Mid-lower bathyal benthic foraminifera of the Campos Basin, Southeastern Brazilian margin: Biotopes and controlling ecological factors

Marine Micropaleontology 61 (2006) 40 – 57 www.elsevier.com/locate/marmicro

Mid-lower bathyal benthic foraminifera of the Campos Basin, Southeastern Brazilian margin: Biotopes and controlling ecological factors Silvia Helena de Mello e Sousa a,⁎, Raquel Fernanda Passos a , Marina Fukumoto a , Ilson Carlos Almeida da Silveira a , Rubens Cesar Lopes Figueira b , Eduardo A.M. Koutsoukos c , Michel Michaelovitch de Mahiques a , Carlos Eduardo Rezende d a

d

Instituto Oceanográfico, Universidade de São Paulo, Praça do Oceanográfico, 191, 05508-900, São Paulo, SP, Brazil b Universidade Cruzeiro do Sul, R.Dr.Ussiel Cirilo, 225, 08060-090, São Paulo, SP, Brazil c PETROBRAS - CENPES, Cidade Universitária, Quadra 7, Ilha do Fundão, Rio de Janeiro, RJ, 21941-598, Brazil Universidade Estadual do Norte Fluminense, Centro de Biociências e Biotecnologia, Laboratório de Ciências Ambientais, Av. Alberto Lamego, 2000, 28.013-602, Rio de Janeiro, RJ, Brazil Received 1 July 2005; accepted 23 May 2006

Abstract The benthic foraminiferal assemblages recovered from 41 surface sediment samples of the Campos Basin, southeastern Brazilian continental margin, were analyzed in order to understand their distribution patterns and ecological preferences. Living and dead specimens of benthic foraminifera were selected for identification and quantitative analysis from the 63 to 125 μm, the > 125 μm and the combined > 63 μm size fractions. Q-mode cluster and canonical correspondence analyses for all size fractions show the dissimilarity between the samples collected on the middle slope (Group 1: 750 to 1050 m water depth) and those collected on the lower slope (Group 2: 1350 to 1950 m water depth). Food supply, energy state (stability) at the benthic/pelagic boundary and the grain size of the substrate seem to be the most important environmental factors determining the distribution pattern of the benthic foraminiferal assemblages in the deep sea. The middle slope (between 750 and 1050 m water depth) is characterized by the dominance of different species of the genus Bolivina, Cassidulina laevigata and Globocassidulina subglobosa. The occurrence of these species in association with Cibicidoides kullenbergi, Epistominella exigua and Uvigerina proboscidea seems to be related to seasonal organic matter fluxes, relatively oxic bottom waters, strong bottom currents and sandy sediments. The lower slope (between 1650 and 1950 m water depth) is inhabited by a microfauna with different characteristics, composed of preferentially epifaunal or shallow infaunal deposit feeding species (e.g., Bolivina spp., Eponides weddellensis, Lenticulina cultrata) and suspension feeders that are adapted to oligotrophic conditions and high dissolved oxygen levels in the bottom waters (e.g., Rhabdammina spp., Rhizammina sp.). © 2006 Elsevier B.V. All rights reserved. Keywords: Benthic foraminifera; Brazilian continental margin; Ecology; South Atlantic

⁎ Corresponding author. E-mail address: [email protected] (S.H. de Mello e Sousa). 0377-8398/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.marmicro.2006.05.003

S.H. de Mello e Sousa et al. / Marine Micropaleontology 61 (2006) 40–57

Fig. 1. Location of the Campos Basin and sampling sites.

41

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S.H. de Mello e Sousa et al. / Marine Micropaleontology 61 (2006) 40–57

Fig. 2. Mean temperature–salinity structure for the Campos Basin region. Upper panel: the T–S diagram exhibiting the mixing triangle from which water masses interfaces are found following classical thermodynamic theorems. TW = Tropical Water; SACW = South Atlantic Central Water; AAIW = Antarctic Intermediate Water; NADW = North Atlantic Deep Water. Lower panel: the temperature and salinity vertical profiles with water mass portions indicated. The synoptic summer–winter data set used for these calculations is derived from the “Dinâmica do Ecossistema de Plataforma da Região Oeste do Atlântico Sul—DEPROAS” Experiment.

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43

climate (e.g., Streeter et al., 1982; Loubere and Banonis, 1987), circulation (e.g., Schnitker, 1974, 1980; Streeter and Shackleton, 1979; Ishman, 1996) or productivity (e.g., Lutze and Coulbourn, 1984; Mackensen et al., 1985; Corliss and Chen, 1988; Loubere, 1996; Schmiedl and Mackensen, 1997; Jorissen et al., 1998; Den Dulk et al., 1998; Martinez et al., 1999). In the 1980s, some authors, such as Kitazato (1984), Corliss (1985) and Corliss and Chen (1988) established that deep-sea benthic foraminifera inhabit a number of different stratified microhabitats on and below the sediment surface (see Schmiedl et al., 1997, for further references). Later workers, however, have shown that

