Dynamics of tropical African climate and marine sedimentation during major climate transitions

Abstract

This thesis presents the late Miocene/early Pliocene climate transition (7-5 Ma) as recorded in marine sediments from Ocean Drilling Programme (ODP) Site 959 in the equatorial Atlantic off tropical West Africa, and a reconstruction of changing Holocene environments of the Eastern Tropical Atlantic (CHEETA) based on surface and core sediments from a transect along the Portuguese and NW African margin. The late Miocene to early Pliocene climate transition had irreversible consequences for atmospheric and ocean circulation leading to global cooling, northern hemisphere glaciations and modern climate conditions. In this study, continental climate, vegetation change and surface ocean dynamics at millennial time scale resolution is investigated from UK37’ alkenone derived SST, leaf wax lipids and organic carbon records (TOC). Despite low TOC (<1%) which was highly variable at cm-scale (~2.5-5 kyr) resolution, ubiquitous evidence from alkenones (C37:2 and C37:3) and leaf wax lipids indicate that the primary climate signal was preserved. The UK37’ based SST estimates (24.8-29°C) showed pronounced warm and cool cycles in the magnitude of 4°C. Elevated leaf wax lipids n–alkanes (C27, C29, and C31) correspond with cool SSTs and indicate a coupled relationship between upwelling and atmospheric ocean circulation patterns which intensified around 5.6 Ma arguing for wind driven deposition from terrestrial sources related to the position and strength of the ITCZ. The high amplitude cyclic patterns in the ODP Site 959 records were investigated by time frequency analyses. The common 41 kyr in all records supports a response to high latitude climate forcing. The 75 kyr variations and lead/lag observed in the leaf waxes and SST records during the late Miocene to early Pliocene are probably related to continental ice volume variations. The study on surface sediments from the Portuguese and NW African margins, confirm the presence of two soil-specific biomarkers, branched GDGTs (expressed as the BIT index) and bacteriohopanepolyols (BHPs), in this region of almost exclusive aeolian export. TOC exceeding 2% and δ13Corg gradients from -22.5‰ off Portugal to -19‰ off W Africa identify areas of upwelling off Cape Blanc and the transition from C3 to C4 vegetation habitats in Northern Africa, respectively. Despite low signals of the molecular records, slightly stronger response of soil-marker BHPs in the sediments is attributed to preferential erosion of the upper, (oxic) part of the soil column in central African source areas, the proposed source for soil BHPs. An alternative explanation is that in situ production of the branched GDGTs may be responsible for the low BIT index. As yet there is no evidence of the soil BHPs, adenosylhopane produced in situ in marine systems and aeolian transport of GDGTs is yet to been proven. More studies on dust samples from continental margins needs to be carried out to validate this transport mechanism of branched GDGTs and BHPs. A compilation of first bulk geochemical and molecular results from a selection of cores from the Portuguese and NW African margin within an integrated chronological framework document the variations in marine sedimentation and constrain regional variations in continental climate and terrigenous supply since the last glacial period. TOC accumulation records document millennial scale variability in response to the African Humid Period. Carbon isotope trends pronounced during the last glacial maximum support organic matter input from C4 type vegetation during colder glacial periods. Preliminary molecular isotopic records of n-alkanes identify gaps in this study that will need further investigation to assess and confirm vegetation sources and continental climatic conditions over Northwest Africa.