Quaternary productivity records from the eastern equatorial Atlantic and the Benguela upwelling system.

摘要

The main purpose of this thesis is to reconstruct the history of upwelling in the eastern equatorial Atlantic and the Benguela Upwelling System through the Quaternary by developing productivity-related proxy records. This work was motivated by the long standing question of whether productivity in the eastern South Atlantic was higher during cold or warm periods. This question persisted because of conflicting evidence from deposition of organic matter and opal. Benthic foraminiferal assemblages are sensitive to the amount of food available and thus provide a means to reconstruct the original flux of organic matter to the sea floor. The total abundance and accumulation rate of benthic foraminifera in the eastern equatorial Atlantic and in the Benguela System (off Angola) suggest the presence of increased upwelling and organic matter supply to the sea floor during cold periods. However, there are major differences in species assemblages between the environments investigated (open ocean versus continental margin). The differences in species composition provide additional information regarding the degree of oxygenation, and the mode of supply and quality of food. Time series analysis was performed to evaluate the relationship of the benthic foraminiferal records to the presumed forcing mechanisms. In the eastern equatorial Atlantic, benthic foraminifers seem to respond to wind-driven upwelling at precessional (23-k.y.) periodicities. In addition to the 23-k.y. periodicity, there is a 15-k.y. signal in the Mid-Angola Basin. This suggests that latitudinal migrations of the Angola-Benguela Front, occurring at 15-k.y. periodicities, may have influenced the productivity fluctuations in this region.; Another widely used proxy for productivity is the content and flux of opal. Between 20° and 30°S, sediments in the Benguela Current System show a distinct diatom maximum within the lower half of the Matuyama reversed polarity chron. Biogenic opal was analyzed to define precisely the Matuyama diatom maximum for future comparison with other records in the region and elsewhere. The results from the opal determinations are quite similar to the overall diatom abundance. The examination of the benthic foraminiferal fauna across this diatom-rich interval, containing Thalassiothrix mat deposits, provides an insight on the effect of episodes of mat flux on the benthic environment.