1. Introduction Several sedimentological investigations have been undertaken in the Campos Basin, on the southeastern Brazilian continental margin (e.g., Caddah et al., 1998; Viana et al., 1998; Kowsmann and Carvalho, 2002). The present work, dedicated to foraminiferal studies, is a part of a PETROBRAS multidisciplinary environmental research project, the so-called “Campos Basin Deep-sea Environmental Program”, in the deep-water exploration and production area of the basin. Benthic foraminifera have been widely used as proxies for oceanic environmental changes, either related to Table 1 Sampling localities, water depth, sedimentological and oceanographic data Station

Latitude (S)

Longitude (W)

Depth (m)

Mud (%)

Sand (%)

CaCO3 (%)

Corg (mg g− 1)

Total phosphate (μmol kg− 1)

45 46 47 48 50 51 52 53 54 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87

22 10 54,3 22 10 55,5 22 11 04,4 22 11 16,6 22 04 33,9 22 04 43,4 22 04 44,3 22 04 46,2 21 57 17,5 21 57 15,5 21 57 15,5 21 57 26,9 21 52 59,6 21 52 50,4 21 52 51,9 21 52 41,9 21 52 44,1 22 36 03,0 22 40 57,8 22 44 48,6 22 46 59,0 22 48 05,3 22 31 12,5 22 35 04,5 22 38 53,6 22 41 03,8 22 41 35,2 22 27 31,6 22 31 28,3 22 34 05,7 22 36 03,4 22 37 02,5 22 19 50,1 22 24 31,6 22 27 18,9 22 28 49,5 22 30 35,3 22 26 27,7 22 29 33,9 22 31 36,0 22 33 10,0

39 39 39 39 39 39 39 39 39 39 39 39 39 39 39 39 39 40 40 40 40 40 40 40 40 40 40 40 40 40 39 39 40 39 39 39 39 39 39 39 39

1050 1350 1650 1950 1050 1350 1650 1950 750 1350 1650 1950 750 1050 1350 1650 1950 750 1050 1350 1650 1950 750 1050 1350 1650 1950 750 1050 1350 1650 1950 750 1050 1350 1650 1950 1050 1350 1650 1950

55 95 97 93 89 80 93 82 91 94 91 86 52 61 61 91 86 91 85 87 94 93 86 87 90 95 93 57 52 91 93 92 31 31 74 94 93 54 52 89 93

45 5 3 7 11 20 7 18 9 6 9 14 48 39 39 9 14 9 15 13 6 7 14 13 10 5 7 43 48 9 7 8 69 69 26 6 7 46 48 11 7

29.4 41.7 34.7 42.6 28.5 31.2 35.9 49.3 33.2 31.8 30.0 43.3 31.6 26.5 31.6 34.4 45.5 37.3 37.2 41.1 32.1 49.9 37.1 38.6 41.1 43.7 48.1 37.2 46.6 42.4 44.6 50.4 24.0 23.9 39.7 41.3 48.8 29.2 42.6 46.2 50.4

6.2 10.4 11.5 9.5 14.9 13.3 12.8 12.3 5.3 14.9 11.8 12.6 8.0 7.0 6.4 11.2 10.8 18.1 15.8 13.7 14.2 14.2 13.8 11.8 12.9 13.7 14.3 9.7 18.0 11.6 12.8 13.1 2.2 4.9 9.1 14.3 15.2 8.4 9.7 13.2 16.4

1.85 1.81 1.76 1.44 1.27 1.8 1.48 1.55 No data 1.56 1.74 1.09 No data 1.82 1.97 1.28 1.3 No data No data No data No data No data No data 2.29 2.05 1.96 1.15 No data 1.89 2.0 1.75 1.45 No data 1.98 2.13 1.85 1.48 1.93 2.01 2.3 1.79

52 19,4 49 00,6 47 04,6 43 44,7 52 04,9 49 08,2 46 31,5 43 02,0 56 01,1 49 37,4 47 43,8 40 33,8 55 30,6 51 42,6 48 11,7 46 17,5 40 45,6 21 45,4 16 30,3 10 07,7 07 49,4 06 38,6 15 11,1 08 53,1 04 14,2 02 29,6 00 45,2 09 23,2 03 50,4 00 10,3 57 54,7 56 20,5 00 35,1 57 28,0 54 50,5 53 24,3 51 45,4 58 51,6 56 17,6 55 15,0 54 22,0

44

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some species change their microhabitat in response to food supply and dissolved oxygen concentration in the bottom and pore waters (Jorissen et al., 1995; Bernhard et al., 1997; Van der Zwaan et al., 1999; De Rijk et al., 2000). Other species seem to be adapted to the input of organic matter to the sea floor (Gooday, 1993). To some authors (e.g., Fontanier et al., 2002), most species are able to adapt to severe dysoxic or even anoxic bottom conditions. Conversely, it has also been suggested that the quantity and quality of food particles within the sediment appear to be important factors determining the vertical distribution of benthic foraminifera (Fontanier et al., 2002). Studies carried out by Mackensen et al. (1995) and Schmiedl et al. (1997) have shown that the composition of deep-sea benthic foraminiferal communities is determined by the combination of oceanographic, trophic and sedimentological parameters of a given region. They have proposed that the generation and distribution of deep-sea foraminiferal assemblages are influenced by four primary but interdependent groups of environmental agents: lateral advection and bottom water ventilation, primary productivity and organic carbon flux rates, bottom water carbonate corrosiveness, and energetic state at the benthic boundary layer. This study presents a survey of the distribution patterns of Recent benthic foraminifera in the Campos

