Upwelling intensity and filament activity off Morocco during the last 250,000 years

https://doi.org/10.1016/S0967-0645(02)00101-7Get rights and content

Abstract

The high-productive upwelling area off Morocco is part of one of the four major trade-wind driven continental margin upwelling zones in the world oceans. While coastal upwelling occurs mostly on the shelf, biogenic particles derived from upwelling are deposited mostly at the upper continental slope. Nutrient-rich coastal water is transported within the Cape Ghir filament region at 30°N up to several hundreds of kilometers offshore. Both upwelling intensity and filament activity are dependent on the strength of the summer Trades. This study is aimed to reconstruct changes in trade wind intensity over the last 250,000 years by the analysis of the productivity signal contained in the sedimentary biogenic particles of the continental slope and beneath the Cape Ghir filament. Detailed geochemical and geophysical analyses (TOC, carbonate, C/N, δ13Corg, δ15N, δ13C of benthic foraminifera, δ18O of benthic and planktic foraminifera, magnetic susceptibility) have been carried out at two sites on the upper continental slope and one site located further offshore influenced by the Cape Ghir filament. A second offshore site south of the filament was analyzed (TOC, magnetic susceptibility) to distinguish the productivity signal related to the filament signal from the general offshore variability. Higher productivity during glacial times was observed at all four sites. However, the variability of productivity during glacial times was remarkably different at the filament-influenced site compared to the upwelling-influenced continental slope sites. In addition to climate-related changes in upwelling intensity, zonal shifts of the upwelling area due to sea-level changes have impacted the sedimentary productivity record, especially at the continental slope sites. By comparison with other proxies related to the strength and direction of the prevailing winds (Si/Al ratio as grain-size indicator, pollen) the productivity record at the filament-influenced site reflects mainly changes in trade-wind intensity. Our reconstruction reveals that especially during glacial times trade-wind intensity was increased and showed a strong variability with frequencies related to precession.

Introduction

Near-shore upwelling of nutrient-enriched subthermocline waters and their offshore transport in eddies and filaments result in high rates of primary production and fixation of carbon. Upwelling regions thus play an important role in global carbon cycling. Changes in upwelling intensity and consequent changes in primary productivity have been proposed to explain part of the changes of climate-sensitive atmospheric CO2 concentrations on glacial–interglacial time-scales (Sarnthein et al., 1987). Both upwelling intensity and the formation and extension of filaments (in the following named filament activity) are dependent on regional climate, especially on the strength and duration of along-shore winds (Speth et al., 1978; Lutjeharms and Meeuwis, 1987; McCreary et al., 1991; Strub et al., 1991; Nykjær and Van Camp, 1994). Thus, detection of past productivity variations in upwelling regions can be used to infer changes in past wind circulation (Müller et al., 1983; Hughen et al., 1996).

A variety of micropaleontological and geochemical methods have been developed to reconstruct past variations in export production and variations in water mass characteristics that depend on upwelling intensity, like sea-surface temperature, salinity, or nutrient availability. A comprehensive overview about the use of these methods in paleoceanography has been given by Wefer et al. (1999).

The influence of sea-level changes on the sedimentary productivity record only recently has been discussed and is poorly understood (Guichard et al., 1999; Martinez et al., 1999; Bertrand et al., 2000). During the last glacial maximum (LGM) sea-level was about 120 m lower compared to present-day conditions (Fairbanks, 1989). At present, upwelling occurs mostly on the shelf (Mittelstaedt, 1991). A retreat of the coastline towards the edge of the shelf during times of low sea-level forces an offshore movement of the upwelling center towards the continental slope. Thus, changes in the sedimentary productivity record might be induced by changes in the upwelling intensity and/or by changes in the distance of the investigation site to the upwelling center. Additionally, lateral particle transport and nutrient dynamics may have changed during times of low sea level compared to present conditions (Fütterer, 1983; Bertrand et al., 2000). Using sedimentary paleoproductivity records as paleoclimatic archives can be facilitated by separating the impact of sea-level changes from wind-induced changes in upwelling intensity. This is hardly achievable, however, when only one site is assumed to be representative for the entire upwelling system, as done in most previous studies.

