Continental shelf upwelling and benthic Ostracoda in the Benguela System (Southeastern Atlantic Ocean)

ELSEVIER

Marine Geology 122(1995) 207-225

Continental shelf upwelling and benthic Ostracoda in the Benguela System (southeastern Atlantic Ocean) R.V. Dingle South African Museum,

P. 0, Box 61, Cape Town, South Africa1

Received 25 March 1994; revision accepted 5 September 1994

Abstract

The distribution of benthic Ostracoda (micro-crustacea) on the continental shelf off southwestern Africa is controlled by sea-floor physical and chemical parameters, which in turn can be correlated with the positions of quasi-permanent upwelling cells of the Benguela System. The linkage between benthic and surface physico-chemical environments (and consequently between benthos and surface parameters) is not direct, however, being modulated by equatorward motion in the upper water column, benthic poleward water-motion, fluvial input and cross-shelf advection of off-shelf water. These phenomena, and in particular the intensity of surface upwelling, show a sharp distinction about - 27”S, so that the ostracod faunas in the Northern Benguela Region are relatively sparse and dominated by Palmoconcha walvisbaiensis, (Hartmann) and Cytherella namibensis, Dingle, while in the Southern Benguela Region the faunas are more diverse and dominated by Pseudokeijella lepralioides, (Brady) and Ruggieria cytheropteroides, (Brady). When minor taxa are also considered, each cell in the Benguela upwelling system is shown to be associated with a unique ostracod fauna which can be related to its in particular environmental characteristics.

1. Introduction Upwelling of near-surface waters is a major component of the oceanic regime over the continental margin off southwestern Africa between Cape Town and the Kunene River (Fig. 1) (e.g. Hart and Currie, 1960; Bang, 1971; Nelson, 1985; Hay and Brock, 1992). A comprehensive synopsis of the large body of literature on this phenomenon allowed Shannon (1985) to present a conceptual image of the Benguela System in which he identified six main centres of oceanic upwelling, with the region off Luderitz forming a “major of cold, high turbulence environmental barrier” water. In a subsequent publication, Lutjeharms

’ Present address: British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 OET, UK. 002%3227/95/$9.500 1995Elsevier Science B.V. All rights reserved SSDZ 0025-3227(94)00098-O

and Meeuwis (1987) demonstrated the quasipermanent nature of the main upwelling cells using sea-surface temperatures (SST) measured by satellites, and identified seven cells between 15”s and the Cape Peninsula (34”s) (Fig. l), (only the six southern cells are considered herein). Previous work locally (e.g. Hart and Currie, 1960; Calvert and Price, 1983; Bremner, 1978; Rogers and Bremner, 1991), and in analogous situations elsewhere (e.g. off northwest Africa: Berger et al., 1978, Sarnthein et al., 1982; Peru: Schrader, 1992; and California: Calvert, 1966), indicates that regions affected by intense upwelling are characterized by sediments rich in a variety of organic and mineral tracers that can be used to suggest the presence of upwelling in palaeoenvironments (e.g. Diester-Haass, 1978; Siesser, 1980; Oberhansli, 199 1) .

R. V. Dingle/Marine

208

NORTHERN WALVIS

BAY

qENGUELA

REGION

25c

SOUTHERN

BENGUELA

Geology 122 ( 199.5) 207-225

and Dingle and Nelson (1993: physical oceanography). The ostracod faunas and their distribution have been related previously to a variety of sediment textural and geochemical parameters which were originally reported by Birch ( 1975), Rogers (1977) and Bremner (1978). The faunas were obtained from 270 continental shelf samples collected using a Van Veen grab. One hundred and twenty-three species of ostracod were isolated, and various analyses of their distribution have been presented by Dingle (1994) and Dingle and Giraudeau ( 1993) (see therein for details of analytical techniques). Averages and standard deviations of values for various sea-floor parameters under each surface cell are presented in Table 1: temperature, salinity, dissolved oxygen, total organic matter (MORG), calcium carbonate, percentage of elemental Fe in the < 63 km fraction (indicative of the terrigenous component, Bremner and Willis, 1993), sand, mud, opal, glauconite and apatite. Values for individual ostracod taxa have been calculated as a percentage of the total number of valves available for study from under each upwelling cell (Table 2).

3. Characteristics

Fig. 1. Upwelling cells of the Benguela system over the continental margin of southwestern Africa and inner shelf areas barren of benthic Ostracoda. Isobaths in kilometres. Based on Lutjeharms and Meeuwis ( 1987) and Dingle (1994).

Here, I discuss the distribution of continental shelf benthic Ostracoda (Crustacea), together with various environmental parameters, under the different upwelling cells of the Benguela System. The question to be investigated is whether upwelling cells with differing physical surface characteristics support significantly different benthic faunas. 2. Data The data sets used are those of Dingle (1992, 1993, 1994: Ostracoda, sediments, geochemistry)

of the Benguela upwelling cells

Lutjeharms and Meeuwis (1987) determined the extent of the upwelling cells by plotting all upwelling events over a three year period (198221985), and concluded that they consistently cluster around centres which “are closely circumscribed and show only limited north-south variation in

Table 1A Environmental Surface waters

Namib Walvis Luderitz Namaqua Columbine Peninsula

characteristics of Benguela upwelling (after Lutjeharms and Meeuwis, 1987)

