Benthic environmental responses to climatic changes during the late Quaternary: a micropalaeontological and geochemical approach

Abstract: There is a limited understanding of how the benthic environment within upwelling regions responded to past rapid climatic changes. Within this thesis, a multiproxy approach is applied to two marine sediment cores from two coastal upwelling sites in the low latitude subtropical Atlantic. With a focus on benthic foraminiferal faunal analyses, the response of the benthic environment to rapid climatic changes and the degree of coupling with surface primary production was reconstructed for the last 35 ka for the Mauritanian upwelling system and 70 ka for the Benguela upwelling system. Benthic foraminiferal faunal composition shifts occurred within both records, in the case of the Mauritanian upwelling site four shifts occurred: during late MIS3 (35-28 ka), across Heinrich event 2 and the Last Glacial Maximum (28 to 19 ka), throughout Heinrich event 1, the Bølling Allerød and the Younger Dryas (18-11.5 ka) and throughout the Holocene (11 ka – present). From the Benguela Upwelling System, six benthic foraminiferal assemblages were documented within the record: the first two during MIS4 and early MIS3 (70-59 and 59-40 ka), late MIS3 (40-30 ka), early-late MIS2 (30-16 ka), the termination of MIS2 to the onset of MIS1 (16 – 12 ka) and the Holocene (12 ka - present). Perhaps the most striking finding from both records was the abundance of low oxygen tolerant benthic foraminiferal species Eubuliminella exilis being so tightly correlated with diatom accumulation rate. From this coupling, low oxygen conditions at the seafloor were inferred to be caused by extreme levels of productivity export which actually hindered the benthos in terms of benthic foraminiferal diversity and accumulation rate; during Heinrich Event 1 and the Younger Dryas within the Mauritanian upwelling system and during late MIS4 and MIS3 within the Benguela upwelling system. In conclusion, major changes in deep-sea benthic foraminiferal faunas over the late Quaternary were attributed not only to upwelling intensity influenced by trade wind strength but also a complex balance between surface water productivity, sea level and deep water circulation. Therefore, this thesis demonstrates the rapidity of the benthic environmental response to these factors induced by global scale climatic change. To investigate the interplay between the surface and bottom water o further, a geochemical approach using the elemental composition of foraminiferal shells (tests) to develop a proxy of bottom water oxygen content was undertaken. The analytical methods of Secondary Ion Mass Spectrometry (SIMS) and Flow- Through Inductively Coupled Plasma Optical Emission Spectroscopy (FT-ICP-OES) were used to measure redox sensitive element manganese (Mn) and the results indicate that foraminiferal Mn/Ca in might prove to be a valuable proxy for oxygen in the bottom and pore waters when influenced by different productivity regimes. Lastly this thesis explores the concept of size fractions used for benthic foraminiferal analyses. By performing size fraction studies on samples from the Benguela record and reviewing the literature, an underrepresentation of opportunistic taxa such as Epistominella exigua occurred when the finer (>63-125 ?m) fraction was not analysed. However, the relative abundances of the benthic foraminiferal species does not alter sufficiently and therefore the palaeoecological interpretation does not change within this specific record. Overall, the findings within this thesis contribute to a gap in the knowledge regarding the seafloor responses to surface productivity dynamics during rapid climate changes, which need to be better understood in order to comprehend upwelling regions and predict future benthic environmental changes.