Upwelling intensity and ocean productivity changes off Cape Blanc (northwest Africa) during the last 70,000 years: geochemical and micropalaeontological evidence

https://doi.org/10.1016/S0025-3227(98)00161-3Get rights and content

Abstract

The accumulation of biogeochemical (organic carbon, calcium carbonates, molybdenum and iodine), micropalaeontological (benthic foraminifera) and terrigenous markers (grain-size, Si/Al, Zr/Al, K/Al) over the last 70,000 years in one core (Sedorqua-11K) on the northwest African margin off Cap Blanc has been used to reconstruct past variations of local upwelling intensity and oceanic productivity. The study demonstrates that productivity in this area increased during Stage 3, particularly between 40,000 and 50,000 yr B.P., and during the last deglaciation between 6000 and 15,000 yr B.P. During most of isotopic Stage 2, and particularly during the last glacial maximum, productivity was much lower. These variations can be attributed to changes in local wind stress and seasonality that are related to variability in monsoon pressure intensity. Because of the establishment of upwelling cells over the shelf due to high sea-level, the conditions of sedimentation during the last 5–6 thousend years on the upper slope (site 11K) are largely dominated by advection from the shelf, leading to strong sorting prior to deposition. Advection seems to have been minor during the other periods of enhanced productivity. A conceptual model is proposed to link the productivity variations to atmospheric circulation, in particular to the wind stress, direction and seasonality.

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)

  • F.E. Grousset et al.

    Saharan wind regimes traced by the Sr–Nd isotopic composition of subtropical Atlantic sediments: Last Glacial Maximum vs. today

    Quat. Sci. Rev.

    (1998)
  • J.C. Herguera

    Deep-sea benthic foraminifera and biogenic opal: glacial to post-glacial productivity changes in the western equatorial Pacific

    Mar. Micropaleontol.

    (1992)
  • F.J. Jorissen et al.

    A conceptual model explaining benthic foraminiferal habitats

    Mar. Micropaleontol.

    (1995)
  • F.J. Jorissen et al.

    Live benthic foraminiferal faunas off Cape Blanc, Northwest Africa: community structure and microhabitats

    Deep Sea Res. (Part I)

    (1998)
  • Ph. Martinez et al.

    An integrated view of inorganic and organic biogeochemical indicators of palaeoproductivity changes in a coastal upwelling area

    Org. Geochem.

    (1996)
  • D.G. Martinson et al.

    Age dating and the orbital theory of the ice ages: development of a high resolution 0 to 300,000-year chrono-stratigraphy

    Quat. Res.

    (1987)
  • J.D. Milliman

    Effects of arid climate and upwelling upon the sedimentary regime off southern Spanish Sahara

    Deep Sea Res.

    (1977)
  • P.D. Naidu et al.

    Do benthic foraminifer records represent a productivity index in oxygen minimum zone areas? An evaluation from the Oman Margin, Arabian Sea

    Mar. Micropaleontol.

    (1995)
  • D.M. Nelson et al.

    Near-surface dissolution in the upwelling region off Northwest Africa

    Deep Sea Res.

    (1977)
  • W.L. Prell et al.

    Surface circulation of the Indian Ocean during the Last Glacial Maximum, approximately 18,000 years B.P

    Quat. Res.

    (1980)
  • N.B. Price et al.

    The contrasting geochemistry behaviours of iodine and bromine in recent sediments from the Namibian shelf

    Geochim. Cosmochim. Acta

    (1977)
  • P. Rognon et al.

    Paleoclimates off Northwest Africa (28–35°N) about 18,000 yr B.P. based on continental eolian deposits

    Quat. Res.

    (1996)
  • W.F. Ruddiman

    Tropical Atlantic terrigenous fluxes since 25,000 yrs B.P

    Mar. Geol.

    (1997)
  • B.K. Sen Gupta et al.

    Benthic foraminifera in oxygen poor-habitats

    Mar. Micropaleontol.

    (1993)
  • C.P. Summerhayes et al.

    Variability in the Benguela Current upwelling system over the past 70,000 years

    Prog. Oceanogr.

    (1995)
  • L. Van Camp et al.

    Upwelling and boundary circulation off Northwest Africa as depicted by infrared and visible satellite observations

    Prog. Oceanogr.

    (1991)
  • G. Wefer et al.

    Seasonal patterns of vertical particle flux in equatorial and coastal upwelling areas of the eastern Atlantic

    Deep Sea Res.

    (1993)
  • F.F. Abrantes

    Variability of upwelling off northwest Africa during the latest Quaternary: diatom evidence

    Paleoceanography

    (1991)
  • J.M. Barnola et al.

    Vostok ice core provides 160,000-years record of atmospheric CO2

    Nature

    (1987)
  • Ph. Bertrand et al.

    Past sedimentary organic matter accumulation and degradation controlled by productivity

    Nature

    (1993)
  • Bertrand, Ph., Shimmield, G., Martinez, Ph., Grousset, F., Jorissen, F., Paterne, M., Pujol, C., Bouloubassi, I.,...
  • R. Boje

    The phytoplankton stock in the West African upwelling area north of Cap Blanc in January and February 1975

    Rapp. P.-V. Réun. Cons. Int. Explor. Mer

    (1982)
  • E. Boyle

    Chemical accumulation variations under the Peru Current during the past 130,000 years

    J. Geophys. Res.

    (1983)
  • Calvert, S.E., 1976. The mineralogy and geochemistry of near-shore sediments. In: Ryley, J.P., Chester, R. (Eds.),...
  • Calvert, S.E., Pedersen, T.F., 1992. Organic carbon accumulation and preservation in marine sediments: how important is...
  • Caulet, J.-P., Vénec-Peyré, M.-T., Vergnaud-Grazzini, C., Nigrini, C., 1992. Variation of south Somalian upwelling...
  • Chiapello, I., 1996. Les aérosols atmosphériques au-dessus de l'Atlantique nord tropical: approche physico-chimique et...
  • Dale, B., Fjellsa, A., 1994. Dinoflagellate cysts as paleoproductivity indicators: state of the art, potential and...
  • G.J. Demaison

    Anoxia vs. productivity: what controls the formation of organic-carbon-rich sediments and sedimentary rocks? discussion

    Am. Assoc. Pet. Geol. Bull.

    (1991)
  • Cited by (0)

    View full text