Mean temp. ‘C

Wind stress mz sKz

Extension offshore km

“/o time

18.0 16.5 15.9 16.8 17.1 16.9

9000 11,000 16,000 14,000 9000 6500

140 240 280 200 230 130

25 30 90 65 45 10

cells,

Frequency

R. V. DinglelMarine Table 1B Environmental

Namibia Average Maximum Minimum SD Walvis Average Maximum Minimum SD Luderitz Average Maximum Minimum SD Namaqua Average Maximum Minimum SD Columbine Average Maximum Minimum SD Peninsula Average Maximum Minimum SD

characteristics

of Benguela

upwelling

cells. Sea-floor

Temperature

Salinity

10.3 14.0 3.9 2.9

34.94 35.35 34.51 0.25

1.2 2.1 0.6 0.4

6.2 11.3 0.3 2.6

11.0 14.0 5.5 2.5

35.00 35.29 34.41 0.26

1.0 2.1 0.4 0.6

9.1 13.5 3.0 2.6

34.71 35.14 34.39 0.24

10.0 11.8 8.9 0.8

Oxygen

Geology 122 (1995) 207-225

MORG

209

parameters

Fe

Apatite

Mud

Opal

4.0 4.9 2.0 0.5

63.6 98.1 19.2 20.9

31.3 80.8 1.5 21.8

0.2 4.0 0.0 0.9

52.2 75.7 0.4 20.6

0.1 0.7 0.0 0.2

3.4 17.2 0.4 4.7

9.2 20.7 3.8 4.4

1.7 4.9 0.6 0.8

51.4 90.1 10.5 26.6

44.5 89.5 6.5 28.5

26.7 84.0 0.0 35.7

40.8 83.5 1.3 32.8

0.3 4.7 0.0 0.9

5.4 48.8 0.5 9.0

1.8 3.0 0.8 0.6

6.3 14.4 1.1 3.5

3.7 7.6 0.9 1.8

55.1 79.0 22.2 19.2

37.6 77.8 10.0 23.7

0.3 4.0 0.0 1.0

50.7 71.1 10.2 21.3

0.5 1.2 0.0 0.4

12.2 45.9 0.8 13.2

34.82 34.92 34.71 0.05

2.4 3.5 1.5 0.6

3.1 5.9 0.2 1.6

6.2 11.8 3.0 3.3

57.2 87.0 0.7 26.5

42.0 99.1 13.0 26.5

0 0 0 0

42.5 89.3 0.5 32.7

2.3 14.0 0.0 3.3

2.9 11.1 0.3 3.0

8.3 9.4 5.5 0.9

34.70 34.92 34.51 0.10

3.1 4.1 1.5 0.9

3.9 9.9 1.6 2.5

5.5 9.7 2.2 2.3

64.3 84.8 2.2 18.5

35.3 97.1 15.2 18.6

0 0 0 0

29.6 80.3 2.4 26.8

5.3 24.0 0.0 7.5

2.6 11.9 0.6 2.6

8.9 9.7 7.5 0.6

34.63 34.69 34.59 0.03

3.6 4.0 2.3 0.3

1.9 5.0 0.7 1.6

5.2 7.0 3.0 1.1

90.0 97.7 74.0 8.4

10.1 25.7 3.2 8.2

0 0 0 0

15.0 33.5 2.0 10.3

46.0 69.0 1.0 28.0

10.0 15.0 1.1 5.7

Temperature: “C, salinity: apatite: %. SD = standard

%o, dissolved deviation.

oxygen:

ml/l, elemental

position”. Only six isolated events occurred outside the boundaries of the main upwelling cells within three years. Using averaged physical parameters, they also quantified Shannon’s (1985) observation that the principal upwelling centre of the Benguela System lies in the vicinity of Luderitz, and differentiated the Walvis and Luderitz cells on the basis of mean values for surface characters at 22”s and 25”s (Table 1A). Although the individual upwelling sites centred on these two latitudes merge into a single large area when plotted from satellite images (Fig. 1)) I have retained the distinction and have calculated average fauna1 and environmental parameters for separate Walvis and Luderitz cells.

Fe: % ~63 pm fraction,

sand,

CaC03

Glauconite

Sand

mud,

opal,

CaCO,,

glauconite

and

3.1. Sea-surface and sea-floor environmental parameters Four physical measurements were used by Lutjeharms and Meeuwis (1987) to characterise the quasi-permanent upwelling cells off the west coast: average wind stress, frequency of occurrence, offshore extent and low sea-surface temperatures (Table 1A). All four reach their extreme values in the Luderitz cell, which is, consequently, the most intense in the Benguela System. Using the same criteria, a semi-quantifiable measure of upwelling intensity for the other cells gives the following ranking: 1, Luderitz; 2, Namaqua; 3, Walvis; 4, Columbine; 5, Namib; 6, Peninsula

6 (32) 20

Number of barren sites (%I) Total number of species

1

16

1

48

12 (40) 31

82 30

11 21 1

1

I

24

6 1

21

5 4 2

4

6

Walvis

cells and inter-cell

3 3

86 19

Namib

species > 1% under upwelling

Amhostracon sp. 3556 Ambostracon_t7abellicostata Ambosfracon keeleri Ambostmcon levetzovi Argilloecia sp. 3483 Australoecia fulleri Bairdoppilata simplex Garciaella k. knysnaensis Garciaella k. robusta Buntonia namaquaensis Buntonia rogersi Buntonia rosenfeldi Chrysocythere craticula Coquimba birchi Cytherella dromedaria Cytherella namibensis Cytheropteron trinodosum Cytheropteron whatleyi Doratocythere exilis Henryhowella melobesioides Incongruellina venusta Krithe capensis Kuiperiana angulata Macrocypris cf. metuenda MunseyeNa eggerti Neocaudites lordi Neocaudites osseus Neocytherideis boomeri Palmoconcha walvisbaiensis Paracypris lacrimata Poseidonamicus panopsis Propontocypris (P) subreniJornris Pseudokeijella lepralioides Ruggieria cytheropteroides Urocythereis arcana Xestoleberis africana Cumulative percentage Number of sites

Cell Inter-cell

Table 2 Ostracod Percentage

10 (40) 23

84 25

4 26

1

8

10 2 3 2

23 1 2

2

Luderitz

areas.

