Browsing by Author "Tostevin, Rosalie"

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    An investigation of the tufa deposits at Ga-Mohana Hill in the Northern Cape, South Africa: palaeoenvironmental context for Late pleistocene and holocene human occupation in the Southern Kalahari
    (2025) von der Meden, Jessica Johanna; Harris, Chris; Tostevin, Rosalie; Pickering, Pickering
    There is a rich record of human occupation in the southern Kalahari, but the palaeoclimatic backdrop to early human activity in this area is poorly understood. Secondary calcium carbonates, including tufa, are common in the Kalahari, yet their palaeoclimate potential remains relatively unexploited. Tufa deposits at Ga-Mohana Hill, an archaeological site in the southern Kalahari, represent past water availability in a presently semi-arid region. Tufa are useful palaeoclimate proxies that form from freshwater springs and streams. They are amenable to radiometric dating, and conditions during their formation can be investigated through geochemical proxies, such as carbon and oxygen isotopes. In this study, I use the tufa deposits at Ga-Mohana Hill, and their association with the archaeological record of Late Pleistocene and Holocene human occupation, to investigate the interplay between environmental changes and human evolution in this previously understudied region. I show that the Ga-Mohana tufa deposits comprise cascade, barrage, terrace breccia and dome morphologies that formed from a presently inactive perched-springline system, and that these represent past streams, waterfalls, and shallow pools. Based on petrographic observations, bulk X-ray diffraction, and in situ laser ablation analysis, calcite is identified as the main crystalline phase, along with variable inclusions of quartz and other non-carbonate phases rich in Mn, Fe and Al. Element distribution maps constructed from laser ablation analysis were used to identify portions of the tufa samples suitable for uranium-thorium dating. Episodic tufa formation over the last ~114 ka is inferred from thirty-three U-Th ages obtained from twelve tufa samples. These episodes are interpreted to indicate water availability during MIS 5d (~114 - 100 ka), MIS 4 (~73-57 ka), two stages during MIS 3 (~57-48 ka, and ~44-32 ka), and two stages during the Holocene (~15 - 6 ka, and ~4-2 ka). This timing supports the notion that conditions for tufa formation are not restricted to warm and wet interglacial periods. The tufa carbon (-8.83 to +3 .55‰) and oxygen (-6.14 to -0.05‰) isotope values indicate some kinetic fractionation effects, likely driven by evaporation. The carbon isotopes indicate a combined input of soil-derived CO2 and heavier bedrock CO2, along with a high proportion of C4 vegetation, except during MIS 4, during which lower d13C values indicate a higher input of soil-derived CO2 and/or a greater proportion of C3 vegetation. Overall, the carbon and oxygen isotope values indicate a gradual increase in aridity over the last ~70 ka, i.e., warmer temperatures, increased evaporation, and a higher proportion of C4 vegetation, albeit wet enough to sustain tufa formation. Three of the tufa episodes coincide with archaeological units at Ga-Mohana Hill, dated to ~105 ka, ~31 ka, and ~15 ka. I propose that prior to ~70 ka, human occupation in the region coincided with water availability, whereas by ~30 ka, water availability was much reduced, yet human occupation persisted. Following the last glacial maximum (LGM) there is evidence of human occupation despite variable water availability at Ga-Mohana Hill, and in the region. This may point to early human development of arid-adaptive behaviours in the southern Kalahari.
