Thermoluminescence and phototransferred phermoluminescence of synthetic quartz
- Authors: Dawam, Robert Rangmou
- Date: 2020
- Subjects: Thermoluminescence , Quartz
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/145849 , vital:38472
- Description: The main aim of this investigation is on thermoluminescence and phototransferred thermoluminescence of synthetic quartz. Thermoluminescence was one of the tools used in characterising the electron traps parameters. The samples of quartz annealed at various temperatures up to 900̊C and the unannealed were used. The thermoluminescence glow curve was measured at 1̊C s~ 1 following beta irradiation to 40 Gy from the samples annealed at 500̊C and the unannealed consist of main peak at 70̊C and secondary peaks at 110, 180 and 310̊C. In comparison, the thermoluminescence glow curve for the sample annealed at 900̊C have main peak at 86̊C and the secondary ones at 170 and 310̊C. The kinetic analysis was carried out only on the main peak in each case. The activation energy was found to be decreasing with increase in annealing temperatures. The samples annealed at 500̊C and the unannealed were found to be affected by thermal quenching while sample annealed at 900̊C shows an inverse quenching for irradiation dose of 40 Gy. However, when the dose was reduce to 3 Gy the effects of thermal quenching was manifested. The activation energy of thermal quenching was also found to decrease with increase in annealing temperature. Thermally assisted optically stimulated luminescence measurement was carried out using continuous wave optical stimulated luminescence (CW-OSL). The samples studied were those annealed at 500̊C for 10 minutes, 900̊C for 10, 30, 60 minutes and 1000̊C for 10 minutes prior to use. The CW-OSL is stimulated using 470 nm blue LEDs at sample temperatures between 30 and 200̊C. It is measured after preheating to either 300 and 500̊C. When the integrated OSL intensity is plotted as a function of measurement temperature, the intensity goes through a peak. The increase in OSL intensity as a function of temperature is associated to thermal assistance and the decrease to thermal quenching. The kinetic parameters were evaluated by fitting the experimental data. The values of activation energies of thermal quenching are the same within experimental uncertainties for all the experimental conditions. This shows that annealing temperature, duration of annealing and irradiation dose have a negligible influence on the recombination site of luminescence using OSL. Phototransferred thermoluminescence (PTTL) induced from annealed samples using 470 nm blue light was also investigated. The quartz were annealed at 500 _C for 10 minutes, 900̊C for 10, 30, 60 minutes and 1000̊C for 10 minutes prior to use. The glow curves of conventional TL measured at 1 _C s1 following irradiation to 200 Gy shows six peaks in each case labelled I-VI for ease of reference whereas peaks observed under PTTL are referred to as A1 onwards. Only the first three peaks were reproduced under phototransfer for the sample annealed at 900̊C for 60 minutes and 1000̊C C for 10 minutes. Interestingly, for the intermediate duration of annealing of 30 minutes, the only peak that appears under phototransfer is the A1. For quartz annealed at 900̊C for 10 minutes, the PTTL appears as long as the preheating temperature does not exceed 560̊C. All other annealing temperatures, PTTL only appears for preheating to 450 and below. This shows that the occupancy of deep electron traps at temperatures beyond 450̊C or 560̊C is low. The activation energy for peaks A1, A2 and A3 were calculated. The PTTL peaks were studied for thermal quenching and peaks A1 and A3 were found to be affected. The activation energies for thermal quenching were determined as 0.62 ± 0.04 eV and 0.65 ± 0.02 eV for peaks A1 and A3 respectively. The experimental dependence of PTTL intensity on illumination time is modelled using sets of coupled linear differential equations based on systems of donors and acceptors whose number is determined by preheating temperature.