3 (43) 11

95 I

5 58

6 14

I 1 1

1 1

Luderitz/ Namaqua

of ostracod

6 (23) 34

90 26

66 20 1

1

1

I

1

Namaqua

fauna

0 36

82 18

12 56

1

1

3

Namaqua/ Columbine

5 (25) 32

84 20

4 2 I I 59

1 4

1 11

1

2

I

Columbine

3 (18) 30

88 17

4

1

6

3 61

1 2 2

Columbine/ Peninsula

5 (28) 59

5 83 18

22 3

I

1 8

1

2

1

4 4

13

Peninsula

R. V. Dingle/Marine Geology 122 (1995) 207-225

(Table 1A; Fig. 2). There is thus a symmetry of upwelling intensity about the Luderitz cell, with progressive declines to the north and south. The explanation for this is that cells to the south are essentially summer phenomena, while those to the north are mainly active during the winter. The result is that the area immediately north of Luderitz, being the centre about which the temporal variability pivots, experiences year-round upwelling (Lutjeharms and Meeuwis, 1987). To make a comparison between the regional variations in surface and benthic conditions, a similar semiquantifiable measure is made for the extremes in the benthic environment, using the mean values of temperature, salinity, dissolved oxygen and MORG. The results are compared with sea-surface

WALVIS

data in Fig. 2, and the measure of benthic extremes shows a ranking significantly different from that at the sea-surface, with a symmetry about 22”s (Walvis cell) for the three northern cells (Namib, Walvis and Luderitz: Northern Benguela RegionNBR), and a progressive decrease in the three cells to the south (Namaqua, Columbine and Peninsula: Southern Benguela Region-SBR). The practical implications of Fig. 2 for palaeoenvironmental reconstructions are that the ostracod fauna which, for example, inhabits the seafloor with the lowest dissolved oxygen, and highest total organic matter, temperature and salinity values, will represent in the fossil record the facies deposited under only the third most-intense surface upwelling cell off southwestern Africa ( Walvis). In

LUDERllZ 1m

SI~SURFACE

211

NAMAQUA m

COLUMBINE

SEA-FLOOR

Fig. 2. Comparison of measures of intensity of environmental factors for the sea-surface and -bottom of Benguela System upwelling cells. The scale of intensity is a cumulative value of the ranking of various environmental extremes. The values are purely relative and the opposite polarities are merely for cartographic effect. The surface measures are Lutjeharms and Meeuwis’s (1987) values for temperature, wind stress, offshore extension, and frequency (calculated from Table 1A), while the benthic measures are Dingle’s (1994) values for temperature, salinity, dissolved oxygen and total organic matter (MORG) (calculated from Table 1B).

212

R. V Dingle/Mauine

mud

Geology 122 ( 1995) 207-225

a

R. cytheropteroides G.k.robusta/C.d

m m

Fig. 3. Benthic environmental and fauna1 characteristics of the Benguela system upwelling cells. (A) Average values of six sea-floor environmental parameters (from Table 1B). Left scale is for”C (temperature), %O (salinity, x 10 +34), ml/l (dissolved oxygen), and o/o (total organic matter=MORG, Fe). Right scale is % (mud). (B) Main fauna1 elements for each cell (from Table 2). G. k. robustu refers to the Namib and Walvis cells, while Cytherellu dromeduria (C.d) refers to the Peninsula cell.

C. namibensis A. keeleri

m

214

R. V. DingleJMarine Geology 122 (1995) 207-225

contrast, the fauna under the most-intense surface cell (Luderitz) inhabits environments that rank only third in terms of the severity of benthic characters within the Benguela System. Clearly, it seems to be important to recognise that the relationships between “intensity of upwelling” and benthic faunas may not be as straightforward as conventionally assumed. For example, it is commonly held that evidence of lowest values in dissolved oxygen in bottom waters can be equated with zones or periods of maximum upwelling (e.g. Oberhansli et al., 1990; Malmgren and Funnell, 1991), but as the time-averaged data from off Namibia shows, this is not necessarily the case. If the benthic dissolved oxygen map of Dingle and Nelson (1993) is compared with the upwelling intensity data of Lutjeharms and Meeuwis (1987), it is clear that the most intense cell (Luderitz, 25”s) does not coincide with the area of lowest oxygen values (22”-23”s). Sea-floor environments under the cells of the NBR are characterized by several factors: low dissolved oxygen (~2.0 ml/l), high total organic matter (>6.0%), the presence of opal (diatom frustules: 0.2-26.7%), and high temperature and salinity (>9.O”C and >34.7%0) (Table 1B; Fig. 3A). Also they have relatively high carbonate and mud (> 40% and > 30%) and low terrigenous values (~4.0% elemental Fe in the < 63 pm fraction). An important factor determining the composition of sediments in this region is the absence of perennial river discharge between the Kunene (175”S) and Orange (-28.5”s) rivers, and flash floods (e.g. in the Swakop River -30 km north of Walvis Bay) are considered to be of minor regional influence (Bremner, 1983). Averaged values of these parameters are significantly different under the SBR, so that, generally, the sea-floor south of the Luderitz cell is colder and less saline, and has higher terrigenous and lower mud and carbonate values. In particular, there are sharply higher dissolved oxygen and lower MORG values, while opal > 1% does not occur in sediments south of 25”s (Bremner, 1980; Rogers and Bremner, 1991). The generally low average bottom temperature and salinity values for the SBR reflect the advection of outer shelf-