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    Reconstructing local redox conditions, mineral precipitation processes and nutrient availability in seawater during deposition of the Transvaal Supergroup, South Africa
    (2025) Nke, Ansahmbom; Tostevin, Rosalie; Tsikos, Harilaos
    Iron formations (IF) and carbonate rocks serve as geochemical archives, which can be used to reconstruct marine environments in the Archean and Proterozoic Eons. The chemistry of ancient oceans influenced iron redox states and primary mineral precipitation, affecting the cycling of other trace elements and nutrients. While carbonate minerals are commonly used as geochemical archives, reconstructions using IF have proved more challenging, with many studies analysing bulk powders. However, reconstructing ancient conditions is challenging due to post-depositional alteration, fine-grained mineral composition, and complex mineralogy. Recent studies highlight greenalite, an Fe(II)-silicate mineral, as a primary phase in IF and a potential proxy for ancient seawater chemistry. This study combines geochemical reconstructions from carbonate rocks with mineral-specific data from greenalite. The Transvaal Supergroup in South Africa spans the Great Oxidation Event (GOE), and provides an ideal setting for studying ancient marine chemistry due to its exceptional preservation across a range of facies. This study aims to uncover early ocean conditions by investigating carbonate and IF sequences from across a shelf environment using laser ablation inductively coupled plasma-mass spectrometry. Specifically, it explores Archean oxygen oases by studying redox-sensitive rare earth element (REE) data from contorted microbial laminae and adjacent carbonate cement from microbial carbonates, alongside a new compilation of existing data to explore the implications for pre-GOE molecular oxygen availability. A novel methodology that involves cutting along bedding planes to maximize exposure of fine-grained primary seawater phases like greenalite, combined with MATLAB code to isolate greenalite data from the enclosing chert, is used to extract accurate trace metal and REE content from primary precipitates. The trace metal content from Paleoproterozoic greenalite is then used to predict Paleoproterozoic ocean metal content. The findings from this study show that the microbial carbonates from the Campbellrand Subgroup exhibit seawater-like post-Archean Australian Shale patterns, characterized by heavy REE enrichment, small positive Eu and La anomalies, super-chondritic Y/Ho ratios, but lack Ce anomalies, indicating deposition in anoxic marine conditions. These results provide no evidence for an oxygen oasis in the Transvaal basin. However, Fe-speciation data from these carbonates show evidence for pre-GOE free molecular oxygen. This suggests that one of the proxy records is not recording primary conditions, or that cyanobacteria were producing oxygen, but it was either being consumed by aerobic processes or the environmental conditions might be too fine-scaled to be recorded by the laser ablation technique. Greenalite from the Paleoproterozoic Kuruman Formation shows heavy REE enrichment, small positive Eu anomalies, super-chondritic Y/Ho ratios, and low total REE, consistent with formation in a shelf environment. While greenalite precipitation in vent settings is possible, significant formation likely occurred on the shelf, potentially triggered by the presence of minor Fe3+, local pH increases, or warmer temperatures. High resolution data from the greenalite in this study provide quantitative constraints on marine metal concentrations in an ~2.46 Ga shelf environment, revealing drastically different patterns compared to today. Specifically, Zn and V were scarce, Ni levels were similar, Co was enriched, and Mn was highly abundant. These patterns align with thermodynamic models and other geological archives, suggesting significant impacts on early microbial evolution. This supports phylogenomic and proteomic data indicating early microbes' preference for Mn and Co over Zn. This research highlights greenalite's value as a paleo-archive and lays the groundwork for exploring the Archean and Late Proterozoic Eons to reconstruct metal concentrations across various settings and geological epochs.
