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Distributions of water transit times (TTDs), and related storage-selection (SAS) distributions, are spatially integrated metrics of hydrological transport within landscapes. Recent works confirm that the form of TTDs and SAS distributions should be considered time variant-possibly depending, in predictable ways, on the dynamic storage of water within the landscape. We report on a 28 day periodic-steady-state-tracer experiment performed on a model hillslope contained within a 1 m3 sloping lysimeter. Using experimental data, we calibrate physically based, spatially distributed flow and transport models, and use the calibrated models to generate time-variable SAS distributions, which are subsequently compared to those directly observed from the actual experiment. The objective is to use the spatially distributed estimates of storage and flux from the model to characterize how temporal variation in water storage influences temporal variation in flow path configurations, and resulting SAS distributions. The simulated SAS distributions mimicked well the shape of observed distributions, once the model domain reflected the spatial heterogeneity of the lysimeter soil. The spatially distributed flux vectors illustrate how the magnitude and directionality of water flux changes as the water table surface rises and falls, yielding greater contributions of younger water when the water table surface rises nearer to the soil surface. The illustrated mechanism is compliant with conclusions drawn from other recent studies and supports the notion of an inverse-storage effect, whereby the probability of younger water exiting the system increases with storage. This mechanism may be prevalent in hillslopes and headwater catchments where discharge dynamics are controlled by vertical fluctuations in the water table surface of an unconfined aquifer. Plain Language Summary Volumes of water reside within landscapes for varying amounts of time before they are discharged to a stream. That length of time determines how long the water has to interact chemically with soil and rock, and therefore influences the chemistry of water that ends up in stream channels. Quantifying the full range and variability of those travel times remains a challenge. We built an experimental hillslope, which allows us to keep track of all the water that enters and exits the soilsomething that is difficult to accomplish in open environmental systems. We introduced chemically distinct water into the hillslope at specific points in time and followed the movement of that water within, and upon exit from the soil. We discovered that the water being discharged from the hillslope tends to have resided in the landscape for shorter lengths of time when the hillslope is very wet (like a wetted sponge) than when it is very dry (like a dry sponge). This insight helps us understand how different rainfall regimes, and the associated wetness of the landscape, can potentially influence water transit times through the landscape, and their relationship with stream chemistry.

Clinopyroxene, olivine, plagioclase and hydrous fluid lithium partition coefficients have been measured between 800-1100oC at 1 GPa. Clinopyroxene-fluid partitioning is a function of temperature (ln DLicpx/fluid = -7.3 (+0.5) + 7.0 (+0.7) * 1000/T) and appears to increase with increasing pyroxene Al2O3 content. Olivine-fluid partitioning of lithium is a function of temperature (ln DLiol/fluid = -6.0 (+2.0) + 6.5 (+2.0) * 1000/T) and appears to be sensitive to olivine Mg/Fe content. Anorthite-fluid lithium partitioning is a function of feldspar composition, similar to the partitioning of other cations in the feldspar-fluid system. Isotopic fractionation between clinopyroxene and fluid, Licpx-fluid, has been measured between 900-1100oC and ranges from -0.3 to -3.4 ‰ (±1.4 ‰). Lithium diffusion has been measured in clinopyroxene at 800-1000oC and in olivine at 1000oC. The lithium diffusion coefficient is independent of the diffusion gradient as values are the same if the flux of lithium is into or out of the crystal and ranges from -15.19 ± 2.86 m2/s at 800oC to -11.97 ± 0.86 m2/s at 1000oC. Lithium diffusion in olivine was found to be two orders of magnitude slower than for clinopyroxene at similar conditions. Closure temperatures calculated for lithium diffusion in clinopyroxene range from ~400 to ~600oC. These results demonstrate that lithium equilibration between fluids and minerals is instantaneous, on a geological timescales. The confirmation of instantaneous equilibration, combined with min-fluid partition coefficients and values for Licpx-fluid, permits quantitative modeling of the evolution of lithium concentration and isotopic composition in slab-derived fluids during transport to the arc melt source. Our results indicate that fluids migrating by porous flow will rapidly exchange lithium with the mantle, effectively buffering the fluid composition close to ambient mantle values, and rapidly attenuating the slab lithium signature. Fluid transport mechanisms involving fracture flow are required to maintain a slab-like lithium signature (both elemental and isotopic) from the slab to the melt source of island arc basalts. This study demonstrates that mineral-fluid equilibration is rapid, and as a result the lithium content of minerals can only reliably represent chemical exchange in the very latest stages of the sample’s history.