upper slope Antarctic Intermediate Water (Dingle and Nelson, 1993). The N-S sea-floor parameter gradients under the upwell cells are disrupted under the inter-cells regions with sharp reversals. For example, dissolved oxygen values under all three inter-cell regions are higher than in adjacent cells, while there is a similar trend (with reversed polarity) in the MORG, and, south of the Namaqua cell, with and terrigenous temperature. Mud, carbonate components vary less consistently in a regional sense, but the NamaquaaColumbine inter-cell region stands out as carbonateand sand-rich, and, relatively, mud and terrigenous-componentpoor. 3.2. Linkage between upwelling and benthic dissolved oxygen and MORG Strong correlations between the MORG in sediments and dissolved oxygen in bottom waters (e.g. Dingle, 1994) raise the issue of the main factor controlling organic carbon in local continental shelf bottom sediments. Specifically, whether it is the dissolved oxygen content of the bottom waters (as proposed, for example by Demaison and Moore, 1980 and Paropkari et al., 1992), or the rate of primary productivity of plankton (e.g. Bremner, 1983; Pedersen and Calvert, 1990; Calvert and Pedersen, 1992). Bailey ( 1991) addressed this question, and concluded that off southwestern Africa there is a three-way linkage: high plankton production in the surface waters (indicated by concentration of chlorophyll a) sustains high organic carbon values in bottom sediments, the in situ oxidation of which produces dysoxic and anoxic shelf bottom-waters. Bailey, therefore, supported the view of Pedersen and Calvert (1990, p. 457), who stated that on the Walvis shelf “..the dysaerobic conditions in the water are not the cause of high carbon content in the sediments but are the consequence of high productivity in the surface waters.. .” Bailey ( 199 1) went further, however, in equating the extent of dysoxic bottom water with the distribution of organic-rich bottom sediments. Taken with the results of Brown et al (1991) [that spatially and temporally, high primary productivity in the

R. V DinglelMarine Geology 122 (1995) 207-225

Benguela System, as reflected by chlorophyll a concentrations in the euphotic zone (O-30 m), correlates with areas of strong surface upwelling], this indicates that intense upwelling is an important factor in dysoxia. Evidence of nutrient recycling from both sediments and suspensate under conditions of low dissolved oxygen (Calvert and Price, 1971; Bailey, 1987) also suggests that the zones of high productivity will be partially self-sustaining. An additional feature of surface upwelling cells is the production of benthic saline “hot spots” (Dingle and Nelson, 1993). These may result from periodic out-of-phase trapped coastal waves that inject relatively warm, saline surface water into the poleward benthic undercurrent, but whatever their dynamics, they result in localised increases in benthic temperature and salinity. Clearly, the effects of all these phenomena will be most pronounced in the area where upwelling is perennial (Luderitz cell), and less so to the north and south. However, since the Angola Basin is a primary source of advected oxygen-poor water (Hart and Currie, 1960; Stander, 1964; Bubnov, 1972), the effects of dysoxia will be greater under the seasonal northern cells than in the seasonal SBR cells, which are more influenced by advected, offshelf, Antarctic Intermediate Water. Overall, therefore, the combined effects of dysoxia, and high temperatures and salinities will be more pronounced in the NBR. While this body of evidence establishes a prima facie connection at the regional level between surface upwelling and primary productivity on the one hand, and benthic MORG, dissolved oxygen, temperature and salinity values on the other, it does not account for the out-of-phase relationships previously noted between the extreme surface and benthic environmental measures for individual cells (Fig. 2). For these to be explained, there has to be a decoupling of the surface and benthic processes, and in Fig. 4C it is suggested that this is achieved by virtue of the fact that plankton blooms in upwelled waters are generally carried northwards by the equatorward Benguela drift and will settle “upstream” of the main centres of upwelling (e.g. Bremner, 1983; Bailey, 1991), while movement of dysoxic bottom water is primarily southward in the poleward undercurrent (De