Basin, and their ecological preferences. PETROBRAS, the Brazilian state-owned oil company, has undertaken intense oil exploration and production activity in the basin for more than two decades, though it does not constitute an area of natural seepage. Benthic foraminiferal assemblages have been defined and the environmental factors which determine their distribution in the area are discussed. 2. Environmental setting The Campos Basin is located on the southwestern margin of the South Atlantic, between latitudes 21° and 23°S (Fig. 1). The continental slope is 40 km wide and extends from the shelf-break, at the 110 m isobath, to the 2000 m isobath where it merges into the São Paulo Plateau. The average gradient of the slope is very gentle (2.5°), but both the upper (110–600 m) and the lower (1200–2000 m) slopes have steeper gradients. The latter is characterized by a pronounced escarpment, inclined at up to 8°, produced by the surface expression of a Neogene clinoform wedge (Caddah et al., 1998). In the north (at c. 1500 m water depth) the base of the slope is shallower than in the south (c. 2000 m) due to the occurrence of a submarine cone connected to the Almirante Camara submarine canyon (Viana et al., 1998). The study area is also cut by other canyons, such

Fig. 3. Vertical section of the observed current structure in the Campos Basin. The BC is seen in the upper portion with negative (southwestward) velocities; the IWBC is underneath it with positive (northeastward) velocities. Contour intervals are 0.05 m s− 1. Modified from Silveira et al. (2004).

S.H. de Mello e Sousa et al. / Marine Micropaleontology 61 (2006) 40–57

as the São Tomé Submarine Canyon in the south and the Itapemirim Canyon in the north. 2.1. Physical oceanography The water mass dynamics and current structure are linked to the displacement of the South Atlantic western boundary current system, which has been extensively described by several authors (e.g., Evans et al., 1983; Piola and Gordon, 1989; Reid, 1989; Peterson and Stramma, 1991; Silveira et al., 2000, 2004).

45

According to Silveira et al. (2000), it is commonly accepted that there are four main water masses with distinct signatures in the Temperature–Salinity plane (Fig. 2): the Tropical Water (TW), the South Atlantic Central Water (SACW), the Antarctic Intermediate Water (AAIW) and the North Atlantic Deep Water (NADW). The Tropical Water (T > 20 °C, S > 36.2) occupies the oceanic mixed layer and its distribution is restricted to the upper 150 m. Beneath the TW, the SACW (T = 10 to 20 °C; S = 34.8 to 36.2) is defined as the thermocline water mass and extends vertically from 150 to about

Table 2 Faunal counts, number of species, diversity and equity in 63–125 μm and >125 μm fractions Stations Counts (N) Species no. (S) Diversity (H′) Equity (J′) Counts (N) 63–125 μm 63–125 μm 63–125 μm 63–125 μm >125 μm

Species no. (S) >125 μm

Diversity (H′) >125 μm

Equity (J′) >125 μm

45 46 47 48 50 51 52 53 54 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87

52 83 78 79 64 91 83 80 61 87 86 86 56 56 55 59 51 59 55 84 73 81 51 45 72 57 65 42 46 78 60 72 47 56 46 49 46 54 49 52 41

3.379 3.149 3.236 3.022 3.699 3.257 2.925 2.651 3.537 3.065 3.206 3.341 3.469 3.492 3.464 3.609 3.333 3.354 3.491 3.263 2.769 2.968 3.21 3.165 2.937 2.588 2.746 2.924 3.211 3.75 2.772 3.458 3.271 3.346 3.178 3.39 3.323 3.266 3.149 3.357 3.198

0.8553 0.7127 0.7428 0.6916 0.8894 0.722 0.6619 0.605 0.8604 0.6864 0.7198 0.7501 0.8618 0.8676 0.8643 0.8852 0.8478 0.8227 0.8713 0.7365 0.6454 0.6754 0.8165 0.8315 0.6867 0.64 0.6579 0.7824 0.8386 0.8609 0.6771 0.8086 0.8497 0.8312 0.8301 0.8711 0.868 0.8187 0.809 0.8497 0.8612

12,757 5824 2336 2432 5168 4960 5264 4720 5328 2984 6240 2248 13,098 6912 19,392 11,968 10,304 7440 4736 6656 4544 4704 37,248 16,832 4768 2352 2392 38,016 18,880 3424 2360 1384 12,715 12,629 25,536 3168 3115 12,459 21,056 9984 4720

39 43 44 44 40 41 43 39 34 41 43 34 29 33 34 46 44 42 36 50 40 39 30 41 41 37 49 32 45 55 39 50 27 37 39 43 40 38 44 45 32

