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Koun, A. J. 1960a. Ecological notes on Conus (Mollusca: Gastropoda) in the Trincomalee region of Ceylon. Annals and

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Koun, A. J. 19606. Spawning behaviour, egg masses and larval development in Conus from the Indian Ocean. Bulletin of

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ANNALS OF THE SOUTH AFRICAN MUSEUM

ANNALE VAN DIE SUID-AFRIKAANSE MUSEUM

Volume 103 °+#Band

March 1994 Maart

Part 7 Deel

QUATERNARY OSTRACODS FROM THE

CONTINENTAL MARGIN OFF SOUTH-WESTERN

AFRICA. PART HI. OCEANOGRAPHICAL AND

SEDIMENTARY ENVIRONMENTS

R. V. DINGLE

Cape Town Kaapstad

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D2700

QUATERNARY OSTRACODS FROM THE CONTINENTAL MARGIN OFF

SOUTH-WESTERN AFRICA. PART III.

OCEANOGRAPHICAL AND SEDIMENTARY ENVIRONMENTS

R. V. DINGLE

Micropalaeontology Research Unit, South African Museum, Cape Town

(With 34 figures and 7 tables)

[MS accepted 7 December 1992]

ABSTRACT

The distribution of benthic Ostracoda on the continental shelf and upper slope between Cape

Agulhas and the Kunene River is shown to be related to various time-averaged oceanographical and

sedimentary parameters. The microfossils represent mixed modern and relict assemblages, probably dating

from Recent to late Holocene time (c. 7000 yr Bp.). For each of the 36 most abundant species (> 95% of

the total ostracod assemblage, with 123 species) mean values for a range of environmental sea-floor

parameters have been calculated. These relate to water properties (temperature, salinity, dissolved oxygen)

and substrate characteristics (sand, mud, calcium carbonate, total organic matter, elemental Fe and

authigenic mineral contents). Correlation coefficients between these parameters and individual species

indicate which parameters are the most important in determining distributions.

On a regional scale, the various areas of the continental shelf are dominated by a particular species.

North of about 24°S, upwelling-induced low dissolved oxygen and high total organic matter (MORG)

values favour Cytherella namibensis (outer shelf) and Palmoconcha walvisbaiensis (inner to mid-shelf),

respectively. Farther south, the influence of advected, well-oxygenated Antarctic Intermediate Water on to

the uppermost slope and outer shelf controls the distribution of Ruggieria cytheropteroides, whereas on the

mid- and inner shelf, variations in mud and terrigenous components are the main controls for Pseudo-

keijella lepralioides and Bensonia knysnaensis knysnaensis, respectively. In water deeper than about 500 m,

the dominant species along the whole margin is Henryhowella melobesioides, whose distribution is

primarily controlled by temperature/salinity variations. (Closer inshore, mud content of bottom sediments

is more important.) For the other most abundant species, the main environmental controls are substrate-

dominated, with sand and calcium carbonate (mainly negative) the most important. Elemental Fe (which

is used as a gauge of the terrigenous component) is also important (both positively and negatively), with

total organic matter more frequently important than any of the bottom-water properties.

Barren areas (sparse or no ostracod faunas) occur in both shallow and deep water, and are associated

with the effects of upwelling (north of 27°S), fluvial terrigenous input (Namaqualand inshore area), and

isolation from sources of terrigenous and organic matter (either side of the Cape Canyon).

CONTENTS

PAGE

EMCO CUICELO Tse ers a ira eine oO GE EMCI ote eee ata 384

JSeGTPL IIS reisics Bl ees cuca ach oe certain ey ER eae ict Cf RM ToC Aa age 389

Bhysicaltoceanoprap hyn re vasiecnac ey oie eat ae eee 393

Bottommsedimentsiand seochemistnye mer ter eerie ia ae ea 406

BRO pUlatrony structure peer ee. oe eee eee eae to eRe ae eee 411

[SENSI RE NN) eH Ns cata roth o EEO O CULO Pee o Opa Ha b.OOe ceo ou oly ere 413

DY ISCUSSIOTUR eee der a ECE ie dart Maa eens 417

LLINTITTE Ey AGie erg Se 01k Dae ORE oS CAPA Cn aOR mon I Rar te) de DR a IANS hae ary 428

ACKHOWICASEIIEIS Myce eee Ceres n aR ee Ee ee: 431

FRCLETEMICES eet ayeae CRT Ra ate ese nce he OH Re RIO rnc AU NAL 432

Jo of {AA ONDC 1S Petre heehee ey Geet, ola eres Ce es AE RnR, COREE Ae tet EE Red 435

383

Ann. S. Afr. Mus. 103 (7), 1994: 383-441, 34 figs, 7 tables.

384 ANNALS OF THE SOUTH AFRICAN MUSEUM

INTRODUCTION

The taxonomy of the benthic Ostracoda from the continental shelf and upper slope

off south-western Africa has been documented in parts I and II of this study (Dingle 1992,

1993). These supplemented earlier localized accounts by Brady (1880), Miller (1908),

Klie (1940), Benson & Maddocks (1964) and Hartmann (1974). In the present paper,

aspects of the distribution of the whole fauna will be assessed in relation to various

environmental parameters of the bottom waters and sediments.

Ostracoda were isolated from 270 sea-floor sediment samples collected between Cape

Agulhas and the Kunene River in water depths between 15 m and 950 m (Fig. 1). A total

of 123 species, belonging to 54 genera, was recorded (Table 1). The sediment samples were

collected during the period 1967—1985 from the University of Cape Town’s R.V. ‘Thomas

B. Davie’ by personnel of the joint Geological Survey/University Marine Geoscience

Unit.

The regional oceanography off south-western Africa has been summarized by Hart

& Currie (1960), Stander (1964), Shannon (1966, 1985), Chapman & Shannon (1985),

Lutjeharms & Meeuwis (1987), Shannon & Hunter (1988) and Shannon et al. (1990),

amongst others.

Briefly, the essential elements consist of a three-layer deep-water configuration that

abuts the continental margin (Antarctic Bottom Water (AABW), North Atlantic Deep

Water (NADW) and Antarctic Intermediate Water (AAITW)) and a mixed layer on the

continental shelf (Fig. 2). The latter has several complexly related components, and is

subject to considerable variability. Surface waters for the most part emanate from the

South Atlantic gyre and move in a northerly direction, more or less parallel to the coast.

This is the main component of the Benguela Current, and strong wind stress over it

results in quasi-permanent regions of subsurface upwelling of varying intensity (e.g.

Lutjeharms & Meeuwis 1987). Other major features are the intrusion of sub-tropical

Angola Current water adjacent to the north coast, typically as far south as 18°S, and

periodic intrusions of vortices and filaments of warm Agulhas Current water around the

southern tip of the Agulhas Bank from the western part of the Agulhas Retroflexion (e.g.

Shannon et al. 1990). The latter typically extend no farther north than about 33°S,

although there has been considerable debate on their role in large-scale transfer of warm

South-Western Indian Ocean water into the central Atlantic (e.g. Gordon & Haxby

1990). Southward subsurface movement of shelf water has been documented by De

Decker (1970) and Nelson (1989) along most of the west coast, whereas north of 25°S

several authors have postulated a southward moving current just below the shelf break

that transfers oxygen-deficient water from the Angola Basin (Hart & Currie 1960;

Stander 1964; Chapman & Shannon 1985).

Sediment samples used in this study were collected using a Van Veen grab, which

typically penetrates 10cm beneath the sediment—water interface. Ostracod valves were

separated using standard washing and picking techniques, and faunas were examined

from > 125 nw size fractions.

No physical oceanographical measurements were collected from the sample sites but,

because of the mixed Recent—subrecent nature of the ostracod assemblages, this omission

is not critical to the study. Long-term mean values of parameters at each site were

obtained in two ways: by averaging bottom-water data in quarter-degree squares around

QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 385

WWALVIS BAY

LUDERITZ

Orange River

Fig. 1. Bathymetry of the continental margin off south-western Africa with

sediment sample sites indicated. e = ostracod-bearing, 0 = barren samples,

WR = Walvis Ridge, OB = Orange Banks, CB = Childs Bank, CC = Cape

Canyon.

386 ANNALS OF THE SOUTH AFRICAN MUSEUM

TABLE |

Species of Ostracoda recorded from the west-coast continental shelf. Species are

listed alphabetically.

Species Novos

specimens

* Ambostracon (A.) flabellicostata (Brady, 1880) 490

* Ambostracon (A.) keeleri Dingle, 1992 1 022

Ambostracon (A.) levetzovi (Klei, 1940) 19

Ambostracon sp. 3553 1

Ambostracon sp. 3571 2

Ambostracon (Patagonacythere) sp. 3556 14

Argilloecia sp. 3483 14

Aurila kliei Hartmann, 1974 44

* Australoecia fulleri Dingle, 1993 96

Australoecia sp. 3550 1

* Austroaurila rugosa Dingle, 1993 91

* Bairdoppilata simplex (Brady, 1880) 435

?Basslerites (Loculiconcha) sp. 3444 2

Bathycythere vanstraateni Sissingh, 1971 1

* Bensonia k. knysnaensis Benson & Maddocks, 1964 Shit

* Bensonia k. robusta Dingle, 1992 43

Bradleya cf. B. dictyon (Brady, 1880) 1

Bradleya (?Quasibradleya) sp. 3568 8

* Buntonia bremneri Dingle, 1993 79

* Buntonia deweti Dingle, 1993 8

* Buntonia gibbera Dingle, 1993 39

* Buntonia namaquaensis Dingle, 1993 37.

* Buntonia rogersi Dingle, 1993 46

* Buntonia rosenfeldi Dingle, Lord & Boomer, 1990 47

Buntonia sp. 3486 2

Bythocythere sp. 3349 7

Caudites sp. 3329 2)

* Chrysocythere craticula (Brady, 1880) 358

* Coquimba birchi Dingle, 1993 86

* Cytherella dromedaria Brady, 1880 702

* Cytherella namibensis Dingle, 1992 422

Cytherelloidea compuncta Dingle, 1993

?Cytherois sp. 3538

Cytheropteron cuneatum Dingle, 1993

Cytheropteron frewinae Dingle, 1993

Cytheropteron aff. C. frewinae Dingle, 1993

* Cytheropteron trinodosum Dingle, 1993

Cytheropteron whatleyi Dingle, 1993

Cytheropteron sp. 2878

Cytheropteron sp. 2881

Cytheropteron sp. 2882

Cytheropteron sp. 2902

Cytheropteron sp. 3406

Cytherura siesseri Dingle, 1993

* Doratocythere exilis (Brady, 1880)

Doratocythere sp. 3584

?Falklandia sp. 3546

Hemicytherura petheri Dingle, 1993

Hemicytherura sp. 3393

?Hemicytherura sp. 3404

NR DR NORK KH WN REDD RRP RR NOMNeE MB YE

* Henryhowella melobesioides (Brady, 1869) 42

* Incongruellina venusta Dingle, 1993 9

Kangarina hendeyi Dingle, 1993

Kangarina mucronata (Brady, 1880) 3

Kangarina sola Dingle, 1993

Kangarina? sp. 3439

* Krithe capensis Dingle, Lord & Boomer, 1990 143

* Krithe spatularis Dingle, Lord & Boomer, 1990 12

Krithe sp. 8 Dingle, Lord & Boomer, 1990 11

Krithe sp. 9 Dingle, Lord & Boomer, 1990 12

Kuiperiana angulata Dingle, 1992 62

?Kuiperiana sp. 3320 2

QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 387

TABLE | (cont.)