215

Decker, 1970; Nelson, 1989), which is also considered responsible for the southward, “downstream” deflection of coastal saline “hot spots” (Dingle and Nelson, 1993). Statistical corroboration of the decoupling is supported by plotting the latitudinal variations along the 150 m isobath in primary productivity, reflected in euphotic zone chlorophyll a concentrations, (regional mean values from Brown et al., 1991), against benthic dissolved oxygen values (Fig. 4B). A regression analysis on these data points gives an r2 of 0.5267, which is improved to 0.5939 by shifting the dissolved oxygen data points northward by 0.5” (ie simulating the southward drift of the bottom water). The effect of decoupling surface and benthic physicochemical parameters is further modified in the SBR by the influence of fluvial input (low salinity water and terrigenous detritus) to the inshore areas, and large-scale advection of cool, oxygenrich water onto and across the outer shelf. 3.3. Ostracodfaunas The substrate and bottom-water differences between the NBR and SBR have a corollary in their ostracod faunas, where species diversity is heavily skewed towards the SBR. Here, 70 species (which occur as > 1 valve) have been recorded, 39 of which are restricted to this area, while in contrast, a total of 37 species only are recorded from the NBR, a mere six of which do not extend their range farther south (Table 3). In addition, the average number of valves obtained per standard sediment sample (100 g) indicates that the abundance of individuals is much lower in the NBR (27) compared to the SBR (204), a conclusion that is reinforced by the fact that 31% of all samples from the NBR were barren of ostracods (from a total of 81 sites), compared to 19% from the SBR (n = 99). Consequently, because the original sampling grid was comprehensive and systematic (e.g. see Dingle, 1994, fig. 1), it can be concluded that the sea-floor environments of the SBR are, overall, more conducive to the maintenance of diverse assemblages and abundant ostracod faunas. Table 2 and Fig. 3B show the dominant assemblages and secondary species under the various

R. V, DinglelMarine Geology 122 (1995) 207-225

216

latitude

‘p

20

22

NORTHERN upwelling

I

BENGUELA Walvis

Namib

cells

I

26

26

24

I

I

32

30 SOUTHERN

REGION

-Namaqua

Luderitz

369

34

I

I

I

I

I

BENGUELA Columbine -

REGION Peninsula

A

PP

seasonal

seasonal

perennial

N

P

C

N

L

W

*es surface

sea floor relative

9L MORQ-

-

oxygen-poor Angolan basin waler

4

latitude

4

4

4

put

fluvial Input

I

I

I

1

I

I

I

I

I

16

20

22

24

26

28

30

32

34

I 38OS

Fig. 4. Relationships between benthic and surface phenomena under the upwelling cells of the Benguela system. (A) Variations of upper depth limits of key ostracod species with latitude: PW= Palmoconcha walvisbaiensis, CN = Cytherella namibensis, PL = Pseudokezjella lepralioides, RC= Ruggieria cytheropteroides. (B) Variations with latitude of surface primary productivity (as reflected by Chlorophyll a, mg/m3 concentrations) and benthic dissolved oxygen (ml/l) along the 150 m isobath (data from Brown et al 199 1, Dingle and Nelson 1993). The relationship between primary productivty and chlorophyll a in surface waters is best expressed by multiplicative model (y =axb) using Ch. a values integrated over 30 m in the euphotic zone (?=0.48) (Brown et al., 1991). (C) Decoupling mechanisms for surface primary productivity, sediment MORG and dysoxic bottom-water along the length of the Benguela system. It represents a conceptual pattern for the shelf between the coast and 400 m water depth. Stippled areas represent suspended organic matter (denser pattern equates with higher productivity). EC=equatorward Benguela Current, PUC= benthic poleward under current. Advected cool, outer-shelf water is more important under the Southern Benguela Region, while oxygenpoor Angola Basin water extends into the northern part of the Northern Benguela Region.

R. V. DinglelMarine Geology 122 (1995) 207-225 Table 3 Distribution

of ostracod

species within

Benguela

217

system NBR

SBR NamaquaColumbine

Northern Benguela Region only: *GarciaeNa knysnaensis robusta Cytheropteron cf. frewinae Neocaudites punctatus *Palmoconcha walvisbaiensis Palmoconcha walvisridgensis Urocythereis sp. 3567 Northern and Southern Benguela Region: *Ambostracon levetzovi Kuiperiana sp. 3320 *Ambostracon keeleri *Argilloecia sp. 3483 Buntonia bremneri *Buntonia namaquaensis *Buntonia rogersi *Cytherella namibensis *Cytheropteron trinodosum Krithe sp. 9 Krithe spatularis *Kuiperiana angulata Meridionalicythere petricola *Neocaudites lordi Paradoxostoma aff. luederitzensis *Propontocypris (P.) cf. subreniformis *Ambostracon flabellicostata Aurila kliei *Bairdoppilata simplex *Garcia&a knysnaensis knysnaensis *Buntonia rosenfeldi *Cytheropteron whatleyi *Henryhowella melobesioides *Incongruellina venusta *Krithe capensis *Macrocypris cf. metuenda *Paraeypris lacrimata *Pseudokeijella lepralioides *Ruggieria cytheropteroides * Urocythereis arcana *Xestoleberis africana

cells

Peninsula cell

+ + + + + + + + + + + + + + + + + + + + + + $ $ $ $ $ $

+ + + + + +

$

+

$ $ $ $ $ $ $ $

+ + + + + + + +

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

Southern Benguela Region only: Namaqua-Columbine cells only: *Ambostracon sp. 3556 Buntonia gibbera Caudites sp. 3329 Meridionalicythere sp 3.581 *Munseyella eggerti *Australoecia fulleri Krithe sp. 8 *Neocaudites osseus

+ + +

218

R. V. DinglelMarine

Geology 122 ( 1995) 207-225

Table 3 (continued) SBR

NBR

NamaquaColumbine

+ + + + + + + +

*Poseidonamicus panopsus Quadracythere sp. 3333 Stigmatocythere sp. 3479 Austroaurila rugosa *Chrysocythere craticula * Cytherella dromedaria *Doratocythere exilis *Neocytheridies hoomeri Peninsula cell only: Basslerites sp. 3444 Buntonia deM,eti Bythocytherr sp. 3349 Cvtherois? sp. 3538 Cytheropteron cuneatum Cytheropteron,frewinae Cytherura siesseri Falklandia sp. 3546 Kangarina mucronata Mutilus hensonmaddocksorut~3 Mutilus malloryi Palmoconcha subrhomboidea Paradoxostoma aK auritum Paradoxostoma griseum Parakrithella .simpsoni Semicytherura sp. 3382 Semicytherura sp. 3385 Xestoleberis capens& Xestoleberis hartmanni Xestoleheris ramosa

cells

Peninsula cell

+ + + + + + + +

+ + + +

*Ccxptimha birchi Hemicytherura petheri Xestoleheris sp. 3398 Cytherelloidea compuncta Kangarina sola Kangarina hendeyi Kangarina sp. 3439 Semicytherura sp. 3414 NBR = Northern Benguela Region, SBR = Southern Benguela Region. $ =extends south and east of Peninsula cell. Species represented by one valve only, and indeterminate genera have been omitted * S P ecies of > 1%.

upwelling cells. There is a strong contrast between the complexions of the ostracod populations of the NBR and SBR, both in the abundant and minor species, where certain of the latter appear to be preferential indicators of either upwelling

(~42

species)

or non-upwelling conditions. For example, Munseyella eggerti and Doratocytherr exilis prefer inter-cell areas, while Urocythereis arcana, Neocytherideis boomeri and Australoecia filleri more commonly occur under upwelling cells. A

R. V. DinglelMarine Geology 122 (1995) 207-225

particularly useful marker is the genus Buntonia, with B. namaquaensis and B. bremneri (and probably B. gibbera) avoiding areas of upwelling, while B. rogersi and B. deweti occur predominantly under upwelling cells. Plate I illustrates the more important taxa from the continental shelf of the Benguela System. Northern Benguela Region

The most distinctive feature of the ostracod fauna under the windward (inner-mid shelf) zones of NBR cells is the dominance of Palmoconcha walvisbaiensis and Cytherella namibensis, and the relatively large areas which are effectively barren of any ostracods (Figs. 1, 3,4A and 5) (see Basov, 1976, and Bremner, 1983, for similarly low abundances of benthic foraminifera). As mentioned above, this region has the lowest dissolved oxygen, and the highest MORG, temperature and salinity sea-floor values along the west coast continental shelf and the affinity of the two species for this particular habitat is shown by their correlation coefficients: P. walvisbaiensis (MORG, 0.7432, temperature, 0.5493), C. namibensis (oxygen, -0.7109, MORG, 0.5324) (Dingle, 1994). Limiting average parameters for ostracod occupancy in the inner shelf barren zone of the NBR are: temperature, 12.2”C (SD = 1.18); salinity, 35.1%0 (SD = 0.21); dissolved oxygen, 0.76 ml/l (SD=O.45); and MORG, 7.8% (SD=5.0), and P. walvisbaiensis represents the pioneer species in the habitable zone, having higher average temperature [12.5”C(SD=1.26)],salinity[35.14%0(SD=0.13)], and MORG [ 5.78% (SD =4.05)] and lower oxygen (0.87 ml/l [SD=O.37]) tolerances than any other west coast species. It is restricted to a narrow zone immediately seaward of the barren areas, and approaches the coast only where the shelf is not affected by quasi-permanent upwelling between the Namib and Walvis cells at Palgrave Point (Fig. 5). The general southerly decrease in temperature and salinity, and increase in dissolved oxygen in the NBR (Dingle and Nelson, 1983) coincides with the progressive decline in importance of P. walvisbaiensis, and the species suffers a sudden extinction at -27’S, where the 2 ml/l dissolved oxygen contour crosses the shelf (Fig. 4A and B). Cytherella namibensis co-exists at several sites

219

on the seaward edge of the P. walvisbaiensis zone, but prefers cooler water, lower MORG and somewhat higher oxygen values [9.35”C (SD=2.34), 4.22% (,SD=2.04), and 2.5 ml/l (SD= 1.32)]. Nevertheless, relative to the west coast ostracod faunas as a whole (and in a world-wide context), C. namibensis is a species tolerant of warm, organic-rich and oxygen-deficient environments. Fig. 5 suggests that critical values in limiting its distribution are MORG >7% and dissolved oxygen > 2 ml/l. Other, less-abundant species characteristic of the windward side of the NBR cells are Garciaella knysnaensis robusta and Neocaudites lordi (Plate I; Fig. 5). In the NBR, Ruggieria cytheropteroides is the most abundant and widely distributed of the cosmopolitan west coast species (Table 2; Fig. 3B). It is most strongly correlated with dissolved oxygen levels, and, overall, prefers low temperatures (average 9.l“C) and high oxygen values (3.4 ml/l) (Dingle, 1994). Variations in its distribution with depth and latitude (Fig. 4A) shows that in the NBR its upper depth limit (UDL) lies > 200 m on the outer shelf along the distal edges of the upwelling cells, limited by the 12°C and > 1.0 ml/l contours. South of 25‘S, however, where these two contours swing sharply across the shelf, the UDL of R. cytheropteroides encroaches onto the mid-shelf between 100 and 200 m (Fig. 4A). Consequently, while this species does occur relatively abundantly within the Walvis-Luderitz cells, it does so only under the leeward side of the NBR cells (where it is associated with Henryhowella melobesioides and Incongruellina venusta, Table 2), and is more typical of the areas of less intense upwelling farther south. In contrast, the relatively rare Buntonia rogersi, which also occurs on the leeward side of the NBR cells, prefers areas of upwelling. Southern Benguela Region

