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The Canary Current Temperatures from Portugal to Cape Most Reliable Indicators

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The Canary Current Temperatures from Portugal to Cape Most Reliable Indicators V. Sediments and benthos Rapp. P.-v. Réun. Cons. int. Explor. Mer, 180: 315-322. 1982. Sediments in up welling areas, particularly off Northwest Africa Eugen Seibold Geologisch-Paläontologisches Institut der Universität Kiel Olshausenstrasse 40/60, 2300 Kiel 1, Bundesrepublik Deutschland Introduction Oceanic upwelling supplies water from subsurface lay­ with sediments, however, most of these variations, ers to the surface layer and may occur as a persistent together with biological production cycles, are aver­ process anywhere, although it is a particularly con­ aged out, as even the uppermost centimetre of sedi­ spicuous phenomenon along western coasts of conti­ ment normally includes events spanning a century or nents where prevailing winds drive the surface water more. Most of the indicators employed are of organic from the coast (Smith, 1973). origin. It is well established that typical upwelling This paper discusses indicators of coastal upwelling water masses are several degrees cooler than nearby revealed in the underlying sediments off Northwest waters, are less saline, show a relatively low oxygen Africa and makes some comparisons with upwelling content, and have higher nutrient concentrations, effects in sediments from the coastal area off Southwest increasing primary production in the photic zone. At Africa. From these results it will be attempted to present only lowered temperatures as preserved in reconstruct periods of upwelling during the last 20 mil­ planktonic organisms tests can be used as clear indi­ lion years off Northwest Africa. Some of these prob­ cators for upwelling, although higher productivity may lems have been treated generally in the classic paper of provide additional hints. As geologists, we try to recon­ Brongersma-Sanders (1948) and recently by Diester- struct the environment from the sediments, and there­ Haass (1978b). Some observations from Northwest fore have to look upward from them and their buried Africa are reported in Milliman (1977). Many of the biological assemblages towards the surface waters with results, however, are based on the investigations of the their life assemblages. Several filters en route, the marine geological group of the Geological Institute, quantitative effects of which are barely known, alter or University of Kiel, during the past 10 years (Fig. 217). even destroy environmental indicators. (See general The scientific and technical help is sincerely acknowl­ discussion in Berger, 1976). edged. Financial support by the Deutsche Forschungs­ gemeinschaft (DFG) and for the “Valdivia” cruises by the Bundesministerium für Forschung und Technologie Filter problems (BMFT) is very much appreciated. Today, coastal upwelling occurs off Northwest Africa All suspended particles have to pass several filters be­ throughout the year between 20°N and 25°N (Fig. 217). tween the surface waters and their incorporation into Strong nearshore cooling, however, migrates - to­ marine sediments. Most of the effects pertinent to our gether with the trade wind system - from about 15°N, problem depend on the duration of this transfer. near Cape Verde in winter to Portugal in summer (Wooster et al., 1976). Upwelling off Senegal was observed by Schemainda et al. (1975) to occur for less Midwater filter than half the year, and off Sierra Leone for only about one month. Settling rates depend on sizes and shapes of particles In addition, short-term and local variations due to and may reach more than 2000 m/day (empty ptero- meteorological and oceanographical conditions occur. pods, planktonic foraminifera). Rates for diatom frus- (See recent literature, e.g. Barton et al., 1977; tules or coccolithophore plates around 10 m/day have Cruzado, 1974; Herbland et al., 1973; Hughes and Bar- been reported (Berger, 1976; Seibold, 1978). Even ton, 1974; Johnson, 1976; Johnson et al., 1975; and unidirectional, continuous currents with velocities up Mittelstaedt et al., 1975. Schemainda et al. (1975) and to 4 cm/s throughout the water column would result in Shaffer (1976) give a general review.) When dealing lateral shifts of indicators from planktonic foraminifera 315 led diatom tests as observed by Richert (1975) and Mil- liman (1977) and measured by Nelson and Goering (1977) and Friederich and Codispoti (1982) off North­ west Africa, although they may partly bypass this filter M 8 /19 67 if contained within fecal pellets (Schrader, 1971; 1972a,b). Dissolution of aragonite (pteropods) and cal- cite (foraminifera, coccoliths) occurs in deeper waters SALVAGES and only a small percentage of these tests may be dam­ 3 O ° N aged or lost during settling. Saidova (1968) observed well-preserved