chemistry, 190 x 10⁻¹⁵, and on the measured neutron flux, 220 x 10⁻¹⁵. ¹²⁹I and ³⁸C1 were also measured in uranium ores from the Koongarra and Ranger deposits, N.T., Australia. Samples from the oxidized ore zone contain only 6-23% of the ¹²⁹I contents predicted for equilibrium, suggesting preferential loss of ¹²⁹I relative to U during weathering. ³⁶C1 is produced as a result of high neutron fluxes in the ore. Measured ³⁶C1/C1 ratios range from 3 x 10 -12 to 1 x 10⁻¹⁰, corresponding to apparent neutron fluxes of 2 x 10⁵ to 1 x 10⁷/cm²/yr.

Initial published work suggested that Fe isotope fractionations recorded in sediments were a product of biological activity. Experiments and measurements of natural samples now indicate that Fe isotope fractionation can be the product of both biological and inorganic processes. Sedimentary iron sulphides provide unique information about the evolution of early life which developed under anoxic conditions. It is in these sedimentary Fe-S species and in particular in Archean and Proterozoic pyrites that the largest Fe isotope variations (up to a range of ~5‰ for δ56/54Fe) have been measured. Most research has focussed on potential processes responsible for the formation of a 56Fe depleted Fe(II) pool from which iron sulphides would precipitate without additional fractionation, recording the light Fe isotope composition of the pool. Much less attention has been given to the possibility that the iron sulphide forming mechanisms themselves could produce significant fractionations. The Fe-S system constitutes a diverse group of stable and metastable phases, the ultimate Fe sequestrating phase being pyrite. The aim of this study was to examine experimentally where Fe isotope fractionations occur during the abiotic formation of iron sulphides in order to assess whether or not the measured Fe isotope signatures in natural pyrite could be explained by chemical mechanisms only. Both analytical and experimental protocols were developed in order to determine the partition of Fe isotopes for each step towards diagenetic pyrite formation. 56/54Fe and 57/54Fe ratios were measured on an IsoProbe-P Micromass MC-ICP-MS, and all experiments were performed under oxygen-free N2 atmosphere. Supporting previously published data, the results indicate that the precipitation of the nanoparticulate iron(II) monosulphide mackinawite (FeSm) kinetically fractionates lighter isotopes with initial fractionations of Δ56FeFe(II)aq-FeS = 1.17 ± 0.16 ‰ at 25°C and Δ56FeFe(II)aq-FeS = 0.98 ± 0.16 ‰ at 2°C. The rate of isotopic exchange between Fe(II)aq and FeSm decreases as FeSm nanoparticles grow. Fe isotope exchange kinetics are consistent with i) FeSm nanoparticles that have a core–shell structure, in which case Fe isotope mobility is restricted to exchange between the surface shell and the solution and ii) a nanoparticle growth via an aggregation– growth mechanism. Because of the structure of FeSm nanoparticles, the approach to isotopic equilibrium is kinetically restricted at low temperatures. The equilibrium Fe isotope fractionation between Fe2+ aq and FeSm was determined using the three isotope method and is Δ56FeFe(II)-FeS = -0.33 ± 0.12 ‰ at 25°C and Δ56FeFe(II)-FeS = -0.52 ± 0.16 ‰ at 2°C. This suggests that at equilibrium, FeSm incorporates heavier isotopes with respect to Fe2+ aq, and the isotopic composition of most naturally occurring FeSm does not represent equilibrium. During pyrite formation, pyrite incorporates kinetically lighter isotopes with a fractionation Δ56FeFeS-pyrite ~ 2.2 ‰. Because pyrite is sparingly soluble in sedimentary environments, isotope exchange is prevented and pyrite does not equilibrate with its Fe(II) source. Combined fractionation factors between Fe2+ aq, mackinawite (FeSm) and pyrite permit the generation of pyrite with Fe isotope signatures that encapsulate the full range of sedimentary δ56Fepyrite recorded in both Archean and modern sediments. Archean Fe isotope excursions reflect various degrees of pyritisation, extent of Fe(II)aq utilisation, and variations in source composition rather than microbial dissimilatory Fe(III) reduction only. Our results show that sedimentary pyrite is not a passive recorder of the Fe isotope composition of the reactive Fe(II) reservoir forming pyrite. It is the formation process itself that influences pyrite Fe isotope signatures with consequent implications for the interpretation of sedimentary pyrite Fe isotope compositions throughout geological time.