Of the major differences between NBR and SBR cells in averaged benthic parameters, those in MORG and oxygen deficiency are particularly large (Table 1; Fig. 3A), while a further important factor is the presence of three perennial rivers (Orange, Olifants and Berg). As a consequence,

220

PLATE I

R. Y. Dinglr/Marine

Geology 122 ( 1995) 207-225

R. V. Dinglelhfarine Geology 122 (1995) 207-225

the ostracod faunas of two species which have with dissolved oxygen in the Columbine cell) texture and terrigenous

the SBR are dominated by their strongest correlations (Ruggieria

cytheropteroides

and factors related to the component of the substrate (Pseudokeijella lepralioides in the Namaqua and Peninsula cells) (Fig. 3B; Dingle, 1994; Dingle and Giraudeau, 1993). Areas barren of ostracods on the inner shelf of the SBR correspond approximately with the terrigenous mud belt that stretches from the Orange River mouth to St Helena Bay (e.g. Rogers and Bremner, 1991) (Fig. 1). Here, limiting average parameters for ostracod occupancy are: temperature, 9.7”C (SD=O.87); salinity, 34.8%0 (SD= 0.06); dissolved oxygen, 2.15 ml/l (SD = 0.47); and MORG, 4.2% (SO = 3.2). Unlike the cells of the NBR, where there is a large degree of continuity of fauna1 complexion, each of the SBR cells has a distinct assemblage (Fig. 3B). Under the Namaqua cell, only two species individually account for >2% of the total fauna (cumulatively 86%, with 34 species in all: Table 2). Pseudokeijella lepralioides overwhelmingly dominates in an area which has the combination of lowest dissolved oxygen, and highest mud and terrigenous values in the SBR and of these, the relatively low oxygen value probably accounts for the subordinate position of Ruggieria cytheropteroides.

221

In contrast, under the Columbine cell the overall high oxygen and low temperature/salinity sea-floor values of the continental shelf off southern Namaqualand result in it being the only area where R. cytheropteroides is overwhelmingly abundant. However, a further five species (from a total of 32) individually account for >2% of the ostracod fauna (cumulatively 78%), which is a sharp increase in diversity compared to the Namaqua cell. The benthic characteristics of the Peninsula cell stand alone amongst the upwelling cells of the Benguela System. Ten species individually account for >2% of the total fauna, and although Pseudokeijella lepralioides is most abundant, it is closely followed by Ambostracon keeleri and Cytherella dromedaria (Fig. 3B). Sixty-one species have been recorded from this relatively small region (Table 3), where the bottom sediments have low mud, carbonate and MORG contents, but the highest authigenic component of the west coast. The bottom waters are relatively cold and oxygen rich, and no major rivers debouch into the area. Non-upwelling Region

areas of the Southern

Benguela

Higher average dissolved oxygen and lower MORG values under the inter-cell areas suggest that while the poleward undercurrent transports oxygen-poor water southwards from the NBR, sea-floor biochemical processes related to SBR

PLATE I Dominant and typical ostracod taxa from the continental Northern Benguela Region cells: 1. Palmoconcha walvisbaiensis (Hartmann, 1974). 2. Cytherella namibensis Dingle, 1992. 3. Garciaella knysnaensis robusta (Dingle, 1992). 4. Buntonia rogersi Dingle, 1993. 5. Neocaudites lordi Dingle, 1993. Southern Benguela Region cells: 6. Pseudokeijella lepralioides (Brady, 1880). 7. Cytherella dromedaria Brady, 1880. 8. Ruggieria cytheropteroides (Brady, 1880). 9. Ambostracon keeleri Dingle, 1992. Southern Benguela Region inter-cells: 10. Buntonia namaquaensis Dingle, 1993. 11, B. bremneri Dingle, 1993. 12. Munseyella eggerti Dingle, 1993. 13. Doratocythere exilis (Brady, 1880).

shelf under the cells and inter-cell

areas of the Benguela

system (see Table 2).

R. V. DingleJMarine

222

Geology 122 (1995) 207-225

2001 ALGRAVE

POINT

APE

:..

CROSS

:::: :[r3 .:.. WALVlS BAY

C.

namibensis

Fig. 5. Distribution of Pulrnoconcha walvishaiensis, Cytherella nurnibensi.~. Garciuella knysnaensis robusta (GKR) and Neocuudites lord (NL), and inner shelf areas barren of ostracods in the Northern Benguela Region.

upwelling cells enhance the degree of oxygen depletion. Fauna1 manifestations of these differences are most pronounced in the large increases in the importance of Ruggieriu cytheropteroides in the LuderitzzNamaqua and Namaqua-Columbine inter-cell areas, and the local importance of minor taxa in the inter-cell areas of the SBR (e.g.: Chrysocythere craticulu, Doratocythere exilis, Krithe cupensis, Table 2). 4. Discussion

and conclusions

Table 4 and Fig. 4 summarise the benthic fauna1 and environmental characters and processes of the

