The dynamics of the coastal upwelling and the aeolian input off Mauritania
by
Romero, Oscar
Dept of Geosciences and RCOM, University of Bremen, P O Box 330440, 28334 Bremen, Germany
Coauthors: Peer Helmke (Dept of Geosciences, University of Bremen, P O Box 330440, 28334 Bremen, Germany)

Due to the fairly steady trade winds, upwelling off Cape Blanc occurs throughout the year, with periods of stronger intensity in spring, early summer and autumn. The source waters for the upwelling are either salty and relative nutrient-poor North Atlantic Central Water, north of about 23°N, or less-saline and nutrient-rich South Atlantic Central Water, south of 21°N. The upwelled waters near Cape Blanc vary in nutrient concentration depending on their origin, which is most likely, the reason for the corresponding regional differences in primary production. Additionally, off Cape Blanc "giant filaments" of relatively high pigment concentration persist throughout the year with substantial seasonal and interannual variations. The latter is the site of a “giant filament” which extends up to 450-600 km offshore; it is present for most of the year with substantial intra- and interannual variations. The large offshore transport represents a potential mechanism for exporting cool, nutrient-rich water from the coastal region, and is thought to be partly responsible for both enhanced offshore primary production, and the occurrence of coastal phytoplankton found several hundred kilometres offshore.

Because of the relative paucity of the data off Cape Blanc, interannual variations of the upwelling dynamics and export production are hard to determine. Recent observations prove that year-to-year variations are very strong. Whether they might be related to global-scale climatic variations or to a natural level of basin-wide atmospheric and/or oceanic variability is an issue in need of further investigation. With the goal of providing information on export production of marine particulates and fluxes to the seafloor, we have studied the flux of particle using sediment traps deployed off Cape Blanc. In this paper, we present information on intra- and interannual patterns of the flux of bulk components and siliceous primary producers for a four-year sampling period (1988-91).

Significant variations in flux and diatom species composition were observed accompanied by a marked drop in the export of biogenic opal and diatoms from 1988 to 1989; fluxes remained low thereafter (Fig. 2). We hypothesize that this diminution might be related to (a) the decrease in coastal upwelling intensity and offshore spreading of the typical chlorophyll filament, and/or (b) a lesser silicate content of waters being upwelled off Cape Blanc. The first hypothesis is supported by the lessened diatom flux and contribution of neritic diatoms from 1989 through 1991 (Fig. 3). According to satellite observations (Fig. 1), the CB mooring lays occasionally within the chlorophyll filament of cool, pigment-rich waters spreading seaward at ca. 24° and 20°N off NW Africa. Although colour satellite measurements for the period in study are not available, earlier observations show that seaward extension and the pigment concentration of the filament strongly varies on both seasonal and interannual bases. As for hypothesis (b), it has been demonstrated that diatoms play a decisive role in the phytoplankton community off Cape Blanc. Hence, lowered silicate content in surface and subsurface waters could limit the total production and the export of siliceous phytoplankton to the deep sea. Though silicate measurements at or close to the CB trap site for the time period presented are not available, the sharp increase in the flux of organic carbon in 1991, and the enhancement in the relative contribution of calcium carbonate to the total mass flux from 44% in 1988 to 52% in 1991 indicates a change in the composition of the primary producer community off Cape Blanc. In general, the dominance of neritic diatoms reflects the continuous offshore influence of coastal upwelling at the CB trap site, with stronger intensity in spring/summer (Fig. 3). In contrast, the occurrence of pelagic diatoms is linked to inshore transport of oceanic waters, generally in winter.

We have also studied the fluxes of airborne freshwater diatoms (FD), phytoliths (PH), and pollen grains (PO) collected at the CB site from 1988 till 1991 (Fig. 4). Both continental rainfall variations and wind mean strength and direction play a key role in the temporal fluctuations of the fluxes of eolian traces in the pelagic realm. Drier conditions in Northern Africa in 1987 could have preceded the high lithogenic input and moderate FD flux in 1988. The PH peak in summer 1988 was probably caused by increased wind velocity. Wetter rainy seasons of 1988/89 might have promoted a significant pollen production in summer 1989 and FD in late 1989 and early 1990, as well as contributed to the reduction of the lithogenic flux in 1989/90. Decreased fluxes of FD, PH and PO, and higher contribution of the 6-11 µm lithogenic fraction in 1991 would mainly reflect minor intensity and decreased amount of continental trade winds. Air mass backward trajectories confirm that the Saharan Air Layer is predominantly involved in the spring/summer transport. Trade winds play a decisive role in the fall/winter months, but also contribute to the transport during late spring/summer. Origin of wind trajectories does not support a direct relationship between transporting wind-layers and material source areas in Northern Africa. High winter fluxes of eolian tracers and high amount of trade winds with continental origin in summer warn against a simplistic interpretation of the seasonal eolian signal preserved in the sediments off Cape Blanc, and the wind layer involved in its transport.

The preserved diatom assemblage below the sediment trap resembles the spring-early fall trapped flora, dominated by robust to moderately silicified neritic species, and mostly associated with periods of high diatom flux. In contrast, the abundance in the sediment of several pelagic diatom species, such as Nitzschia bicapitata group, scarcely exceeds 1%. For other pelagic species with moderately silicified valves, such as Fragilariopsis doliolus and Planktoniella sol, abundances in the traps are similar to those found in the surface sediments, and their occurrence in the preserved record may be used as an indication of inshore incursions of open-ocean waters into the coastal area of Cape Blanc. Apparently, an overwhelming neritic signal has been the rule throughout the late Quaternary. Chaetoceros spores, accompanied by Thalassionema nitzschioides var. nitzschioides, representing high coastal upwelling productivity, especially during glacial times, dominate Downcore sediments younger than 80 kyr.

Date received: January 27, 2004