. No. of

SISSIES specimens

Macrocypria sp. 3471 >)

* Macrocypris cf. M. metuenda Maddocks, 1990 102

Meridionalicythere petricola (Hartmann, 1974) 13

?Meridionalicythere sp. 3581 4

Munseyella eggerti Dingle, 1993 36

Mutilus bensonmaddocksorum Hartmann, 1974 2

Mutilus malloryi Dingle, 1993 17

* Neocaudites lordi Dingle, 1993 25

* Neocaudites osseus Dingle, 1993 142

Neocaudites punctatus Dingle, 1993 7,

* Neocytherideis boomeri Dingle, 1992 511

Palmoconcha subrhomboidea (Brady, 1880) 39

* Palmoconcha walvisbaiensis (Hartmann, 1974) 475

?Palmoconcha walvisridgensis Dingle, 1992 >)

* Paracypris lacrimata Dingle, 1992 533

Paracytheridea sp. 3339 1

Paradoxostoma griseum Klie, 1940 4

Paradoxostoma aff. P. auritum Klie, 1940 23

Paradoxostoma aff. P. luederitzensis Hartmann, 1974 16

Parakrithella simpsoni Dingle, 1993 68

?Parakrithella sp. 3468 2

* Poseidonamicus panopsus Whatley & Dingle, 1989 Ay

Propontocypris cf. P. (P.) subreniformis (Brady, 1880) 66

Propontocypris (?P.) sp. 3345 2

Propontocypris (?Ekpontocypris) sp. 3434 1

Propontocypris (?Schedopontocypris) sp. 3535 i

* Pseudokeijella lepralioides (Brady, 1880) 8 181

?Quadracythere sp. 3333 12

* Ruggieria cytheropteroides (Brady, 1880) 5 298

Semicytherura clausi (Brady, 1880) |

Semicytherura sp. 3379 I

Semicytherura sp. 3382 4

Semicytherura sp. 3385 5

Semicytherura sp. 3414 4

Stigmatocythere sp. 3479 a

Trachyleberis sp. 3586 il

* Urocythereis arcana Dingle, 1993 166

?Urocythereis sp. 3310 2

?Urocythereis sp. 3472 1

?Urocythereis sp. 3567 2

?Urocythereis sp. 3570 1

* Xestoleberis africana Brady, 1880 500

Xestoleberis capensis Miller, 1908

* Xestoleberis hartmanni Dingle, 1992

Xestoleberis ramosa Miller, 1908

Xestoleberis sp. 3398

Xestoleberis sp. 3524

Indet. sp. 3306

Indet. sp. 3308

Indet. sp. 3343

Indet. sp. 3412

Indet. sp. 3426

Indet. sp. 3429

Indet. sp. 3447

Indet. sp. 3481

Indet. sp. 3539

Indet. sp. 3543

Indet. sp. 3568

Indet. sp. 3574

Indet. sp. 3576

Indet. sp. 3578

ie)

i N

me SND SE OO RR eS SE ELD OD

*—thirty-six most-abundant species; these account for more than 95 per cent

of total population, and have been used for most of the statistical analyses.

388

ANNALS OF THE SOUTH AFRICAN MUSEUM

Angola Current

} Walvis Bay

Luderitz

Warm water

} Cape Town

N f

Main centres —_

A ee \

of upwelling — OE !) ‘\C. Agulhas

Shelf break

Benquela flow line

Axis of Walvis Ridge Agulhas

Retroflexion

Fig. 2. Main oceanographical elements in relation to the position of the

continental shelf edge off south-western Africa. Based on Shannon (1985),

Lutjeharms & Meeuwis (1987) and Lutjeharms (1989). AAIW = Antarctic

Intermediate Water, NADW = North Atlantic Deep Water, AABW = Ant-

arctic Bottom Water, SMZ = salinity minimum zone, CCD = carbonate

compensation depth, ¢ = southward motion, 6 = northward motion.

QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 389

sample sites, and by reading values at sites from regional maps constructed specifically for

the purpose.

Physical oceanographical data were obtained from the South African Data Centre

for Oceanography (SADCO), for temperature and salinity, and Sea Fisheries Research

Institute, for dissolved oxygen. Dingle & Nelson (1993) provided a preliminary account

of the bottom temperature, salinity and dissolved oxygen distributions, as well as details

of the data processing and reliability. Briefly, this consisted of screening the 30000

SADCO records to obtain 2 869 temperature and salinity readings. To construct regional

maps, these measurements were averaged in 391 quarter-degree rectangles over the west-

coast continental margin. A similar technique was used to produce | 314 bottom-water

dissolved-oxygen values, which were combined with results from the survey of De Decker

(1970). ;

The texture and geochemistry of sea-floor sediments on the margin between the

Kunene River and Cape Agulhas have been analysed in three doctoral theses by Birch

(1975), Rogers (1977) and Bremner (1981). These workers used the same set of samples as

in the present study. Their results have been summarized and refined in Birch et al.

(1986), Bremner er a/. (1986) and Rogers & Bremner (1991). Additional analytical details

of the sedimentary geochemistry off Namibia have been presented by Bremner (1980,

1983) and Bremner & Willis (1993).

Geochemical and textural data for each site utilized in the present study were

extracted from these publications, either as analyses of specific sediment samples or

extractions from regional contoured maps. Reference should be made to Birch (1975),

Rogers (1977) and Bremner (1981) for details of analytical techniques. Elemental analyses

were performed on the < 63 y fractions of sediments, which Bremner & Willis (1993)

have shown provide a good estimation of overall sediment geochemistry.

RESULTS

Descriptive statistics (means, standard deviations and ranges) and Pearson product-

moment correlation coefficient analyses have been performed on the 36 most abundant

species of ostracods for a variety of environmentally relevant parameters (Table 2). The

latter relate to the physical oceanography (bottom-water temperature, salinity and

dissolved oxygen, water depth and geographic latitude) and nature of the bottom

sediments (organic matter, texture and elemental geochemistry). The most abundant

species account for 95.47 per cent of the total available ostracod fauna, and are illustrated

in Figures 3—S.

These results allow me to supplement the distributional data presented in Parts I and

II (Dingle 1992, 1993). The numerical data presented in the Appendix comprise what I

believe to be a unique published compilation of environmental information for a modern

ostracod fauna from such a large area of continental shelf (approximately 420 000 km/7).

The correlation coefficients are used to supplement and highlight relationships

between and within elements of the fauna. It should be remembered that strong corre-

lation coefficients indicate which species are most strongly influenced (positively or

negatively) by changes in the parameters and will not necessarily be those that have the

highest (or lowest) mean values. In this sense, the correlation coefficient is a measure of

ANNALS OF THE SOUTH AFRICAN MUSEUM

390

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QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 39]

Fig. 3. Most abundant ostracod species on the continental shelf off south-western Africa arranged in

order of latitudinal centre of distribution (mean of all observed sites, see Figure 6). Vertical bars are

degrees of latitude (S); horizontal scales = 100 wu. A = Palmoconcha walvisbaiensis, B = Bensonia k.

robusta, C = Cytherella namibensis, D = Neocaudites lordi, E = Incongruellina venusta, F = Buntonia

rogersi, G = Krithe spatularis, H = Cytheropteron whatleyi, | = Bensonia k. knysnaensis, J = Buntonia

rosenfeldi, K = Cytheropteron trinodosum, L = Ambostracon flabellicostata, M = Ruggieria cytheropter-

oides, N = Buntonia gibbera, O = Buntonia namaquaensis.

ANNALS OF THE SOUTH AFRICAN MUSEUM

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Fig. 4. The most abundant ostracod species on the continental shelf off south-western Africa arranged in

order of latitudinal centre of distribution (mean of all observed sites, see Figure 6). Vertical bars are

degrees of latitude (S); horizontal scales = 100 u. A = Pseudokeijella lepralioides, B = Urocythereis

arcana, C = Ambostracon keeleri, D = Krithe capensis, E = Poseidonamicus panopsus, F = Buntonia

bremneri, G = Henryhowella melobesioides, H = Doratocythere exilis, | = Paracypris lacrimata,

J = Chrysocythere craticula, K = Xestoleberis africana, L = Bairdoppilata simplex, M = Macrocypris

cf. M. metuenda, N = Neocytherideis boomeri, O = Austroaurila rugosa.

QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 393

Fig. 5. The most abundant ostracod species on the continental shelf off south-western Africa arranged in

order of latitudinal centre of distribution (mean of all observed sites, see Figure 6). Vertical bars are

degrees of latitude (S); horizontal scales = 100 uw. A = Australoecia fulleri, B = Cytherella dromedaria,

C = Neocaudites osseus, D = Xestoleberis hartmanni, E = Buntonia deweti, F = Coquimba birchi.

the sensitivity of the species to change in the parameter. In addition, correlation

coefficients based on simple regression analyses are presented to show the relationships

between the environmental variables (Table 3).

To aid reliability, descriptive statistics and correlation coefficients were performed

only on samples containing > 100 valves (n = 45). Exceptions to this standard were

regional latitudinal and depth distributions, and averages for environmental parameters

for the following species, whose ranges into deeper water precluded its use: Krithe

capensis, K. spatularis, Buntonia rosenfeldi and Henryhowella melobesioides.

PHYSICAL OCEANOGRAPHY

Latitudinal and depth distribution

Figure 6 shows the total and averaged north—south distribution of the most abun-

dant species. Most species (19; 53%) have their northern limits straddling the Walvis

Ridge, whereas others occur in the vicinity of Walvis Bay (7), Orange River (7) and the

Cape Peninsula (3). In contrast, 33 species (92%) have their southern limits south of the

Cape Peninsula.

The averaged position for each species is an indication of its centre of distribution

(based on the number of observed sites). With the exception of three species, these all lie

south of 27°S (Liideritz), and only Bensonia knysnaensis robusta and Palmoconcha walvis-

baiensis have their centres of distribution north of 23°S (Walvis Bay). Figures 3—5

illustrate each of the most abundant species, arranged in order of their southward

latitudinal distribution.

ANNALS OF THE SOUTH AFRICAN MUSEUM

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QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 395

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Fig. 6. Mean and ranges of the maximum latitudinal distribution of the most abundant species. Vertical

scale is degrees of latitude (S). WR = Walvis Ridge, WB = Walvis Bay, LUD = Liideritz,

OR = Orange River, CC = Cape Columbine, CP = Cape Peninsula. The mean values are calculated

on the number of sample sites; thus they represent weighted centres of distribution and not average

positions between northern and southern limits.

Figure 7 shows the total and averaged across-shelf distribution of the most abundant

species. With the exception of two species (Krithe spatularis and K. capensis), all the most

abundant species have their upper depth limits (UDL) shallower than 200 m (i.e. in the

inner—mid-shelf area), whereas, with the exception of three species (the least abundant of

this category), they all have their LDL deeper than 200 m. The curve of averaged depth

distributions has gradient changes separating two shelf faunas (at 250 m, I and II), and

upper and mid-slope faunas (350 m, III, and 450 m, IV).

Figures 6 and 7 indicate that, with few exceptions (Coquimba birchi, Buntonia deweti,

B. gibbera, Bensonia k. robusta and Xestoleberis africana), the most abundant species are

relatively cosmopolitan in their distribution along and across the shelf (unlike many of

the rarer taxa).

Regional variations in the abundances of several of the most abundant species were

briefly considered by Dingle (1992), who presented along-shelf variations of the dominant

396 ANNALS OF THE SOUTH AFRICAN MUSEUM

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Fig. 7. Mean and ranges of maximum and minimum depth distribution of the most abundant species and

barren sites. UDL = upper depth limit, LDL = lower depth limit. The mean values are calculated on the

number of sample sites; thus they represent weighted centres of distribution and not average positions

between upper and lower limits. I-IV on the upper border demarcate species grouped between gradient

changes in the curve of mean values.

taxa within various latitudinal sectors. A more comprehensive analysis has been carried

out, and is summarized in Figures 8 and 9. These represent projections of abundance

values (as smoothed percentages of the total fauna) on to across-shelf (depth), and along-

shelf (latitudinal) axes, respectively. A plan of the distribution of dominant taxa (> 20%

total fauna) on the shelf and slope (Fig. 10) was constructed using Figures 8 and 9, and

additional depth/abundance profiles computed at intervals of 5° latitude. A simple calcu-

lation of regional dominance gives the following abundances in order of rank: areas north

of 24°S— Palmoconcha walvisbaiensis = 32 per cent, Cytherella namibensis = 21 per

cent; south of 24°S — Pseudokeijella lepralioides = 36 per cent, Ruggieria cytheropter-

oides = 22 per cent, Bensonia knysnaensis knysnaensis = 6 per cent; in water > 500 m —

Henryhowella melobesioides = 43 per cent.

