Department of Earth Sciences

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36 Chlorine isotope systematics in saline groundwater in the Buffels River Valley
(Stellenbosch : Stellenbosch University, 2018-03) Van Gend, Jani; Miller, Jodie A.; Clarke, Catherine E.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: It is typical for coastal aquifers in arid regions to be affected by salinisation, populations in these areas that rely upon groundwater resources are often directly afflicted by this phenomenon. Large parts of the western coast of South Africa are affected by variably saline groundwater as a result of varying degrees of salinisation, primarily driven by evaporative processes. Initial stable δ2H and δ18O isotopic investigations suggest rainfall and groundwaters carry evaporative signatures, further investigation has suggested that there are other significant salt contributors in the region, although these sources and pathways are poorly constrained. One such alternate salt source may arise from paleo-termite mounds, called heuweltjies, that are found in areas along the west coast. These structures typically consist of salt- and nutrient-rich sediments and the recorded sediment EC values for these structures are an order of magnitude higher than that of the adjacent interheuweltjies. Furthermore, sediment EC values of heuweltjies increase with depth and the difference between deep and shallow EC values in heuweltjies are between 1000 and 3000 μS/cm, with the deeper samples having the higher EC value. The possibility of heuweltjie salts entering the groundwater system and contributing to groundwater salinisation in this area has not been investigated as yet. The town of Buffels Rivier in the western region of the Northern Cape has limited surface water resources and is dependent on local groundwater for subsistence. For this reason, it is an ideal site to investigate these atypical salinisation drivers. In order to effectively isolate the additional salt sources, several geochemical, isotopic and geophysical methods were implemented. Highly variable soil and groundwater EC values were observed in the field area. Furthermore, 36Cl isotope ratios together with noble gas data provides some evidence that aquifer mixing occurs. This is further evidenced by the spatial discontinuity in groundwater 87Sr/86Sr ratios. The geology of this area consist of basement granite gneisses and the contribution of rock salt to the groundwater cannot be excluded. 87Sr/86Sr ratios in groundwater in the Buffels River Valley are elevated, ranging between 0.73030 and 0.78240, which is typically associated with the water-rock interaction granitic rocks. The extent of the salt contribution from the various systems in the Buffels River Valley is still not fully understood but it is evident that conventional, semi-arid salinisation through evaporation is not the only driver in this regional system.
Aerosol trace metal concentration and dissolution characteristics from known dust emitters in southern Africa
(Stellenbosch : Stellenbosch University, 2017-12) Kangueehi, Kaukurauee Ismael; Fietz, Susanne; Frank, Eckardt; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: Dust can be a source of micronutrients to surrounding areas such as oceans and terrestrial regions. The deposition of dust can provide trace elements to the open oceans, which can increase primary production and ultimately remove carbon dioxide from the atmosphere, therefore reducing global warming. Previous remote sensing studies have shown that southern African is a prominent dust emitting region and can potentially provide micronutrients to oceanic regions which might be depleted in some bioactive trace elements. Hysplit modeling software was used to estimate the long distance transport of dust emissions observed in southern Africa between January 2005 and December 2008. The observations revealed that most of the dust emissions occur during spring and winter seasons, with very little emissions in autumn. Most of the dust emissions tend to travel off the Namibian coastline towards the north-west Africa regions and are mainly influenced by strong south easterly trade winds. A strong air mass migrates towards the Indian Ocean and as far as the Australian continent due to the effects of the westerlies. Fewer air masses travel towards the nutrient-limited regions of the Atlantic Southern Ocean and central eastern Indian Ocean. The locations further north of the southern Africa preferentially travel towards the north west Atlantic Ocean, because the westerlies are not strong enough to transport air-masses towards the southern oceanic regions. This study also revealed that the prominent dust emitting sites in southern Africa are two ephemeral rivers, Kuiseb and Omaruru River as well as two ephemeral pans, the Etosha Pan in Namibia and Makgadikgadi Pan in Botswana. Emissions from these sources tend to travel towards north west Atlantic Ocean and south east Indian Ocean, with the exception of the Etosha Pan, which has emissions that travel towards the northern regions. These emitters were investigated for particle size distribution, mineralogical characteristics and trace elemental concentrations. The role of ephemeral rivers in southern Africa as potential sources of micronutrients to marine environments has not been previously investigated extensively. Most previous studies focussed on the ephemeral pans. Particle sizes can be an indicator of how far the sediments can potentially travel and of the trace elemental solubility. Etosha Pan has the finest grain sizes, while the Makgadikgadi had the coarsest grain size. Omaruru and Kuiseb River showed medium grain size variation. Our dissolution experiments showed, however, that the dissolution of the sediments is mostly influenced by the mineralogy rather than the particle sizes. The two pans appeared to be enriched in calcite, silica oxide and quartz, while the two rivers were more enriched in kaolinite, quartz, illite and muscovite. High trace element solubility in the Etosha Pan is most probably attributed to the high calcite content, which is highly soluble. A continuous flow through method proved to be effective and inexpensive. This study is one of the few in southern Africa which aimed at modelling the air mass pathways from dust emissions that have been observed instead of just creating simulations. Our findings highlight the importance of additional studies to prove the dissolution and quality of dust in dry regions as potential contributors to marine primary production.
The anatectic history of Archaean metasedimentary granulites from the Ancient Gneiss Complex, Swaziland
(Stellenbosch : Stellenbosch University, 2012-03) Taylor, Jeanne; Stevens, Gary; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: This study is an investigation of the anatectic history of high-grade paragneisses from the Ancient Gneiss Complex (AGC) in Swaziland. The work involved an integrated field, metamorphic, geochemical, geochronological and structural study of metasedimentary granulites from three separate, but spatially related areas of outcrop in south-central Swaziland, which were subjected to multiple high-grade partial melting events throughout the Meso- to Neoarchaean. The project has aimed to constrain the age(s) and conditions of metamorphism, so as to contribute to the understanding of geodynamic processes in the Barberton and AGC granite-greenstone terranes, as well as to investigate certain physical and chemical aspects of anatexis in the migmatites. The metamorphic record retained in these rocks, constrained by phase equilibria modelling as well as zircon and monazite SHRIMP and LA-ICP-MS geochronology, informs on the state of the mid- to lower-crust of the southeastern Kaapvaal Craton during key events associated with early lithosphere assembly and crustal differentiation. It also suggests that the region is comprised of more than one high-grade terrane. Two of the areas investigated experienced high-temperature metamorphism at ca. 3.23-3.21 Ga, in addition to a major 830-875º C, 6.5-7.6 kbar anatectic event at ca. 3.11-3.07 Ga. Intermediate and younger high-temperature events are recorded at ca. 3.18 Ga, ca. 3.16 Ga and 2.99 Ga. The timing of these metamorphic events coincided with the amalgamation of the eastern domain of the proto-Craton via subduction and accretion of micro-continental fragments at ca. 3.23 Ga, including the Barberton Greenstone Belt (BGB) and AGC terranes, as well as discrete episodes of crustal differentiation and potassic granitic magmatism between ca. 3.23 and 3.10 Ga. The third area investigated holds no record of Mesoarchaean metamorphism, but instead experienced a 830- 855 ºC, 4.4-6.4 kbar partial melting episode at ca. 2.73 Ga. This broadly coincided with the formation of a large continental flood basalt province, the ca. 2.71 Ga Ventersdorp LIP, and widespread intracratonic granitic magmatism on the Craton towards the end of the Neoarchaean. An explanation for the contrast in metamorphic record in the two terranes may be that the 2.71 Ga granulites represent a much younger sedimentary succession, and that granulites from the older terrane were left too restitic, after substantial partial melting during the Mesoarchaean, to record subsequent high-grade events. Finally, this study documents the details of S-type granitic magma production and extraction from a typical metapelitic source. Using the 2.73 Ga granulites from the AGC as a natural field laboratory, a case is made for the selective entrainment of peritectic garnet to the magma as a mechanism for generating relatively mafic, peraluminous S-type granite compositions. The work demonstrates the evolution of entrained peritectic garnet in such magmas, and is in strong support of a ‘peritectic phase entrainment’ process by which relatively mafic granite magmas are produced from melts which, in theory, should be highly leucocratic.