Clinopyroxene, olivine, plagioclase and hydrous fluid lithium partition coefficients have been measured between 800-1100oC at 1 GPa. Clinopyroxene-fluid partitioning is a function of temperature (ln DLicpx/fluid = -7.3 (+0.5) + 7.0 (+0.7) * 1000/T) and appears to increase with increasing pyroxene Al2O3 content. Olivine-fluid partitioning of lithium is a function of temperature (ln DLiol/fluid = -6.0 (+2.0) + 6.5 (+2.0) * 1000/T) and appears to be sensitive to olivine Mg/Fe content. Anorthite-fluid lithium partitioning is a function of feldspar composition, similar to the partitioning of other cations in the feldspar-fluid system. Isotopic fractionation between clinopyroxene and fluid, Licpx-fluid, has been measured between 900-1100oC and ranges from -0.3 to -3.4 ‰ (±1.4 ‰). Lithium diffusion has been measured in clinopyroxene at 800-1000oC and in olivine at 1000oC. The lithium diffusion coefficient is independent of the diffusion gradient as values are the same if the flux of lithium is into or out of the crystal and ranges from -15.19 ± 2.86 m2/s at 800oC to -11.97 ± 0.86 m2/s at 1000oC. Lithium diffusion in olivine was found to be two orders of magnitude slower than for clinopyroxene at similar conditions. Closure temperatures calculated for lithium diffusion in clinopyroxene range from ~400 to ~600oC. These results demonstrate that lithium equilibration between fluids and minerals is instantaneous, on a geological timescales. The confirmation of instantaneous equilibration, combined with min-fluid partition coefficients and values for Licpx-fluid, permits quantitative modeling of the evolution of lithium concentration and isotopic composition in slab-derived fluids during transport to the arc melt source. Our results indicate that fluids migrating by porous flow will rapidly exchange lithium with the mantle, effectively buffering the fluid composition close to ambient mantle values, and rapidly attenuating the slab lithium signature. Fluid transport mechanisms involving fracture flow are required to maintain a slab-like lithium signature (both elemental and isotopic) from the slab to the melt source of island arc basalts. This study demonstrates that mineral-fluid equilibration is rapid, and as a result the lithium content of minerals can only reliably represent chemical exchange in the very latest stages of the sample’s history.

Mass dependent isotope fractionation of stable isotopes between meteorites and planetary materials has been used to assess processes that occurred during formation of Earth and its core. However, thus far little is known about the mass dependent isotope fractionation of Mo in the solar system, and at high temperatures in the Earth, in particular during mantle processes. Molybdenum is a refractory and moderately siderophile element. The processes that might have fractionated Mo in the early solar system include condensation and evaporation of dust grains, metal-silicate segregation, core crystallization, silicate and sulphide melting and aqueous alteration. In order to investigate the processes fractionating Mo isotopes, it is first necessary to assess how much fractionation takes place during mantle melting, estimate the isotopic composition of the bulk silicate Earth, and then make comparisons with primitive and differentiated meteorites. I present double spike Mo isotope data for forty-two mafic and seven ultramafic samples from diverse locations, and nineteen extra-terrestrial samples. The delta^98/95Mo values of all the terrestrial samples (normalized to NIST SRM 3134) exhibit a significant range from +0.53±0.21 to -0.56±0.09‰. The compositions of mid-ocean ridge basalts (MORBs) (+0.03±0.07‰, 2s.d.) and ultramafic rocks (+0.38±0.15‰, 2 s.d.) are relatively uniform and well resolved, providing evidence of fractionation associated with partial melting. In contrast intraplate and ocean island basalts (OIBs) display significant variability within a single locality from MORB-like to strongly negative (-0.56‰). The most extreme values measured are for nephelinites from the Cameroon Line and Trinidade, which also have anomalously high Ce/Pb and low Mo/Ce relative to normal oceanic basalts. The observed relationships between delta^98/95Mo and Ce/Pb, U/Pb and Mo/Ce provide evidence that sulphide plays a critical role in retaining Mo in the mantle and fractionating its isotopic composition in basaltic magmas. If residual sulphides are responsible the Mo isotopic composition, Mo budget of the bulk silicate Earth will be misrepresented by values estimated from basalts. On this basis a revised best estimate of the Mo content in the bulk silicate Earth (BSE) ranging between 251 to 268 ppb is derived, approximately 6 times higher than previously assumed, and similar to the levels of depletion in refractory siderophile elements such as W, Ni and Co. This significantly ameliorates the argument for Mo removal via late stage sulphide extraction to the core. The Mo isotopic composition of the BSE (0.35‰) is distinct from the delta^98/95Mo values found in primitive and iron meteorites. Although Mo isotopic fractionation varies between different phases within a single iron meteorite, and occurs during fractional crystallization in asteroidal cores, most iron meteorites have ddelta^98/95MoSRM3134 (-0.14 to -0.06‰) that are similar to ordinary and CI carbonaceous chondrite (-0.12 to -0.09‰). This range of delta^98/95Moo is not only significantly lighter than the BSE, but also enstatite chondrites, which have delta^98/95Mo values of 0.04 to 0.13‰. Several possible explanations are proposed. (A) Core-mantle differentiation fractionates Mo isotopes. The recently proposed Mo effect of sulphide liquid removal is likely to be minor because this should have generated a light Mo isotope composition for the BSE. However, isotopic fractionation associated with metal-silicate partitioning may be responsible for the heavy Mo in the BSE. (B) A distinct isotopic composition for the late material that contributed Mo to the BSE. Enstatite chondrites (or other putative groups of chondrites with a heavy Mo isotope composition) and sulphur-rich components form the cores of impacting bodies are the most likely candidates that could deliver heavy Mo to Earth. (C) The Mo isotopic composition of the Solar System is heterogeneous in a mass dependent fashion such that heavier Mo isotopes are enriched in the section of the disk from which Earth accreted. There are some difficulties behind each of these models and further work is needed to determine which is correct.

Ice cores are a valuable component with regards to paleoclimate reconstruction due to the ability to use stable water isotopic concentrations in ice as a proxy for paleo-temperature records. It is therefore important to understand the processes and conditions under which isotopic concentrations can be altered after ice has formed. Historically, sublimation has been considered to only have a trivial impact on the isotopic record in glacial ice due to the low diffusivity of solid ice (~10-15 m2 s-1). Recent publications have shown that diffusion of impurities through ice can occur at much faster rates than the diffusivity of solid ice would imply, and have proposed that networks of unfrozen liquid (premelt) between ice grains may expedite the diffusion process. However, the application of this mode of diffusion to isotopic concentrations in ice under non-equilibrium conditions has been largely unexplored. Here I model changes in isotopic concentrations in ice using a two-dimensional diffusion mechanism, which incorporates premelt, coupled with a sublimation flux at the surface. Model results show an increase in δD at the ice surface and in near-surface ice. Concentrations exponentially decrease from the surface value to the initial concentration at depth. These results are consistent with recent experimental results.

Stable isotope of natural gas hydrates provides useful information of their gas sources. We investigated the isotopic fractionation of gas molecules during the formation of synthetic gas hydrates composed of methane and ethane. The gas hydrate samples were experimentally prepared in a pressure cell and isotopic compositions (δ13C and δD) of both residual and hydratebound gases were measured. δD of hydrate-bound molecules of methane and ethane hydrates was several per mil lower than that of residual gas molecules in the formation processes, while there was no difference in the case of δ13C. Effect of temperature on the isotopic fractionation was also investigated and it was found that the fractionation was effective at low temperature.

The ozone formation reaction O+O2+M→O3+M is a unique example of a chemical reaction that leads to an anomalous isotopic composition of the products, most likely due to symmetry - breaking effects. So far, results on other chemical systems that might show similar effects are spurious, even though such claims concerning reactions other than the formation of ozone have been made repeatedly.This applies in particular to the spin forbidden O+CO+M→CO2+M reaction, where two studies report a mass-independent fractionation of about 8%. Nevertheless, the presence of ozone in these experiments raises questions as to the validity of this assertion. We thus make a new attempt to study the O+CO+M reaction in the photoreactor at CCAR (University of Copenhagen) where reagents and contaminants are monitored on-line by FTIR spectroscopy. This study combined with the analysis of the spectral distribution of the employed lamps and isotope kinetic modeling lead to a complete re-interpretation of previous experiments. We conclude that available measurements are more compatible with the hypothesis that there is no mass-independent isotope fractionation in the O+CO reaction. We propose that all observations can be completely explained by an isotope transfer from ozone, involving photolytic production of O(1D) that in turn leads to OH radicals, which then rapidly form CO2 from reaction with CO. We also present a method to produce pure ozone samples and derive an upper limit on nitrogen oxide contaminations based on mass spectrometer measurements. These values will serve as benchmarks values for future studies of ozone absorption cross sections in the IR and UV.

