Deep Sea Research Part II: Topical Studies in Oceanography
Upwelling intensity and filament activity off Morocco during the last 250,000 years
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/
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