2.25 2.756 2.973 2.612 2.583 2.985 2.448 2.29 1.692 2.674 3.021 2.806 1.692 1.75 1.88 2.13 2.16 1.724 2.142 2.993 2.46 2.777 1.64 1.788 2.556 2.399 2.591 1.876 2.233 3.517 2.599 3.163 1.562 2.011 1.809 2.338 2.302 2.139 2.282 2.315 2.117

0.6141 0.7329 0.7857 0.6903 0.7001 0.8039 0.6508 0.6252 0.4797 0.72 0.8033 0.7957 0.5025 0.5006 0.5331 0.5563 0.5707 0.4614 0.5976 0.7652 0.6669 0.758 0.4821 0.4814 0.6884 0.6642 0.6658 0.5414 0.5867 0.8778 0.7095 0.8086 0.4739 0.557 0.4939 0.6215 0.624 0.5881 0.6031 0.6082 0.6108

4139 3146 2521 2554 786 5314 2815 4931 327 3004 6244 2844 2544 2976 4656 795 668 315 570 3420 3571 4655 1204 1176 2601 2179 1900 13,936 9280 1511 2186 1199 2280 6251 2312 285 472 3240 2384 1568 502

46

S.H. de Mello e Sousa et al. / Marine Micropaleontology 61 (2006) 40–57

Table 3 Selected (frequencies > 10%) foraminifera species identified in the 63–125 μm size fraction Stations/species

45

46

47

48

50

51

52

53

54

56

57

58

59

60

61

62

63

64

65

66

Split

3/128

1/16

1/8

1/8

1/16

1/16

1/16

1/16

1/16

1/8

1/16

1/8

3/128

3/64

1/64

1/32

1/32

1/16

1/16

1/16

Specimens/10 cc

12,843

6400

2432

2480

5312

5440

5472

4928

5440

3376

6784

2456

13,440

7040

19,776

12,032

10,592

7600

4816

7392

Angulogerina angulosa Bolivina a. multicostata Bolivina albatrossi Bolivina doniezi Bolivina simplex Bolivina spp. Bolivinella daggaziensis Brizalina danvellinensis Brizalina spp. Cassidulina laevigata C. parkerianus Cibicides sp. Cibicidoides kullenbergi Cibicidoides sp. Cymbaloporetta bradyi Epistominella exigua Eponides weddellensis G. subglobosa Haplophragmoides spp. Hormosina spp. Karrerulina conversa Laticarinina pauperata Miliolinella sp. Nonionella turgida Oridorsalis umbonatus Rhabdammina spp. Rhizammina sp. Uvigerina peregrina Uvigerina proboscidea Verneuilinulla sp.

