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 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 in the . 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 (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- California (Soutar, 1971). Even off Northwest Africa tion. Therefore dissolution effects increase, partly in a interstitial waters in sediments, with 1-4 % Corg and dramatic way. sedimentation rates of generally less than 50-100 In general, below 1-3 km at present no aragonitic mm/103 years, reducing conditions prevail below the tests are preserved (ACD - Aragonite Compensation uppermost centimetres. The regular decrease of the Depth), but fragmentation by dissolution begins in interstitial Ca2+ concentrations together with increas­ even shallower depths. Off Northwest Africa the Cal­ ing alkalinities in these cores indicate even carbonate cite Compensation Depth (CCD) is recorded at about 5 precipitation (Hartmann et al., 1973; 1976). Again dis­ km (Berger, 1976), but dissolution again begins at shal­ solution is possible within the uppermost oxic sediment lower depths (within the range of the “lysocline”). Car­ centimetres, in addition to the above-mentioned losses bonate dissolution there depends partly on the contents on the sea bottom. of organic material and carbonates together with the “Organic matter” is an extremely simplified term for sedimentation rate. Bacterial decomposition of organic a chemically very complex material. Nevertheless some matter produces C 02 and therefore an upward H C03~ generalizations may be possible. After the reported concentration gradient, a driving mechanism for the rates of primary production off Northwest Africa escape of these ions across the water/sediment inter­ (0-27-0-67 g C/m2/day, Wooster and Reid, 1963; 0-28- face. Carbonate particles can be dissolved there by this 0-59 g C/nr/day, Schemainda et al., 1975; or even 1-3 g mechanism or directly by near-bottom seawater if not C/m2/day off Cape Blanc, after Huntsman and Barber, protected by a sediment input, covering them. 1977) and calculations from surface sediments, only Dissolution of opaline silica on the sea bottom and 0-1-2 % pass our different filters (Müller and Whiticar, therefore recycling (see Rowe et al., 1977) may be even 1978). more effective. Richert (1976) analysed surface sedi­ Bacterial decomposition rates of organic matter in ment samples in the standard profiles (Fig. 217) be­ sediments, depending on several factors including tween Cape Bojador and Dakar from the shelf to the sedimentation rates, decrease sharply below 5-10 cm rise. He found diatoms only in some samples off Cape (see direct measurements as in Christensen and Pack­ Blanc and northwest of Nouakchott, but mostly off ard, 1977, and indirect geochemical conclusions as in Cape Barbas. The assemblage consisted of 90 % Chae- Hartmann et al., 1973; 1976). High sedimentation rates toceras resting spores. Samples barren with regard to seem to preserve chemically more labile fractions of diatoms, sporadically contained fecal pellets with well- the organic matter. preserved diatom frustules. Sometimes fibrils were pre­ Scanty direct measurements in interstitial waters sent which possibly originated from Thalassiosira col­ tend to indicate phosphate dissolution, such as fish onies. This result is very disappointing, because in debris within the uppermost few sediment centimetres plankton samples off Northwest Africa he found off Peru (Suess, 1978). (Richert, 1975) up to 14xl05 diatom cells/1. Many of Quantitative effects of bioturbation are poorly the dominant species therein have strongly silicified known although the uppermost few centimetres off valves or resting spores. Northwest Africa are homogenized by this process. This indicates substantial activity of epi- and infauna in these sediments relatively rich in organic matter. Sediment filter Underneath, individual biogenic tracks are arranged in Interstitial silica gradients also indicate dissolution of tiered groups down to several centimetres. Therefore siliceous material. However these concentrations are bioturbation may damage particles mechanically and higher and the pH-values are lower than in near-bot­ favour dissolution in the uppermost centimetres but tom waters. Therefore dissolution within the sediments may protect them through downward mixing. Upward is less effective than directly at the interface. Addition­ mixing also occurs, complicating the recognition of ally, more-resistant tests such as thick-walled com­ short-period events even in the surface sediments (A. pared with thin-walled diatoms, or in general radiola­ Wetzel, personal communication; see also general dis­ rians compared with diatoms, may be protected by se­ cussion in Berger, 1976).

