Since the seminal works of Wegener and Darwin the notion that things evolve, and the how and the why of it, has generated intense debate. The surface of the Earth, and the creatures that live on it, are not static entities. Landscapes evolve. Organisms evolve. Understanding the how and the why requires a firm understanding of a myriad of interdependent and complex variables such as (but not limited to) climate, ecology, and tectonics. Unravelling the complexities though which landscapes and ecosystems evolve requires a broad interdisciplinary approach, where multiple investigative tools are simultaneously brought to bear on a given question. The study of old lake sediments, or paleolimnology, is a marquee example of a powerful interdisciplinary methodology that has been used extensively in reconstructing the Earth's past. This work showcases two examples where the discipline of paleolimnology advances our understanding of evolution on a landscape scale and on a human scale. In the southwestern United States, a record of the processes involved during the late Miocene and early Pliocene (~ 5 Ma) evolution of a major continental river drainage - the Colorado River – is partially preserved along the southern border of Arizona and California as the enigmatic Bouse Formation. And in southern Kenya, nearly 170 meters of lake and wetland sediments that have accumulated in the Koora Plain preserve a one-million-year long record of the environmental conditions against which our species, Homo sapiens, evolved. My research allows me to conclude that the depositional environment of the Bouse Formation was lacustrine; a fully marine interpretation that has been previously proposed is untenable. I also demonstrate that over the past 1.0 Ma, Homo sapiens in southern Kenya evolved against a backdrop of increasing regional aridity.

Abstract RF-gradients produced utilizing RF-field inhomogenity of conventional receiver/transmitter coil of NMR-probe can be used to mimic the effects of B0-gradients. This is done by placing long inhomogenous pulse in between two 90° pulses of appropriate phases (z-rotation cluster). B0-gradient based excitation sculpting can be converted into RF-gradient version. Selective one-dimensional TOCSY and NOESY using RF-gradient based excitation sculpting are described. In addition, non-selective two-dimensional experiments, TOCSY and NOESY, with RF-gradient based coherence selection are presented. Excitation sculpting using BIRD or BIRDR as inversion element results in isotope filter. Pre-suppression of non-13C-bound protons using RF-gradient BIRD prior to HSQC enables recording of spectrum of comparable quality to B0-gradient selected HSQC. This is beneficial for spectrometers lacking B0-gradient capabilities. Excitation sculpting using BIRDR can be used efficiently as low-pass filter in HMBC experiment. Furthermore, simultaneous elimination of protons bound to 15N and 13C can be accomplished with BIRDR based method.

Climate change is expected to have significant and uncertain impacts on methane (CH4) emissions from northern peatlands. Biogeochemical models can extrapolate site-specificCH(4) measurements to larger scales and predict responses of CH4 emissions to environmental changes. However, these models include considerable uncertainties and limitations in representing CH4 production, consumption, and transport processes. To improve predictions of CH4 transformations, we incorporated acetate and stable carbon (C) isotopic dynamics associated with CH4 cycling into a biogeochemistry model, DNDC. By including these new features, DNDC explicitly simulates acetate dynamics and the relative contribution of acetotrophic and hydro-genotrophic methanogenesis (AM and HM) to CH4 production, and predicts the C isotopic signature (delta C-13) in soil C pools and emitted gases. When tested against biogeochemical and microbial community observations at two sites in a zone of thawing permafrost in a subarctic peatland in Sweden, the new formulation substantially improved agreement with CH4 production pathways and delta C-13 in emitted CH4 (delta C-13-CH4), a measure of the integrated effects of microbial production and consumption, and of physical transport. We also investigated the sensitivity of simulated delta C-13-CH4 to C isotopic composition of substrates and, to fractionation factors for CH4 production (alpha(AM) and alpha(HM)), CH4 oxidation (alpha(MO)), and plant-mediated CH4 transport (alpha(TP)). The sensitivity analysis indicated that the delta C-13-CH4 is highly sensitive to the factors associated with microbial metabolism (alpha(AM), alpha(HM), and alpha(MO)). The model framework simulating stable C isotopic dynamics provides a robust basis for better constraining and testing microbial mechanisms in predicting CH4 cycling in peatlands.