2 14 9 5 9 1 0 6 0 8 5 0 6 0 5 27 0 151 2 0 1 0 0 1 5 0 0 2 4 0

2 4 6 0 0 13 3 2 18 4 14 8 0 17 19 0 2 136 4 1 0 17 4 1 6 0 0 8 12 3

6 4 1 0 0 12 3 3 10 4 3 3 0 6 9 0 8 89 0 6 0 24 0 4 3 14 2 6 5 12

0 3 2 0 0 7 7 1 12 6 1 3 0 1 8 0 38 115 0 15 0 0 3 1 0 2 14 4 0 13

3 22 5 20 25 1 14 2 0 0 0 0 0 0 0 14 5 124 7 0 3 0 0 19 4 0 0 2 4 0

2 12 14 0 0 16 8 1 21 20 8 18 0 10 18 0 0 75 0 1 0 13 1 3 5 0 0 7 5 5

7 7 2 0 0 36 2 4 19 3 0 0 0 3 3 0 9 142 0 7 0 6 0 2 0 2 1 1 0 23

1 5 2 0 0 29 3 2 11 2 0 0 0 4 4 0 18 143 0 2 0 5 6 5 0 0 0 2 2 9

4 13 5 5 14 4 6 4 0 5 0 0 1 0 0 0 10 221 2 0 1 0 0 5 0 0 0 3 2 0

7 24 9 0 0 47 5 3 41 9 2 12 0 3 31 0 8 107 0 2 0 7 3 2 0 0 0 5 0 2

2 13 5 0 0 43 15 10 38 7 4 17 0 7 41 0 11 66 0 13 0 24 2 5 1 0 0 5 3 1

12 6 1 0 0 29 10 6 24 1 0 11 0 0 45 0 7 57 0 6 0 13 9 2 2 2 0 0 1 4

3 13 1 11 11 1 1 0 0 10 0 0 9 0 0 0 10 197 0 0 0 0 0 0 0 1 0 2 3 0

1 10 1 10 3 2 8 3 0 7 1 0 5 0 0 5 6 212 0 0 1 0 0 2 0 1 0 0 0 0

2 10 10 13 7 4 0 0 0 3 5 0 4 0 4 5 1 188 1 0 1 0 0 3 5 0 0 0 5 0

2 13 8 16 5 6 6 2 0 1 0 0 2 6 7 29 0 208 9 0 5 0 0 3 0 1 0 2 3 0

0 5 5 14 5 1 4 3 0 4 4 0 1 0 7 6 11 184 8 0 6 0 0 1 0 1 0 0 0 0

1 11 13 31 10 6 4 1 0 0 1 0 5 0 2 2 1 305 11 0 0 0 0 7 1 0 0 4 1 0

8 9 24 2 2 2 0 2 0 5 1 0 4 0 5 9 0 160 9 0 1 0 0 0 5 0 1 5 6 0

9 11 9 0 0 92 10 4 45 15 0 6 0 4 21 0 5 63 0 7 0 19 5 6 0 0 0 8 4 6

67

68

69

70

71

72

73

74

75

76

77

78

79

80

81

82

83

84

85

86

87

1/16

1/16

1/128

1/32

1/16

1/8

1/4

1/128

1/64

1/8

1/8

1/4

3/128

3/128

1/64

3/32

3/32

3/128

1/64

1/32

1/16

4896

5040

38,656

17,184

5120

2528

2568

38,784

19,264

3552

2488

1444

12,928

12,715

26,112

3264

3243

12,843

21,248

10,112

4832

500 m depth. The AAIW (T = 4 to 10 °C; S = 34.2 to 34.8) is found between 500 and 1000–1200 m and presents a marked salinity minimum, as well as relatively high total phosphate values (see Table 1). The NADW (T = 3 to 4 °C, S = 34.6 to 35) reaches depths of 3000 m and is characterized by both low nutrient (see Table 1 for phosphate values) and high oxygen contents. Between latitudes 21° and 23°S, the AAIW exhibits a relatively lower dissolved-oxygen content (approximately 4.2 ml l − 1 ) than the NADW, which presents values of dissolved-oxygen content up to 5.6 ml l− 1 (Tsuchiya et al., 1994). In terms of current structure, the Campos Basin region comes under the influence of three boundary currents: the Brazil Current (BC), the Intermediate Western Boundary Current (IWBC) and the Deep Western Boundary Current (DWBC) (Silveira et al., 2000). From the surface down to intermediate waters, the BC flows south–southwestward with maximum speeds of 0.5–0.8 m s− 1 (Fig. 3), and transports the TW and SACW. Below 500 m, there is a flow direction reversal associated with the IWBC. This intermediate-level undercurrent transports the AAIW, and mixtures of SACW-AAIW in its upper portion and of AAIW-NADW in its lower part. It has a swift narrow core

of velocities that exceed 0.30 m s− 1 centered at 800 m (Böebel et al., 1999; Silveira et al., 2004). The IWBC can extend vertically for more than 1200 m. The DWBC occupies depths greater than 2000 m and transports basically the NADW. Within the Campos Basin, the DWBC is displaced offshore due to the occurrence of the São Paulo Plateau and becomes decoupled from the BC-IWBC system. As depicted in Fig. 3, the resulting current scenario in the upper 2000 m is marked by a strong vertical shear with the BC and IWBC flowing in opposite directions. Intense eddy activity is a dynamic consequence of this current structure and leads to occasional clockwise ring shedding into the interior waters of the South Atlantic Subtropical gyre. 2.2. Sediments The inner to middle shelf in the Campos Basin is covered with siliciclastic to bioclastic sands. The modern outer shelf is dominated by quartzo-feldspathic sand with secondary carbonate, derived from red and green algae. The base of the shelf-edge scarp constitutes an important depositional site for shelf