Apatite, allanite, titanite and monazite characteristics in S-, I- A-type Cape Granites
(Stellenbosch : Stellenbosch University, 2001-12) Spicer, Esme M. (Esme Marelien); Scheepers, R.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: This study focussed on the comparison of accessory mineral chemistry and paragenesis in the S-, I- and A-type granites of the Cape Granite Suite. The objective of the study was to use differences in accessory mineral chemistry and petrography to give insight in the evolution, recycling and formation of continental crust as affected by the Cape Granite Suite. Because of the high partition coefficients of the REE and trace elements into accessory minerals these minerals play an important role to explain granite evolution. The accessory mineral features are used as discriminators between barren and mineralized S-, I- and A-type granites in the suite. The petrography of the suite reflects the allanite-monazite dichtonomy with allanite and titanite occurring in the I -type granites while monazite occurs in S-type granites. Monazite becomes unstable in high Ca melts such as I-type granites. Apatite occurs in all the plutons which reflects its stability over a wide range of geological conditions. Rounded crystal habits of apatite and monazite in S-type granites indicate they are relics of sedimentary source rocks. Concentric growth- and sectoral zoning, as observed with CL and SEM, are common features in minerals that crystallized in barren plutons. The overprinting of magmatic textures reflects secondary processes, such as those that occurred in mineralized plutons, by "patchy" zoning and irregular alteration rims (coronas) in the mineralized plutons' accessory minerals. CL and SEM observations revealed that REE are redistributed into these coronas. Mineral chemistry of the accessory minerals reflects mostly the whole rock chemistry and physical conditions of the magmas. (Al~ Fe) substitution in titanite is controlled by P-T conditions, together with Ca, Mn and Mg substitution which is controlled by whole-rock chemistry, are good discriminators in S- and I-type granites. LREE and Sr content in allanite discriminate between the plutons and reflect the whole-rock chemistry. Apatite, because it occurs in all the plutons, is the most useful accessory mineral for discriminating between the plutons. From previous studies it is known that ASI controls the two main substitutions in apatite: Ca+P~Si+REE and Na+REE~2Ca, Fe and Mn content in apatite (0,1 pfu Mn and 0,05 pfu Fe contents are the cut-off between S-and Itype granites) are controlled by oxidation state of the magma and Sr, REE and Mg reflect whole-rock chemistry. The content of these elements in apatite can be used as discriminators between the plutons as their ASI, oxidation states and whole-rock chemistry differ. REE patterns of monazite and allanite are LREE enriched without exception, while apatite and titanite REE patterns are mostly birdwing profiles with occurrences of LREE or HREE enrichment. These patterns are influenced by crystallization of coexisting REE-bearing phases, fractionation history of the pluton and by crystallization sequence of the accessory minerals. Phase relationships were investigated experimentally for monazite and allanite under magmatic conditions (870 °C, 1,8 kbar) in peraluminous to metaluminous granitic melts. Monazite became unstable when aqueous CaCh solutions of 0.7-7 g CaCh/10cc H20 where added to peraluminous melts (ASI> 1 ). Monazite broke down to Cl-apatite and corona textures were observed. Allanite was tested in peraluminous (ASI> 1) and metaluminous (ASI=1) melts with different P20 5 (0.08 - 0.25 wt%) concentrations. Allanite became unstable at high phosphorus and peraluminous melt conditions and broke down to LREE-P± Al, Ca, K phases. Corona (kelyphitic) textures were observed. It is also clear that phosphorus played an important role, with Al, in the melt structure as can be seen from the absence or presence of crystals in the glasses of the different melts. This is possible because adding of phosphorus to the melt results in a depression of the T of the granitic melts' liquidus. Because of an interaction of phosphorus with Si networks and formation of complexes it also depolymerize aluminosilicate melts. The solubility of monazite was also tested in aqueous solutions under atmospheric conditions and low T (100-350 °C) to investigate low TIP alteration. Solutions ofCaCb +NaCl (1:1) chlorides were very reactive and dissolved the monazite completely, while solutions of CaCb were less reactive and only partly dissolved the monazite. These experiments demonstrate the concentrations required in hydrothermal solutions to destabilize monazite and explain textures found in natural rocks. Accessory minerals are useful discriminators between S-, I- and A-type granites and also their mineralized counterparts. Discrimination does depend on what accessory minerals are present and therefore apatite is the best mineral because it occurs in all the plutons. Petrography of these minerals is an indicator of primary or secondary processes.