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- Date Issued: 2020
A physiological study on a commercial reef fish to quantify the relationship between exploitation and climate change resilience
- Authors: Duncan, Murray Ian
- Date: 2019
- Subjects: Chrysoblephus laticeps -- Climatic factors , Chrysoblephus laticeps -- Physiology , Sparidae -- South Africa -- Climatic factors , Reef fishes -- South Africa -- Climatic factors , Fish populations -- South Africa -- Climatic factors , Fish populations -- Measurement , Fish populations -- Monitoring , Fisheries -- South Africa -- Environmental aspects , Ocean temperature -- Physiological effect -- South Africa
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/76541 , vital:30599
- Description: The persistence of harvested fish populations in the Anthropocene will be determined, above all, by how they respond to the interacting effects of climate change and fisheries exploitation. Predicting how populations will respond to both these threats is essential for any adaptive and sustainable management strategy. The response of fish populations to climate change is underpinned by physiological rates and tolerances, and emerging evidence suggests there may be physiological-based selection in capture fisheries. By quantifying important physiological rates of a model species, the endemic seabream, Chrysoblephus laticeps, across ecologically relevant thermal gradients and from populations subjected to varying intensities of commercial exploitation, this thesis aimed to 1) provide the first physiologically grounded climate resilience assessment for a South African linefish species, and 2) elucidate whether exploitation can drive populations to less physiologically resilient states in response to climate change. To identify physiologically limiting sea temperatures and to determine if exploitation alters physiological trait distributions, an intermittent flow respirometry experiment was used to test the metabolic response of spatially protected and exploited populations of C. laticeps to acute thermal variability. Exploited populations showed reduced metabolic phenotype diversity, fewer high-performance aerobic scope phenotypes, and a significantly lower aerobic scope curve across all test temperatures. Although both populations maintained a relatively high aerobic scope across a wide thermal range, their metabolic rates were compromised when extreme cold events were simulated (8 °C), suggesting that predicted future increases in upwelling frequency and intensity may be the primary limiting factor in a more thermally variable future ocean. The increment widths of annuli in the otoliths of C. laticeps from contemporary and historic collections were measured, as a proxy for the annual growth rate of exploited and protected populations. Hierarchical mixed models were used to partition growth variation within and among individuals and ascribe growth to intrinsic and extrinsic effects. The best model for the protected population indicated that the growth response of C. laticeps was poorer during years characterised by a high cumulative upwelling intensity, and better during years characterised by higher mean autumn sea surface temperatures. The exploited population growth chronology was too short to identify an extrinsic growth driver. The growth results again highlight the role of thermal variability in modulating the response of C. laticeps to its ambient environment and indicate that the predicted increases in upwelling frequency and intensity may constrain future growth rates of this species. A metabolic index (ϕ), representing the ratio of O2 supply to demand at various temperatures and oxygen concentrations, was estimated for exploited and protected populations of C. laticeps and used to predict future distribution responses. There was no difference in the laboratory calibrations of ϕ between populations, and all data was subsequently combined into a single piecewise (12 °C) calibrated ϕ model. To predict the distribution of C. laticeps, ϕ was projected across a high-resolution ocean model of the South African coastal zone, and a species distribution model implemented using the random forest algorithm and C. laticeps occurrence points. The future distribution of C. laticeps was estimated by predicting trained models across ocean model projections up to 2100. The best predictor of C. laticeps’ current distribution was minimum monthly ϕ and future predictions indicated only a slight range contraction on either edge of C. laticeps’ distribution by 2100. In order to provide policy makers, currently developing climate change management frameworks for South Africa’s ocean, with a usable output, the results of all research chapters were combined into a marine spatial model. The spatial model identified areas where C. laticeps is predicted to be resilient to climate change in terms of physiology, growth and distribution responses, which can then be prioritised for adaptation measures, such as spatial protection from exploitation. While these results are specific to C. laticeps, the methodology developed to identify areas of climate resilience has broad applications across taxa. From a global perspective, perhaps the most salient points to consider from this case study are the evidence of selective exploitation on physiological traits and the importance of environmental variability, rather than long-term mean climate changes, in affecting organism performance. These ideas are congruent with the current paradigm shift in how we think of the ocean, selective fisheries, and how they relate to organism climate resilience.