The inner—outer-shelf region (0—300m) is dominated by three species (Fig. 10).

North of 23°S, Palmoconcha walvisbaiensis occurs on its own but, south of 25°S, is

replaced, respectively, by Bensonia k. knysnaensis on the inner shelf and Pseudokeijella

, QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 397

inner — outer shelf — mid —- upper slope

mid-shelf uppermost

Per cent

" Gh

LAL \N A

LE NNA

WSS

70 iN NS a Ah , ——

Depth (km)

Palmoconcha walvisbaiensis

Cytherella namibensis

Bensonia k. knysnaensis

Pseudokeijella lepralioides

Ruggieria cytheropteroides

Henryhowella melobesioides

Fig. 8. Variation of species dominance with depth across the continental margin. Constructed by

projecting all data points on to a single axis, and smoothing each curve with a five-point running mean.

lepralioides on the mid—outer shelf. Immediately south of Walvis Bay, there is a mixed

assemblage containing Palmoconcha walvisbaiensis and Bensonia k. knysnaensis. A further

mixed zone occurs between c. 31.5° and 34°S, where the two dominant taxa are ‘diluted’

by the relatively diverse and abundant faunas off the south-western Cape (which contain

many of the rarer taxa described by Dingle 1993).

Outer-shelf and uppermost-slope areas are dominated by two species: Cytherella

namibensis in the north and Ruggieria cytheropteroides in the south. Upper and mid-slope

areas are dominated by Henryhowella melobesioides, with a narrow mixed zone contain-

ing abundant Krithe (mainly K. capensis) and the deeper-water species of Buntonia

(B. rosenfeldi, B.bremneri and B.namaquaensis) intervening between the Cytherella

namibensis— Ruggieria cytheropteroides upper-slope assemblage and the Henryhowella

melobesioides upper—mid-slope assemblage.

All three inner—outer-shelf dominant species typically constitute 40—50 per cent of

the local populations; projecting their abundances on to a cross-shelf axis (Fig. 8) empha-

sizes that each taxon reaches its individual maximum at different depths: Bensonia k.

knysnaensis, 50 m; Palmoconcha walvisbaiensis, 80-110 m; and Pseudokeijella lepralioides,

398 ANNALS OF THE SOUTH AFRICAN MUSEUM

Central

Namib

“lassociation

SW Cape

association

Per cent

Palmoconcha walvisbaiensis

Cytherella namibensis

Bensonia k. knysnaensis

Pseudokeijella lepralioides

Ruggieria cytheropteroides

Fig. 9. Variation of species dominance, expressed as a percentage of total ostracod population, with

latitude. Constructed by projecting all data points on to a single axis, and smoothing each curve with a

five-point running mean. WR = Walvis Ridge, WB = Walvis Bay, LUD = Liideritz, OR = Orange

River, CC = Cape Columbine, CP = Cape Peninsula.

130—180 m. In contrast, a similar degree of dominance on the outer-shelf and uppermost

slope is only reached south of 25°S (Ruggieria cytheropteroides), whereas north of Walvis

Bay, Cytherella namibensis constitutes only 20—30 per cent, with other taxa being rela-

tively more important. Below a transitional zone (450—550m), Henryhowella melo-

besioides progressively increases its dominance, reaching > 70 per cent in water deeper

than 900 m. Its eventual maximum (> 80%) occurs at 1 200 m on the middle slope, before

rapidly declining below 1 500m (Dingle et a/. 1989, 1990; Dingle & Lord 1990).

Finally, summaries of regional simple population diversity (expressed as number of

species/sample) show a preponderance of inner—mid-shelf species south of the Orange

River (Figs 11, 12). Latitudinally, there is a progressive increase in population diversity

from < 10 species north of the Walvis Ridge to > 50 species off the south-western Cape

(Fig. 11). The increase in numbers is particularly high across the Walvis Ridge and in the

vicinity of Walvis Bay, whereas between the latter and the Orange River, there is a

plateau (33 species). A maximum is reached off the southern Namaqualand coast

(45 species), south of which the diversity decreases slightly, reaching a low at 33°S (Cape

QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 399

Central Namib SW Cape

Association Association

ORANGE R.

LUDERITZ | C. COLUMBINE

25° 30

WALVIS C. PENINSULA

WALVIS RIDGE BAY

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Palmoconcha walvisbaiensis

Cytherella namibensis

Bensonia k. knysnaensis

Pseudokeijella lepralioides

Fig. 10. Latitude—depth plan of areas dominated by various species (> 20%). Constructed using profiles

similar to Figure 8 at 5° intervals of latitude.

Columbine: 40 species), before rising very rapidly in the vicinity of the Cape Peninsula

(55 species).

Across the shelf, maximum diversity (40 species) occurs between 160m and 200m

(Fig. 12). There is a rapid increase from the inner shelf, with a subsidiary maximum

(33 species) at 100 m, and an equally rapid decline into water between 200 m and 300 m. A

diversity plateau (24 species) extends to 500 m, below which there are two further declines

in species numbers (530m and 710m) to 10 species between 800 m and 900 m.

Temperature and salinity

The correlation coefficient between temperature and salinity is high (R = 0.8960;

Fig. 13A) at all 270 continental-shelf sites, so that these two parameters vary sympath-

etically. The correlation between temperature and dissolved oxygen in the bottom waters

is lower (R = —0.7432; Fig. 13B), whereas with other parameters (e.g. CaCO3 and

organic matter MORG) it is < 0.5000 (see Table 3).

The distribution of temperature and salinity preferences has three well-defined

categories (Figs 14, 15). Two species prefer high temperature and high salinity (> 11°C,

> 34.90%.) — Palmoconcha walvisbaiensis and Bensonia k. robusta — and, in both cases,

their means are markedly different from those of other species.

400 ANNALS OF THE SOUTH AFRICAN MUSEUM

s ° i

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oi = a

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Fig. 11. Variation of simple species diversity of total fauna with latitude. The complete latitudinal range

of each species has been used and the assumption is that the species occurs at all sample sites between these

limits. The curve is a five-point running mean through the sample sites plotted on to a single N—S axis.

Horizontal axis is in degrees of latitude. W. Ridge = Walvis Ridge, WB = Walvis Bay, LUD =

Liideritz, OR = Orange River, CC = Cape Columbine, CP = Cape Peninsula.

T T T T T T T T

mid — upper slope

Number of species

outer shelf

mid-shelf uppermost

slope

0.5

Depth (km)

Fig. 12. Variation of simple species diversity of total fauna with depth. The complete depth range of each

species has been used and the assumption is that the species occurs at all sample sites between these limits.

The curve is a five-point running mean through the sample sites plotted on to a single E—W axis.

401

QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA

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Best-fit regression curves for sea-floor parameters (using all sample sites, n

A. Temperature against salinity (linear,

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(exponential, R = —0.74323). Inner dashed lines

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(0%) Ayues

Fig. 14. Mean and standard deviation (SD) of sea-floor temperature for species at each site containing

> 100 specimens.

402 ANNALS OF THE SOUTH AFRICAN MUSEUM

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Fig. 15. Mean and standard deviation (SD) of sea-floor salinity for species at each site containing

> 100 specimens.

At low temperatures/salinities, three species have means below 7°C and 34.6%bo:

Krithe spatularis, K. capensis and Buntonia rosenfeldi, with Henryhowella melobesioides

closely associated with this group.

The remainder of the most abundant species fall within the following ranges of mean

temperatures and salinities: 10.17—8.73°C, and 34.89—34.70%o, respectively.

Correlation coefficients between the various species, and temperature and salinity are

shown in Table 2. Palmoconcha walvisbaiensis correlates most positively with temperature

and Buntonia gibbera with salinity, whereas Ambostracon flabellicostata correlates

strongly with both. Four species correlate negatively, with Buntonia namaquaensis

returning the largest values for both parameters.

Abundance and species diversity trends of the whole ostracod population (i.e. most

abundant species plus rarer species in all samples) with temperature and salinity are

shown in Figure 16.

QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 403

600

TEMPERATURE

400- ra

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ne}

200-

SALINITY sf

@®

72)

34.4 34.6 34.8 35.0 35.2 35.4 %o

Fig. 16. Relationship of diversity (top panel, light curve: number of species) and

abundance (bottom panel, dark curve: number of valves/100 g sediment) for whole

ostracod assemblage. A. Sea-floor temperature. B. Sea-floor salinity.

Maxima in both abundance and diversity lie within the temperature range 7°—10°C

and salinity range 34.6%0—34.9%o. There are further minor peaks in the high temperature/

high salinity areas of the graphs, although these are not in phase, implying that at the

higher values, one or other of the factors is dominant in determining the distribution

patterns.

Dissolved oxygen

The distribution of species’ means for sea-bottom dissolved oxygen is skewed

towards a preference for high values (Fig. 17), although it must be remembered that,

404 ANNALS OF THE SOUTH AFRICAN MUSEUM

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(o)

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SS SRST SR ES STS SSS SS s esses eysasessesee

ES Ow 4235 QEGFCQCsaesogssxprsg RS) =

SSSSFBSZFICSCSEREKGSS GRE sc oegge sos ggHgdgcg ~gasgs

SSeS o 9 & Se Qa SCe ss ggssseaguaasa

eget Seo er gl SO 2a © Sia S45 S18 Sa Oo Soe teceNpORseecEG

SSF®a "FoF GESGHRASHSSEG OT Bets 4 > SS Go Gl Sion

SSeS Cy ae ee SF SSgwHstcecRaeaSsesxsagss LC.

3 = 2 < Ea y,re®PeEvorserotocal ds Cees > x

SSE SsSHSO>G4Y¥y SEES EOQRSCsspFrsr~sFsass aé& 2

SE®stSntseseasssssesgEes SFVe FL wa eSSsNAe oes Bo

x E See = SOS) Seo SiS a S78 fe oo ats SY SE THEO CLT c 2'E

= = = or Sao eS SO ® ror 5 4 3x is} c

LHsHS SSeS eos esse sa“ SSSSSLSES SA a SS$su v8

ay ~ —

Gy, Ia cr, as oO [s) HimQagxeers oO (d= < of

SfIsF FS Ha Q= 2a SeesssSsSs 5 ceca

Sra) Sig AO PS Xm SF FRD 2 ~) 26 iS S

<> 2 ro) x ote ed = s2) 5 =} iS)

=8 = rm) ) BeOS § Q = g

S S

5S a ras) es S =

ts <7 a

Fig. 17. Mean and standard deviation (SD) of sea-floor dissolved oxygen for species at each site

containing > 100 specimens.

according to the terminology of Chapman & Shannon (1985), the whole of the west-coast

continental shelf falls within the category ‘oxygen-depleted’ (< 5 ml/l). Correlation

coefficients between dissolved oxygen and other water parameters are greatest between

salinity (—0.8130) and temperature (— 0.7332), whereas between oxygen and sediment

parameters, the closest links are with organic matter (— 0.5795), glauconite (0.4525) and

CaCO; (—0.3471) (Table 3).