Application of 87Sr/86Sr ratios to discerning groundwaters from different lithostratigraphic units in the Southwest coastal region of South Africa
(Stellenbosch : Stellenbosch University, 2022-04) Gcakasi, Monis Nolitha; Miller, Jodie A.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: South Africa has been significantly affected by climate change and has experienced reoccurring drought conditions induced by low precipitation levels and resulting in declining surface freshwater levels. This is further exacerbated by a rapid rise in population and increasing urbanization and demand for freshwater supply. The Western Cape Province is one of the most affected regions in the country to the point where in 2018, The City of Cape Town was close to “Day Zero” when all water reserves were set to run out. This led to a search for alternative water supply options and groundwater was identified as a major source of freshwater for residents. Western Cape groundwater is sourced from fractured lithostratigraphic rock units and the principal aim of this study was to constrain the 87Sr/86Sr ratio signatures of five of these aquifers. These are namely, the Malmesbury, Cape Granite, Alluvial, Table Mountain Group (TMG) and Bokkeveld Aquifers. A total of 83 borehole sites were sampled and groundwater samples were analysed for EC, pH, alkalinity, hydrochemistry, 87Sr/86Sr ratios and stable isotopes. The TMG Aquifer had the lowest EC values ranging between 9.90 – 422 μS/cm with an average of 199.55 μS/cm while the Malmesbury Aquifer had the highest average EC 3223.73 μS/cm with values ranging between 129 – 8870 μS/cm. Alluvial groundwaters had EC values ranging between 370 – 10810 μS/cm with an average of 2147.29 μS/cm. Cape Granite Aquifer samples had groundwater ranging between 43.40 – 5230 μS/cm with an average of 743.77 μS/cm. The Bokkeveld Aquifer had an average EC of 470.20 μS/cm and a range between 268 - 919 μS/cm. In terms of pH, TMG Aquifer groundwaters had the lowest pH with an average of 5.24 and values ranging between 4.03 – 6.29. The Cape Granite Aquifer had values ranging between 5.83 – 8.06 and an average of 6.50. Malmesbury and Alluvial groundwaters had similar average pH values of 6.74 mg/L and 6.40 mg/L respectively. However, the Malmesbury Aquifer samples had a narrower pH range of 6.06 – 7.66 compared to the wider range of 4.47 – 7.60 observed for the Alluvial Aquifer. Bokkeveld Aquifer samples had a pH range of 6.29 – 6.88 and an average of 6.52. Alluvial Aquifer alkalinity values ranged between 2.10 - 371.70 mg/L with an average of 107.84 mg/L while Cape Granite Aquifer values ranged between 11.19 – 159.01 mg/L with an average of 57.54 mg/L. The Malmesbury Aquifer had the highest average alkalinity at 143.66 mg/L with a range of 60 - 418 mg/L while the TMG Aquifer had the lowest alkalinity values with a range of 1.80 – 34.20 mg/L and an average of 8.00 mg/L. The Bokkeveld Aquifer had an average alkalinity of 74.60 mg/L and values varied between 31 - 108.10 mg/L. In general, the dominant cations and anions for the groundwater samples are, Na+ < Ca2+ < Mg2+ < K+ and Cl-< HCO3-< SO42- < NO3- < PO43-. Most of the groundwaters are saline with Na+ as the dominant cation and Cl- as the dominant anion (NaCl-type). Malmesbury groundwaters generally had the highest major ion concentrations, especially Na+, Ca2+, Mg2+, K+, Cl- , SO42-, NO3- and HCO3- . This is followed by the Alluvial Aquifer which also has high concentrations of the same ions. Groundwater from the TMG Aquifer had the lowest concentrations in Na+, Ca2+, Mg2+, K+, Cl- , SO42-, NO3- and HCO3-. Cape Granite groundwaters did not exhibit a distinct ionic composition but generally had higher concentrations of Na+ and Cl- compared to Ca2+, Mg2+ and K+. The Cape Granite Aquifer also had higher concentrations of HCO3- and SO42- compared to NO3-. The Bokkeveld groundwater samples had low concentrations of Ca2+, Mg2+ and Cl- but slightly higher Na+ and K+ concentrations with low SO42- and NO3- concentrations. The TMG and Bokkeveld Aquifers had similar δD ranges of -25.1 to -10.90 ‰ and - 29.4 to -12.8 ‰ respectively. The average δD for the TMG Aquifer was -20.3 ‰ and - 23.4 ‰ for the Bokkeveld Aquifer. The δ18O range of the TMG Aquifer was -5.33 to -2.89 with an average of -4.40 ‰ and -5.23 ‰ to -2.55 ‰ with an average of -4.40 ‰ for the Bokkeveld Aquifer. The Malmesbury and Alluvial Aquifers had average δD values of -17.4 ‰ and -15.4 ‰ respectively and the δD values had ranges between - 20.2 to -11.3 ‰ and -19.8 to -6.00 ‰ respectively. The δ18O values for Malmesbury groundwaters ranged from -4.32 to -3.07 ‰ with an average of -3.74 ‰ while the Alluvial Aquifer had an average δ18O value of -3.44 ‰ and values ranged between - 4.26 to -1.98 ‰. The Cape Granite Aquifer had a δD range of -27.3 to -12.3 ‰ and a δ18O range of -4.44 to -2.96 ‰ with an average δD of -18.6 and an average δ18O of - 4.06 ‰. Sixty samples were chosen for analysis of the 87Sr/86Sr ratio and 35 additional 87Sr/86Sr ratios were obtained from Costaras (2019). The 87Sr/86Sr ratios for the aquifers were found to have distinct ranges. The Cape Granite Aquifer had 87Sr/86Sr ratios ranging between 0.71812-0.73939. The Alluvial and Malmesbury Aquifers had 87Sr/86Sr ratios ranging between 0.71166 – 0.71988 and 0.70936 – 0.71852 respectively. The Bokkeveld Aquifer had a 87Sr/86Sr ratio range between 0.71720 – 0.72214 while the TMG Aquifer exhibited the most variability in 87Sr/86Sr with ranges between 0.71152 and 0.72955. The assigned 87Sr/86Sr ratio was 0.72620±0.0049 for the Cape Granite Aquifer, 0.71510±0.0035 for the Malmesbury Aquifer, 0.71311±0.0029 for the Alluvial Aquifer, 0.71580±0.0068 for the TMG Aquifer and 0.71892±0.0028 for the Bokkeveld Aquifer. Assessment of the spatial distribution of groundwaters showed that most coastal samples are associated with the Alluvial Aquifer and have 87Sr/86Sr ratios close to modern seawater. Further inland, groundwaters had progressively higher 87Sr/86Sr ratios. A temporal variability study showed that the 87Sr/86Sr ratio remains largely constant over shorter and longer term periods. The isotopic ratio for samples taken three-hourly showed little variation while longer-term samples take monthly showed more but not significant variation. This indicates that influence by recharge and dilution processes is unlikely to significantly affect the 87Sr/86Sr ratio over long-periods. In future studies, the study of the spatial distribution of 87Sr/86Sr ratios would require a more well-distributed sampling area to get a better sense of the change in 87Sr/86Sr ratios with geographical location. A challenge faced in this study was a lack of boreholes in certain areas or the occurrence of boreholes where sampling could not be done due to issues like lack of a take-off point or boreholes with no pumps. For the longer-term temporal variability study, sampling would need to take place over a more extensive period to further ensure the stability of the 87Sr/86Sr ratios.
Application of GIS-based suitability modelling to predict fossil sites in high-risk/high-reward geographic settings
(Stellenbosch : Stellenbosch University, 2021-04) Lines, Harry Alexander; Tucker, Ryan T.; Govender, Romala; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
This thesis explores the application of GIS-based predictive modelling to the field of Palaeontology. A universal methodology based on a multi-variate analysis of five environmental variables is presented. The incorporated environmental datasets were successfully acquired from a variety of publicly accessible online sources related to each study area. Our methodology was based on previous studies and expanded upon previous approaches to suitability modelling in the context of fossil prospecting. All stages of modelling were conducted within the ArcGIS software package and could be closely reproduced in alternative free software packages such as QGIS. Environmental variables were reclassified to a universal suitability scale of 1 to 20 and then amalgamated into a singular suitability analysis using the Weighted Overlay tool. The produced model was tested on two well documented fossil localities – The West Coast Fossil Park near Langebaanweg (South Africa) and the Cedar Mountain Formation of Central Utah (USA). After successfully classifying known fossil sites in both study areas, the model was applied to a largely unexplored and undocumented region – The Skeleton Coast of North-West Namibia. From the resultant model, 5 sites were identified and suggested for the 2021 expedition season. All models resulted in significant reductions in the prospective ground of each study area, ranging between 54% and 99%.
Application of hydrochemistry and residence time constraints to distinguish groundwater systems in the Karoo Basin prior to shale-gas exploration
(Stellenbosch : Stellenbosch University, 2016-03) Swana, Kelley; Miller, Jodie A.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: The possibility of shale-gas development in the semi-arid Karoo Basin, South Africa has created the need to develop a hydrochemical baseline for deep Karoo groundwater. However, it is uncertain how the process of hydraulic fracturing (fracking) will affect the surrounding groundwater and environments, particularly shallow groundwater sources. Therefore, it is important to have a good understanding of the characteristics of the deep groundwater and its connectivity to the shallower groundwater systems which are of utmost importance to the farmers and local communities in the region. As a result of the requirement for information regarding the deeper groundwater in the Karoo Basin, this project was initiated in order to create a baseline of the deeper groundwater and assess how it differentiates from the shallow groundwater. Nineteen groundwater samples were collected from 8 locations throughout the Karoo Basin. At each site a deep sample was collected from a warm spring or borehole, and a corresponding shallow site was collected from a nearby shallow borehole. Following an initial assessment of the groundwater samples, three groups were identified; deep, shallow and mixed. The shallow samples could be identified by temperatures less than 25°C, high alkalinities, the presence of Mg2+, NO3- and U, as well as higher δ2H and δ18O values. Warmer temperatures usually above 25°C, low alkalinities less than 80 mg.L-1 HCO3-, elevated Na+ and F- as well as lower δ2H and δ18O values were characteristic of the deep samples. The results of the mixed samples consistently feel between those of the deep and shallow samples, indicates that natural mixing occurs between the deep and shallow aquifer systems. This has important implications as upward migration of poor quality deep groundwater and contaminants resulting from fracking activities have been known to occur in the USA. Significant differences are observed between the deep and shallow samples, resulting from different controls on the groundwater chemistry. For example, the process of nitrification occurs in the shallow groundwater resulting in elevated NO3- concentrations, whereas the dissolution of fluorite results in elevated F- concentrations in the deep groundwater. Furthermore, increased contact time with the host lithologies results in increased Na concentrations in the deep groundwater. Residence times of the groundwater samples were calculated using four isotopic tracers, namely, tritiogenic helium, radiocarbon, helium-4 and chrlorine-36. Overall, the deep groundwater samples showed much longer residence times than shallow groundwater, with radiocarbon producing the most probable residence times. Pollution and contamination of shallow aquifers as a result of fracking is possible. Therefore, continuous baseline monitoring is essential before, during and after fracking activities in order to identify abnormalities that could be associated with fracking.
The application of Radon-222 in constraining zones of recent groundwater recharge in the Table Mountain Group aquifer in the City of Cape Town and its surrounding areas
(Stellenbosch : Stellenbosch University, 2020-03) Agyare-Dwomoh, Yaa; Miler, Jodie Ann; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: The world’s population is expected to increase by 2.14 billion people by the year 2050, and therefore finding sustainable water resources to satisfy humanity’s water demand has become one of the most urgent challenges of the 21st century. Due to population growth, poor water management practices and climate change, water scarcity has become a critical issue in arid regions like South Africa. Drought conditions affected the available water resources in the Western Cape during 2015-2018, and groundwater was considered as a sustainable water resource to mitigate recurrent water stresses. The Table Mountain Group (TMG) aquifer that surrounds the City of Cape Town is considered an excellent target for large scale abstraction to supplement the municipal supply. Although the high quality of the water (judged on the basis of very low TDS) makes this aquifer suitable for domestic abstraction purposes, for long term sustainability of abstraction still needs to be evaluated. 222Rn is an inert radioactive noble gas with a half-life of 3.82 days. This isotope is present in very low concentrations in precipitation but much higher concentrations in groundwater. Therefore, changes in the radon activity concentrations in groundwater might reflect dilution due to rapid recharge. Thus radon has the potential to be a means of evaluating groundwater sustainability where sustainable groundwater is defined as groundwater that is regularly recharged by modern precipitation. Analysis of 222Rn as undertaken in groundwater from different aquifer systems that surround the City of Cape Town in order to understand the groundwater recharge dynamics of each system. The groundwater systems examined were the Table Mountain Group aquifer, the Malmesbury Group aquifer, the Cape Granite Suite aquifer, the Bokkeveld Group aquifer, the Witteberg Group aquifer and the Quarternary sediments aquifer. As the groundwater was not further differentiated into specific formations or rock units within each of these stratigraphic units, they are referred to as aquifer systems. The hydrochemistry including stable isotopes of each groundwater sample was used to assign each groundwater sample to a host aquifer system. These host aquifer groupings were then used to characterise radon activity concentrations in each aquifer. Radon activity concentrations were variable in each aquifer system and these significant ranges meant that each aquifer system did not have a distinct radon activity concentration character. The two exceptions to this were: (1) the TMG aquifer where the lack of U and Ra in the aquifer host rocks, meant that the activity concentration of radon was lower in this aquifer than the other aquifers; and (2) the Cape Granite Suite aquifer system, where the high concentrations of U and Ra in the host rocks resulted in higher radon activity concentrations in groundwater hosted by these rocks. The radon activity concentration in groundwater in different locations changed as a consequence of groundwater recharge. As rainwater contains negligible radon activities, a dilution effect was noted in response to groundwater recharge in some of the aquifer systems. Three radon activity concentration trends were noted: (1) an immediate dilution in the radon activity concentration was recorded due to direct recharge; (2) a delayed dilution in the radon activity concentration was recorded due to a lag time in the recharge; and (3) radon activity concentrations were stable indicating little or no recharge response within a period of ~ 20-25 days after the recharge event (precipitation event). The radon data was compared with radiocarbon data (collected as part of a separate parallel study) for the same sample locations. The 14C data was consistent with the three radon activity concentration trends above being associated with groundwaters of different ages. The groundwater samples with the stable radon activity concentrations were associated with lower 14C activities, implying older residence times and hence a disconnection from modern recharge. In utilising the groundwater radon activity concentrations, sites of rapid groundwater recharge were delineated and mixing behaviour between surface water and groundwater was evaluated. This contributed to a better understanding of the groundwater recharge dynamics and allowed assessment of which aquifer systems were more sustainable. Groundwater from the TMG aquifer system has low radon activity concentrations. After precipitation events, these values dropped rapidly implying a direct recharge response. 14C data for the same groundwater samples, indicates the groundwater is typically young (± ≥ 100 pMC) and thus its sustainability is directly linked to current precipitation patterns. Hence, during periods of little or no rain, the aquifer is vulnerable to overexploitation and should be closely monitored and used sparingly. The results presented here introduce new perspectives in the application of groundwater isotopic tracers to understanding the TMG aquifer system and how it is recharged.