S.H. de Mello e Sousa et al. / Marine Micropaleontology 61 (2006) 40–57

47

Table 3 (continued) 67

68

69

70

71

72

73

74

75

76

77

78

79

80

81

82

83

84

85

86

87

1/16 4896

1/16

1/128

1/32

1/16

1/8

1/4

1/128

1/64

1/8

1/8

1/4

3/128

3/128

1/64

3/32

3/32

3/128

1/64

1/32

1/16

5040

38,656

17,184

5120

2528

2568

38,784

19,264

3552

2488

1444

12,928

12,715

26,112

3264

3243

12,843

21,248

10,112

0 1 1 0 0 6 6 3 9 11 3 3 0 1 14 0 7 132 0 6 0 2 0 3 6 1 5 6 1 13

4832

3 8 0 0 0 18 8 7 17 9 0 9 0 3 31 0 9 94 0 3 0 3 3 8 0 0 0 6 0 5

0 15 9 11 6 1 0 0 0 1 4 0 4 0 2 7 0 193 4 0 0 0 0 1 6 0 0 0 4 0

9 17 6 36 23 9 2 2 0 5 1 0 1 0 0 28 7 326 0 0 1 0 0 6 0 0 0 4 3 0

6 10 8 0 0 19 8 2 32 3 0 4 0 5 5 0 1 122 0 1 0 6 2 3 0 7 1 8 1 6

1 6 4 0 0 15 7 0 19 5 0 4 0 2 8 0 11 138 0 2 0 2 3 4 0 6 4 5 6 13

4 3 0 0 0 17 10 2 30 17 0 13 0 38 29 0 36 244 0 5 0 25 17 4 5 5 1 1 1 16

3 17 10 24 7 2 3 3 0 1 4 0 3 0 11 4 0 174 1 0 1 0 0 3 1 0 0 0 1 0

3 7 18 17 3 2 1 3 0 6 1 0 6 0 3 15 0 157 2 0 1 0 0 1 4 0 0 2 2 0

5 16 11 0 0 31 9 0 29 8 9 15 0 19 23 0 6 33 1 11 0 34 4 19 8 7 9 4 3 15

1 6 6 0 0 19 1 6 17 3 2 0 0 2 0 0 19 108 1 11 0 9 1 1 1 4 15 7 0 25

0 6 2 0 0 19 12 5 28 6 0 1 0 6 5 0 34 59 0 25 0 6 13 6 0 0 0 4 3 27

4 15 3 11 4 4 4 4 0 6 7 0 4 0 1 5 0 203 0 0 0 0 0 5 0 0 0 0 0 0

2 7 6 10 8 2 4 4 0 5 3 0 8 0 5 13 1 176 3 0 1 0 0 2 3 0 0 0 0 0

6 8 8 30 13 0 5 0 0 7 6 0 1 0 6 10 2 251 4 0 2 0 0 3 0 2 0 2 1 0

2 3 4 9 8 3 4 1 0 6 3 0 2 0 2 19 10 151 8 0 7 0 0 7 2 1 0 1 3 2

1 15 2 14 3 1 0 0 0 5 1 0 0 0 0 3 29 142 8 0 6 0 0 1 2 1 0 0 2 2

2 12 6 22 3 3 2 1 0 6 4 0 5 0 2 7 5 161 3 0 0 0 0 1 2 0 0 0 0 0

2 14 11 24 11 3 3 2 0 4 7 0 3 0 2 5 2 171 2 0 5 0 0 2 6 2 2 2 3 2

3 16 1 26 8 0 3 2 0 2 4 0 2 0 8 5 8 158 0 0 6 0 0 3 4 0 0 5 1 1

4 12 7 11 7 4 2 1 0 6 2 0 9 0 20 0 0 159 4 0 1 0 0 3 1 0 1 0 0 3

spill-over sands. These deposits are composed of fineto-coarse grained sands to the south, and of coarser sands to the north of the São Tomé canyon. The middle slope (550–1200 m depth) is characterized by indurated finely laminated iron-rich sands (generally 10 cm thick) and deep-water coral mounds that overlie silty-mud to sandy laminated muds, disrupted in some places by mass-flow processes (Viana et al., 1998). The occurrence of sandy crusts rich in iron oxides probably reflects oxic conditions in the ocean bottom, although the dissolved oxygen content in the AAIW is considered relatively low in the area by Tsuchiya et al. (1994). Below the 1200 m isobath, the NADW flows southwards. At this depth, a thin (< 10 cm thick) calcareous nannoplankton-planktonic foraminifera ooze overlies the iron-rich crust with a sharp nonerosive contact (Viana et al., 1998). 3. Material and methods Forty-one samples were collected with a box corer (50 cm long × 50 cm wide × 50 cm high) along transects ranging from 750 to 1950 m water depth (see Table 1 for location of the sampling stations). Sections (10 cm

long × 10 cm wide × 10 cm high) were taken from each box core using a stainless steel box, and the core top (0– 2 cm interval) of each section was collected. All samples were, upon collection, immediately put into a plastic bottle containing 4% formalin solution of Bengal Rose (Walton, 1952). An aliquot of 10 cm3 was removed from the preserved material and washed through 125 μm and 63 μm mesh screens in order to study the >125 μm and the 63–125 μm fractions separately. These residues were dried and at least 300 tests of both living and dead specimens of benthic foraminifera were selected for identification and quantitative analysis. Samples were split when there was a large number of benthic foraminiferal specimens. Faunal counts were multiplied to give values for absolute abundance of each species per 10 cc. Classification was based mainly on Boltovskoy et al. (1980), van Morkoven et al. (1986), Loeblich and Tappan (1988), Jones (1994), and Barbosa (2002). In the study area, the abundance of living specimens was very low and for this reason the statistical analysis and numerical indices of faunal diversity and equitability were applied to the total fauna (living + dead specimens).

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S.H. de Mello e Sousa et al. / Marine Micropaleontology 61 (2006) 40–57

Table 4 Selected (frequencies > 10%) foraminifera species identified in the >125 μm size fraction Stations/species

