Geochemical insights into the influence of Holocene sea level change on the evolution of the Mkhuze River Delta, Lake St Lucia, northen KwaZulu- Natal
Higgs, Caldin Grant
The Mkhuze River discharges into the most northern part of Lake St Lucia, via a contemporary bayhead delta. The delta formed in response to sea level rise during the last deglaciation and today exerts great influence on the functioning of Lake St Lucia, one of the largest estuarine systems in Africa and a globally important conservation area. A sediment core (11.5 m) was extracted from the distal end of the delta to examine the geomorphic evolution of the Mkhuze River Delta and links with variations in Holocene sea level and climate. Radiocarbon and optically-stimulated luminescence dating show that the core captured the entire Holocene infill and documents changes in sedimentation over the last ~13.8 kyr. Grain size and high resolution XRF analysis indicates that initiation of the modern delta occurred since ~7200 cal yr BP , when deglacial sea-level rise reached present-day level. Initial Holocene aged sediments are dominated by clay and silt material that was deposited when seawater intruded into Lake St Lucia via a palaeo-river connection to the ocean at Leven Point. The influx of silt and clay material was accompanied by the emergence of an onshore proto-barrier that created a sheltered lagoonal environment and promoted the accumulation of fine fluvial sediment. The presence of discrete, coarse-grained horizons enriched in zircon identifies a period of increased marine palaeostorm activity between 4700 and 2500 cal yr BP. This period is characterised by the presence of discrete shell fragment accumulations and is interpreted to reflect a strongly positive Indian Ocean dipole anomaly, which resulted in warmer sea surface temperatures and an increase in regional cyclone activity and frequency. The upper part of the core is characterized by generally fine silt and is marked by a decrease in sedimentation rate that corresponds to a phase of lateral delta progradation. The last ~1700 cal yr BP years of the record identify with subtle changes in grain size that can be attributed to a strengthening in El Niño–Southern Oscillation (ENSO) activity, which is known to be associated with prolonged drought and wind erosion in eastern South Africa. This study highlights the usefulness of coastal geochemical records in identifying environmental changes and related climate signals at a regional scale.
Thesis submitted in fulfilment of the academic requirements for the degree of Master of Science. Department of Chemistry University of Witwatersrand, Johannesburg April 2017.
Higgs, Caldin Grant (2017) Geochemical insights into the influence of Holocene sea level change on the evolution of the Mkhuze River Delta, Lake St Lucia, northen KwaZulu- Natal, University of the Witwatersrand, Johannesburg, <http://hdl.handle.net/10539/23745>