Geochemical insights into the influence of Holocene sea level change on the evolution of the Mkhuze River Delta, Lake St Lucia, northen KwaZulu- Natal
Date
2017
Authors
Higgs, Caldin Grant
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Abstract
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.
Description
Thesis submitted in fulfilment of the academic requirements for the degree of Master of
Science.
Department of Chemistry
University of Witwatersrand, Johannesburg April 2017.
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Citation
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>