In this study we use geochemical methods to show how productivity variations over the last 250,000 years are preserved in marine sediments off Morocco (NW Africa). We try to distinguish between the influence of sea-level change and variations in trade-wind intensity on the sedimentary paleoproductivity record, by investigating four different sites (Fig. 1). Two sites are located at the continental slope near the coastal upwelling area. The other two sites are located further offshore, one off Cape Ghir, a region of intensive present-day filament activity. We show that the climatic signal is strongly overprinted by the impact of sea-level variations at the upwelling-influenced coastal sites. The filament-influenced site is highly sensitive to climatic-induced changes in productivity during glacial times. The summer NE trade-wind circulation that controls upwelling activity and filament intensity off Morocco, seems to be strongly controlled by precessional forcing mechanisms.

Section snippets

Present-day climate and oceanography off NW Africa

The NE trade-wind belt is the prevailing wind system off NW Africa. Seasonal variations are caused by the latitudinal shift of the subtropical high-pressure system currently known as the Azores High and of the tropical deep-pressure system related to the Inter-Tropical Convergence Zone. During the boreal winter, the trade-wind belt is mainly located between 10°N and 25°N. During boreal summer, trade winds blow prevalently between 20°N and 32°N.

The main direction of the trade winds along the NW

Materials and methods

The four sediment cores investigated in this study were recovered during Meteor cruise M37/1 in December 1996 (Wefer et al., 1997). Two sites were located at the continental slope (GeoB 4223, GeoB 4240), and two farther offshore (GeoB 4216, GeoB 4228) (Fig. 1; Table 1). Upwelling occurs mostly on the shelf. However, most biogenic fluxes derived from the upwelling are deposited at the continental slope between 1000 and 1500 m, due to remobilization and transport across the shelf (Fütterer, 1983).

Stratigraphy

Age control at sites GeoB 4216, GeoB 4223, and GeoB 4240 was obtained by correlating the δ18O records with the SPECMAP stack (Imbrie et al., 1984) (Fig. 2). For this purpose the planktic δ18O records were used, since they have been measured with high resolution at all three sites using the same species (G. bulloides). In our study, the major transitions, stadials, and interstadials could be easily identified in all three cores, besides oxygen isotope event 3.0. For oxygen isotope event 2.0 we

Discussion

Productivity variations in the upwelling area off NW Africa between 13°N and 32°N during glacial–interglacial cycles have been investigated by various methods using micropaleontological and geochemical proxies (Müller et al., 1983; Sarnthein et al., 1987; Marret and Turon, 1994; Martinez et al., 1996; Guichard et al., 1999; Abrantes, 2000). With the exception of sites located at 20°N (Harris et al., 1996; Guichard et al., 1997; Martinez et al., 1999) increased upwelling was observed during

Conclusions

Despite early diagenesis and lateral particle transport, sedimentary geochemical properties reflect mainly productivity gradients in the investigation area adjacent to the upwelling area off Morocco, allowing at least a qualitative reconstruction of late Quaternary productivity variations. The productivity records at the four sites investigated in this study showed both similarities and differences. The observed general pattern, of higher marine productivity during glacial times, is similar at

Acknowledgements

The authors thank the crew aboard R.V. METEOR for the help during coring of sediments. We are grateful to D. Hebbeln for intensive discussions and helpful suggestions on an earlier version of this manuscript. We thank W. Berger and an anonymous reviewer for helpful comments. We acknowledge the assistance in the laboratory work of M. Segl, H. Buschoff, J. Thiele, A. Eberwein and P. Franke. Data presented in this study are available at the CANIGO data center and under //www.pangaea.de/home/tfreudenthal/

References (71)

  • J.I. Hedges et al.

    Sedimentary organic matter preservationan assesment and speculative synthesis

    Marine Chemistry

    (1995)
  • H. Hooghiemstra et al.

    Vegetational and climatic changes at the northern fringe of the Sahara 250,000–5000 years BPevidence from 4 marine pollen records located between Portugal and the Canary Islands

    Review of Paleobotany and Palynology

    (1992)
  • R.A. Jahnke et al.

    The influence of organic matter diagenesis on CaCO3 dissolution at the deep-sea floor

    Geochimica et Cosmochimica Acta

    (1994)
  • J. Johnson et al.

    A fine resolution model of the eastern North Atlantic between the Azores, the Canary Islands and the Gibraltar Strait

    Deep Sea Research I

    (2000)
  • M. Knoll et al.

