Browsing by Author "Walsh, Jared Tanner"

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    Plio-pleistocene palaeoenvironment of the south western continental shelf of South Africa: a foraminiferal perspective
    (Stellenbosch : Stellenbosch University, 2023-12) Walsh, Jared Tanner; Fietz, Susanne; Bergh, Eugene; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: The oceanography and ecology on the western continental shelf of South Africa (Orange Shelf) were highly influenced by the initiation and intensification of the Benguela Up- welling System (BUS) during the Plio-Pleistocene. On the Orange Shelf, literature sub- stantiating the e↵ect of the Plio-Pleistocene climate transition on foraminiferal assemblages is sparse. Foraminifera proved reliable as palaeo-proxies, and by assessing the response of foraminifera to the Plio-Pleistocene climate transition, a new perspective emerges for the changing oceanography in the BUS during this time. Furthermore, interpolations from the well-studied Namibian shelf and slope are often relied upon to interpret the palaeoceano- graphic conditions in the BUS, therefore supporting the need for new foraminifera-based palaeoceanographic studies from South Africa. Sediments and microfossil foraminifera from vibracores on the inner and outer Orange Shelf (200–400 meters below sea level) were anal- ysed using faunal analyses, foraminiferal biostratigraphy, and Strontium Isotope Stratigra- phy. The dating methods used provided conflicting results for various components. For example, in one unit, phosphorite was dated to 5.70–5.98 Ma, but Pleistocene indicator species Globorotalia truncatulinoides yielded ages approximately 2.58 Ma from its first ap- pearance. This study observed sediment reworking within Plio-Pleistocene sediment and, to a lesser extent, foraminiferal assemblages across the continental shelf. The cooling of sea surface temperatures (SST) in the BUS following the initiation of Northern Hemisphere glaciation during the Plio-Pleistocene transition was observed by the increase of temperate SST indicators Globigerina bulloides and Globoconella inflata, and the subpolar SST indi- cator Neogloboquadrina pachyderma. The influence of Agulhas leakage was prevalent on the outer shelf, and to a lesser extent on the inner shelf during the Pliocene, and was inferred from high abundances of Orbulina universa and trace abundances of Globorotalia menardii and Globigerinoides ruber. Regarding primary productivity, planktic indicators Globigerina bulloides and Neogloboquadrina pachyderma decline with the onset of the Pleistocene on both the inner and outer shelf. The inner shelf seemed to undergo less drastic changes in terms of primary productivity than the outer shelf during this time. This study suggests an external nutrient influx with increasing proximity to the coast, supporting the persistence of planktic primary productivity during the Pleistocene and the decline in upwelling intensity. Benthic primary productivity indicator Uvigerina peregrina increased during the Pleistocene. While upwelling intensity appears to decrease over the Plio-Pleistocene transition, the influx of nutrient-rich bottom water to benthic foraminifera on the outer shelf remained sufficient to support primary productivity. This study contributes to a greater understanding of the relationship between productivity, nutrient supply, and the upwelling intensification of BUS in the context of foraminifera assemblages on the Orange Shelf over the Plio-Pleistocene.