Reconstructing Late Quaternary environmental change on the southern Cape coast of South Africa: A 30,000-year geochemical record from Pearly Beach
Date
2023-11
Authors
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Publisher
University of the Witwatersrand, Johannesburg
Abstract
The southern Cape coast of South Africa has undergone complex and dynamic climate changes due to its position at the boundary of major atmospheric and oceanic systems. However, climate changes that occurred in the region during the Last Glacial Maximum (30,000 – 18,000 cal yr BP) and the deglaciation period that followed (18,000 – 11,000) are still not fully understood. Climatic change along the southern Cape coast is of archaeological, cultural and environmental significance given the ecological diversity and hominin fossil, richness of the region. Moreover, predictions of future climate change and water scarcity management issues in the southern Cape make it vital to understand and response to future climate scenarios. To address this gap, this study investigated the geochemical composition of a 5.42 m sediment record (KSV 1) extracted from a freshwater coastal wetland near Pearly Beach on the southern Cape coast. The wetland, situated in the winter rainfall zone, is influenced by the southern westerly storm track, the South Atlantic anticyclone, and Agulhas-Benguela interactions, making it an ideal location to investigate changes to large-scale atmospheric and oceanic circulation systems. Using a combination of inorganic elemental (X-ray fluorescence, XRF) and stable carbon isotope (δ13C) analysis, this study presents a 30,000 year record of palaeoenvironemental change from Pearly Beach and considers these in relation to broad-scale climatic perturbations that occurred in southern Africa over this period. A chronology for the core was derived from radiocarbon dating of 8 bulk sediment samples. Sedimentation rates over the record averaged 0.02 cm/yr, giving the XRF analyses which were conducted at 2 cm intervals, an average temporal resolution of ~ 110 years. Sediments deposited 30,000 – 15,000 cal yr BP consist primarily of quartz-rich (60 – 90% SiO2) silt and fine sand with relatively low quantities of clay material. An increase in the relative proportion of Al2O3 from ~15,000 cal yr BP, coupled with a rise in sedimentation rate, marks an increase in the accumulation of fine-grained material within the wetland that is associated with a rise in global sea levels following deglaciation. A distinct shift to higher δ13C values around this time points to a change in vegetation community, with an increase in contribution from C4 drought-tolerant plants. The presence of calcium carbonate in sediments provides a strong indicator for marine intrusions at site and a pronounced increase in marine influence is inferred from ~8,500 cal yr BP, corresponding with the stabilisation of sea levels near to present-day levels. Variations in elements typically associated with heavyminerals (titanium and zirconium) are used to infer changes in depositional energy at the site. Enrichments in Ti and Zr suggest a period of enhanced transport energy between 18,800 and 14,500 cal yr BP. This is coupled with an increase in the Si/Al ratio and linked to enhanced aeolian activity. An increase in wind intensity during this period is attributed to the intensification in the Southern Hemisphere westerly winds at this time. This corresponds with available fossil pollen records, which suggest an increase in moisture at Pearly Beach 18,500 to 15,000 cal yr BP that was possibly linked to a slowdown in the Atlantic Meridional Overturning Circulation (AMOC) and build-up of heat in the southeast Atlantic. It is possible that a build-up of heat in the southeast Atlantic brought on by a slowdown in the AMOC increased moisture uptake in the frontal storms, resulting in stronger low-pressure systems and intensified storms, as reflected in the KSV-1 record. Comparisons with other records from South Africa, including Mfabeni, Cold Air Cave and Cango Caves, indicate that the central and eastern parts of the country experienced a cooling event ~18,500 and 14,500 cal yr BP. New geochemical data from Pearly Beach suggests that this cooling may have been linked to intensification of westerly winds at this time. An apparent lag effect in the warming of South Africa after deglaciation may be attributed to the enhanced influence of westerly winds during the glacial-interglacial period, which pushed cold air masses across the interior of the country. The complex interplay between local sea level changes, westerly wind dynamics, and regional climatic conditions underscores the significance of the geochemical record from Pearly Beach in understanding past environmental changes and their broader implications for southern Africa.
Description
A dissertation submitted in fulfilment of the academic requirements for the degree of Master of Science, to the Faculty of Science, School of Chemistry, University of the Witwatersrand, Johannesburg, 2023.
Keywords
Geochemistry, Palaeoclimate, Southern Africa, UCTD
Citation
Mey, Rachel Tess. (2023). Reconstructing Late Quaternary environmental change on the southern Cape coast of South Africa: A 30,000-year geochemical record from Pearly Beach. [Master's dissertation, University of the Witwatersrand, Johannesburg]. https://hdl.handle.net/10539/42345