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    The palaeoredox condition and morphological variation of microbialites from the Nama Group, South Africa
    (2023) Wilton, Aidan; Tostevin, Rosalie
    Microbialites are the fossilised traces of microbial communities and are present in the rock record since the Paleoarchean, 3.5 billion years ago, making microbialites some of the earliest direct evidence for life on Earth. Microbialites exhibit a wide variety of macrostructures and fabrics, including laminated sheets, domes and clotted thrombolites, and formed under a range of conditions. Microbialites also vary on fine scales whereby individual layers in some cases vary within a single microbialite. The ubiquity of microbial reefs across the Precambrian means they were some of the most abundant deposits during major developments in Earth's history, namely the inception of photosynthesis, the oxygenation of the atmosphere and oceans, the development of the earliest metazoans and the origin of skeletonization. While Earth's atmosphere has contained oxygen since the GOE, the oceans only became persistently oxic during the mid-Palaeozoic. Oscillatory redox conditions occurred throughout the Cryogenian and Ediacaran Periods and the early Palaeozoic Era before this stability was reached. While oxic surface waters were present throughout the Proterozoic, there are open questions as to the maximum dissolved oxygen levels, and where oxic waters were located. Microbial reefs, typically composed of photosynthetic cyanobacteria, could have played a vital role in shaping this oxygen landscape. It is possible that cyanobacteria within the reefs created local oxygen oases within a generally low oxygen environment. Here, I test this hypothesis using a suite of microbialites from a section of the Nama Group that crops out in the Northern Cape, South Africa. I have studied the texture and mineralogy using XRD, SEM, petrography, and polished surfaces. I have also analysed high-resolution rare-earth element patterns from microdrilled microbial laminae to study local redox conditions. Analysis of the mesostructure and microstructure of the microbialites indicates that they were columnar stromatolites that grew into an extensive reef of domal stromatolites within the subtidal zone. Small-scale spatiotemporal feature variations indicate there was widespread turbulence across the reef, potentially indicating rapid fluctuations in water depth or the creation of topographic heterogeneity by the reef. The laminae of the microbial material are composed of micritic calcite peloids, while the intercolumn material is largely microspar calcite. The presence of peloids coupled with the ubiquitous columnar growth patterns suggest an association with microbial activity. In concert with the proposed relatively shallow water depositional environment, it is likely that photosynthetic cyanobacteria are the primary microbes that created these structures. The REE data record primary seawater compositions and are in line with bulk rock analysis from the Nama Group. However, there are no cerium anomalies, suggesting these microbial reefs grew in anoxic waters. This is contrary to the cyanobacterial affinities of the microbialites as well as the presence of in-situ animal fossils in similar microbial reefs in other parts of the Nama Group. We propose three main models to reconcile these observations. First, the microbial communities may not have included oxygenic cyanobacteria, and the micro- and macro-environment surround the reef may have been anoxic. Second, the microbial reefs may have been oxic micro-environments, but this may not be reflected in the REE patterns due to microbial influence on the partitioning behaviour of Ce, or timescales of oxygen production that were much shorter than depositional timescales. Thirdly, the REE may reflect the surrounding water column, rather than the oxic reef environment, if the carbonate was trapped and bound, rather than precipitated in situ.
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    Using high resolution Sr isotope data from the Nama Group, South Africa, to constrain global stratigraphic relationships and continental weathering rates in the terminal Ediacaran
    (2023) Mtonda, Mcdonald Takondwa; Tostevin, Rosalie
    Although life on Earth evolved in the Archean, complex hard bodied animals only emerged in shallow marine environments during the late Ediacaran (~550 Ma). The trigger for the appearance of these hard-bodied animals is still debated but it was probably related to ecological and/or environmental change. Continental weathering fluxes into the oceans can influence redox conditions and seawater chemistry and can be tracked using the strontium isotope ratio of seawater, which can be faithfully captured by carbonate minerals. However, analysis of strontium isotopes in ancient carbonates can be complicated by diagenetic alteration. To ensure that the 87Sr/86Sr ratios generated in this study were reflective of primary seawater signals, a sequential leaching procedure, first proposed by Bailey et al., (2000), was tested on Ediacaran bulk rock carbonate samples. The successful implementation of the sequential digestion technique was verified through careful examination of samples for diagenetic alteration using petrography, trace element ratios and  18O. The sequential digestion technique was then applied to a high-resolution stratigraphic transect through terminal Ediacaran carbonate rocks from the Nama Group, collected in South Africa and Namibia. The isotopic signature of these rocks is consistent with other terminal Ediacaran age rocks, with typically slightly positive  13C and relatively high 87Sr/86Sr ratios (~0.7085). Correlation with other Ediacaran basins reveals a drop in 87Sr/86Sr from ~0.7090 to ~0.7085 ca. 550 Ma. The drop was likely due to location of palaeocontinents compounded by palaeoclimatic cooling during the terminal Ediacaran. These changes provide important context for a critical period in metazoan evolution and may have influenced the cost-benefit ratio of producing hard body parts.