    The Eastern Boundary Current System between the Canary Islands and the African coast

    Deep-Sea Research II

    (2002)
  • A. Mariotti et al.

    Natural isotopic composition of nitrogen as a tracer of origin for suspended organic matter in the Scheldt estuary

    Geochimica et Cosmochimica Acta

    (1984)
  • F. Marret et al.

    Paleohydrology and paleoclimatology off Northwest Africa during the last glacial–interglacial transition and the Holocenepalynological evidences

    Marine Geology

    (1994)
  • P. Martinez et al.

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

    Organic Geochemistry

    (1996)
  • P. Martinez et al.

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

    Marine Geology

    (1999)
  • H. Meggers et al.

    Assessment of geochemical and micropaleontological sedimentary parameters as proxies of surface water properties in the Canary Islands region

    Deep-Sea Research II

    (2002)
  • J.D. Milliman et al.

    Biologically mediated dissolution of calcium carbonate above the chemical lysocline?

    Deep-Sea Research I

    (1999)
  • E. Mittelstaedt

    The ocean boundary along the northwest African coastcirculation and oceanographic properties at the sea surface

    Progress in Oceanography

    (1991)
  • A. Moreno et al.

    Orbital forcing of dust supply to the North Canary Basin over the last 250 kyrs

    Quaternary Science Reviews

    (2001)
  • P.J. Müller et al.

    Productivity, sedimentation rate, and sedimentary carbon content in the oceans

    Deep-Sea Research

    (1979)
  • B. Ransom et al.

    Organic matter preservation on continental slopesimportance of mineralogy and surface area

    Geochimica et Cosmochimica Acta

    (1998)
  • M. Schulz et al.

    Spectrumspectral analysis of unevenly spaced paleoclimatic time series

    Computers and Geosciences

    (1997)
  • R.E. Sweeney et al.

    Natural abundances of 15N as a source indicator for near-shore marine sedimentary and dissolved nitrogen

    Marine Chemistry

    (1980)
  • L. Van Camp et al.

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

    Progress in Oceanography

    (1991)
  • M.A. Altabet et al.

    Testing models of past ocean chemistry using foraminifera 15N/14N

    Global Biogeochemical Cycles

    (1989)
  • M.A. Altabet et al.

    Sedimentary nitrogen isotopic ratio as a recorder for surface ocean nitrate utilization

    Global Biogeochemical Cycles

    (1994)
  • P. Bertrand et al.

    Sea level impact on nutrient cycling in coastal upwelling areas during deglaciationevidence from nitrogen isotopes

    Global Biogeochemical Cycles

    (2000)
  • K. Billups et al.

    Surface ocean density gradients during the Last Glacial Maximum

    Paleoceanography

    (2000)
  • R. Davenport et al.

    Seasonal and interannual pigment concentration in the Canary Islands region from CZCS data and comparison with observations from the ESTOC

    International Journal of Remote Sensing

    (1999)
  • L.M. Dupont

    Pollen and spores in marine sediments from the East Atlantic. A view from the ocean into the African continent

  • R.G. Fairbanks

    A 17,000 year glacio-eustatic sea level recordinfluence of glacial melting rates on the Younger Dryas event and deep-ocean circulation

    Nature

    (1989)
  • Cited by (55)

    • Uranium-series ages of corals, sea level history, and palaeozoogeography, Canary Islands, Spain: An exploratory study for two Quaternary interglacial periods

      2014, Palaeogeography, Palaeoclimatology, Palaeoecology
      Citation Excerpt :

      However, the interaction of the Canary Current and upwelling with the morphology of the continental shelf and coast cause cold-water meanders, or filaments to develop and migrate farther off the African coast, bathing parts of Lanzarote, Fuerteventura, and Gran Canaria (Johnson and Stevens, 2000; Freudenthal et al., 2002). These cold-water filaments, like the coastal upwelling waters closer to shore, are driven by the trade winds (Freudenthal et al., 2002). The southern boundary of the seasonal upwelling zone at Cape Vert is likely the most critical factor determining the boundary between the Mauritanian and Senegalese/Guinean marine invertebrate faunal zones (compare Figs. 1 and 14).

    View all citing articles on Scopus
    View full text