There are four gradient changes in the mean oxygen curve (Fig. 17), isolating five

unequally-sized groups of species. These occur at 3.4, 3.1, 2.8 and < 2.5 ml/l, with the

bulk (21; 58%) plotting above > 3.4 ml/l, where Krithe spatularis, Henryhowella melobes-

ioides and Macrocypris cf. M. metuenda occupy the top three rankings. Only two species

have a preference for oxygen-deficient water (< 2 ml/l): Palmoconcha walvisbaiensis and

Bensonia k. robusta. Their mean values (< 1 ml/l) are markedly lower than the next

lowest groups, in which only two fall below 3.0 ml/l (Buntonia namaquaensis and

QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 405

800

| OXYGEN

600 5

Diversity

Abundance

ORGANIC MATTER

Diversity

200

Abundance

0 5 10 15 20

Per cent

Fig. 18. Relationship of diversity (top panel, light curve: number of species) and

abundance (bottom panel, dark curve: number of valves/100 g sediment) for whole

ostracod assemblage. A. Sea-floor dissolved oxygen. B. Organic matter in sea-floor

sediments.

Neocaudites lordi). Cytherella namibensis, Bensonia k. knysnaensis and Buntonia rogersi

constitute a further group clearly able to tolerate a degree of oxygen depletion.

Correlation coefficients between species and dissolved oxygen are listed in Table 2.

Two taxa strongly correlate with fluctuations in this parameter: Cytherella namibensis

(negatively) and Buntonia namaquaensis (positively). A further three species (Palmo-

concha walvishaiensis, Bensonia k. knysnaensis and Neocaudites lordi) also correlate nega-

tively with dissolved oxygen values.

406 ANNALS OF THE SOUTH AFRICAN MUSEUM

Abundance and species diversity trends of the whole ostracod fauna (in all samples),

with dissolved oxygen values, are shown in Figure 18A. These are similar to those

displayed for the most abundant species data, and have a distinctly trimodal distribution,

with maxima at 0.6 ml/l, 2.2—2.5 ml/l and the main maximum between 3.0—4.2 ml/l.

BOTTOM SEDIMENTS AND GEOCHEMISTRY

Correlations between the physical oceanographical and sedimentary parameters are

shown in Table 3 (based on samples with > 100 specimens). The only relatively strong

correlations are the negative relationships between temperature/salinity and calcium

carbonate, and between oxygen and organic matter. Within the sediments, the only

relatively high correlations are between mud and sand, and organic matter.

I have investigated the correlations between the overall ostracod abundance (number

of valves/100 g sample), simple diversity (number of species/sample), and various par-

ameters using both the whole data set (including and excluding barren sites), and only

those samples with > 100 specimens (Table 4). In both cases, only the dissolved oxygen

values showed relatively strong positive correlations, with the diversity having greater

dependence than the abundance (to a maximum of 0.5575). Mud content showed the

second-strongest correlation (to a maximum correlation of 0.3839). Correlations with

both temperature and MORG are weak.

TABLE 4

Correlation coefficients (based on simple regression analyses) between environmental parameters

and ostracod populations.

Temp. Oxygen Mud MORG

WHOLE DATA SET, INCLUDING BARREN SITES

Abundance -0.056 0.239 -0.129 -0.105

Simple diversity -0.197 0.489 -0.283 -0.268

WHOLE DATA SET, EXCLUDING BARREN SITES

Abundance 0.011 0.187 -0.080 -0.074

Simple diversity -0.165° 0.458° -0.281° -0.306°

SITES WITH > 100 SPECIMENS

Abundance -0.1642 0.2167 0.3839 0.1075

Simple diversity -0.2189 0.5575 0.2945 -0.0493

CaCO, reflects the biogenic component

Fe reflects the terrigenous component

MORG = organic matter

Abundance = number valves/100 g sample

Simple diversity = number species/100 g sample

Organic matter (MORG)

The distribution of mean values of organic matter in the bottom sediments plotted

against species distribution is shown in Figure 19. Most species (23; 64%) have a

preference for organic matter values within the range 2.7—3.9 per cent. Only one species

QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 407

Per cent

\ |] |

| SD MORG

}

| /|

ea |

| | | |

» | | :

14 1 41] | CTY T V7

( siceat Saale ese — rm

CBSE SERBZDBDOWVLHAAGACTHHKE DA QBAHADMWHAHS TS HBowYW4OH HD x

SSS TO See eT ee SSK ES SGT IQS sTggrgg_ssass

SS Se SSS Ee RES BESSSSSLSESSESSECESS

SSSES ES SSSR SS sels sso gees ssssree ss esse

= = = ise) ~Fok Dn oa .G Q Qgatd

eQqgok HORYAOTQM cs ano) 6s 3 SS EQ %

~ : ~ Saas Qeq QeeQ TBAQ %

ASS TS SS Sy a a) RS SES IG) (OS SID Co GS oS es

= i) 2 ~ Qe erstrds~_.4 XL ofa Ff THR

SSLEPSSLT SESS SSS TS SEF ees FS sees Ss ssa

3 9 SOogo068tUCUS KX 2) 35o 8 SSS SHesesE <sees

MPOLHOssle aS ssSG&SsosSoeeseseseVgateos

is) = C19) 7-2 2QoO On £€ S oo is) 3°90 TKD

= SSFSC SHOLHKESCHRSS gH SLuesQgcs £8ees

g mae&s S$ SBEnsS og sVegrengers ges

is) Q a SG S76) SS x Ss SS c aT

2 S *< o eSGdsea #8 SRO

o =x 5) oo

real ie) & rei SS ss a9 OG 6 be

eo Sy Tee ae ES

S 0 gc S To

=? z 8 &

Fig. 19. Mean and standard deviation (SD) of organic matter (MORG) in sea-floor sediments for species

at each site containing > 100 specimens.

(Austroaurila rugosa) has a low tolerance of organic matter (< 2.0%), whereas five others

have mean values < 3 per cent (Neocytherideis boomeri, Bairdoppilata simplex, Poseidon-

amicus panopsus, Palmoconcha walvisbaiensis and Buntonia gibbera). The inclusion of

Palmoconcha walvisbaiensis in this group may be anomalous, as the mean for this

species — based on all sample sites ——is 5.78 per cent. The species most tolerant of

MORG (> 5.0%) are Coquimba birchi, Buntonia deweti and Xestoleberis hartmanni.

Although, in general, the correlation between oxygen and organic matter in the sediments

of the west coast is only moderately strong (R = —0.5795; Tables 3 and 4), the relation-

ship is borne out by the mean preferences of Austroaurila rugosa and Neocytherideis

boomeri (low MORG), and Bensonia knysnaensis robusta, Neocaudites lordi and Buntonia

rogersi (high MORG).

Correlation coefficients between species and organic matter in bottom sediments are

listed in Table 2. Two species correlate negatively with organic matter: Macrocypris cf.

408 ANNALS OF THE SOUTH AFRICAN MUSEUM

M.metuenda and Henryhowella melobesioides, whereas 12 species correlate positively,

with Cytheropteron trinodosum showing the highest value (0.8102).

Abundance and species diversity trends of the whole ostracod fauna with organic

matter are shown in Figure 18B. Although the curves are relatively complex, they are

essentially bimodal: maximum abundances and diversity occur between one and 4.5 per

cent organic matter in the sediments. These values are similar to those for the majority of

the most abundant species (Fig. 19). In terms of species diversity, the maximum MORG

values lie at the lower end of this range (c. 1.5%), whereas maximum population

abundance occurs at somewhat higher values (3.0—3.5%). The effective cut-off maximum

values for significant population abundance and species diversity are 7.0 and 7.5 per cent,

respectively.

Terrigenous sediments

Variations in the elemental Fe content can be used to characterize the terrigenous

component in marine sediments on the continental shelf off south-western Africa (e.g.

ee ee

|e ea |r| al eC | a || Sa

mean Fe (= terrigenous) |

SSeS Sse es

Per cent

(ae)

}

eae

ee La ab + + -

1 Hee

| |

eal | |

Her Pi ieile |

| | (met [ee es tna

Oaeclepetigee! T- LLL a Lm Ue SUI aL Lea ed Daa ea tT 5 ><

SESHSEGRSSSESLES SEL SESSSELLSS SE Seeese a

SSSSSICRSSLSESS SHES SS GESS lS SEssszeseess

S “aS Gs. BS SSHSGALSFS CO Sp 208 G2 OO 2 SoS

SESSSESTSSSSSHTLESEES SSS ese ase Ses eee

SSoSeBHSasP RSF sys segs Sesces tester age

RESO CSises SSS Pate oo o242R Gx s ox STRKAX D

= » 2D Ss Q.6 255 = & eS SS SSS oa iS Se

SSESSESSSECSTHSSESLHARSE SSP SESCESLSVASS cg

O20 + 5 SST Sse © SS DRS = = oS = 0 5 2 Se = Seer

oO o c ~ Oo) 2 S o MS = od Sees Cc

SS UBMBERSTSSSESSSESSSS SS Ses sols owes

eos oat CQyHyOCTELxHEVS x SsSGZGaee SS 3

West (Sion “Sos & Seoefs a Ley Rew Sw Ss Ss

x Sia “5 as NM SX mo aS) = se asees

3 Sfiac S > =< £50055

S OR 4 S 3 9

= Océ a 5 ek

S os

Q = =

Fig. 20. Mean and standard deviation (SD) of Fe (= terrigenous component) in sea-floor sediments for

species at each site containing > 100 specimens.

QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 409

Bremner & Willis 1993). Figure 20 shows the mean values associated with the most

abundant ostracod species. Three species lie at the upper end of the Fe (> 5%) curve:

Coquimba birchi, Xestoleberis hartmanni and Neocaudites osseus. At the opposite end of

the graph, the species associated with values of Fe < 2 per cent (i.e. terrigenous-poor

environments) are Buntonia gibbera, Krithe spatularis and Buntonia rosenfeldi.

Correlation coefficients for Fe (Table 2) are strong only for Buntonia namaquaensis

and Buntonia rogersi (negative), and Xestoleberis africana and Australoecia fulleri

(positive).

Biogenic sediments

Variations in average values of CaCO3 are used to express the biogenic component

in bottom sediments (Fig. 21).

90 ] can ea al lien nal ] 79

PPE Seen asta

| th ae Pel aly 2]

Pa Reali ite pelestetin

\ |

70- : Ea Se

60> oe ' ia LL,

fe) mean terrigenous (Fe) || | | | | |

| lt il] |

O Ts ea et

SEN e st peal

wo 20> ale ieee lteaanl 5 ne

O | 4a

~ €

c 40 5 4 0

seb) iS)

) c te

a ®

@ 307 7 34a

a | L

} bg

| ;

| m carbonate

10- Ria Z 1

\ elena

led Hees | tt \

Qo0—__——— — = r@)

2) 2) nro SOHKTOHAHHTEVHOEKXBVHVHYHYYOHDSZREEES

SSR eee SASS SS CERES SESS SRSAR2AGESSSEES

Q2ESFCS EH QV Qs = ose%og Qre ny oad Pir

SSR RT CS SREESSSS SST LEL EES SSS SS Ry SSsEs

S25 ess GglasrgCoMe yt grVVoeE ices Fo ama

i da) oe i ee eS Sie ee Soe NOES oon Cl is ee Oa eS pene

ZRClix~awaeecert Wc gn FY oS? o o4%2,8 3569 9 Sos eee

SS ge aos CQeuwlg TS SRERRCERLSCHKSLsStssacgse

SSTLST TT Fc SSeS RSET SSFP SGs Ra Sis se See sey

c = >= 2 =

SeseS PLES SSRs SSls ss sss seas sa ge saacs

=£ce¢E LSter Ogg Stat = ow’ 3 is)

Sf £9 CES: <> % a2) ra aS) £& Oe <

Singers sss Qo cee ee Se lao sik oe %

S cD Boxes sS a) Q x QNEOGQ iz 6) x

So SS Se es SL > SS HS oS

gs a ASS) ES se re

= EO ES

Q Ss SC re

Fig. 21. Mean and standard deviation (SD) of CaCO; in sea-floor sediments for species at each site

containing > 100 specimens. This factor is a good indicator of the biogenic component of bottom

sediments. To illustrate the antipathetic relationship between biogenic and terrigenous components, the

mean per cent of Fe is also plotted.

