Variations in the upwelling intensity and sedimentation regime during Holocene and Latest Pleistocene
Sub-project within the Collaborative Project EVAR ("The Benguela Upwelling System under climate change - Effects of variability in physical forcing on carbon and oxygen budgets")
Despite many studies in and on the Benguela upwelling system (BUS), there is sparse knowledge on the low-frequency variability of the geochemical (redox) conditions on longer (i.e. > decadal) time scales and how the responsible changes in physical driving factors are triggered. Based on the existing time series, it is clear that the inspection of a longer period is essential for the reliable identification of trends and drivers. The lack of such information is due to an insufficient number of high-resolution marine sediment cores and the foci of former scientific projects. For the time period since the last glacial, several significant local climate changes have been documented by records of different proxies in terrestrial and continental margin archives. Most of these studies reconstructed the history of upwelling intensity primarily by means of calculations of sea surface temperatures and accumulation rates. Furthermore, whilst some studies discuss variations in the atmospheric circulation, none attempted to derive the variability and intensity of anoxia. An answer to the question of whether oxygen is available, limited or absent and for how long is crucial for nearly all geochemical processes, regardless of whether they are microbially catalyzed or inorganic. In this sub-project, our aim is to fill this gap by focusing on the question:
Is the intensity of anoxia documented in high-resolution sediment archives?
For this purpose, fresh, long sediments cores both from the shelf and from the continental slope are required. While in deeper waters some locations with suitable deposits are already known, acoustic profiles from former cruises will help us to locate undisturbed sediments despite the highly dynamic sedimentation processes on the shelf. After radiocarbon dating, selected sediment cores will be analyzed first for several key parameters (grain size distribution, organic carbon and carbonate contents, etc.). Then we will use redox proxies to reconstruct variations of prevailing geochemical conditions in the BUS. We will especially concentrate on Fe-C-S based proxies, excess concentrations of redox sensitive elements (e.g. Fe, Mn, Mo) and ratios of these elements to components which are diagenetically largely conservative. To check the potential for preservation, we will use a biogeochemical model approach.