317 productivity of more than 200-300 g C/m2/year off Consequences of sediment filters Cape Blanc around 20° to 23°N. Upwelling during only Due to these filters, upwelling indicators in sediments a few months in the south around 10°-12°N, however, off Northwest Africa generally represent only a small results in similar maxima owing to additional river fraction of the input from surface waters. input of nutrients (Schulz et al., 1978). Mechanical destruction is highest on the shelf. Sort­ Diagenetic products, such as pyrite or phosphate ing of particles may also occur on the slope. Larger and enriched in sediments with higher organic matter con­ denser particles are the more resistant ones. tent, could not be used as upwelling indicators in This also holds true for chemical destruction. A suc­ younger sediments off Northwest Africa. cession of increasing dissolution effects seems to be: ? Fish debris (Phosphate)? < Benthonic foraminifera < Planktonic foraminifera Metals < Coccoliths (Calcite) < Pteropods (Aragonite) and Radiolarians < Diatoms (Opaline silica) Boström et al. (1974) compared average contents of It may be expected that dissolution decreases with elements in planktonic matter, shales, and sediments increasing sedimentation rates. from the high productivity region of the equatorial Pacific, and they deduced the importance of biological contributions of elements like Cu, Ni, and Ba. Zn and Pb have a possibly biogenic origin, too. Hartmann et Upwelling indicators in sediments al. (1976) could not detect in surface sediments off Northwest Africa higher contents of these elements nor It is well known that upwelling combined with high any correlations, for example, between Cu and Zn and productivity is not restricted to coastal areas. On the organic matter. Schöttle (1977) reports relative enrich­ other hand, in coastal areas fertility may also increase ment of Zn off the Senegal mouth, probably explain­ near river mouths. This creates additional complica­ able by a higher input of clay minerals together with tions off Northwest Africa. organic matter. According to Lange (1975) these clays show high contents of montmorillonites and high specific surface values. Slightly increased contents of non-detritic Cu and Ni off Cape Barbas could perhaps Organic matter be attributed to higher productivity. Relations between Organic matter content in sediments per se can be only planktonic matter and metals, the influence of clay an approximate indicator of higher productivity be­ minerals and hydroxides during their transfer to the cause this depends on sedimentation rates in a compli­ sediments, and diagenetical changes are insufficiently cated way. High rates may dilute the content but they well known for these metals to be used as upwelling protect the organic matter better against decomposi­ indicators off Northwest Africa. tion. Furthermore, it is a well-known geological fact Barium, however, may have a possible significance that fine-grained sediments are generally richer in as an upwelling indicator. The well-known relationship organic matter due to greater adsorption capacities and with siliceous planktonic organisms (Arrhenius, 1963; the tendency to be deposited under more quiet condi­ diatoms: E. Suess, personal communication) and the tions. Accumulation rates of organic matter generally fact that barium is easily precipitated together with sul­ could be better indicators of higher productivity pro­ phate should be investigated off Northwest Africa. vided that sediments with similar grain-size distribu­ tions and sedimentation and diagenetical situations can be compared. Off Northwest Africa there is a trend Biological hard particles towards increasing accumulation rates of organic mat­ ter from north to south, for example in Holocene sedi­ As mentioned above, coarser shells in the sediments ments of 2000-2600 m water depths from about 10 mg are in general better suited to the investigation of poss­ Corg/cm2/1000 years at latitude 25°N to 150 mg at 16°N ible upwelling relationships because of their increased (Müller, 1976, and personal communication). This resistance to mechanical or chemical destruction. seems to indicate an increase of productivity from Therefore most of the results from the Kiel group are north to south, but certainly input of clay minerals by based on quantitative coarse-grain component analysis the Senegal River and better preservation of organic (fractions > 63 um, Sarnthein, 1971; Diester-Haass et matter from plankton or from land plants in these fas­ al., 1973; Diester-Haass, 1975; 1976; 1977; 1978). ter accumulating fine-grained sediments are additional Some additional information from the silt fractions (2- factors. The possible relations between organic matter 63 |im) are given by Fütterer (1977). Sand contents contents or even accumulation rates and upwelling are vary considerably, between more than 95 % on the even more complicated, bearing in mind that coastal shelf and the upper slope, to less than 5 % on the lower upwelling during the entire year results in a primary slope and rise, but within these environments, too. Silt