planktonic calcite tests in gut contents I. C AN AR I AS of benthic organisms living in water depths of more C.Jubi than 4000 m. Berger and Piper (1972) calculated some 25/1971: loss values after field experiments. C Bojador Predators may damage tests mechanically, but pro­ tect them chemically in fecal pellets. C Barbas M2S/1971 C. Blanc 20° / Bottom-water filter Waves and currents normally affect the sea bottom on M39/197! the shelf with maximal effects. Erosion and non-depos­ ition is reported from many areas off Northwest Africa (Einsele et al., 1977; Masse, 1968; McMaster and Lachance, 1969; Newton et al., 1973; Summerhayes et A.DE C.VERDE al., 1972; 1976). All signs of present upwelling are VA/19 absent there, while in many other areas fine particles together with most of our indicators are winnowed out or damaged within the sands or coarser materials. Reworking mixes indicators of different periods and areas. However, patches of muddy sediments (>75 % < 63 um) were mapped by Domain (1977a,b; 1982) in Figure 217. Northwest African continental margin standard water depths between 20 and 80 m from 16°30'N to profiles. Geological Institute, University of Kiel, cruises with FS “Meteor" (8-1967, 25-1971, 39-1975) and RFS “Valdivia” 15°15'N, in water depths between 15 and 100 m from (1975). the Casamance estuary to 11°35'N and near some 1-5 = Types of natural vegetation and climates. canyon heads. In general these patches contain up to 1 Mediterranean scrub: Temperate-warm, summer dry, about 2 % Corg and an increased benthic biomass. Both winter rainy. 2 Steppe: Hot, summer dry. factors vary seasonally (up to 1:4), which is not easily 3 Desert: Hot, dry. explained. Bearing in mind all these complications on 4 Steppe: Hot, winter dry. the shelf, we concentrated our sedimentological inves­ 5 Savannah: Tropical, winter dry. tigations on slope and rise stations, hoping that parti­ 6 Tropical rain forest: Hot, humid. Dashed boundary indicates central area of dust falls during cles indicating upwelling reach these regions directly winter. from mixed surface waters extending 50-100 km off­ shore or by near-bottom downslope transport. The lat­ ter can be proved by small amounts of glauconite, shelf of only a few kilometres at continental margins. Smal­ relict material, thick-shelled benthonic pelycopods and ler particles could he laterally transported some tens of benthonic foraminifera from shallow water incorpo­ kilometres; however, and many of them are incorpo­ rated in surface sediments on the slope and rise (Dies- rated in fecal pellets with settling rates of some 100 ter-Haass, 1975; Bein and Fütterer, 1977; Lutze, 1980; m/day. Downwelling as discussed in Thiel (1978) may Lutze et al., 1979). Furthermore, longer cores for accelerate sinking, while upwelling and pycnoclines paleo-oceanographic studies could more easily be ob­ (Bishop et al., 1977) retard sinking. Nevertheless this tained in water depths below about 500-1000 m. lateral translation is neglible for our biological indi­ Poleward-flowing contour currents in water depths cators in continental margin depths. between about 100 and 600 m were observed on the Long transfer times of small particles, however, are Northwest African slope from Senegal up to Cape important for chemical effects. Higher temperatures Bojador by Johnson et al. (1975) and Mittelstaedt and lower silica concentrations within the uppermost (1976). In 150-200 m water depths, daily mean speeds few hundred metres (“silica corrosion zone”, Berger, of 7-20 cm/s were measured. Off Northwest Africa 1976) are responsible for dissolution of empty thin-wal­ Fahrbach and Meincke (1978) measured near-bottom 316 velocities resulting from internal waves with maxima lective dissolution and combined buffering. Neverthe­ between 10 and 40 cm/s in water depths from 250 to less, Schrader (1972a) observed complete dissolution 1000 m. Such velocities are sufficient to bring fine par­ of diatoms and radiolarians below the uppermost 30 cm ticles into suspension, as are also the activities of of a sediment core off Morocco (2811 m water depth), benthic organisms. Accordingly it is necessary to con­ with sponge spicules being more resistant. sider transport and sorting effects. From a mixture of Interstitial waters of strongly anoxic sediments with planktonic foraminifera and radiolarians the latter may high organic matter contents and accumulation rates be partly winnowed out, for example. As mentioned tend to preserve carbonates because they are saturated above, sinking of most biogenic particles in the water or even oversaturated with C aC 03 within the sulfate column lasts several days to weeks. On the sea bottom reduction zone and below. A good example for excel­ they are exposed to near-bottom waters for several lent preservation are the varved basin sediments off years, dependent on sedimentation rates and bioturba-
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