Upwelling intensity and ocean productivity changes off Cape Blanc (northwest Africa) during the last 70,000 years: geochemical and micropalaeontological evidence
Introduction
It is well known that continental shelves and slopes play a significant role in the marine biogeochemical cycle of carbon and associated elements such as nitrogen (Walsh, 1991). Due to large inputs of both continental and oceanic nutrients via rivers and upwelling, high primary production and high particulate carbon standing stocks are found in these regions. Sedimentation of biogenic particles leads to some carbon being stored in sediments and thus removed to the atmosphere.
Ice-cores have revealed that atmospheric CO2 concentrations have varied by up to 40% over the last climatic cycle, with a dramatic decrease during the last glacial maximum (Barnola et al., 1987). Past variations in oceanic productivity (biological activity) have been proposed as a possible explanation for such changes. For example, Mix (1989)has shown that the whole Atlantic was more productive during glacial times and could have played a major role in regulating past atmospheric CO2 variations. On a more local scale, it has been proposed that productivity of the northwest African coastal upwelling system increased during the same period and could explain part of the atmospheric changes (Sarnthein et al., 1988). Recently, Bertrand et al. (1996)indicated that this feature was not as simple in the northwest African margin and that large heterogeneities in organic carbon (OC) contents and fluxes were recorded between two sites at two different latitudes (core 20bK and core 11K; see Fig. 1); these authors hypothesized that these differences could be attributed to combined wind stress and sea-level changes.
Here, we present new data about one core located in the coastal upwelling of northwest Africa, i.e., core 11K, at the latitude of Cape Blanc. The main purpose of this paper is to understand how biological productivity has been recorded in the sediments at this site located on the upper slope. A multi-proxy approach is used: (1) to characterize the variations of the quantity and the quality of the biogenic and organic input to the sediment (organic carbon, calcium carbonate, benthic foraminifera accumulation rates); (2) to characterize the benthic system response to these variations (redox conditions: molybdenum, iodine, deep infauna of benthic foraminifera), and in turn, to evaluate the quality of the organic input (metabolisable versus refractory). Other markers specific of the lithogenic fraction (Zr/Al, Si/Al, mean grain-size, percentages of the coarse and fine terrigenous fractions) are used to reconstruct atmospheric circulation changes. Finally, a comparison is made among all the results in order to propose a conceptual model which summarizes our findings and hypotheses.
Section snippets
Present-day climatic, hydrologic and sedimentological features off the northwest African coastal upwelling
The strength and persistence of the coastal upwelling system off northwest Africa are coupled with the large-scale variations of the trade winds. The variations observed in both winds and upwelling are related to the migration of the Inter-Tropical Convergence Zone (ITCZ), which is controlled by changes in the maximum of insolation over both hemispheres. During the boreal winter, the ITCZ is centred at approximately 5°N. During this time, trade winds are well developed between 10 and 25°N.
Materials and methods
Core 11K was collected from the northwest African upwelling system during the SEDORQUA cruise in March 1994 (see Fig. 1). This core (725 cm in length) is located at 21°28.9′N, 17°57.2′W, at mid-slope (1200 m water depth) offshore of Cap Blanc where at present the most intense biological production of the northwest African margin occurs (Schemainda et al., 1975).
Visual observation and X-ray radiographs (M. Cremer, pers. commun.) indicate that core 11K does not contain any abrupt sedimentological
Organic carbon
This is the most traditional proxy of palaeoproductivity in sediment cores. Organic carbon (OC) contents and fluxes are shown in Fig. 3. Optical investigations revealed that organic matter is mainly composed of structureless flakes which are typical of a planktonic and/or bacterial origin (Boussafir et al., 1995), and that continental organic matter (mainly ligno-cellulosic fragments) is a minor contributor. This is confirmed by biomarker molecular composition (Ternois, 1996). OC contents
Productivity variations off Cap Blanc: present and past
We know from present-day conditions that the area off Cap Blanc is where productivity on the northwest African margin between 16 and 25°N is highest (Schemainda et al., 1975), and that biogenic sedimentation is controlled by different processes that include: (1) productivity in the overlying waters, essentially over the shelf and shelf-break (Van Camp et al., 1991); (2) export of organic and biogenic constituents from the shelf onto the slope from surface-water filaments (Gabric et al., 1993);
Conclusion
In this study, we have shown that different processes are involved in controlling the productivity-induced sedimentation on the slope offshore of Cap Blanc on the northwest African margin. These processes are: productivity in the overlying waters, which occurs essentially over the shelf and the shelf-break, export of organic and biogenic constituents from the shelf onto the slope via surface-water filaments, and resuspension from the shelf of the finest and lightest fraction of the sediments
Acknowledgements
We thank the officers and crew of the R/V Le Suroı̂t who helped to recover sediment-cores during the SEDORQUA cruise (March 1994). The SEDORQUA program is financially supported by French research institutions (MENRT, CNRS, CEA, National Programs `Geosciences Marines' and `DYTEC') and the British NERC (to GBS). We thank Martine Paterne (CFR, Gif-sur-Yvette, France) who provided the δ18O time scale record, G. Hanaut and G. Chabaud for technical assistance in grain-size measurements. We express
References (78)
- et al.
Carbon budget for the mid-slope depocenter of the Middle Atlantic Bight
Deep Sea Res. II
(1994) Benthic foraminiferal distribution and biomass related to porewater oxygen content: central California continental slope and rise
Deep Sea Res.
(1992)The deposition of molybdenum in anoxic waters
Mar. Chem.
(1972)- et al.
Electron microscopy and pyrolysis of kerogens from the Kimmeridge Clay Formation, UK: source organisms, preservation processes and origin of microcycles
Geochim. Cosmochim. Acta
(1995) - et al.
Geochemistry of recent oxic and anoxic marine sediments: implications for the geological record
Mar. Geol.
(1993) Sulfate reduction and oxic respiration in marine sediments: implications for organic carbon preservation in euxinic sediments
Deep Sea Res.
(1989)- et al.
Distribution of rose bengal stained deep-sea benthic foraminifera from the Nova Scotian continental margin and the Gulf of Maine
Deep Sea Res.
(1990) - et al.
Anoxic environments and oil source bed genesis
Org. Geochem.
(1980) A study of the regulation of the concentrations of some trace metals (Rb, Sr, Zn, Cu, V, Cr, Ni, Mn and Mo) in Saanich Inlet sediments, British Columbia, Canada
Mar. Geol.
(1988)- et al.
Origins of peri-Saharan dust deposits traced by their Nd and Sr isotopic composition
Palaeogeogr., Palaeoclimatol., Palaeoecol.
(1992)