410 ANNALS OF THE SOUTH AFRICAN MUSEUM

Twenty-six (72%) of the most abundant species occur in sediments with average

calcium carbonate values > 50 per cent. Seven species occur in sediments with average

values > 70 per cent, with Buntonia gibbera having a mean value > 85 per cent. Palmo-

concha walvisbaiensis is the only species to occur in opal-rich sediments (mean value of

28% for all sample sites). The three species having the lowest affinity for carbonate-rich

sediments (< 45%: Xestoleberis hartmanni, Buntonia deweti and Coquimba birchi) all have

a high affinity for terrigenous material. This expresses the general relationship between

CaCO; and terrigenous means (Fig. 21), which shows that, as the former decreases, the

mean for Fe increases. A simple regression analysis between the values on this curve gives

a correlation coefficient of —0.6264.

Correlation coefficients for CaCO3 and various species are listed in Table 2, where

the strongest relationships are between Austroaurila rugosa (positive) and Cytheropteron

trinodosum (negative).

_| mean authigenic

Per cent

(0) ere cago ee a eae Sd ae

aa so, mo) a) Te ty eS pos om eS PR. OS

See eT eee SST SSSR STS ES ESSESRSSSERSE

SS GS % ty Pees TS Scie SS aS cD aS) $ ox

SOD GS ry SUS SS SeSsET OS oo o 2 6 } oa PRES

wee SS SESS PSST SESRESESES SESS GS, less

‘oa “ go 0 TS) th aa) Bw = 2 4 oF mw Qo so S

P&S ee ses sse estes seen e sees eee

Ok eS CS ROS TS a SS a ee) A) RS as) SS Ls 4

= o v of > oS x= co 3b 2 a & 8S

B=S RESSSESSSSESSES LESH Ss 2S S Sos Seceseeecee

Bee SSESSSSESPSSe cess S28 Pee Sa Cpe

353.0 6 SBNGHEHOTHESGETRSBAVSosSSseQ se $ Sse

SS iS QO Soa: S

Iausx ~ SO gaaacy Sto. x ~ a od

SP Gs) Shee 205 Gi SeS ioee sus Ser cronmnomoms 7 8

go a ASS a RS) SS S ak a ® x & iS)

Ss 2 2) 6) gaGos 5 G92 = iS)

% > a Q S

) S ) x 1S) S

S Q ce S r)

g SE Q

Fig. 22. Mean of authigenic minerals (phosphorite and glauconite) in sea-floor sediments for species at

each site containing > 100 specimens.

QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 41]

Authigenic sediments

Authigenic minerals are relatively abundant on the continental margin off south-

western Africa (e.g. Birch ef a/. 1986; Bremner ef al. 1986). To express the relationship

between species distribution and authigenic bottom sediments, the combined average

values of apatite (in the form of phosphorite) and glauconite have been plotted (Fig. 22).

Note that data for Krithe spatularis, K. capensis and Henryhowella melobesioides have

been omitted (too few samples contained authigenic minerals for reliable means).

The distribution falls into two clear groups: species with means > 4 per cent and

those with < 3 per cent. In the higher category, Australoecia fulleri (9%) occurs apart

from the other means, which lie approximately linearly between 7 and 4.5 per cent. Other

species favouring authigenic-rich sediments are: Macrocypris cf. M. metuenda, Cytherella

dromedaria and Bairdoppilata simplex. At the lower end of the curve, 11 species lie within

a narrow preference band of 2.2—1.8 per cent.

Plotting phosphorite values separately (lower curve in Fig. 22) shows them to lie

along an approximately sympathetic curve, with glauconite forming a somewhat greater

proportion of the total at the higher value end of the curve.

Coefficient analyses for apatite and glauconite show that several species have strong

correlations with both minerals: Buntonia namaquaensis, Urocythereis arcana and Austra-

loecia fulleri (positive), and Bensonia knysnaensis knysnaensis and Neocaudites lordi

(negative). Of these, only Australoecia fulleri and Neocaudites lordi prefer sediments with

high and low mean values of authigenic minerals, respectively.

Sediment texture

Affinities for bottom-sediment types have been expressed in average values of sand

(> 63 w) and mud (< 63 y: silt + clay) (Fig. 23). In the mud-rich sediments (> 30%

mud), there is strong representation by deeper-water taxa, with four of the first six species

in ranking having average depth occurrences > 400m (Krithe spatularis, K. capensis,

Buntonia rosenfeldi and Henryhowella melobesioides: Fig.7). The remaining two,

Coquimba birchi and Buntonia deweti, are mid—inner-shelf taxa. Most species (92%)

occur in sediments with average mud values > 20 per cent, and only three are strongly

displaced off the curve at the mud-poor end of the graph: Austroaurila rugosa and

Bensonia k. robusta.

The plot of average sand values is almost complimentary (the differences

representing relatively small gravel components) and all species lie between approxi-

mately 60 and 90 per cent sand. The two main exceptions are Palmoconcha walvisbaiensis

and Buntonia namaquaensis. Both values have high standard deviations and probably

result from variance in the data set.

Correlation coefficients for sand and mud (Table 2) indicate that Austroaurila rugosa

is the most sensitive indicator of changes in the ratio of the textural parameters (positive

for mud, negative for sand).

POPULATION STRUCTURE

Brouwers (1988) and Whatley (1988) have both recently discussed the question of the

structure of ostracod populations in assessing environments. Most podocopid ostracods

moult eight times to reach maturity (Brouwers 1988), so that complete preservation of an

412 ANNALS OF THE SOUTH AFRICAN MUSEUM

100

Per cent

Sais eS Se ee ee af |

barren

Krithe capensis

Coquimba birchi

Buntonia deweti

Krithe spatularis

Henryhowella melobesioides

Buntonia rogersi.. ——$_—_$_—— oo

Neocaudites osseus |——_|—___

Urocythereis arcana

Buntonia gibbera

Neocytherideis boomeri

Neocaudites /ordi

Australoecia fulleri

Palmoconcha walvisbaiensis

Buntonia bremneri

Cytheropteron trinodosum

Ambostracon flabellicostata |__|

Buntonia rosenfeldi

Macrocypris cf. M. metuenda

Paracypris lacrimata

Bensonia k. knysnaensis

Xestoleberis africana 4 ——_+——

Doratocythere exilis

Xestolebris hartmanni

Ambostracon kee/er;_..——_—— SS Sa

Cytherella dromedaria

Incongruellina venusta |__| -_| —

Pseudokeijella lepralioides | |__|}

Ruggieria cytheropteroides 4~_|____|—

Buntonia namaquaensis

Poseidonamicus panopsus

Cytheropteron whatleyi

Bairdoppilata simplex

Cytherella namibensis

Austroaurila rugosa

Bensonia k. robusta

Fig. 23. Mean mud and sand content of sea-floor sediments for species at each site containing

> 100 specimens.

ostracod population would give a juvenile: adult valve ratio of 8:1. Theoretically, any

post-mortem partitioning by bottom currents will disturb this ratio, so that values < 8:1

will indicate populations from which early instars have been winnowed, and values

> 8:1 environments into which currents have carried fine suspensate, including early

instars. Brouwers (1988) considers that the ideal 8:1 ratio is unlikely to be achieved in

most natural environments, where the smallest instars are destroyed through predation,

dissolution and crushing, and in her work she concluded that the ‘ideal’ ratio is likely to

bel Gira or 5 le

Figure 24 is a histogram of the juvenile: adult ratio of the samples with

> 100 ostracod valves and was constructed following the same technique employed by

Brouwers (1988). Fifteen per cent of the sites have a ratio 7:1, and one site has the

theoretically ‘ideal’ ratio 8: 1. In addition, 44 per cent of the sites have a ratio between 6

and 5:1. Consequently, at 59 per cent of the sample sites, sedimentation has occurred

under relatively low energy conditions (according to criteria used by Whatley 1983 and

QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 413

105 z

| j

B | 7,7

= ey

ea ill

a Ti

| Til

ALLL

7777

; mii 7

Tiiiill

FORO oo

1 AS 5) 6

"4011.12 13 14

ba |

-+ ©

i<e)

Juvenile : adul

Fig. 24. Histogram of juvenile : adult ratios in samples containing > 100 specimens.

Brouwers 1988). In contrast, 39 per cent of the sites have juvenile: adult ratios that

indicate some post-mortem disturbance, and the majority of these (29% of the total)

suggest removal of instars, presumably by currents.

The problem with this type of approach is that the instars of larger species are larger

than adults of some smaller taxa, and that methods relying on the juvenile : adult ratio or

a detailed population age structure, using all the various instar stages (e.g. Whatley 1988),

take no account of size differences between species. Nevertheless, the results have been

presented for comparative purposes, and possible implications for sea-floor conditions

will be mentioned further in the discussion section.

BARREN SAMPLES

In understanding the distribution of the ostracod assemblages, a knowledge of the

environments in which they are not found is almost as important as knowing the

circumstances under which they do occur.

Of the 270 samples examined for ostracods, a surprisingly large number (81; 30.0%)

were barren. These sites extend from west of the Cape Peninsula to just south of the

Kunene River (Fig. 25) but are concentrated in two main inner—mid-shelf areas: north

and south of the Orange River, and in deep water. In detail (Fig. 26), both shelf areas are

further subdivided: a small cluster of sites occurs west of the Cape Peninsula between

200m and 300m, separated from the Namaqualand inner-shelf zone, whereas the

extensive northern zone, which becomes progressively deeper south of Walvis Bay, is

414 ANNALS OF THE SOUTH AFRICAN MUSEUM

15° 20°E

PAC 3A

20° Ne

Ne 4

‘| 3B

a ee

Al ++ + WALVIS BAY

1 \ +HH

a tk em

a A

25° 4 ma

+ :

i ? LUDERITZ

=| +. .

300

+ 4 (— ™ Cape Columbine

(moO at

Stgicere POUR ns

i Ree ——

sa ee

coal 7 gt

Fig. 25. Distribution of sites barren of ostracods (black squares) in relation to ostracod-

bearing sediments (crosses). Areas numbered 1—5 are also shown in Figure 26, and

described in the text.

separated from a small number of barren sites in deeper water on the Walvis Ridge

Abutment shelf. I have numbered these areas 1—5 on Figures 25 and 26. The extensive

barren zone on the upper slope lies in progressively deeper water from north to south

(550—1 000 m). Within the barren areas, the ratio of barren to ostracod-bearing sites is

high, reaching 2.8: 1 off Namaqualand (mean of 1.8: 1; Table 5).

In relation to the regional distribution of ostracod valves, there are frequent rapid

transitions from barren areas to regions of relatively high abundance (> 20 valves/

sample — Fig. 26). In particular, this occurs along the western edge of the Walvis Bay—

QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 415

TABLE 5

Statistics of barren and ostracod-bearing samples.

Barren Ostracod-bearing Rati

Area* sites sites ae

(B) (O) i

| 7 9 0.7

2 7 3} 2S

3 49 20 ZED

4 11 4 2.8

5 8 0.9

Total 81 44 1.8

*—areas indicated on Figure 25

Walvis Ridge sector (area 3), immediately west of the Orange River (area4), and

immediately south and inshore of the Cape Peninsula (area 5).

Environmental parameters of the barren sites

Mean values of various parameters within the areas 1—5, into which the barren

samples are concentrated (Fig. 26), as well as overall means for all barren sites, are shown

in Table 6.

In comparison with the mean values for the various ostracod species, the means for

all barren samples show the following features:

1. High temperature (10.8°C) and high salinity (35.0%). Only Palmoconcha walvis-

baiensis and Bensonia knysnaensis robusta are higher (Figs 14, 15).

2. Low dissolved oxygen (1.5 ml/l). Only Palmoconcha walvisbaiensis and Bensonia k.

robusta are lower (Fig. 17).

3. High organic matter (6.6%). This value is higher than the average for any individual

ostracod species (Fig. 19).