Abstract Dissimilatory microbial sulfate reduction (MSR) The specific objectives of the study were to provide the first measurements of sulfur isotope fractionation associated with acidophilic sulfate reducing-microorganisms, and to examine whether pH influences sulfur fractionation during MSR. The fractionation associated with the strains investigated was comparable to that of neutrophilic strains with similar metabolisms (4-12‰), but varied with pH. Two fractionation regimes were identified: one regime is consistent with fractionation during exponential growth, while the other – not identified previously - is not linked to active sulfate reduction and may result from internal sulfate accumulation. This would represent the first measurement of sulfur fractionation during sulfate uptake, the first step of MSR. Geological processes at the Cretaceous-Paleogene (KPg) boundary The KPg boundary is associated with one of the largest biological extinctions in the history of our planet. Two major geologic events - the Chicxulub bolide impact with evaporite terrane and the eruption of the Deccan continental flood basalts - coincide with the KPg boundary and have been identified as possible triggers for the extinctions, but their relative timing remains unresolved. The objectives of this study were to identify the contribution of these processes to the sulfur burden in the sedimentary environment of two freshwater KPg sections, and to determine their relative timing. The results demonstrate that the peak of Deccan volcanism post-dates the Chicxulub impact and the associated abrupt KPg mass extinction, thus precluding a direct volcanic causal mechanism, but shedding light on the underlying causes for the delayed recovery of ecosystems in the early Paleogene.

Le nitrate, produit de la fin de chaîne de réaction des oxydes d’azotes de l’atmosphère (NOx = NO +NO2), est l’un des ions le plus abondant de la neige et de la glace polaire. Ses rapports isotopiques stables (δ18O, δ15N et Δ17O) ont été abondamment utilisés pour contraindre ses sources et les chemins réactionnels. De plus, le nitrate archivé dans les carottes de glace profondes peut apporter de nouvelles contraintes sur les conditions climatiques passées. Cependant, le dépôt de nitrate dans les régions polaires à faible accumulation est réversible en raison des processus post-dépôts, compliquant l’interprétation des enregistrements. Actuellement, il existe des enregistrements de nitrate issus de carottes de glace profonds couvrant de l’information climatique sur plusieurs milliers d’années dont leur interprétation dépend d’une quantification précise ces phénomènes post-dépôts. Nous avons étudié expérimentalement le transfert d’excès-17O de l’ozone durant la réaction en phase gaz de NO2 + O3  NO3 + O2, qui est une réaction importante de la chimie nocturne de formation du nitrate. De cette étude nous avons déterminé la fonction de transfert du 17O donnée par : ∆17O(O3*) = (1.23 ± 0.19) × ∆17O(O3)bulk + (9.02 ± 0.99). Nous avons aussi évalué la distribution intramoléculaire des isotopes de l’oxygène de l’ozone et observé que l’excès d’enrichissement résidait de manière prépondérante sur les atomes terminaux de l’ozone. Ces résultats auront une implication importante sur la compréhension de la formation du nitrate via les mécanismes d’oxydation des précurseurs NOx. L’impact de la photolyse sur les concentrations et les compositions isotopiques stables du nitrate est étudié dans ce travail de thèse sur la base d’étude de laboratoire et de terrain. Une étude de laboratoire a été conduite en irradiant de la neige naturelle de Dôme C avec une lampe UV à xénon et en utilisant différents filtres UV (280 nm, 305 nm et 320 nm). Sur la base des mesures des rapports isotopiques de l’oxygène et de l’azote, la dépendance aux longueurs d’onde des fractionnements isotopiques a été déterminée. En conséquence, en présence de lumière UV de haute énergie, le fractionnement isotopique est décalé vers des valeurs moins négatives et vice versa. Sur la base des fractionnements isotopiques obtenus en laboratoire, nous avons dérivé un décalage apparent de la valeur du zéro point d’énergie (ZPE) qui apporte une meilleure contrainte sur la section efficace d’absorption du 15NO3-. Ce décalage apparent est obtenu en minimisant les écarts entre les observations et les fractionnements isotopiques calculés basés sur un modèle de décalage ZPE, modèle qui inclut outre le décalage ZPE, le changement des largeurs, de