318 is normally represented between 40 and 60 %; clay west Africa indicate lower ratios in areas with higher again varies between less than approximately 5 % (on productivity (Berger et al., 1978; Diester-Haass, the outer shelf and upper slope) and 50 %. 1978b). This may be attributed to higher food supply to Coarse-grain analysis of biogenic constituents must benthonic organisms, and, indeed, sometimes other therefore take into account errors, if only small quan­ benthonic groups like echinoderms also seem to be tities of sand grains are available in surface samples. more abundant there. Once again, higher organic mat­ Siliceous tests, for example, maximally reach 6 % of ter contents may influence the ratio by dissolving the sand fractions off Northwest Africa (Diester- planktonic foraminifera tests more easily than benth­ Haass, 1975) and contribute less than 1 % to the silts onic ones. (Fütterer, 1977). Thus single abundance figures are less reliable than trends.

Fish debris Species analyses Suprisingly, few fish remains were found off Northwest Diatom life assemblages off Northwest Africa unfortu­ Africa (Diester-Haass) with a possible exception of nately include no index species for upwelling areas in around 18°N, where Miro Orell (1973) found 2-3 % spite of high diversity values (Shannon factors around fish debris in surface sediment fractions > 40 um. 2-5; see Richert, 1975). Most of the species are cos­ There seems to be no other explanation for this scarcity mopolitans with high ecological tolerances. However, than chemical destruction within the uppermost sedi­ none of these features could be evaluated from surface ment centimetres. sediment samples because of the losses passing through the above-mentioned filters. Siliceous shells Planktonic foraminifera life assemblages were inves­ Similar dramatic losses of opaline silica shells off tigated in detail by Thiede (1975a; 1977) and were com­ Northwest Africa were noted while considering the dif­ pared with burial assemblages on the continental mar­ ferent filters. Therefore, the presence of radiolarians gin from western Europe to about 20°N. He tried to or diatoms alone may indicate an originally increased silica input, i.e. increased productivity. Dissolution in interstitial waters then could be diminished by buffer­ ing effects protecting more resistant forms. Diester- Table 66. Coastal upwelling and sediments Haass (1978b) reports several per cent of diatoms and Indicator Northwest Southwest Africa radiolarians in the fractions > 40 um of some surface Africa (see Diester- samples between 18° and 22°N. Dinoflagellate cysts (This paper) Haass, 1978b) await detailed investigation. Organic matter ? + (Extremely content high) Aragonite shells Pteropod shells are nearly absent in Holocene sedi­ Phosphorites - + ments off Northwest Africa. Accordingly, only in Metals ? Cu ?Ni ?Zn ?Ni ?Zn +U exceptional cases could the relations between water temperatures and volumes of protoconchs or dominan­ Barium Not investigated No information ce/diversity ratios of pteropod assemblages be used (Diester-Haass and van der Spoel, 1977). C-Isotopes ô13C lower No information in planktonic foraminifera tests Calcitic shells Increased productivity of planktonic foraminifera is Fish debris - + indicated by higher sedimentation rates of their tests on Opaline silica tests ?(+) + (Extremely the Northwest African upper slope between 20°N and high diatom 24°N (Diester-Haass, 1976). An attempt has been contents) made by Diester-Haass (1977) to use the radiolarian/ planktonic foraminifera ratios in surface samples. Calcitic tests ?(+) No information Sedimentation rates Again this area was singled out. This ratio, however, cannot be used uncritically, because calcite dissolution Radiolarian/planktonic ? ( + ) + also influences the ratio and depends on organic matter foraminifera ratio and carbonate contents as well as on sedimentation Planktonic/benthonic ?(+) ? rates. Sorting effects may also be involved. foraminifera ratio Planktonic/benthonic foraminifera ratios generally increase with the water depth of the sediments, mostly Cool water assemblages owing to decreasing benthos abundance. Variations of Planktonic foraminifera + Coccoliths ?"(+) No information this ratio in similar water depths occurring off North­