45

46

47

48

50

51

52

53

54

56

57

58

59

60

61

62

63

64

65

Split

9\28

Total

Total

Total

3\8

Total

Total

Total

7\8

Total

Total

Total

1\8

1\8

1\16

3\8

7\16

Total

1\2

Specimens/10cc

4281

3146

2521

2554

816

5314

2815

4931

352

3004

6244

2844

2608

3040

4816

824

698

324

602

4 1 3 7 0 0

1 14 28 42 91 24

17 42 28 8 84 21

64 0 22 15 53 50

0 0 3 5 0 2

6 32 184 222 242 123

16 49 50 16 252 14

49 15 76 31 438 46

5 0 4 1 0 1

4 49 173 66 329 37

7 30 196 81 645 226

20 96 48 10 233 80

1 2 2 4 0 1

4 0 0 10 0 0

0 0 0 2 0 0

9 1 3 5 0 1

12 0 0 0 0 0

0 0 2 14 0 0

1 1 0 5 0 0

0 6 9 0 11

126 21 33 88 14

70 0 33 35 9

85 14 46 38 8

0 4 8 0 5

315 22 310 311 10

134 2 22 8 12

165 17 32 9 10

0 0 4 0 6

291 9 69 88 0

572 18 108 266 6

192 0 9 96 19

0 8 10 0 19

0 26 4 0 28

0 4 14 0 17

0 1 3 0 18

0 0 1 10 16

0 6 6 0 5

0 0 12 0 13

19

18

8

2

2

24

1

10

1

2

3

5

23

17

22

11

11

18

15

0 7 0 2 0 7 46 4 2 0 8 0 0 21 0 0 0 6 0 1 7 5 4 3 0 8 3 0 20 18 0

174 0 138 0 14 41 986 32 7 7 2 133 0 0 28 90 44 11 0 65 25 0 3 0 20 0 79 39 98 86 24

64 0 63 0 56 16 630 4 12 43 28 174 7 10 7 50 21 2 0 193 147 0 0 0 56 0 3 5 51 40 87

17 0 58 0 266 25 824 6 19 108 30 1 0 35 39 2 3 4 2 54 128 0 0 1 43 0 6 0 32 5 94

0 9 5 6 10 5 20 8 0 0 4 0 0 7 0 0 0 4 0 8 14 2 5 1 7 6 25 0 18 24 0

194 0 273 0 0 68 1152 0 4 17 8 196 0 13 17 34 80 7 0 29 28 0 18 0 6 0 44 19 162 111 105

33 0 24 0 63 16 1009 5 9 58 48 47 0 30 8 9 8 1 0 119 85 0 0 0 29 0 11 0 20 8 165

88 0 61 0 270 8 2161 1 17 32 11 77 15 44 101 34 1 8 0 53 61 1 2 0 12 0 12 2 40 37 152

1 1 11 4 13 4 11 13 2 0 1 0 0 13 0 0 0 1 0 2 23 0 5 8 1 7 2 0 8 44 0

31 0 221 0 56 20 749 1 13 21 5 58 0 5 26 79 1 2 0 75 33 0 1 0 20 0 4 1 46 39 21

117 0 619 0 165 48 1000 1 25 195 4 362 0 7 37 105 18 1 0 68 78 0 2 0 24 0 5 18 82 52 28

6 1 361 0 56 29 468 0 50 50 9 105 17 25 87 20 18 9 5 71 88 0 2 5 71 0 12 0 6 10 36

0 0 3 0 6 9 23 11 1 0 0 0 1 26 0 0 0 6 2 1 3 2 7 18 4 8 3 0 11 19 0

0 11 1 2 0 7 26 10 10 0 17 0 0 22 0 0 8 10 6 0 7 25 0 6 3 1 9 0 27 8 0

4 6 0 3 0 7 40 7 8 0 7 0 0 8 0 0 9 2 1 0 6 13 2 0 3 3 8 0 21 11 0

1 13 1 9 1 4 22 24 4 0 15 0 0 12 1 0 4 3 2 2 4 10 0 0 9 11 6 0 16 12 0

2 0 2 0 1 1 26 45 12 0 14 0 0 22 1 0 6 2 6 1 4 9 0 4 9 1 3 0 1 5 0

0 4 2 10 2 2 45 21 5 0 5 0 0 4 2 0 1 7 3 0 2 3 22 0 1 0 3 2 31 29 0

0 0 2 14 4 5 31 19 7 0 13 0 0 0 0 0 11 5 0 0 4 9 1 0 3 3 16 12 11 0 7

Ammovertelina spp. Angulogerina angulosa Bolivina a. multicostata Bolivina albatrossi Bolivina spp. Bolivinella daggaziensis Brizalina spp. Bulimina aculeata Cassidulina laevigata Cibicides sp. Cibicidoides kullenbergi Cibicidoides wuellerstorfi Cibicidoides sp. C. subglobosus Cymbaloporetta bradyi Epistominella exigua Eponides weddellensis Fissurina orbignyana G. subglobosa Haplophragmoides spp. Hoeglundina elegans Hormosina spp. Lagenammina sp. Lenticulina cultrata Lenticulina sp. Miliolinella subrotunda Miliolinella sp. Nonion spp. Oridorsalis umbonatus Pullenia bulloides Pyrgoella irregularis Rhabdammina spp. Rhizammina sp. S. schlumbergeri Trifarina bradyi Triloculina laevigata Trochammina advena Trochamminita spp. Uvigerina bradyana Uvigerina crassicostata Uvigerina peregrina Uvigerina proboscidea Verneuilinulla sp. 66

67

68

69

70

71

72

73

74

75

76

77

78

79

80

81

82

83

84

85

86

87

Total

Total

Total

1\4

1\4

Total

Total

Total

1\52

1\32

Total

Total

Total

1\8

3\64

1\8

Total

5\8

1\8

1\8

3\16

5\8

3420

3571

4655

1264

1216

2601

2179

1900

15236

9664

1511

2186

1199

2440

6505

2504

303

495

3376

2448

1654

516

Cluster and Canonical Correspondence analyses were applied to the total faunal data using the MVSP Software (Kovach Computing), version 3.11. Euclidean Distance Index was used with clustering undertaken by the Ward method. Variables were standardized as proportions prior to the multivariate statistical analyses. Species which occurred in less than 10% of the samples and at percentages lower than 2% were removed from the total faunal assemblages.

