Palynological investigation of Holocene climatic and oceanic variability in South Africa and the southern Benguela upwelling system
|Other Titles:||Palynologische Untersuchung von Klima- und ozeanischen Variabilität des Holozäns in Südafrika und des südlichen Benguela Auftriebssystems.||Authors:||Zhao, Xueqin||Supervisor:||Wefer, Gerold||1. Expert:||Wefer, Gerold||2. Expert:||Behling, Hermann||Abstract:||
South Africa is very sensitive to climate change, because it is located between two atmospheric systems (subtropical and warm-temperate systems) and two oceanic systems (Benguela Current and Agulhas Current). Presently, the seasonal changes of atmospheric and oceanic systems induce a pronounced rainfall seasonality comprised of three different rainfall zones (summer rainfall zone, SRZ; year-round rainfall zone, YRZ and winter rainfall zone, WRZ) over South Africa. However, the seasonality development during the Holocene in South Africa is poorly understood and the driving forces of the climate change are debated. Therefore, this study aims to provide a detailed reconstruction of Holocene climate and vegetation variability of South Africa, palaeoceanographic changes in the southern Benguela upwelling system as well as the land-ocean linkages. In the first part of this thesis, the spatial distribution of pollen in marine surface sediments was investigated based on a transect of 12 marine surface sediment samples retrieved from north (29.12AAAAAAAdegreeS) to south (32.50AAAAAAAdegreeS) of the Namaqualand mudbelt off western South Africa. The distinct pollen spectra reflect vegetation communities on the adjacent continent with a marked north-south gradient of pollen concentration. The distribution of specific pollen taxa suggests that the Orange River is a major contribution of pollen to the northern mudbelt declining southwards. Whereas the seasonal inputs of pollen from offshore berg winds and local ephemeral Namaqualand rivers have a great contribution to the central mudbelt. In the southern mudbelt, the dominated Fynbos elements indicate a main pollen source from the Fynbos vegetation in the southwestern Cape of South Africa. The approach in the first part of this thesis allows for climate reconstructions of the SRZ and WRZ using specific pollen taxa from a single marine archive. Therefore, in the second part of this thesis, the Holocene vegetation and climate variability in South Africa was reconstructed using pollen and microcharcoal records of two marine core sites GeoB8331 and GeoB8323 from the Namaqualand mudbelt offshore of the west coast of South Africa covering the last 9900 and 2200 years, respectively. Three different climate periods were described with apparently contrasting climate developments between the SRZ and WRZ: during the early Holocene (9900-7800 cal. yr BP), a minimum of grass pollen suggests lower summer rainfall in the SRZ, while relatively wet conditions in the WRZ were indicated by the initial presence of Renosterveld vegetation. During the middle Holocene (7800-2400 cal. yr BP), a maximum in grass pollen suggests an expansion of rather moist savanna/grassland. This is probably associated with higher summer rainfall in the SRZ resulting from increased austral summer insolation. In the WRZ, a decline of Fynbos vegetation accompanied by an expansion of Succulent Karoo vegetation indicate warmer and drier conditions, which possibly suggests a southward shift of the southern westerlies. Comparing the results of the two sites for the last 2200 years show a more stable climate in the WRZ than in the SRZ. In addition, the a Little Ice Agea (LIA) event (ca. 700-200 cal. yr BP) was detected with colder and drier conditions in the SRZ but colder and wetter conditions in the WRZ. To assess the land-sea linkages in this region, in the third part of this thesis, the Holocene palaeoceanographic and palaeoenvironmental changes in the southern Benguela upwelling system was investigated based on the organic-walled dinoflagellate cyst analysis of 12 marine surface sediment samples and gravity core GeoB8331-4 from the Namaqualand mudbelt offshore of the west coast of South Africa. The results were compared with pollen and geochemical records from the same samples. Three main phases were identified with significantly distinct oceanographic conditions during the Holocene: during the early Holocene (9900-8400 cal. yr BP), warm and stratified conditions were indicated by high percentages of autotrophic taxa suggesting reduced upwelling likely due to a northward shift of the southern westerlies. In contrast, during the middle Holocene (8400-3100 cal. yr BP), cool and nutrient-rich waters with active upwelling were indicated by a strong increase in heterotrophic taxa at the expense of autotrophic taxa. This is probably caused by a southward shift of the southern westerlies. During the late Holocene (3100 cal. yr BP to modern), strong river discharge with high nutrient supply between 3100-640 cal. yr BP was implied by high percentages of Brigantedinium spp. and other fluvial-related taxa such as Protoperidinium americanum and Lejeunecysta oliva.
|Keywords:||Holocene, marine sediments, pollen, microcharcoal, organic-walled dinoflagellate cyst, paleoenvironment, paleoceanography, South Africa||Issue Date:||4-May-2017||URN:||urn:nbn:de:gbv:46-00105891-19||Institution:||Universität Bremen||Faculty:||FB5 Geowissenschaften|
|Appears in Collections:||Dissertationen|
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