M.Sc. / Please refer to full text to view abstract

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Reactions between CO2-charged brines and reservoir minerals might either enhance the long-term storage of CO2 in geological reservoirs or facilitate leakage by corroding cap rocks and fault seals. Modelling the progress of such reactions is frustrated by uncertainties in the absolute mineral surface reaction rates and the significance of other rate limiting steps in natural systems. This study uses the chemical evolution of groundwater from the Jurassic Navajo Sandstone, part of a leaking natural accumulation of CO2 at Green River, Utah, in the Colorado Plateau, USA, to place constraints on the rates and potential controlling mechanisms of the mineral-fluid reactions,under elevated CO2 pressures, in a natural system. The progress of individual reactions, inferred from changes in groundwater chemistry is modelled using mass balance techniques. The mineral reactions are close to stoichiometric with plagioclase and K-feldspar dissolution largely balanced by precipitation of clay minerals and carbonate. Mineral modes, in conjunction with published surface area measurements and flow rates estimated from hydraulic head measurements, are then used to quantify the kinetics of feldspar dissolution. Maximum estimated dissolution rates for plagioclase and K-feldspar are 2x10-14 and 4x10-16 mol·m-2·s-1, respectively. Fluid ion-activity products are close to equilibrium (e.g. DGr for plagioclase between -2 and -10 kJ/mol) and lie in the region in which mineral surface reaction rates show a strong dependence on DGr. Local variation in DGr is attributed to the injection and disassociation of CO2 which initially depresses silicate mineral saturation in the fluid, promoting feldspar dissolution. With progressive flow through the aquifer, feldspar hydrolysis reactions consume H+ and liberate solutes to solution which increase mineral saturation in the fluid and rates slow as a consequence. The measured plagioclase dissolution rates at low DGr would be compatible with far-from-quilibrium rates of ~1x10-13 mol·m-2·s-1 as observed in some experimental studies. This suggests that the discrepancy between field and laboratory reaction rates may in part be explained by the differences in the thermodynamic state of natural and experimental fluids, with field-scale reactions occurring close to equilibrium whereas most laboratory experiments are run far-from-equilibrium. Surface carbonate deposits and cementation within the footwall of the local fault systems record multiple injections of CO2 into the Navajo Aquifer and leakage of CO2 from the site over ca. 400,000 years. The d18O, d13C and 87Sr/86Sr of these deposits record rapid rates of CO2 leakage (up to ~1000 tonnes/a) following injection of CO2, but rates differ by an order of magnitude between each fault, due to differences in the fault architecture. Elevated pCO2 enhances rates of feldspar dissolution in the host aquifer and carbonate precipitation in fracture conduits. Silicate mineral dissolution rates decline and carbonate precipitation rates increase as pH and the CO2 charge dissipate. The Sr/Ca of calcite cements record average precipitation rates of ~2x10-6 mol/m2/s, comparable to laboratory derived calcite precipitation rates in fluids with elevated Mn/Ca and Fe/Ca, at cc of ~1 to 3. This suggests that far-from-equilibrium carbonate precipitation, which blocks fracture conduits and causes the leaking system to self-seal, driven by CO2 degassing in the shallow subsurface, can be accurately modeled with laboratory derived rates. Sandstones altered in CO2 leakage conduits exhibit extensive dissolution of hematite grain coatings and are chemically bleached as a result. Measurements of Eh-pH conditions in the modern fluid, and modeling of paleo-Eh-pH conditions using calcite Fe and Mn concentrations, suggests that the CO2-charged groundwaters are reducing, due to their low dissolved O2 content and that pH suppression due to high pCO2 is capable of dissolving and transporting large concentrations of metals. Exhumed paleo-CO2 reservoirs along the crest of the Green River anticline have been identified using volatile hosting fluid inclusions. Paleo-CO2-charged fluids mobilized hydrocarbons and CH4 from deeper formations, enhancing the reductive dissolution of hematite, which produced spectacular km-scale bleached patterns in these sediment.