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- Date Issued: 2019
Large scale spatio-temporal forcing of pelagic-coastal coupling: disentangling the effects of environmental change on intertidal invertebrate recruitment
- Authors: Muñiz, Carlota Fernández
- Date: 2019
- Subjects: Dinoflagellates -- South Africa , Coastal ecology -- South Africa , Climatic changes -- South Africa , Benthic ecology -- South Africa , Agulhas Current , Ocean temperature -- Agulhas Current , Ocean temperature -- Physiological effect
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/151541 , vital:39140
- Description: Marine systems are driven by the relationships among organisms and environmental conditions. Anthropogenic-induced changes during the past decades have started to alter climatic drivers which have the potential to alter the physical, chemical and biological environment. In coastal systems, biogeography is influenced by the temporal variability in the conditions of the water mass. In addition, many marine benthic organisms develop in the water mass and rely on the conditions that link the pelagic and benthic systems for population maintenance. Such pelagic-coastal coupling indicates that changes in the trophic system during development can be transferred to the adult populations through changes in propagule supply. Thus, changes in environmental conditions can influence benthic populations directly (e.g. through larval advection) or indirectly, through their influence on the phytoplankton community (e.g. through the development of HABs). The South African coastline shows clear alongshore patterns of faunal biomass and species richness. On the south coast, strong longitudinal patterns of recruitment of intertidal organisms exist, with areas of particularly high recruitment. HABs of unprecedented spatio-temporal magnitude have recently developed along the south coast, including the areas where benthic recruitment is most intense. The present thesis used these blooms to study changes in intertidal recruitment directly or indirectly associated with their occurrence. Using a combination of remote sensing data to study the environmental conditions of the water mass in the innermost part of the Agulhas Bank, and estimates of mussel and barnacle recruitment rates to integrate the effects of conditions in the water mass during larval development, this thesis aimed to: (1) understand the conditions that triggered the development of an HAB of the dinoflagellate Lingulodinium polyedrum during summer of 2014, (2) determine the direct or indirect effects of that bloom on recruitment of intertidal organisms, and understand the factors that affect recruitment along the coast, (3) determine if the environmental factors during bloom development produced any carryover effects on recruit growth and mortality, and (4) determine the factors that drive changes in community biomass and composition along the south coast, the long-term trends in those factors, and possible changes experienced in recent years. Water column stability during spring, before the development of the red tide, followed by alternating periods of upwelling and relaxation during summer and autumn, seemed to promote the development and persistence of L. polyedrum. Recruitment of mussels and barnacles was estimated during the reproductive season of mussels in 2014, coinciding with the red tide, and during the following year. Alongshore patterns in recruitment were found, with higher mussel recruitment in the absence of the red tide and the opposite pattern in barnacles. Alongshore patterns in SST and chlorophyll matching those of recruitment were also found, with higher SSTs and lower chlorophyll during the red tide than the following year. Growth and mortality rates in barnacles did not differ between years during the first five months after settlement. This suggests that the factors which produced differences in recruitment between years did not produce carryover effects detectable at the temporal scales studied. Further analysis of 15 years of satellite-derived environmental data showed significant cooling trends potentially driven by a long-term seasonal acceleration of the Agulhas Current in autumn around two upwelling centres on the south coast, coinciding temporally with the reproductive period of mussels and barnacles, and spatially with the areas of highest recruitment. In addition, the comparison of SST and chl-a conditions during the first and the second half of the period of study showed that seasonality of both variables has changed in large areas over the shelf, with increasing importance of shorter-term variability, which would in turn decrease environmental predictability. Thus, the conditions observed during the present study, particularly during 2015, when upwelling seemed to be more intense, may presage the potential effects of identified long-term cooling trends at the upwelling centres. Although the general trend shows cooling around those areas, conditions can vary greatly among years, favouring different taxa. Changes in the Agulhas Current System are affected by changes in distant areas in the Indian Ocean basin. Such tele-connection is unlikely to be unique to this system and indicates the importance of understanding trends in major large scale climatic drivers and their regional effects in order to make predictions about coastal systems.
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- Date Issued: 2019