4. High mud content (47.3%). This value is 5 per cent higher than for any individual

ostracod species (Fig. 23).

5. Low carbonate content (15.7%). This value is 10 per cent less than the lowest for any

individual ostracod species, and less than half the mean for the rest of the most abundant

species (Fig. 21).

6. Moderate average Fe values (nominally representing the terrigenous component of the

sediments) (Fig. 20).

7. Moderate to high average authigenic mineral content (Fig. 22).

When the mean values of parameters in barren areas 1—5 are considered, however, it

is clear that no single factor is responsible for the absence of ostracods from regions of the

west-coast margin. Nevertheless, there are several factors in common between some of

the areas.

Area 1. Upper—mid-slope (mean water depth 763 m). Characterized by low tempera-

ture, low salinity, relatively high organic matter and carbonate contents.

416 ANNALS OF THE SOUTH AFRICAN MUSEUM

Depth (km)

1.0 Slope 0.5 Shelfbreak Shelf (6)

: N

20° — WR

— WB

25°

— LUD

— OR

30°

— CC

— CP

35°S

>20

ial

>500

[aa]

barren areas

Fig. 26. Latitude—depth plan showing barren sites (black squares: areas 1—5 with thick outlines) and

ostracod abundance (crosses: specimens/100g sample). Vertical scale is degrees of latitude.

WR = Walvis Ridge, WB = Walvis Bay, LUD = Liideritz, OR = Orange River, CC = Cape

Columbine, CP = Cape Peninsula.

TABLE 6

Mean environmental parameters for barren areas.

Baeamaie? Barren areas* All

| 5) 3 4 5 samples

Depth (in) 763 287 94 107 218 182

Temperature (°C) 4.5 Piel 12-2 9.7 8.9 10.8

Salinity (%c) 34.47 35 35 ez 34.8 34.6 35

Dissolved oxygen (ml/I) 2.8 l 0.8 22 3.8 IES)

Organic matter (%) 6.5 6.9 7.6 4.2 0.98 6.6

Mud (%) 64 24.6 52.6 57.8 6.8 47.3

CaCO, (biogenic) (%) 64. 1 25 6.3 TS 10.1 I5)57/

Fe (terrigenous) (%) 33 4.4 IT) 8.6 553) 3.9

Opal (%) 0 0 32.1 0 0 19.2

Authigenic (%) 0.6 5) 3}5) 4 Swe WS

*—areas indicated on Figure 25

QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 417

Area 2. Mid—outer shelf on the Walvis Ridge Abutment (mean water depth 287 m).

Characterized by warm, oxygen-depleted water and mixed carbonate/terrigenous, muddy

sands with a high organic matter content.

Area 3. This is the most extensive of the barren areas. It lies on the inner—mid-shelf

(mean water depth 93 m) and is characterized by warm, saline, oxygen-deficient waters

over sediments that have high organic matter and high opaline silica contents, The sedi-

ments are depleted in carbonate, Fe (= terrigenous component) and authigenic minerals.

This area can be subdivided into a smaller coastal zone between 19 and 20°S (area 3A)

and the main zone centred on Walvis Bay, which extends from 21°S to the vicinity of

Liideritz (area 3B).

Area 4. A long, narrow, inner-shelf zone off the Namaqualand coast (mean water

depth 107m) that is characterized by high-terrigenous and low-carbonate muds. In

contrast to area 3, the muds contain no opal and relatively little organic matter. The

mean oxygen value of the bottom water is depleted (2.2 ml/l).

Area 5. On the mid—outer shelf (mean water depth 218 m) either side of the Cape

Canyon. Characterized by water with high dissolved oxygen values over authigenic-rich,

low-carbonate sands with a very low organic content. These sediments have particularly

low mean mud values.

Sea bottom sediments in three of the areas (1, 3 and 4) have in common particularly

high mean mud contents, but each differs in its mineralogical and/or oceanographical

characteristics: the slope area | is a carbonate mud; the Walvis shelf area 3 is an organic-

rich, low-oxygen mud; and the Namaqua area 4 is terrigenous mud. The other two areas

(2 and 5) have low mud contents. In the case of the Walvis Ridge Abutment (area 2), it

shares in common with area 3 low oxygen and high organic matter contents, whereas on

the outer shelf off the Cape Peninsula (area 5) the very low organic matter—high

authigenic (especially glauconite) contents are probably limiting.

In summary, barren areas coincide with one or more of the following limiting factors:

High mud content of bottom sediments (> 57.8%).

Low dissolved oxygen in bottom waters (< 1.0 ml/l).

High (> 6.9%) or low (< 1.0%) organic matter in bottom sediments.

Low salinity bottom waters (< 34.47%).

. High authigenic content (51.5%) of bottom sediments (which may equate with

Pricilarly low terrigenous supply).

A a are

DISCUSSION

‘Modern’ and ‘relict’ faunas

The ostracod assemblages used in the study were mixtures of living and dead

specimens. The latter category presumably consisted of recently dead material, which are

essentially the same age as the living specimens, and older dead specimens that represent a

relict, sub-fossil fauna. In earlier accounts (Dingle 1992, 1993), a distinction was made

between these ‘modern’ (living and recently dead) and ‘relict’ specimens, primarily on the

basis of valve preservation. Opaque, corroded, stained and abraded valves were con-

sidered ‘relict’ in contrast to transparent, pristine specimens, some containing fragments

of internal organs, which were considered only recently dead (i.e. ‘modern’).

418 ANNALS OF THE SOUTH AFRICAN MUSEUM

The logic applied to the analysis of these two categories was that the sea-floor

sediments off south-western Africa represent a quasi-equilibrium deposit developed since

sea-level reached its present position, approximately 7000 years ago (Miller 1990).

Consequently, the ‘relict’ ostracod fauna is a mixed assemblage of specimens ranging in

age from 0 to 7000 years—the so-called post-glacial category, in contrast to the

‘modern’, extant category. I am sure that essentially this logic is sound, but recent

examination of material from a box core west of Walvis Bay (at 132m water depth,

personal data) has cast doubts on the use of this technique to differentiate the two

categories of differing ages.

Consequently, in this report I have adopted a conservative approach by considering

the fauna as a whole, so that the species and assemblage distribution data relate to

‘average’ oceanographical and other environmental parameters, typified by present

conditions, but in reality representing means over the period 7 000 years to the present.

The available time series for assessing such parameters is, naturally, very short.

Oceanographical data

Modern mean annual sea-floor temperature, salinity and dissolved oxygen values for

the west-coast continental shelf were compiled for the present study from a 60-year data

base by Dingle & Nelson (1993). Their maps are summarized in Figure 27, on to which

have been superimposed the mean annual positions of the upwelling cells of the Benguela

system (from Lutjeharms & Meeuwis 1987). The salient points from Figure 27 will be

briefly mentioned.

The high correlation coefficient between temperature and salinity values (R = 0.896

for all stations) means that the structure of the two maps Is very similar, although there

are some differences in detail. Four main features are evident:

Walvis

Luderitz

\ EAS

4\ P=*—~Z Namaqua

———\

\ \\ \=

4.8

\ b——,_ Columbine

——— \

‘ ! \ We aN ==)

Upwelling — \ aN i SO

system iW \ \ A

\ at 9 \VERY Peninsula

Np Ss \ 6\O4 34.8

\.0 Lo:

TEMPERATURE INST SALINITY \O\ DISSOLVED OXYGEN

(°C) Dee \Nee (%o) 34.8 \ (ml/l)

Fig. 27. Bottom water parameters and upwelling cells. A. Temperature. B. Salinity. C. Dissolved

oxygen. After Dingle & Nelson (1993) and Lutjeharms & Meeuwis (1987).

QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 419

1. The steep gradient along the shelf edge, where the temperature gradient in the Benguela

region is typically 1°C/100m between 400 and 1000m. This is related simply to the

bathymetry at the shelf edge.

2. The large intrusion of 9°C/34.7%o water on to the shelf off Namaqualand. This is

attributed to a combination of bathymetry and wind stress. The main topographic

features are the generally wide, deep shelf south of 30°S, and a superimposed transverse

depression that crosses the shelf at 31.5°S (Fig. 1). The latter probably funnels all the cold

water that wells up on to the shelf in this southern region (Peninsula and Columbine cells;

Fig. 27), driven by the pumping action of the large cross-shelf wind divergences that are

common in this area.

3. Sudden meridional shoaling of isotherms/isohalines. This effect is attributed to a flow

of 6—8°C/34.48—34.68%o water across the shelf as compensation for surface Ekman drift

caused by the perennial equatorward winds. This colder water becomes entrained in the

poleward undercurrent, resulting in a progressive decrease in the meridional temperature

and salinity as far south as 30°S.

4. Localized ‘hot spots’ of warm, saline water in the form of south-westerly tongues

stretching from nearshore across the shelf. In the extreme south (c. 35°S) this is caused by

the intrusion of Agulhas Bank water, but elsewhere they correlate with upwelling cells to

the extent that they indicate downwelling events following periods of intense upwelling.

Nelson (1989) has suggested that this occurs via wind-generated continental-shelf waves

that enhance or suppress upwelling. The upwelled surface waters are carried northward

by wind action, whereas the bottom waters are deflected southward by the poleward

undercurrent. Differences in temperature and salinity patterns can be partly accounted

for by their respective diffusion rates (10: 1).

The distribution of dissolved oxygen in the bottom waters is most closely identified

with upwelling in the northern areas. There are two sources of oxygen-depleted (< 5 ml/l)

water in the Benguela region. There is an offshelf (300 m) water mass that originates off

Angola and is carried southward by the poleward undercurrent (Bubnov 1972; Chapman

& Shannon 1985); this extends to about 25°S (occasionally 29°S) and wells up on to the

shelf in the manner described above for the 6—8°C water (item 3). The Angolan low-

oxygen water is further depleted by sea-floor biochemical action under the influence of

the major Walvis Bay—Liideritz upwelling cell, resulting in a large, shelf-wide oxygen-

deficient (< 2.0 ml/l) zone, north of 25°S (e.g. Basov 1976; Bailey et al. 1985; Shannon

1985; Dingle & Nelson 1993). The southern limit of this area is sharply outlined by the

southern 1.5ml/l contour, which corresponds with the edge of the upwelling cell

(Fig. 27C). A stream of this oxygen-deficient water leaks southward under the influence

of the poleward undercurrent, and forms a nearshore zone as far south as St Helena Bay.

Further small zones of depletion occur by biochemical action off southern Namaqualand.

In this connection, De Decker’s (1970) observation of seasonality in oxygen depletion

correlates with the seasonality of the poleward undercurrent (Dingle & Nelson 1993).

Relationships between ostracods and environmental parameters

Having looked at the mean values of the various parameters for each species, what

can be said about their overall correlations? Bearing in mind that a wide range of

environmental parameters influences the distribution of Ostracoda (see, for example,

Neale 1965; Whatley 1983; Brouwers 1988; Athersuch et al. 1989), it is possible that

420 ANNALS OF THE SOUTH AFRICAN MUSEUM

variations in any one parameter will be insufficient to control the geographical range of a

taxon completely. Nevertheless, several authors have concluded that certain parameters

are likely to be more important than others. In this category, temperature has been

singled out as a major factor, so much so that it was the only parameter considered by

Cronin & Dowsett (1990) along the continental shelf off eastern North America, whereas

Valentine (1976) concluded that faunal distribution along the Pacific coast of North

America was primarily controlled by water temperature related to upwelling. Athersuch

et al. (1989) considered temperature to be the main ecological control (along with salinity)

in distribution around the British Isles, and commented that variations in dissolved

oxygen were of little significance. Brouwers (1988), on the other hand, believed that

temperature, salinity and dissolved oxygen are the main physico-chemical controls for

ostracod distribution off Alaska. For comparison, Brouwers (1988) recorded the follow-

ing ranges in parameters on the Alaskan shelf (south-western African values in

parenthesis): temperature, 5—5.5°C (3.0—14.0); salinity, 33.00—34.00%bo (34.39—35.5); and

dissolved oxygen, 3—7 ml/I (0.29—4.8).