Intraplate volcanism that has created the Hawaiian-Emperor seamount chain is generally thought to be formed by a deep-seated mantle plume. While the idea of a Hawaiian plume has not met with substantial opposition, whether or not the Hawaiian plume shows any geochemical signal of receiving materials from the Earth’s Outer Core and how the plume may or may not be reacting with the overriding lithosphere remain debatable issues. In an effort to understand how the Hawaiian plume works I report on the first in-situ sulfides and bulk rock Platinum Group Element (PGE) concentrations, together with Os isotope ratios on well-characterized garnet pyroxenite xenoliths from the island of Oahu in Hawaii. The sulfides are Fe-Ni Monosulfide Solid Solution and show fractionated PGE patterns. Based on the major elements, Platinum Group Elements and experimental data I interpret the Hawaiian sulfides as an immiscible melt that separated from a melt similar to the Honolulu Volcanics (HV) alkali lavas at a pressure-temperature condition of 1530 ± 100OC and 3.1±0.6 GPa., i.e. near the base or slightly below the Pacific lithosphere. The 187Os/188Os ratios of the bulk rock vary from subchondritic to suprachondritic (0.123-0.164); and the 187Os/188Os ratio strongly correlates with major element, High Field Strength Element (HFSE), Rare Earth Element (REE) and PGE abundances. These correlations strongly suggest that PGE concentrations and Os isotope ratios reflect primary mantle processes. I interpret these correlations as the result of melt-mantle reaction at the base of the lithosphere: I suggest that the parental melt that crystallized the pyroxenites selectively picked up radiogenic Os from the grain boundary sulfides, while percolating through the Pacific lithosphere. Thus the sampled pyroxenites essentially represent crystallized melts from different stages of this melt-mantle reaction process at the base of the lithosphere. I further show that the relatively low Pt/Re ratios of the Hawaiian sulfides and the bulk rock pyroxenites suggest that, upon ageing, such pyroxenites plus their sulfides cannot generate the coupled 186Os-187Os isotope enrichments observed in Hawaiian lavas. Therefore, recycling of mantle sulfides of pyroxenitic parentage is unlikely to explain the enriched Pt-Re-Os isotope systematics of plume-derived lavas.

abstract: During the early Solar System many physiochemical processes were taking place that would shape the formation and evolution of rocky bodies. Growth of these rocky objects was rapid, with some growing to sizes of 10s – 1000s km (“planetesimals”) in the first few million years. Because these objects formed early, they contained sufficient 26Al (an isotope of Al with a short half-life of ~705,000 yrs) to heat the interiors to melting temperatures, resulting in the formation of the first igneous rocks in nascent Solar System. Depending on the size and time of accretion, some bodies experienced high degrees of melting (with some having global magma oceans) while others experienced lower degrees of partial melting, and yet others did not experience any melting at all. These varying degrees of heating and melting processes on early-formed planetesimals produced a variety of achondritic meteorite types. These achondrites have bulk compositions ranging from ultramafic to basaltic, with some rare types having more highly “evolved” (i.e., high-SiO2) compositions. Determining the detailed chronology of their formation with fine time resolution is key for understanding the earliest stages of planet formation, and there are high resolution chronometers that are ideally suited for this application. Three such chronometers (i.e., the 26Al-26Mg, 53Mn-53Cr, and 207Pb-206Pb chronometers) are the focus of this work. Based on investigations of these chronometers in several achondritic meteorites, the implications for the formation and evolution of planetesimals in the early Solar System will be discussed. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2020

Geochemical measurements of OIB (ocean island basalt) samples have demonstrated that the Earth's mantle is compositionally heterogeneous, but the cause of this heterogeneity is a point of debate within the scientific community. One such OIB location is Mangaia, Cook Islands (Polynesia). Its lead isotopic composition defines the HIMU (high μ = high ^238U/^204Pb) mantle endmember, which many attribute to recycled oceanic crust being present in the mantle source. If true, this endmember represents an important vehicle for returning surface material to the mantle and an opportunity to study volatile element cycling through the mantle. Sulfur isotopic measurements were made on sulfides hosted in melt inclusions from Mangaia. Prior to 2.45 Ga, the Earth's atmosphere was not oxygenated, allowing photochemical cycles to fractionate sulfur isotopes. This form of fractionation results in a mass independently fractionated (MIF) sulfur isotopic signature in surface materials containing sulfur. We have found such a signal in sulfide inclusions from Mangaia, indicating that the material erupted at this young (~19 Ma) ocean island was once at the surface over 2.45 Ga. This finding confirms the recycled origin hypothesis for the generation of the HIMU mantle endmember. Lead isotopes and major elements were measured in olivine hosted melt inclusions from the island. Previous studies by Saal et al. (1998) and Yurimoto et al. (2004) have revealed large lead isotopic variability, spanning half of the global range for OIBs. A more recent study by Paul et al. (2011) has shown much reduced lead isotopic variability using a different analysis technique. We find the lead isotopic variability in glassy melt inclusions to be less than previously found and attribute much of the earlier observed variability to contaminant lead. Volatile and trace elements were measured in the same olivine hosted melt inclusions, providing the first ever coupled lead isotopes, major, trace, and volatile elements in glassy melt inclusions from the island. We observe some of the highest water and carbon dioxide contents found in OIBs globally. This allows us to constrain volatile abundances in the HIMU mantle source as well as volatile cycling in the mantle, from subduction zones to hotspots.