Applying phase equilibria modelling to igneous systems by coupling trace element partitioning and accessory phase saturation to compositionally variable thermodynamic modelling in Rcrust
(Stellenbosch : Stellenbosch University, 2023-03) Hoffman, Sean; Mayne, Matthew Jason; Stevens, Gary; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: Phase equilibria modelling techniques can constrain the equilibrium conditions of major rock forming minerals, however current thermodynamic databases and activity-composition models cannot accommodate minor and trace elements as chemical components in modelled systems. This thesis presents the integration of trace element partitioning routines between melt and solid phases in the thermodynamic modelling tool Rcrust in order to effectively estimate the trace element composition of stable phases when melt is present. Additionally, trace elements occur in large abundances in certain accessory phases, for which we do not currently have activity-composition models. These trace element-rich accessory phases may have significant influence on the trace element chemistry of a magmatic system. For this constraint, accessory phase saturation routines for apatite and monazite, two commonly occurring accessory phases in granitic rocks, are newly integrated in this thesis to the existing Rcrust modelling tool. This provides a novel methodology for approximating trace element distribution among thermodynamically constrained phases with thermodynamically unconstrained accessory phases, apatite and monazite. The results highlight the importance of major element components that also contribute to accessory phase formation, such as calcium in apatite (which can accommodate a non-trivial quantity of the available calcium) and the resultant effect on phase equilibria of major phases. Saturating apatite can alter phase equilibria, mainly for calcium-bearing phases, that is equivalent to up to 20 °C change in temperature. A case study of the Peninsula Pluton granodiorite, South Africa, is used to validate the use of accessory phase saturation and trace element partitioning with phase equilibria modelling. The model predicts LREE concentrations of apatite and monazite that match the natural sample at temperatures below 725 °C, corroborating previous findings for emplacement P-T constraints. However, phase assemblages predicted through phase equilibria modelling show that the rock equilibrated to P-T below the emplacement estimates of previous studies. By modelling both major and accessory phases, constraints are provided on the formation and crystallisation of crustal magmas which has important implications on geothermobarometry and thermochronology in the crust.
Archaeal membrane lipid-based paleothermometry for applications in polar oceans
(The Oceanography Society, 2020-06) Fietz, Susanne; Ho, Sze Ling; Huguet, Carme
To establish whether ongoing climate change is outside the range of natural variability and a result of anthropogenic inputs, it is essential to reconstruct past oceanic and atmospheric temperatures for comparison with the modern world. Reconstructing past temperatures is a complex endeavor that employs indirect proxy indicators. Over the past two decades, promising paleothermometers have been developed that use isoprenoidal glycerol dialkyl glycerol tetraethers (isoGDGTs) from the membrane lipids of archaea preserved in marine sediments. These proxies are based on the observed relationship between lipid structure and temperature. As with all proxy indicators, observed relationships are often complex. Here, we focus on the application of isoGDGT paleotemperature proxies in the polar oceans, critical components of the global climate system. We discuss the application of and caveats regarding these archaeal membrane lipid-derived proxies and make recommendations to improve isoGDGT-derived polar ocean temperature reconstructions. We also review initial successes using hydroxylated (OH) isoGDGTs proxies in cold Arctic and Southern Ocean regions and recommend that multi-proxy approaches, including both hydroxylated and non-hydroxylated isoGDGTs, be used to contribute to the robustness of paleotemperature reconstructions.
Aspects of the filament activity within the Benguela upwelling system
(Stellenbosch : Stellenbosch University, 1988-12) Stockton, P. L. (Philip Leslie); Lutjeharms, J. R. E.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: The Benguela upwelling system off southern and south western Africa is a zone of strong and extensive upwelling. Owing to the greater fIsh numbers found along the front between the upwelling and South East Atlantic Ocean the frontal zone is a key element in the ecology of the upwelling area. This discourse focuses on the perturbations of this front. The major data source used was the daily Meteosat satellite imagery for the years 1978, 1979 and 1982 to 1985 in the visible and infrared wavebands. These data enable the investigation of cross frontal activity for the entire Benguela Upwelling System at various spatial and temporal scales. NOAA satellite infrared imagery was also used. At the macroscale two upwelling zones are described. The fIrst is the upwelling core along the coast which exhibits well-developed and persistent upwelling. Offshore of this is an area in where the more transient fllament activity predominates. This outer zone is one of constant change and presents highly variable frontal boundary locations. As far as the persistent upwelling is concerned, the northern boundary closely approximates that suggested by Shannon (1985) of l7oS. Cape Agulhas was the effective southern boundary of any regular upwelling. The mean offshore extent of the outer upwelling is 270 kilometres off Liideritz and 45 kilometres off Cape Town. An analysis of the seasonal location of the front shows that the greatest upwelling extent at both Liideritz and Cape Town is observed in winter. Although the winter upwelling extent is the greater of the two seasons, the summer frontal location at Cape Town, in turn, exhibits remarkable stability. The upwelling off Liideritz, on the other hand, is prone to almost constant frontal location fluctuations. . Most of the variation occurs as a result of the growth and decay of fllaments. Filaments were seen along the entire upwelling zone from Cape Agulhas to Cape Frio. On average, the fllament sector was 270 kilometres wide off Liideritz. Between Cape Point and Cape' Agulhas the southwards extent of the upwelling rarely exceeded 40 kilometres, while the maximum fllament off Cape Point was about 200 kilometres. A fllament at Cape Point generally extends about 40 kilometres westwards and the upwelling zone off Walker Bay stretches about 20 kilometres southwards, onto the Agulhas Bank. Along the coast between Cape Agulhas and Cape Point the fllaments rapidly react to changes in the wind speed and direction. Mesoscale weather systems are therefore important factors in determining fllament activity there. This also true to some extent at Liideritz. The berg wind can induce rapid fllament growth off Liideritz. Despite an upwelling positive wind direction it is the wind speed that determines whether upwelling will develop at Cape Town at all. At Liideritz the wind speeds appear to determine the cross frontal temperature gradient. The greater the wind speed, the steeper the temperature gradient. Cross frontal temperature gradient of between 0.020C and .006oC per kilometre were calculated for Liideritz, which compares well with readings in the Californian upwelling zone (Koblinsky et al. 1984). In the way fllaments extend the length of the frontal zone greatly and the manner in which these cold water streams react to the changing winds, they are complex upwelling frontal features of great variability and importance in the Benguela upwelling system.
Assessment of C, N, and Si Isotopes as tracers of past ocean nutrient and carbon cycling
(John Wiley & Sons, Inc., 2021) Farmer, J. R.; Hertzberg, J. E.; Cardinal, D.; Fietz, S.; Hendry, K.; Jaccard, S. L.; Paytan, A.; Rafter, P. A.; Ren, H.; Somes, C. J.; Sutton, J. N.
Biological productivity in the ocean directly influences the partitioning of carbon between the atmosphere and ocean interior. Through this carbon cycle feedback, changing ocean productivity has long been hypothesized as a key pathway for modulating past atmospheric carbon dioxide levels and hence global climate. Because phytoplankton preferentially assimilate the light isotopes of carbon and the major nutrients nitrate and silicic acid, stable isotopes of carbon (C), nitrogen (N), and silicon (Si) in seawater and marine sediments can inform on ocean carbon and nutrient cycling, and by extension the relationship with biological productivity and global climate. Here, we compile water column C, N, and Si stable isotopes from GEOTRACES-era data in four key ocean regions to review geochemical proxies of oceanic carbon and nutrient cycling based on the C, N, and Si isotopic composition of marine sediments. External sources and sinks as well as internal cycling (including assimilation, particulate matter export, and regeneration) are discussed as likely drivers of observed C, N, and Si isotope distributions in the ocean. The potential for C, N, and Si isotope measurements in sedimentary archives to record aspects of past ocean C and nutrient cycling is evaluated, along with key uncertainties and limitations associated with each proxy. Constraints on ocean C and nutrient cycling during late Quaternary glacial-interglacial cycles and over the Cenozoic are examined. This review highlights opportunities for future research using multielement stable isotope proxy applications and emphasizes the importance of such applications to reconstructing past changes in the oceans and climate system.