The question of dissolved oxygen and the distribution of particular marine taxa has

been raised by several authors and this is a factor that is especially relevant off south-

western Africa, where the whole of the continental shelf is oxygen depleted (< 5.0 ml/I),

and large areas are deficient (< 2.0 ml/l) (e.g. Shannon 1985). Briefly, the structure of the

vestibula of the genus Krithe has been linked to variations in dissolved oxygen levels (e.g.

Peypouquet 1977; McKenzie et al. 1989; Riha 1989; Zhou & Ikeya 1992; but for an

alternative viewpoint see Whatley & Quanhong 1993), whereas the physiological

adaptations of certain taxa (particularly the genus Cytherella) have given them

advantages in colonizing low-oxygenated environments (e.g. Whatley 1991).

Other factors have generally received less attention but, nevertheless, some authors

have strongly asserted their importance (e.g. Whatley & Wall 1969; Whatley 1976). In this

category fall factors such as substrate types (animal, plant and mineral, as well as textural

variations), pH, food supply, light levels, turbulence (i.e. energy of the boundary layer),

and so on. Athersuch ef al. (1989) have reviewed the distribution of all the major taxa

around the British Isles and concluded that certain taxa have preference for different

substrates: Xestoleberis is primarily a phytal genus, whereas Urocythereis, Palmo-

concha, Cytheropteron, Cytherura and all Trachyleberididae and Cytherideidae live on

sand. However, many taxa appear to have no preference (e.g. Aurila, Loxoconcha and

Semi-cytherura).

Clearly, it would be an unrealistically complex operation to acquire from the whole

margin off south-western Africa time-averaged data on all the factors mentioned above,

even if it were certain that there were no others of significance. Spot measurements during

sample collection would have served little purpose and, unfortunately, data bases on most

parameters are not available. Also, the circumstances in specific geographical areas will

strongly bias the likelihood of particular factors playing crucial roles. In the present case,

the large range in dissolved oxygen values, the overall lack of terrigenous input, the

locally high contents of authigenic minerals, and the overall intensity of oceanic upwell-

ing, make the continental margin off south-western Africa, if not unique, at least one of

only five localities world-wide with similar conditions (the others being California, Peru,

north-west Africa and the Gulf of Arabia). Adaptation to these conditions can be

expected to have played an important role in the composition of the local faunas, and

QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 421

significant variations from the ‘norm’ of what has been described from, say north-western

Europe or north-eastern North America, can be anticipated. In this connection, the large

number of barren sites must be viewed as a response to the unusual environments, and

not merely an aberration of the sampling and laboratory techniques.

Considering, firstly, the relationships between the environmental parameters them-

selves, Table 3 shows the correlation coefficients based on simple regression analyses

between the parameters in those samples from which > 100 specimens were extracted.

Here there will be a bias against deeper-water sites and areas unfavourable for ostracod

colonization, where, in both cases, valve numbers are low. Some strong relationships are

obvious (sand-mud, and temperature-salinity-oxygen), but the positions of MORG and

CaCO; are less so.

To draw out the more subtle relationships, these values have been plotted (Fig. 28)

on a similarity dendrogram, using an unweighted arithmetic average clustering technique

)

SIMILARITY

100

mud sand MORG glauc. CaCO, 0, _ salin. temp. Fe apatite

Fig. 28. Similarity dendrogram (unweighted arithmetic average) of environmental parameters.

422 ANNALS OF THE SOUTH AFRICAN MUSEUM

(e.g. Legendre & Legendre 1983). This shows two main groupings at the 35—45 per cent

similarity level. The strong antipathetic relationship between sand and mud (—0.9306) is

linked to MORG through a positive correlation (0.4906) with mud, and to glauconite

through a positive correlation with sand (0.3788).

The second grouping is based on the strong positive correlation between temperature

and salinity (0.9092) and their negative correlation with dissolved oxygen (— 0.8130 with

salinity). These relationships are representative of the regional trends discussed by Dingle

& Nelson (1993) and reflect properties of the major water masses and upwelling cells

along the west coast (e.g. Fig. 27). They are negatively correlated with the calcium

carbonate content of the bottom sediments (—0.4902 with salinity), implying that

carbonate-rich sediments are less likely to occur in the warmer, more saline, sea-floor

environments, as well as in outer-shelf areas where more oxygenated waters occur.

Similar conclusions were reached by Rogers & Bremner (1991), who showed that the

areas of most intense upwelling (between about 28° and 24°S, where bottom temperatures

and salinities are particularly high) are underlain by sediments with low carbonate

contents. Similarly, south of 29°S, where dissolved oxygen levels on the shelf are at their

highest, low carbonate values characterize the whole shelf south and west of the Cape

Peninsula. No comprehensive explanation for the latter situation has yet been advanced.

Elemental Fe (= terrigenous component) and apatite values have no close links with

the other two groupings (< 20% similarity). The terrigenous input to the west coast is

controlled by four major factors: Kunene River input north of the Walvis Ridge; the

combined input of the Orange River and of the Olifants and Berg rivers on to the

Orange—Namaqua shelf; and aeolian input between the Orange River and Walvis Bay.

Whereas the latter phenomenon is to some extent linked to upwelling through regions of

strong wind stress, there are no direct relationships between the terrigenous sources and

the environmental factors investigated.

Tables 2 and 7 show which parameters are most strongly correlated with particular

ostracod species. A simple gauge of which parameters are most effective in determining

the distribution of species can be made by totalling the number of species most strongly

correlated with each parameter. Fe and calcium carbonate rank joint first (S species each),

followed by sand (4species). Amongst these, there is a preponderance of negative

correlations, particularly with sand and, to a lesser extent, CaCO 3 (this conclusion is

reinforced if a tally is made of the strongest positive and negative correlations for each

species: sand = 13, calcium carbonate = 8, Fe = 8, and MORG = 8). The implication is

that the abundance of the majority of ostracod species in the study has an antipathetic

relationship to sandy and/or carbonate-rich substrates. In addition, neither temperature,

salinity, nor oxygen is as important as MORG.

Considering the importance of environmental parameters in terms of the regionally

dominant species (Figs 10, 29), however, presents a somewhat different picture. South of

24°S, the two dominant inner-shelf species Pseudokeijella lepralioides and Bensonia

knysnaensis knysnaensis correlate with the mud (positively) and carbonate (negatively)

content of bottom sediments, respectively. In the case of the latter, the strongest positive

correlative is Fe, indicating that increases in abundance of Bensonia k. knysnaensis are

dependent on decreasing carbonate, coupled with increasing terrigenous components.

Farther offshore, Ruggieria cytheropteroides is positively correlated with dissolved

oxygen. North of 24°S, the two dominant species are Palmoconcha walvisbaiensis (mid—

QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 423

TABLE 7

Summary of strongest correlations between species and environmental parameters.

These parameters are those that most negatively or positively influence the abundance

of a particular species.

Species Positive Negative

Cytherella namibensis MORG oxygen

Bensonia k. knysnaensis Fe CaCO,

Ruggieria cytheropteroides oxygen —

Henryhowella melobesioides sand mud (temperature)

Pseudokeijella lepralioides mud sand

Palmoconcha walvisbaiensis MORG oxygen/Fe

Urocythereis arcana apatite =

Macrocypris cf. M. metuenda CaCO, mud

Australoecia fullert Fe —

Bairdoppilata simplex sand apatite

Paracypris lacrimata Fe CaCO,

Cytheropteron whatleyi salinity sand

Cytheropteron trinodosum MORG CaCO,

Cytherella dromedaria MORG sand

Neocytherideis boomeri sand mud

Poseidonamicus panopsus glauconite CaCO,

Neocytherideis lordi MORG apatite

Incongruellina venusta — CaCO,

Buntonia rogersi MORG Fe

Ambostracon flabellicostata salinity sand

Buntonia bremneri mud sand

Xestoleberis africana Fe sand

Chrysocythere craticula CaCO, Fe

Austroaurila rugosa CaCO, sand

Doratocythere exilis apatite temperature

Neocytherideis osseus mud sand

Ambostracon keeleri MORG sand

Buntonia gibbera MORG sand

Buntonia namaquaensis glauconite Fe

Note: items in bold are the overall strongest correlatives.

For H. melobesioides, temperature is probably a better indicator for the deep-water

assemblages.

inner shelf) and Cytherella namibensis (outer shelf—upper slope), and here the strongest

correlations are with MORG (positive) and dissolved oxygen (negative), respectively. The

latter species’ strongest positive correlative is also with MORG, whereas the strongest

negative correlative of Palmoconcha walvisbaiensis is dissolved oxygen. Consequently,

both dominant taxa on the shelf off northern Namibia respond positively to increases in

the quantity of organic matter in bottom sediments and negatively to increases in

dissolved oxygen. Henryhowella melobesioides is the dominant species over the length of

the margin in water deeper than 500 m (to about | 500 m) and this species correlates most

strongly with mud (—0.5360), with sand its strongest positive correlative. However,

because of the relatively small sample sizes in deep water, these correlations are biased

424 ANNALS OF THE SOUTH AFRICAN MUSEUM

Kunene River

P. walvisbaiensis

(+ MORG)

C. namibensis Central Namib

(05) association

Luderitz

B. k. knysnaensis

30°

P. lepralioides

(+ mud)

H. melobesioides ————~

(— mud, — temp.)

association

R. cytheropteroides s KC. Columbine

(+ O>) i

Fig. 29. Distribution of dominant ostracod assemblages (shaded, from Figure 10)

and barren areas (thick outline, 1—5 from Figure 26). Dashed line is the shelf break.

Parameters in parentheses are strongest environmental correlatives for dominant

species (positive or negative) from Table 7.

425

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QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 427

towards the shallower water sites at which H. melobesioides occurs and in which it is a

less-important component of the assemblage. Correlation coefficients based on the whole

sample suite suggest that this species, on a regional basis, most strongly correlates with

temperature (— 0.5143) which, together with salinity, generally decreases with increasing

latitude and depth (Fig. 27).

It is convenient to consider the distribution of ostracod assemblages in relation to

regional environmental parameters in terms of the dominant taxa. Figure 29 shows the

geographical distribution of the dominant taxa, together with the environmental

parameters with which they are most strongly correlated, whereas Figures 30—32 show

regional latitude—depth plans of variations in all the parameters investigated. To make a

more sophisticated assessment of the relationships between individual species requires a

multivariate approach, and this has been presented in a separate publication (Dingle &

Giraudeau 1993).

500

Depth (m)

[seca

1000 4 & SAND

500

Depth (m)

1000

Fig. 32. Latitude—depth plans of dominant ostracod assemblages and bottom-water and

sea-floor sediment parameters. A. Sand. B. Mud.

428 ANNALS OF THE SOUTH AFRICAN MUSEUM

The general conclusions from the discussion above are that the distributions of

dominant taxa in the deeper-water areas are most strongly correlated with parameters

related to the water-mass properties, whereas the mid- and inner-shelf taxa most strongly

correlate with sedimentary characteristics (in particular Fe (= terrigenous component),

calcium carbonate and sand). These correlations do not imply that such factors are the

only controls but that they are, statistically, the most important for individual species.

To understand these correlations, it is important to remember that the oceanography

and climate off south-western Africa are influenced by phenomena that, on a global scale,

are not widely developed: intense oceanic upwelling, large-scale development of oxygen-

depleted bottom water (< 5 ml/l), and low terrigenous input.

SUMMARY

Figure 33 shows the conceptual relationships between the distribution of the

regionally dominant ostracods and the main oceanographical components of the west

coast. In deep water, small-scale phenomena are subordinate to the regional water-

column structure (see Fig. 2), so that the upper limit of the mid—lower-slope assemblage

(dominated by Henryhowella melobesioides) is controlled by the position of the base of

the salinity minimum zone in the AAIW (500—600 m) (see also Dingle et al. 1990).

Higher up the slope, the effects of shelf upwelling and the intrusion of shelf currents

are felt. South of 28°S, the cold, low-salinity, oxygen-rich upper section of the AATW

sustains the upper slope—outermost shelf Ruggieria cytheropteroides-dominated assem-

blage, but northwards this gives way to the less abundant and diverse Cytherella

namibensis-dominated assemblage that is influenced by warmer, more saline, oxygen-

depleted water just beyond the shelf edge moving into the region across the Walvis Ridge

from the Angola Basin. Deflection of uppermost AAIW around the major bathymetric

re-entrant along the northern side of the Orange Banks (27—28°S: Fig. 1) is probably a

critical control in the location of this oceanic/faunal boundary. As Whatley (1991) has

discussed, platycopid ostracods are particularly adapted to competing in lower-oxygen

environments and, clearly, C.namibensis (in marked contrast to its more southern

relation C. dromedaria), has taken advantage of this in areas that are unfavourable to

most other species.

Moving farther on to the shelf, the influences of water mixing and upwelling are

more pronounced. In the north, the off-shelf Angolan water mass advects oxygen-

depleted waters on to the Walvis—Liideritz shelf, where intense upwelling, followed by

further, biologically-induced oxygen extraction on the sea-floor, creates a reservoir of

strongly oxygen-depleted shelf waters and organic enrichment in bottom sediments,

which is the preferred environment of Palmoconcha walvisbaiensis. Under the influence of

the poleward undercurrent, strong southward temperature, salinity and oxygen depletion

gradients are created (Fig. 27), which progressively support different ostracod associ-

ations as the properties of the southward-moving water change, and it interacts with

other, colder shelf waters in the vicinity of the Orange River. Hence, the Palmoconcha

walvisbaiensis-dominated assemblage passes via a mixed zone (Central Namib

association) into the coast-parallel assemblages dominated by Pseudokeijella lepralioides

(positively correlated with mud) and Bensonia k. knysnaensis (negatively correlated with

mud and positively correlated with Fe).

depth (km).

OXYGEN-POOR WATER

ooo

Off shelf edge

Angolan source

Depletion under

upwelling cells

Warm saline

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Cold, low-salinity

water

Low juvenile :

ratio

adult

top of salinity

minimum zone: AAIW

incursion of cold

water onto

Namaqua Shelf

association, CB = Childs Bank.

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Fig. 33. Conceptual relationships between ostracod associations and the main oceanographic regimes and

features plotted on a latitude—depth plan. Vertical scale is degrees of latitude, horizontal scale is water

Abbreviations: BKK = Bensonia k. knysnaensis, CN = Cytherella namibensis,

CNA = Central Namib association, HM = Henryhowella melobesioides, PL = Pseudokeijella lepra-

lioides, PW = Palmoconcha walvishaiensis, RC = Ruggieria cytheropteroides, SWCA = South West Cape

430 ANNALS OF THE SOUTH AFRICAN MUSEUM

The southern limit of oxygen-deficient water, together with intense upwelling of

upper-level AATW off the Cape Peninsula and intrusions of warm, Agulhas water

filaments (Lutjeharms 1989), combine to produce the particularly diverse and abundant,

but geographically small, South West Cape association.

Other aspects of interest are the high degree of strongest negative correlation with

the sand content of the bottom sediments (13 (negative) : 3 (positive)), and the high degree

of strongest positive correlation with the MORG values (8: 0) (Table 7). These contrast

with more equitable correlations with the terrigenous (Fe) and biogenic (CaCO3)

components (4: 4 and 3:5, respectively).

The implications from these relationships are that the abundances of a large

minority of species (34% in Table 7) are affected adversely by increases in the ambient

sand content of their favoured substrates, the exceptions being Henryhowella melobes-

ioides, Bairdoppilata simplex and Neocytherideis boomeri. Although few details have been

published on bottom currents on the west-coast margin, indications are that they are

generally low and poleward. Nelson (1989) suggested that, between Cape Point and the

Orange River, a vector-averaged poleward velocity of 3.8 cm/sec exists, but noted that at

66 m off Chamais Bay (28°S) a current meter has recorded a long-term average of 1.7 cm/

sec north-west. Only along the outer Namaqua shelf, between 31°S and the vicinity of

Cape Columbine (33°S), is there possible evidence for relatively high-velocity currents in

the Benguela system (the Shelf Edge and Columbine jets), with surface velocities up to

40 cm/sec north-west (e.g. Shannon 1985; Nelson 1989). However, although Shannon

(1985) suggested that these may have subsurface effects, Nelson (1989) indicated that they

are merely high-velocity streams (40—50 cm/sec) in the general northward Benguela

surface flow pattern (which itself varies from 5 cm/sec over the shelf to 30 cm/sec beyond

the shelf edge). Strong and turbulent bottom currents are not, therefore, anticipated over

any large part of the west-coast shelf, and few species can be expected to have adapted to

such conditions. Perhaps this explains the susceptibility to increases in sand content (if it

denotes somewhat higher bottom-water energy).

A further assessment of areas with high-velocity sea-floor currents can be related to

possible post-mortem valve transportation. Figure 34 shows the latitudinal distribution

of sample sites plotted against their juvenile : adult ratios. Sites with the lowest ratios are

intuitively taken as those most subjected to higher sea-floor winnowing, and these all

occur south of 30°S. The most-affected sites lie in water depths between 205 and 271 m in

the vicinity of Childs Bank (see Figs 1, 33), where Nelson (1991 pers. comm.) has

suggested that an extension of the Shelf Edge Jet could operate, although it should be

emphasized that other sites on the mid—outer shelf in this area have ratios between 6 and

8 : 1. Winnowing is further suspected at two sites on the inner shelf, immediately west of

the Cape Peninsula (120—140 m at 34°S, Figs 33, 34) and at an inshore site (42 m) near

Cape Columbine (32.5°S, Fig. 34). In contrast, all sites north of Childs Bank as far as the

Walvis Ridge, suggest quiet sea-floor conditions, as do the bulk of the sites off the Cape

Peninsula.

No species has the organic content of bottom sediment as the parameter most

adversely affecting its distribution, whereas an increase in the MORG value favourably

affects 28 per cent of the taxa in Table 7, with Palmoconcha walvisbaiensis, Ambostracon

Keeleri and Buntonia gibbera particularly sensitive in this regard. On a continental shelf

where the organic content of sediments is generally high and, overall, the most organic

QUATERNARY OSTRACODS FROM SOUTH-WESTERN AFRICA 43)

14 =

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x = = a ne oo) a

= = =

= a aaa ca

= = = =

ae 4 =

CHILDS BANK

18 20 22 24 26 28 30 32 34 36

Latitude (°S)

Fig. 34. Latitudinal distribution of juvenile: adult ratios in samples containing > 100 specimens.

Sediment partitioning by bottom currents is progressively indicated by ratios < 5 (see Brouwers 1988).

This phenomenon is suggested at the sites with water depth > 200 m in the vicinity of Childs Bank.

rich in the Atlantic Ocean (Yemel’yanov 1975, quoted in Rogers & Bremner 1991), the

ability not to be affected adversely by increases in MORG is clearly an ecological

advantage.

Finally, in the distribution of barren samples (see Fig. 27), the direct and indirect

effects of upwelling are probably the dominant influence north of 27°S (dissolved-oxygen

values < 1.0 ml/l, MORG values > 6.9%), whereas fluvial input (area 4: terrigenous mud

values > 58%) and topography (area 5 either side of the Cape Canyon, which shelters

areas from organic-matter sources, and perhaps creates locally unfavourably strong sea-

floor currents) are important in the south.

ACKNOWLEDGEMENTS

This work was funded by research grants from the Foundation for Research

Development and South African Museum, for which I am grateful. I have benefited from

discussions with numerous colleagues, but in particular would like to acknowledge

Dr J. M. Bremner (University of Cape Town) and Mr G. Nelson (Sea Fisheries Research

Institute). Professors Lord (University College London) and Whatley (University College

Aberystwyth), and Dr Oertli suggested improvements to the manuscript. Oceanographic

data, upon which many of my conclusions are based, were originally supplied by the

South African Data Centre for Oceanography, and I thank the Director, Dr M.

Grundlingh, for his generous assistance. I also thank Mrs J. Woodford who drafted most

of the figures, and Ms L. Bisset for photography in connection with the plates.

432 ANNALS OF THE SOUTH AFRICAN MUSEUM

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435

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441

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6. SYSTEMATIC papers must conform to the /nternational code of zoological nomenclature (particu-

larly Articles 22 and 51).

Names of new taxa, combinations, synonyms, etc., when used for the first time, must be followed

by the appropriate Latin (not English) abbreviation, e.g. gen. nov., sp. nov., comb. nov., syn. nov.,

ete.

An author’s name when cited must follow the name of the taxon without intervening punctuation

and not be abbreviated; if the year is added, a comma must separate author’s name and year. The

author’s name (and date, if cited) must be placed in parentheses if a species or subspecies is trans-

ferred from its original genus. The name of a subsequent user of a scientific name must be separated

from the scientific name by a colon.

Synonymy arrangement should be according to chronology of names, i.e. all published scientific

names by which the species previously has been designated are listed in chronological order, with all

references to that name following in chronological order, e.g.:

Family Nuculanidae

Nuculana (Lembulus) bicuspidata (Gould, 1845)

Figs 14-15A

Nucula (Leda) bicuspidata Gould, 1845: 37.

Leda plicifera A. Adams, 1856: 50.

Laeda bicuspidata Hanley, 1859: 118, pl. 228 (fig. 73). Sowerby, 1871: pl. 2 (fig. 8a—b).

Nucula largillierti Philippi, 1861: 87.

Leda bicuspidata: Nicklés, 1950: 163, fig. 301; 1955: 110. Barnard, 1964: 234, figs 8-9.

Note punctuation in the above example:

comma separates author’s name and year

semicolon separates more than one reference by the same author

full stop separates references by different authors

figures of plates are enclosed in parentheses to distinguish them from text-figures

dash, not comma, separates consecutive numbers.

Synonymy arrangement according to chronology of bibliographic references, whereby the year is

placed in front of each entry, and the synonym repeated in full for each entry, is not acceptable.

In describing new species, one specimen must be designated as the holotype; other specimens

mentioned in the original description are to be designated paratypes; additional material not regarded

as paratypes should be listed separately. The complete data (registration number, depository, descrip-

tion of specimen, locality, collector, date) of the holotype and paratypes must be recorded, e.g.:

Holotype

SAM-—A13535 in the South African Museum, Cape Town. Adult female from mid-tide region, King’s Beach, Port Eliza-

beth (33°51’S 25°39’E), collected by A. Smith, 15 January 1973.

Note standard form of writing South African Museum registration numbers and date.

7. SPECIAL HOUSE RULES

Capital initial letters

(a) The Figures, Maps and Tables of the paper when referred to in the text

e.g... . the Figure depicting C. namacolus ...’: ‘. . . in C. namacolus (Fig. 10)... .’

(b) The prefixes of prefixed surnames in all languages, when used in the text, if not preceded by

initials or full names

e.g. DuToit but A.L.du Toit; Von Huene but F. von Huene

e.g. Therocephalia, but therocephalian

Punctuation should be loose, omitting all not strictly necessary

Reference to the author should preferably be expressed in the third person

Roman numerals should be converted to arabic, except when forming part of the title of a book or

article, such as

‘Revision of the Crustacea. Part VIII. The Amphipoda.’

Specific name must not stand alone, but be preceded by the generic name or its abbreviation to initial

capital letter, provided the same generic name is used consecutively. The generic name should

not be abbreviated at the beginning of a sentence or paragraph.

Name of new genus or species is not to be included in the title; it should be included in the abstract,

counter to Recommendation 23 of the Code, to meet the requirements of Biological Abstracts.