319 Table 67. Neogene upwelling periods off Northwest Africa

Million Intensity References years

Pleistocene Berger et al. (1978), Crowley (1976), Diester-Haass (1978a), Glacial + + Diester-Haass et al. (1973), Gardner and Hays (1976), Lutze

Interglacial + (1978)

Pliocene DSDP Leg 47a Site 397 (Diester-Haass. 1979)a Late Decreasing

Early - DSDP Leg 47a Site 397 (Diester-Haass, 1979)a J Miocene DSDP Leg 47a Site 397 (Diester-Haass, 1979)“ Late - It* Middle -

Early + + DSDP Leg 41, Site 369 (Diester-Haass, 1978a; Sarnthein, 1978)“ ------22-5------a Deep Sea Drilling Project Sites 369 and 397 are situated off Cape Bojador.

Isotopes quantify relationships between these surface sediment assemblages and sea surface winter and summer temp­ Berger et al. (1978) used the stable carbon composition eratures (and winter salinities), using the statistical of the planktonic foraminifera Globigerinoides ruber in methods from Imbrie and Kipp (1971), together with sediments off Cape Barbas (2066 m water depth). the taxonomic framework from Kipp (1976). ô13CPDB-values are lower during periods of increased In spite of 1) the faunal specializations in marginal upwelling. water masses, 2) the complicated hydrography in East­ ern regions, including seasonal dif­ ferences caused by trade winds, etc., and 3) the filter Regional and geological comparisons effects mentioned above, several climatic assemblages could be identified: polar to temperate assemblages, Comparing sediments from coastal upwelling areas off together with subtropical and transitional assemblages Northwest Africa and off Southwest Africa (Table 66), and negatively tropical assemblages, illustrate cooler cool-water planktonic foraminifera assemblages are the Canary Current temperatures from Portugal to Cape most reliable indicators. Many other sedimentological Blanc. Special features between Cape Blanc and about and biological indicators are applicable if upwelling is 25°N point to additional upwelling effects. persistent and highly effective, as off Southwest Africa. Pflaumann (1975) used temperature-dependent as­ However, in other areas such as off Northwest Africa, semblages from Bé and Hamlin (1967) and Bé and Tol- these indicators are only convincing if many factors can derlund (1971) in surface sediment fauna around 16°N. be combined after careful examination of sedimenta­ He found low temperature assemblages in water mas­ tion regimes. At present it appears to be easier to ses around the shelf edge, again a possible indication of derive special features in sediments indicating upwell­ upwelling effects. ing from hydrographically known areas than to deduce Once again filters may disturb some results. Plank­ less dramatic upwelling areas from sediments only. tonic foraminifera living in cooler waters normally are Nevertheless the criteria mentioned in Table 66 thick shelled and therefore better equipped to escape could be applied in sediments from gravity cores and dissolution. from the Deep Sea Drilling Project off Northwest No detailed floral analysis of coccoliths exists from Africa. Table 67 illustrates that increased upwelling the Northwest African continental margin. One of the occurred at the continental margin in glacial periods of cool-water species, Coccolithus pelagicus, is concen­ the Pleistocene owing to lowered sea level and there­ trated in the coarse silt fraction (20-63 ^m) and fore a narrower shelf, together with possibly stronger enriched in surface sediments on the lower slope and wind and surface water circulation and upwelling with rise off Cape Barbas and Cape Blanc. Again it is dif­ approximately the same intensity as at present during ficult to discriminate between effects of the Canary interglacial periods. Tertiary sediments off Cape Current and true upwelling (Fütterer, 1977). Bojador revealed less intense upwelling during the Late Pliocene, but a peak during the Early Miocene.

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