Assessment of GIS-interpolation techniques for groundwater evaluation : a case study of the Sandveld, Western Cape, South Africa
(Stellenbosch : University of Stellenbosch, 2004-12) Munch, Zahn; Zietsman, H. L.; University of Stellenbosch. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: The Sandveld, a potato growing area of the Western Cape is subject to significant groundwater abstraction for both municipal and agriculture purposes. The climate is arid and sensitive and important ecosystems in the area are showing varying degrees of impact. Management measures are needed to ensure ongoing sustainable development of the area. In this study, different interpolation techniques were evaluated to calculate values for unsampled variables rainfall and groundwater elevation. Local deterministic techniques as well as geostatistical techniques were used. It was found that geostatistical techniques, especially with collateral information, such as topography, provided a more accurate result. For environmental studies of this nature, Kriging is recommended as interpolation technique. The underlying data will determine the selection of the particular type of Kriging. Data was extracted from a customized relational database, geoMon, used for data capture, retrieval, processing and reporting. Ease of data extraction facilitated analysis. The interpolated grids were applied in two scenarios: Recharge calculations and quantification as well as a new classification approach according to Resource Directed Measures (RDM). Management classes were defined based on GIS-derived data.
Atmospheric deposition fluxes over the Atlantic Ocean : a GEOTRACES case study
(Copernicus Publications, 2019-04-11) Barraqueta, Jan-Lukas Menzel; Klar, Jessica K.; Gledhill, Martha; Schlosser, Christian; Shelley, Rachel; Planquette, Helene F.; Wenzel, Bernhard; Sarthou, Geraldine; Achterberg, Eric P.
Atmospheric deposition is an important source of micronutrients to the ocean, but atmospheric deposition fluxes remain poorly constrained in most ocean regions due to the limited number of field observations of wet and dry atmospheric inputs. Here we present the distribution of dissolved aluminium (dAl), as a tracer of atmospheric inputs, in surface waters of the Atlantic Ocean along GEOTRACES sections GA01, GA06, GA08, and GA10. We used the surface mixed-layer concentrations of dAl to calculate atmospheric deposition fluxes using a simple steady state model. We have optimized the Al fractional aerosol solubility, the dAl residence time within the surface mixed layer and the depth of the surface mixed layer for each separate cruise to calculate the atmospheric deposition fluxes. We calculated the lowest deposition fluxes of 0.15±0.1 and 0.27±0.13 g m−2 yr−1 for the South and North Atlantic Ocean (>40∘ S and >40∘ N) respectively, and the highest fluxes of 1.8 and 3.09 g m−2 yr−1 for the south-east Atlantic and tropical Atlantic Ocean, respectively. Overall, our estimations are comparable to atmospheric dust deposition model estimates and reported field-based atmospheric deposition estimates. We note that our estimates diverge from atmospheric dust deposition model flux estimates in regions influenced by riverine Al inputs and in upwelling regions. As dAl is a key trace element in the GEOTRACES programme, the approach presented in this study allows calculations of atmospheric deposition fluxes at high spatial resolution for remote ocean regions.
Austral summer and winter trace metal distributions in the Southern Ocean and Antarctic seasonal sea ice
(Stellenbosch : Stellenbosch University, 2021-03) Loock, Jean Christian; Roychoudhury, Alakendra N.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: The metabolic efficiency of carbon (CO2) sequestering microorganisms, such as marine phytoplankton, is subject to the availability of trace metal nutrients such as Manganese (Mn), Cobalt (Co) and Cadmium (Cd) among others. However, much of the Southern Ocean is characterised by a paucity of trace metals, resulting in low productivity despite excess availability of the macronutrients Nitrate (NO3), Phosphate (PO4) and Silicate (SiO4). Areas exhibiting this paradoxical condition are classified as High Nutrient Low Chlorophyll (HNLC). The paucity is related to the regional remoteness, which increases vulnerability to weak inputs via traditional sources (e.g. atmospheric dust, riverine discharge, and continental margins). Thus, regional productivity may also rely on alternative supply pathways such as internal recycling, hydrothermal vent inputs, winter resupply, and indirect inputs from the seasonal sea ice inventories. However, our understanding of these processes, and by association their impact on global carbon cycling, remains relatively limited with large gaps in the database, particularly concerning the winter state and trace metals in sea ice. This study endeavoured to constrain these poorly understood facets by reporting observations from a novel trace metal dataset spanning three research cruises (1 summer, 2 winter). This includes the first winter data for Cd (Chapter 2), and Co and Mn (Chapter 3) from within the HNLC Southern Ocean (GEOTRACES GIPY05_e transect), and the first multi-element trace metal (Mn, Fe, Co, Cu, Ni, Cd and Pb) and sympagic (sea ice) phytoplankton community data for pancake sea ice in winter (WOCE IO6 transect). Concentrations are reported for the total dissolvable (T, unfiltered) and dissolved (D, <0.2 μm) fractions. Analogous distributions of TCd and DCd were found in the austral summer across the sector, albeit in the surface (and mixed layer) where TCd > DCd owing to depletion of DCd. This results in partitioning of the Cd fractions (ΔTM = total – dissolved) as DCd is incorporated into the TCd fraction. Larger ΔCd values correlated to productivity, but further dependent on the phytoplankton community structure (diatom links) and metabolically related trace metal concentrations. Winter reoccupation of the Antarctic Circumpolar Current (ACC) revealed that TCd and DCd concentrations had increased within the mixed layer, while ΔCd values decreased (consistent with lower winter productivity). Moreover, despite elevated mixed layer concentrations in winter, and weak uptake, DCd maintained a highly predictable linear correlation to PO4 (summer = 592x – 539, R2 = 0.93; winter = 584x – 515, R2 = 0.87). I propose that the relationship is dominated, or set, by mixing with the upwelling UCDW within the Antarctic Zone (AAZ) but may ultimately be susceptible to uptake in the northernly advecting surface waters. In summer, LCo concentrations ranged from ca. 5.15 pmol/kg in surface waters in the Sub-Tropical Zone (STZ) up to ca. 37.7 pmol/kg proximal to the Sub-Antarctic Front (SAF). High concentrations in the surface near the SAF were linked to the Antarctic Intermediate Water (AAIW). In summer, surface DMn concentrations ranged from ca. 0.034 nmol/kg at 60°S in the Weddell Gyre to ca. 0.96 nmol/kg in the STZ. Extremely low concentrations (<0.125 nmol/kg) also characterised surface waters near the Polar Front (PF) at 50°S and at 65°S in the Weddell Gyre, and may co-limit productivity in the region. Thus, supplementation of Mn even via sporadic hydrothermal inputs, such as at 54°S (DMn = 0.67 nmol/kg at 300 m) may be hugely beneficial. LCo and DMn concentration increased significantly in winter in the surface and mixed layer along with decreased ΔCo and ΔMn. LCo and DMn remained strongly coupled in summer and winter, albeit with linear correlation plots unique to the season. Ice cores drilled from pancake ice samples collected within the Antarctic marginal ice zone (MIZ) at 61°S, 30°E revealed that DFe concentrations, and to an extent DMn, were heavily elevated (up to 40 times) in sea ice relative to their open seawater concentrations. Cores were analysed in segments (3 x ± 10 cm). Two trace metal cores were melted using different methods (direct melting and matrix matched melting), but the results were consistent in all segments (no melting method bias) and compared well to ranges reported in external studies. Chlorophyll-a (Chl-a) concentrations were also significantly elevated (>10 times) in sea ice relative to under ice seawater. Trace metal and Chl-a concentrations followed a high (top), low (middle), high (bottom) distribution within the core segments (c-shaped distribution). The phytoplankton community structure was dominated by diatoms and Phaeocystis.
