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Title: Oceanic, climatic and vegetation variability in Western Equatorial Africa since the penultimate glaciation
Author: Lem, Rachael Elizabeth
ISNI:       0000 0004 7659 0713
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
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Global temperatures are expected to rise by 2-2.5°C by the middle of the 21st century, posing a significant threat to human populations and the natural environment. Tropical West Africa has been classified as a highly vulnerable region; naturally-driven multi-decadal droughts are predicted to become more frequent and intense 20 years ahead of the global average. Examination of past records of large scale climate change is critical in order to validate climate models and mitigate against future global warming. This thesis examines the land-ocean interactions of a small-scale Western Equatorial African (WEA) river catchment and explores its potential as a record of regional and global climatic change during the Late Quaternary Period. Previous research has documented large-scale WEA climate change using marine cores offshore the Niger and Congo Rivers, however the response of the smaller, medially positioned, Ogooué River catchment remains to be investigated. In 2003, the IMAGES programme recovered 34 m of sediment, spanning the last 150 ka, from the Ogooué Fan, offshore Gabon. Geochemical, sedimentological, isotopic (δ¹⁸O and δ¹³C), trace element and palynological analyses were carried out. Three research objectives were established: (1) reconstruct the hydrological variability of the Ogooué River catchment through the identification of terrigenous fluvial discharge events; (2) explore the glacial and interglacial control of surface and bottom water δ¹³C in the Eastern Equatorial Atlantic (EEA); and (3) examine WEA vegetation biome changes in order to infer the drivers of past regional hydrological and climatic change. Sedimentological and geochemical investigations demonstrated the strong potential of using a Fe/Ti ratio and foraminiferal planktic δ¹⁸O as proxies for Ogooué River discharge variability. Ogooué discharge events were synchronous with the neighbouring Sanaga and Congo Rivers and were also concomitant with precessional maxima. A preliminary sea surface temperature (SST) reconstruction evidenced coeval SST and salinity responses to riverine discharge variability and highlights an opportunity to undertake further trace element analysis on the study material. Planktic and benthic δ¹³C stratigraphies were constructed in order to produce the first Δδ¹³C gradient record from a region of EEA not dominated by upwelling. Thermocline - bottom water Δδ¹³C evidenced an alternation of nutrient-poor North Atlantic Deep Water influx during glacial periods and nutrient-rich Antarctic Intermediate Water masses during interglacial periods. Locally, planktic d18O data support that precessionally driven riverine discharge events contributed to the δ¹³C of the planktic foraminifera Globigerinoides ruber (white variety). Finally, vegetation change in the Ogooué River catchment, and wider WEA, was documented through palynomorph reconstructions. Pollen data evidences a transition from grassland vegetation in glacial marine isotope stage (MIS) 6, through to more precipitation dependent lowland rainforest and swamp biomes from interglacial MIS 5 (~ 100 ka) to present day. High abundances of rainforest taxa in MIS 4-2 suggests rainforests remained as refugia facilitated by a weaker monsoon. The mangrove pollen, Rhizophora, is interpreted as an indicator of sea level change, with high abundances in MIS 5 and 1 evidencing sea level rise. Lastly, the Afromontane pollen Podocarpus, shows a strong correspondence with precession minima, suggesting alternating dominance of trade wind and monsoon intensity over the 23 ka precessional cycle.
Supervisor: Marret-Davies, Fabienne ; Marshall, Jim Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral