Paleoceanographic Reconstruction of Gulf of Guinea Productivity Variability During the Holocene

Understanding how oceanographic productivity and circulation in the Gulf of Guinea varied during the Holocene provides a deep-time context for interpreting observed modern changes and projecting future responses to anthropogenic climate forcing. This dissertation reconstructs the variability of biological productivity, thermocline depth, and surface ocean circulation in the eastern Gulf of Guinea during the past 11,000 years using multi-proxy analysis of sediment cores recovered from the Nigerian continental slope between 400 and 1,200 metres water depth. Three sediment cores with sedimentation rates of 15 to 25 centimetres per thousand years are analysed at decadal to centennial resolution for a suite of proxies including foraminifera assemblages (for sea surface temperature estimation using the modern analogue technique), alkenone unsaturation indices (Uk'37 paleothermometry), stable oxygen and carbon isotopes of benthic and planktonic foraminifera (for thermocline structure and bottom water oxygenation), biogenic opal and barium accumulation rates (for export productivity), and n-alkane and compound-specific isotope analysis (for terrestrial vegetation and river runoff signals). Age models are constructed from accelerator mass spectrometry radiocarbon dates and correlated with the Marine20 calibration curve. Spectral analysis identifies dominant periodicities in the proxy records corresponding to insolation forcing, Atlantic Multidecadal Oscillation cycles, and Bond events. The dissertation contributes original paleoceanographic records from the poorly sampled Nigerian shelf margin, providing the first Holocene productivity reconstruction for this region of the Gulf of Guinea. Implications for understanding the sensitivity of West African monsoon-driven upwelling to insolation and ocean-atmosphere coupling are discussed. Keywords: paleoceanography, Holocene, Gulf of Guinea, sediment cores, paleoproductivity proxies