Basinfill of The Permian Tanqua depocentre, SW Karoo basin, South Africa
(Stellenbosch : Stellenbosch University, 2012-03) Alao, Abosede Olubukunola; Mikes, Daniel; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: Basin subsidence analysis, employing the backstripping method, indicates that fundamentally two different basin-generating mechanisms controlled Tanqua depocentre development in SW Karoo Basin. The subsidence curves display initial dominantly decelerating subsidence, suggesting an extensional and thermal control possibly in a strikeslip setting during the depocentre formation; on the other hand, subsequent accelerating subsidence with time suggests that the dominant control on the depocentre formation in SW Karoo was flexure of the lithosphere. Based on these observations on the subsidence curves, it is possible to infer that the first stage of positive inflexion (~ 290 Ma) is therefore recognised as the first stage of Tanqua depocentre formation. Petrographic study show that most of the studied sandstones of the Tanqua depocentre at depth of ~ 7.5 Km were subjected to high pressure due to the overlying sediments. They are tightly-packed as a result of grains adjustment made under such pressure which led also to the development of sutured contacts. It is clear the high compaction i.e. grain deformation and pressure solution occurred on the sediments; leading to total intergranular porosity reduction of the quartz-rich sediments and dissolution of the mineral grains at intergranular contacts under non-hydrostatic stress and subsequent re-precipitation in pore spaces. Furthermore, siliciclastic cover in the Tanqua depocentre expanded from minimal values in the early Triassic (Early to Late Anisian) and to a maximum in the middle Permian (Wordian -Roadian); thereby accompanying a global falling trend in eustatic sea-level and favoured by a compressional phase involving a regional shortening due to orogenic thrusting and positive inflexions (denoting foreland basin formation). The estimate of sediment volume obtained in this study for the Permian Period to a maximum in the middle Permian is therefore consistent with published eustatic sea-level and stress regime data. In addition, this new data are consistent with a diachronous cessation of marine incursion and closure of Tanqua depocentre, related to a compressional stress regime in Gondwana interior during the late Palaeozoic.
The biogeochemistry of bioactive trace elements cadmium and cobalt in the Southern Ocean, Atlantic Sector
(Stellenbosch : Stellenbosch University, 2017-03) Loock, Jean Christian; Roychoudhury, Alakendra N.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: The biogeochemical cycling of the bio-active dissolved trace metals cadmium (DCd) and cobalt (DCo), are reported on a meridional transect of the Southern Atlantic, spanning 36°S to 68°S. Seawater samples were collected using an adaptation of the GEOTRACES clean sampling method concurrent to the austral mid-summer (2015). A multi-element inductively coupled plasma mass spectrometry (ICP-MS) technique, coupled with the commercially available seaFAST pre-concentration module was set up and validated for 8 trace elements (Mn, Fe, Co, Ni, Cu, Zn, Cd, Pb). The method proved highly precise (≤10%RSD) in the repeated analysis of an in-house control (TM4A), and accurate in the analysis of the SAFe (D2) international natural seawater reference material. Iron (Fe) quantification proved precise (≤5%RSD) and accurate under repeat analysis. Furthermore, two GEOTRACES standards, GSP and GSC, were analysed for contributing to the determination of their consensus values. DCo concentrations within the oligotrophic sub-surface waters (500m) of the Sub-Tropical Zone (STZ) are heavily depleted (12.57 pmol/kg) owing to strong bio-utilization. Conversely, sub-surface Antarctic Intermediate Waters (AAIW) near the Sub-Antarctic Front (SAF) were elevated (36.1 pmol/kg). Concentrations subsequently decline meridionally through the Antarctic Circumpolar Current (ACC) steadying within the Weddell Gyre (20.3 pmol/kg). STZ DCd sub-surface concentrations are notably depleted (74.1 pmol/kg). However, sub-surface concentrations increase within the ACC at the SAF (450 pmol/kg), continuing into the Weddell Gyre (700.8 pmol/kg). DCd exhibits a typical nutrient-like profile, with bio-utilization in the surface (MLD), followed by regeneration and conservation. The nutrient like behaviour of bio-utilized DCd was exemplified by an excellent Cd/P sub-surface correlation in the ACC, waning somewhat in the sub-optimal productivity of the Weddell Gyre. However, abiotic factors may have contributed to DCd concentrations. The complete water column concentrations ranged from 8.4 ± 0.05 pmol/kg to 902 ± 22.71 pmol/kg. Conversely, DCo demonstrated a typical hybrid-type vertical distribution – nutrient uptake and remineralization in the surface succeeded by scavenging controlled by heterogeneous oxidising bacteria. The complete water column concentrations ranged from 4.1 ± 0.02 pmol/kg to 38.9 ± 0.3 pmol/kg. Elevated DCo concentrations in the sub-surface and a poor but significant DCo/Salinity correlation, suggest DCo flux in the expanse surrounding 46°S and 68°S. Keywords: Biogeochemical cycling, Dissolved Co and Cd, GEOTRACES, ICP-MS, Iron (Fe), seaFAST, Southern Atlantic.