l’asymétrie et de l’amplitude des sections efficaces d’absorption lors de la substitution isotopique. Nous avons validé le nouveau ZPE apparent en conduisant une étude de terrain à Dôme C, Antarctique. Dans cette étude, un dispositif expérimental a été construit sur le site et l’effet du rayonnement solaire UV sur la photolyse du nitrate de la neige investigué. Cette étude était basée sur la comparaison de deux puits remplis par de la neige soufflée homogénéisée dont l’un des deux puits n’était pas soumis aux rayonnements UV. Le fractionnement isotopique de 15N pour la neige exposée aux UV (-67.9 ± 12 ‰) est en bon accord avec le modèle de décalage ZPE estimé au cours de ce travail de thèse (-55.4 ‰). Ces valeurs sont aussi dans la gamme des fractionnements isotopiques apparents observée précédemment au Dôme C. Nous pensons que l’inclusion des ces nouvelles connaissances dans un modèle prédisant l’enrichissement du 15N dans les carottes de glace permettra une interprétation quantitative de l’information préservée dans la glace. / Nitrate, the end product of oxidation of atmospheric NOX (= NO + NO2), is one of the most abundant anions present in polar snow and ice. Its stable isotope ratios (δ18O, δ15N and Δ17O) have been widely used to constrain its sources and oxidation pathways. In addition, the nitrate archived in deep ice cores may be an important metric to constrain past climatic conditions. However, deposition of nitrate in polar regions with low snow accumulation is reversible due to post-depositional processes, and interpretation of this record is complicated. Currently, there exist deep ice core records of nitrate encompassing climatic information of millennial time scales, and their interpretation relies on careful quantification of post-depositional effects. We have experimentally studied the 17O-excess transfer from ozone during the gas phase NO2 + O3 → NO3 + O2 reaction, which is an important nighttime nitrate formation pathway. From this study, we have determined the ∆17O transfer function given by: ∆17O(O3*) = (1.23 ± 0.19) × ∆17O(O3)bulk + (9.02 ± 0.99). We have also evaluated the intramolecular oxygen isotope distribution of ozone and have observed the excess enrichment resides predominantly on the terminal oxygen atoms of ozone. The findings from this study will have an important implication for understanding nitrate formation pathways via different NOX oxidation mechanisms. The impact of photolysis on the amount and stable isotope enrichments of nitrate is investigated in this PhD study based on laboratory and field experiments. A laboratory study was conducted by irradiating a natural snow from Dome C with a Xe UV lamp and a selection of UV-filters (280 nm, 305 nm and 320 nm). Based on the oxygen and nitrogen isotope ratio measurements, wavelength dependent isotopic fractionations were determined. Accordingly, in the presence of high-energy UV light, isotopic fractionation is shifted towards less negative values and the reverse for lower energy UV photons. Based the isotopic fractionations obtained in the laboratory study, we derived an apparent ZPE-shift value, which better constrains the absorption cross-section of 15NO3-. This apparent shift is derived from the best fit between the experimental observations and calculated fractionations based on existing ZPE-shift model and it includes actual ZPE-shift and changes in width, asymmetry and amplitude in absorption cross-section during isotopic substitution. We have validated the newly derived apparent ZPE-shift by conducting a field study at Dome C, Antarctica. In this study, an experimental setup was built on-site and the effect of solar UV photolysis on snow nitrate was investigated. This study was based on a comparison of two snow pits filled with locally drifted snow and by allowing/blocking the solar UV. The 15N fractionation for the UV exposed samples (-67.9 ± 12 ‰) was in fairly good agreement with the ZPE-shift model estimate from this study (-55.4 ‰). These values are also within the range of apparent isotopic fractionation observed at Dome C in previous studies. Further calculations to better constrain the absorption cross-section of 15NO3- with the ZPE-shift are underway, and we propose that the newly derived apparent ZPE-shift value should be used in future studies. We believe that incorporating these new findings in models predicting the enrichments of 15N nitrate in ice cores will allow a quantitative interpretation of the information preserved in ice.