Marine mammals breeding in sympatry use resources differently depending on their time-activity budgets and prey distribution. We measured isotopic values and patterns of δ15N and δ13C in keratinous whiskers of satellite-tagged adult female Antarctic (Arctocephalus gazella) - and Subantarctic (Arctocephalus tropicalis) fur seals from three colonies at Marion Island in the Southern Indian Ocean. The three sympatric colonies were Mixed Pickle (HD_MP), Rockhopper Bay (LD_RhB), and Watertunnel (HD_WT). A comparison between two colonies of differing A. tropicalis densities will also be investigated. We investigated resource use in breeding mature females over varying time and spatial scales during summer and winter. The aim was to understand the variation in resource and habitat use at population and individual levels. The whiskers of A. gazella and A. tropicalis grow constantly at a rate of 0.16 ± 0.05 day-1 and 0.12 ± 0.05 day-1, respectively. To compare and relate behaviours, the stable isotope data and satellite tracks were grouped into winter and summer (pooled for 2010 - 2014). In summer, A. gazella from HD_WT had a larger isotopic niche width (0.85 ± 0.67‰2) compared to A. tropicalis from both HD_MP (0.18 ± 0.19‰2) and LD_RhB (0.21 ± 0.15‰2). However, the isotopic niche width of A. gazella reduced in size by more than half (0.40 ± 0.40‰2) during winter when the mature females were no longer constrained by the lactation mandate. The mean δ15N values in whiskers of both A. tropicalis and A. gazella in summer were similar (11.3 ± 0.4‰ and 11.0 ± 0.9‰, respectively). The A. gazella broadens its’ isotopic niche feeding at different trophic levels in summer (lactation period). We used a Repeatability index (R) to assess levels of intra- and inter-individual consistency. Repeatability indicated that the bigger isotopic niche width for A. gazella is attributed to high inter- and intra-individual variability in their resource use. The A. gazella individuals ranging closer to the colony consistently consumed myctophid fish. Other individuals foraged on low 15N content euphausiids (i.e., Euphausia spp. and Thysanoessa spp.) south of Marion Island in both winter and summer. The A. tropicalis from LD_RhB were repeatable in their δ15N values (trophic level) (Rglobal = 0.63; [95% CI: 0.29 – 0.80]) and δ13C values (habitat) (Rglobal = 0.69; [95% CI: 0.35 – 0.84]). At HD_MP, the A. tropicalis colony, diet was moderately repeatable (Rglobal = 0.33; [95% CI: 0.11 – 0.51]) while habitat was repeatable (Rglobal = 0.58; [95% CI: 0.33 – 0.73]). Consistency in diet (Rglobal = 0.27; [95% CI: 0.08 – 0.44] and foraging habitat (Rglobal = 0.45; [95% CI: 0.21 – 0.63] was moderate at HD_WT, the A. gazella colony. The diet varied with habitat at the individual level, across seasons. Segregation in diet and foraging habitat minimises resource-use overlap between sympatric fur seals. The behaviour of A. gazella from HD_WT (a high-density colony) might have a direct or indirect impact on the A. tropicalis from LD_RhB (a low-density colony) (i.e., competitive exclusion). Seasonality, at-sea movement, and intrinsic factors shaped the foraging behaviour and diet of adult female fur seals breeding at Marion Island. Some breeding mature females changed diets between seasons and depending on foraging habitat. / Dissertation (MSc (Zoology))--University of Pretoria, 2021. / NRF, Grantholder linked / Zoology and Entomology / MSc (Zoology) / Unrestricted