Abstract
Processes that control the hydrological balance in eastern South Africa on orbital to millennial timescales remain poorly understood because proxy records documenting its variability at high resolution are scarce. In this work, we present a detailed 270,000 year-long record of terrestrial climate variability in the KwaZulu-Natal province based on elemental ratios of Fe/K from the southwest Indian Ocean, derived from X-ray fluorescence core scanning. Eastern South African climate variability on these time scales reflects both the long-term effect of regional insolation changes driven by orbital precession and the effects associated with high-latitude abrupt climate forcing over the past two glacial-interglacial cycles, including millennial-scale events not previously identified. Rapid changes towards more humid conditions in eastern South Africa as the Northern Hemisphere entered phases of extreme cooling were potentially driven by a combination of warming in the Agulhas Current and shifts of the subtropical anticyclones. These climate oscillations appear coherent with other Southern Hemisphere records but are anti-phased with respect to the East Asian Monsoon. Numerical modelling results reveal that higher precipitation in the KwaZulu-Natal province during precession maxima is driven by a combination of increased local evaporation and elevated moisture transport into eastern South Africa from the coast of Mozambique.
Highlights
Processes that control the hydrological balance in eastern South Africa on orbital to millennial timescales remain poorly understood because proxy records documenting its variability at high resolution are scarce
Orbital precession leads to anti-phased summer insolation maxima between the hemispheres, resulting in an interhemispheric temperature contrast and latitudinal shifts of the Inter Tropical Convergence Zone (ITCZ) and associated regions that are affected by summer monsoonal precipitation[3]
Beyond the limits of the radiocarbon method we established an initial age model based on the graphic correlation of the benthic δ 18O (Cibicidoides spp.) record of core CD154-10-06P to the global benthic stack LR0430, (Fig. 2D)
Summary
Long-term variability in eastern South African hydroclimate. 06P is based on ten accelerator mass spectrometry radiocarbon dates in the upper part of the record (Methods). The latter contrasts recent findings arguing that the significance of direct insolation forcing on the summer rainfall zone only developed during the Holocene[43] This might imply that the southern African summer rainfall zone was rather heterogeneous with distinct areas responding predominantly to precession forcing (Fig. 3) whereas in others the precipitation amount was potentially more tightly linked to changes in the Indian Ocean temperatures during MIS 3 and 2 It has been demonstrated that abrupt changes to more humid conditions in southeast Africa and the Eastern Cape province were associated with Greenland stadials and periods of weak EAM, resulting in an anti-phasing between rainfall variability in the two hemispheres on a millennial timescale over the last glacial cycle[19,33]. While recurrent shifts between long-term droughts and humid phases in eastern South Africa during the past 270,000 years arose from natural causes, climate models project that this region will undergo progressive aridification in the future as part of a general drying and poleward expansion of the subtropical dry zones driven by the human-induced rise in GHGs18
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