sea-floor under the upwelling cells off the west coast and emphasise the contrasts between the NBR and SBR. From Table 4 it is evident that each upwelling cell in the Benguela System is associated with a unique benthic ostracod fauna, and that this can be related to specific benthic environmental parameters. In summary: in the NBR south of 2O”S, where upwelling intensity (and primary production) is greatest, the faunas are adapted to conditions of high MORG and low dissolved oxygen (related to primary productivity) and high temperature and salinity (related to “downwelled” surface water): dominant Palmoconchu walvisbuiensis, Cytherella numibensis, and secondary Neocaudites lordi, Garciaella knysnuensis robustu. In contrast, under the SBR, lower primary productivity results in lower benthic MORG values which contribute to less extensive and significantly lower values of bottom water dysoxia. Consequently, a combination of advected cool, oxygen-rich outer shelf water and terrigenous input controls the ostracod assemblages, with Pseudokeijellu lepralioides dominant off the Orange River (Namaqua cell) and in the extreme south (Peninsula cell), while the influence of the advected water is strongest in the central Namaqua shelf (Columbine cell) where Ruggieriu cytheropteroides dominates. Fig. 4A graphically shows the across-shelf effects of these phenomena: the dramatic suppression of P. wulvisbuiensis at the southern end of the Luderitz cell, the progressive southward on-shelf advancement of R. cytheropteroides, and the coastward encroachment of P. leprulioides under each of the SBR cells. To return to the question posed at the beginning of this contribution. A linkage between ostracod faunas and benthic parameters can be established, but the relationship between faunas and seasurface conditions can be understood only once aspects affecting physico-chemical parameters (in particular the surface/bottom decoupling phenomenon illustrated in Fig. 4C) is appreciated. In other words, benthic environmental parameters (and hence ostracod distributions) in the Benguela System can be related to the intensity of surface upwelling via primary productivity and the formation of sinks for relatively warm, saline water, but

R. V. DingleJMarine Geology 122 (1995) 207-225 Table 4 Summary

of main components

of ostracod

faunas

and environmental

Cells

Namibia

Characteristic species

P. walvisbaiensis C. namibensis C. namibensis P. walvisbaiensis *R. cytheropteroides G. k. robusta G. k. robusta N. lordi N. lordi Bu. rogersi

Minor, or typical species

Temperature Oxygen MORG Fe Mud CaCO, Authigenics

High Low High Low High

Walvis

High Low High Low High

parameters

under

223

upwelling

cells

Luderitz

Namaqua

Columbine

Peninsula

C. namibensis P. walvisbaiensis *R. cytheropteroides I. venusta K. angulata

Ps. lepralioides R. cytheropteroides

R. cytheropteroides

Ps. lepralioides A. keeleri C. dromedaria Ne. boomeri X. africana N. osseus Co. birchi U. arcana

Ps. lepralioides PO.panopsus A.JEabellicostata

High Low High Low Low High High

High High

Low High

Low High Low

Low Low

Low Low High

* = leeward side of cells only. Abbreviations: N= Neocaudites; A = Ambostracon; Bu = Buntonia; C= Cytherella; Co = Coquimba; G= Garciaella; I= Incongruellina; K= Kuiperiana; Ne = Neocythereis; P = Palmoconcha; PO= Poseidonamicus; Ps = Pseudokeijella; R = Ruggieria; U= Vrocythereis; X= Xestoleberis. Temperature (“C), high: > 10, low: t9; oxygen (ml/l), high: >3, low: ~2; MORG (%), high: >6, low: ~2; Fe (%), high: >6, low: ~4; mud (“XI), high: >40, low: ~40; CaCO, (%), high: >50, low: <30; authigenics (%), high: > 10.

the effects of the surface phenomena are modulated by several factors: equatorward motion in the upper water column; benthic poleward motion; fluvial input; and cross-shelf advection of offshelf water. A final important conclusion is that none of the ostracods in the NBR appears to be capable of inhabiting truly anoxic environments. The lowest dissolved oxygen value from a site containing valves is 0.4 ml/l (Palmoconcha walvisbaiensis: range 0.4-1.6 ml/l), while the species with the narrowest range is Garciaella knysnaensis robusta (0.6-0.9 ml/l). Sen Gupta and Machain-Castillo (1993), who recently examined the distribution of benthic foraminifera in oxygen-poor environments (2.0-O ml/l), concluded that while the same species are also found at higher levels of dissolved oxygen, it is their ability to utilise the additional organic matter found in dysoxic conditions as a trophic resource that gives them an ecological advantage. Accordingly (Sen Gupta and Machain-Castillo, 1993, p. 195) there is “no modern species...whose mere presence can be taken to reflect a low-oxygen bottom water..” milieu. An analogy can be drawn

with P. walvisbaiensis and G. knysnaensis robusta in the NBR, and their success in inhabiting inhospitable mid-shelf areas implies that they are more adapted to the high-MORG, low-oxygen levels than Cytherella namibensis. This conflicts with the conclusion of Whatley ( 1991), who argued that platycopid ostracods have been more successful in these environments than podocopids because of a respiratory advantage inherited from their filterfeeding habit. The contradiction may reflect the misconception that in the geological record, contrary to the evidence of Pedersen and Calvert (1990), high organic-carbon values in sediments reflect benthic, low dissolved-oxygen habitats. An ability to cope metabolically with high organic carbon values seems more likely to hold the key to success in these environments than physiological adaptation.

Acknowledgements This work was funded by research grants the Foundation for Research Development,

from and

224

R. Y. DinglejMarine Geology 122 (1995) 207-225

the South African Museum, which I gratefully acknowledge. I thank Drs J. Rogers and J.M. Bremner (University of Cape Town), Dr W.W. Hay, Dr. H. Oberhgnsli and an anonymous referee for improving earlier drafts, Linda Bisset who made the photographic plate and Judy Woodford who drew several of the figures.

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