Fixed N (nitrate, nitrite, and ammonium) is a limiting nutrient for photosynthesis in the surface ocean. The rates and relative importance of N cycling processes, however, are temporally and spatially complex, which hamper their direct measurement and quantification. The South Atlantic subtropical front separates the Atlantic Ocean and the subantarctic, an area which can elucidate information about water masses both entering and leaving the basin. Through the GEOTRACES programme, an oceanographic section across 40°S in the South Atlantic is used to investigate biogeochemical cycling of nitrogen and carbon in this region. Hydrographic data, in combination with the isotopic composition of nitrate (NO3-), particulate organic carbon and particulate nitrogen (δ15NNO3, δ18ONO3, δ13CPOC, δ15NPN), is used to provide integrative measurements for temporally and spatially variable processes of the marine N-cycle and C-cycle. A thorough examination of the stable isotope cycling of particulate and dissolved N across the subtropical front is used to quantify the supply of fixed N to the mixed layer. The relative importance of nitrate from the subsurface, N2 fixation, terrestrial input and atmospheric deposition in supplying production is determined. Typically, 30-50% of the export flux in the subtropical water masses is sourced from N2 fixers and up to 75% within the Brazil Current. This finding suggests that diazotrophs may be abundant in the South West Atlantic providing a source of new N to this region. To assess the basin scale N-cycling processes, the deep water masses were analysed to reveal the origin and history of NO3-. Intermediate waters formed in the subantarctic are enriched in δ15NNO3 and δ18ONO3 from partial utilisation by algae. This enrichment in δ15NNO3 is not present in the subtropical North Atlantic or the return flow of the North Atlantic Deep Water (NADW), which decreases from ~5.9‰ in the newly formed intermediate waters to ~4.8‰ in the NADW at 40°S. The modification of isotopic signatures through the subtropical Atlantic can be calculated as an incorporation of 26-36 Tg N yr-1 of newly fixed N from an isotopic source of -1‰ (N2 fixation). The extent of N addition is higher than estimated rates of N loss within the Atlantic and surpasses the amount of N deficit supplied to the basin. Fixed N inputs and losses through the global ocean are investigated by the assessment of remineralised nitrate added to the ocean interior. A lower δ15N is observed in Atlantic remineralised nitrate in comparison to the Pacific. The relative importance of N2 fixation and pelagic denitrification within each ocean basin is quantified and through this approach, N2 fixation rates are estimated at 92-116 Tg N yr-1 in the Pacific and 24-32 Tg N yr-1 in the Indian Ocean. Combining Atlantic N2 fixation of ~32 Tg N yr-1 with Indo-Pacific, global N2 fixation rates can be estimated at 142-184 Tg N yr-1. The high inputs in the Pacific suggest that excess P is the dominant control on the success of N2 fixers. However, estimates of new N addition to the Atlantic indicate other mechanisms such as the recycling efficiency of P and supply of Fe to the surface ocean increase N2 fixation rates above this threshold. The organic matter supplied to sediments is principally derived from phytoplankton across the subtropical front. High organic content is associated with the productive Brazil-Malvinas Confluence region where a diverse supply of nutrients sustains elevated biomass. The Rio Plata outflow is characterised with high δ15NNO3 and δ15NPN, suggesting denitrification processes occur in the estuary. A low δ13C source associated with high Al concentrations is identified on the western slope, indicating a supply of terrestrial derived C to the deep ocean. The fractionation of C uptake by phytoplankton is assessed in subtropical and subantarctic waters. In the subantarctic, CO2[aq] and growth rates determine the extent of C isotope fractionation. In this region, low species diversity and a small range in cell size enable the fractionation from CO2[aq] and growth rate to be expressed in phytoplankton. In subtropical water masses a larger range of cell size is the principal determinant of C fractionation. Increased surface area to volume is the main mechanism for increasing C uptake, arguing against the use of δ13CPOC as a palaeoproxy. The low δ13CPOC and δ15NPN observed in the subtropics (from C fractionation and N2 fixation) contrast the heavier signatures in the subantarctic. These observations are propagated to the sediments, wherein organic matter shifts are determined by changes in the subtropical front over time. The results of this study have greatly improved knowledge of N and C cycling within the South Atlantic, providing new insight into the cycling of these two important elements in the surface and deep ocean, on a regional and global scale.

No description available.