Bulk geochemical, biomarker and leaf wax isotope records of Mfabeni peatland, KwaZulu Natal, South Africa since the late Pleistocene
(Stellenbosch : Stellenbosch University, 2016-03) Baker, Andrea; Routh, Joyanto; Roychoudhury, Alakendra N.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: Southern Africa is a topographically diverse region that is influenced by temperate, sub-tropical and tropical climates encompassing varying rainfall zones. The core regional contemporary climatic drivers are the large sea surface temperature (SST) gradients between the Atlantic and Indian Oceans, and seasonal fluctuations in the Inter Tropical Convergence Zone (ITCZ). Our understanding of how these two mechanisms interacted in the past and how ecosystems responded to these climate drivers is ambiguous, mainly due to a lack of continuous archives as a consequence of the region‘s semi-arid climate. The Mfabeni peatland is a 11 m thick continuous peat sequence that has been dated to ca. 47 kcal yr BP. It is the only known coastal peatland record in the summer rainfall zone of Southern Africa to transcend the Last Glacial Maximum (LGM), and gives us the opportunity to reconstruct high-resolution palaeoenvironment records under both glacial and interglacial conditions on the south eastern coastline of the African continent. A diverse set of geochemical techniques and analysis (bulk C and N elemental and stable isotopes; different biomarkers and leaf wax δ13C isotope) was undertaken on a 810 cm long core to reconstruct primary productivity, organic matter (OM) sources, rates of OM remineralisation, peatland hydrology and relative contributions of C3 and C4 plant matter into the peat deposit. These geochemical climatic indicators were used to infer precipitation intensities and relative temperatures at time of sedimentation and, in conjunction with other regional archives, the dominant mechanisms (Indian Ocean SST changes versus changes in the position of ITCZ) driving climatic fluctuations since the late Pleistocene were explored. We established the Mfabeni peatland to be a well-preserved and unique palaeoecological archive that recorded both environmental and climatic signals throughout the depositional history of the peatland. Even though the dominant OM source of the peat was terrestrial and emergent plants, there were definitive periods of predominant submerged macrophyte input, suggesting elevated water levels. A general positive trend was observed between the temperature and moisture proxies, however the local plant physiology (n-alkane chain lengths; ACLalk) and plant types (terrestrial vs aquatic and their influence on OM lability; CPIalk) was dominated by moisture availability as opposed to temperature variations, arguably due to the relatively moderate cooling experience in the sub-tropics during the LGM. The leaf wax C isotope data set established variability in the proportional balance of C3 and C4 plants, with interchanges between plant clades and inter family C3 and C4 switches in response to changes in environmental conditions. However, plant assemblage shifts were absent during some of the more ephemeral climatic events which we concluded was due to local hydrological overprinting. The Mfabeni archive correlates strongly with Mozambique Channel SST records, suggesting the dominant climate forcing factor in south eastern Africa to be the evaporation potential and advection of moisture from the adjacent Indian Ocean since the late Pleistocene. It was also noted that the Mfabeni record exhibited overall opposite environmental responses to Northern Hemisphere climatic events, suggesting an anti-phase coupling between the two hemispheres.
Bunker Cave stalagmites : an archive for central European Holocene climate variability
(Copernicus, 2012) Fohlmeister, J.; Schroder-Ritzrau, A.; Scholtz, D.; Spotl, C.; Riechelmann, D. F. C.; Mudelsee, M.; Wackerbarth, A.; Gerdes, A.; Riechelmann, S.; Immenhauser, A.; Richter, D. K.; Mangini, A.
Holocene climate was characterised by variability on multi-centennial to multi-decadal time scales. In central Europe, these fluctuations were most pronounced during winter. Here we present a record of past winter climate variability for the last 10.8 ka based on four speleothems from Bunker Cave, western Germany. Due to its central European location, the cave site is particularly well suited to record changes in precipitation and temperature in response to changes in the North Atlantic realm. We present high-resolution records of δ18O, δ13C values and Mg/Ca ratios. Changes in the Mg/Ca ratio are attributed to past meteoric precipitation variability. The stable C isotope composition of the speleothems most likely reflects changes in vegetation and precipitation, and variations in the δ18O signal are interpreted as variations in meteoric precipitation and temperature. We found cold and dry periods between 8 and 7 ka, 6.5 and 5.5 ka, 4 and 3 ka as well as between 0.7 and 0.2 ka. The proxy signals in the Bunker Cave stalagmites compare well with other isotope records and, thus, seem representative for central European Holocene climate variability. The prominent 8.2 ka event and the Little Ice Age cold events are both recorded in the Bunker Cave record. However, these events show a contrasting relationship between climate and δ18O, which is explained by different causes underlying the two climate anomalies. Whereas the Little Ice Age is attributed to a pronounced negative phase of the North Atlantic Oscillation, the 8.2 ka event was triggered by cooler conditions in the North Atlantic due to a slowdown of the thermohaline circulation.
The cenozoic stratigraphy and associated heavy mineral palaeo-placer deposit on Geelwal Karoo : West Coast, South Africa
(Stellenbosch : Stellenbosch University, 2005-03) Elferink, Lisa; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: The farm Geelwal Karoo is situated some 16km north of the Olifants River mouth on the West Coast of South Africa and hosts fluvial, marine and aeolian deposits of post-Gondwana age. The oldest basal fluvial succession, unconformably overlies Proterozoic and Palaeozoic basement rocks and is in turn capped by aeolianite and littoral packages representing two transgressive cycles. The fluvial channel clay succession is deposited in shallow bedrock-incised channels, has a wedge-shape and is deposited parallel to the present coastline. The flow direction is along the coast and the northwardtapering, angular, poorly sorted basal vein-quartz lag indicates a northward palaeo-flow direction. Less than 1% total heavy minerals (THM) is found in the matrix of these gravel units and the heavy mineral suite is distinguished by zircon, pseudorutile and kyanite. The channel clay unit is dominated by an upper, mediumgrained quartzose sand and kaolin clay facies, which shows advanced post-depositional weathering. The fluvial unit is correlated with the channel clay unit of Hondeklip Bay and a Cretaceous age is proposed for the initial channel incision. The two shallow marine successions have been correlated with the Late Miocene, Early Pleistocene, +30m and +50m packages respectively. These marine sediments were first described by John Pether (1994) in the Hondeklip Bay area and were named according to their transgressive maxima. They are transgressive successions arranged en echelon down the coastal bedrock gradient, from oldest and highest to youngest. The offshore environment of the +50m package consists of fine silty sand, which is moderately sorted. The mineral assemblage is dominated by quartz and the average THM is 18%. The inshore environment is distinguished by a single poorly sorted basal cobble lag which shows an overall fining upward succession. The beachface environment is composed of medium to fine-grained sand, which is moderate to well-sorted. Mineral diversity is greatest in the inshore and beachface environments and the average THM for these two units is greater than 35%. The +30m package has been extensively eroded due to its lower erosion and outcrops were sporadic along the coast. The +30m offshore sediments are recognised by fine sediments with high concentrations of glauconite and organic matter. The inshore environment is distinguished by numerous poorly sorted pebble lags with fining upward successions. Both the inshore and beachface units have higher feldspar concentrations than the corresponding +50m units. The average THM for these two units is less than 3%. The aeolianite unit, which comprises several distinct units, extends over the entire length of the study area and is characterized by calcrete and red bed horizons. Colour variations in the otherwise homogeneous unit are due to heavy mineral enrichment and/or different degrees of in situ weathering and cementation. The unit is composed exclusively of fine- to medium-grained sand and the THM concentration averages 9%. This unit is composed of more than one generation of aeolian sand and forms part of an aeolian transport corridor which transported sand from the beach to the interior. The oldest unit has been equated with the Upper Miocene Prospect Hill Formation, whereas the more recent yellow dune sand is equated with the Pleistocene Springfontyn Formation. At Geelwal Karoo, only the heavy sand placer in the +50m package was deemed to be of any economic significance. The average THM of this placer was calculated to be 40% and some 150 thousand tons of Tibearing material can be expected from this succession. This relatively small volume of heavy minerals and extensive cementation however, make this placer a less attractive prospect than the neighbouring Namakwa Sands operation.

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