Abstract : The Atlantic southeastern deep margin is characterized by a very significant accumulation of organic carbon
during the last climatic cycles: the contents are among the highest of the world for deep oceanic sediments (from 5 to 18-20% at 1000m depth, from 0.5% to 8% at 3500m depth). These accumulations are related to the intense activity of upwelling cells associated with the Benguela Current system. These cells are related to the trade winds intensity blowing along coast.
A study was carried out on 13 cores located along the margin, in a stratigraphic interval included between the
Last Glacial Maximum (LGM) and the Holocene Climatic Optimum (HCO). The aim of this study is to
understand the different factors associated with this important deep organic sedimentation. The results obtained by various methods (Rock-Eval pyrolysis, Infra-Red Fourier Transform spectrometry, observation of palynofacies and smear slides...) show a general enrichment of the organic contents in the sediments during the LGM with respect to the HCO.
This study pointed out (i) an enrichment of organic content in LGM sediments with respect to HCO, with the
maximum values recorded below the most active upwelling cells (Lüderitz and Walvis Bay), (ii) a negative
correlation between the total organic percentages carbon and carbonates ones due to an organic matter dilution by the mineral fractions and carbonate dissolution during the organic matter degradation processes, (iii) the distribution of biogenic silica is more dependant to the external factors variations than to the differential productivity of the upwelling cells.
Optical studies of organic components allow the identification of two amorphous organic matter types: a
granular one, and a gel-like one. The gel-like one is related to the most active upwelling cells (Lüderitz and
Walvis Bay) and is characterized by a preservation process by natural sulphurization, the granular one, is
associated with a preservation process by argillaceous particles adsorption, and is found below the relatively less active upwelling cells.
These results highlight the variability of the organic record during the LGM/HCO transition, which is controlled by internal and external processes of the upwelling system. The two main parameters are the variability of the productivity intensity occurring in the photic layer, and the deposit migration during the marine transgression associated with the global warming. External processes are mainly related to the contribution of the different water masses from North (Angola Current) and South (Agulhas Current, Antarctic Intermediate Water).
The extrapolation of the oil potential of these sediments shows that the high productivity Lüderitz cell can be
regarded as a future source rocks deposits among the most productive in the world.
A comparison with an older similar climatic cycle (Pleistocene) has been realized after the study of a core from the ODP site 175-1084. Our results, completed by biomarkers data, allow supposing a very different functioning type with respect to the Late Quaternary one. This functioning type is associated with the important detrital
contribution in organic and inorganic sedimentation on the Lüderitz site, and with the existence of a major
climatic change, characterized by a productivity peak associated with siliceous organisms (diatoms) then