Species diversity (Shannon–Wiener index, using loge, H(S)) and equitability of the total fauna data were calculated, using the Primer Software (Primer-E Ltd.). Grain size analyses were undertaken using the sieving and pipetting methods. Total organic carbon and total carbon contents were determined in a CHNS/O Perkin Elmer analyzer (2400 series II), prior to and after acidification of the sediment samples with 1.0 N HCl solution. The carbonate content (i.e. inorganic carbon)

S.H. de Mello e Sousa et al. / Marine Micropaleontology 61 (2006) 40–57

49

Table 4 (continued) 66

67

68

69

70

71

72

73

74

75

76

77

78

79

80

81

82

83

84

85

86

87

Total

Total

Total

1\4

1\4

Total

Total

Total

1\52

1\32

Total

Total

Total

1\8

3\64

1\8

Total

5\8

1\8

1\8

3\16

5\8

3420

3571

4655

1264

1216

2601

2179

1900

15236

9664

1511

2186

1199

2440

6505

2504

303

495

3376

2448

1654

516

3 63 77 68 645 70

3 0 11 13 66 66

2 45 121 0 270 120

0 3 0 14 0 0

4 0 0 6 0 0

6 42 71 60 133 56

12 7 42 28 105 49

11 12 9 0 51 30

1 0 1 20 0 0

1 0 0 14 0 0

3 15 48 33 93 27

6 7 42 42 133 7

53 0 18 6 57 36

0 0 7 8 0 0

2 0 2 5 0 0

3 0 0 4 0 0

10 0 0 1 0 0

12 0 0 0 0 0

1 0 1 21 1 0

1 0 1 4 0 0

4 2 1 0 0 0

0 0 0 0 0 2

315 33 114 57 5

100 33 127 44 22

225 1 146 146 8

0 9 6 0 8

0 2 2 0 30

224 33 23 47 21

133 0 35 28 5

90 1 51 40 1

0 52 1 0 6

0 27 5 2 16

87 6 24 49 10

119 0 21 7 0

84 3 18 7 3

0 23 20 0 8

0 22 6 0 39

0 3 4 0 24

0 0 2 4 7

0 0 0 0 21

0 26 2 5 31

0 8 2 4 16

0 0 3 0 11

0 0 2 6 15

22

8

3

31

28

22

11

4

50

13

18

5

13

14

17

28

20

10

27

39

29

16

78 4 149 0 35 30 524 1 13 50 14 147 1 0 35 25 10 13 0 12 3 0 0 0 10 0 42 4 103 35 46

32 0 155 0 77 31 1493 5 15 66 11 27 49 5 14 37 71 63 1 56 112 0 0 2 13 0 11 0 86 11 148

58 3 465 0 135 122 1421 0 34 46 5 47 105 3 48 47 3 11 3 60 8 0 0 3 10 0 7 0 90 1 80

0 1 2 20 1 1 43 8 10 0 5 0 0 3 0 0 2 6 4 0 0 8 25 1 1 3 3 2 2 30 0

0 7 0 12 1 10 36 12 6 0 8 0 0 2 0 0 11 4 1 0 3 8 0 0 0 1 20 0 34 0 0

97 3 35 0 7 38 910 0 7 9 3 44 2 1 14 14 5 8 1 90 53 0 0 0 20 0 34 10 95 10 55

14 2 56 0 77 31 972 0 5 14 20 14 0 7 21 0 2 2 4 53 31 0 14 2 33 0 4 0 35 42 93

114 0 87 0 108 12 740 0 21 15 0 76 0 17 52 30 15 17 1 43 4 0 0 3 24 0 3 0 5 3 48

2 8 1 5 0 0 0 5 1 0 0 0 0 7 0 0 2 2 0 6 8 5 14 5 0 0 0 0 5 20 0

0 5 0 1 0 1 42 3 7 0 0 0 0 14 0 0 12 2 5 1 3 17 2 1 0 0 21 6 11 18 0

57 2 69 0 18 26 111 3 6 33 7 102 0 6 12 42 35 22 4 25 33 0 0 1 14 0 32 8 38 10 48

16 2 0 0 133 10 762 7 17 77 19 63 0 14 7 0 7 0 3 50 107 0 0 1 15 0 3 0 52 0 176

19 1 15 0 102 16 183 0 17 75 16 18 0 13 39 3 4 2 5 24 2 0 4 0 27 0 6 0 12 9 81

0 1 5 4 0 2 38 1 0 0 0 0 0 4 0 0 3 6 3 0 1 0 15 7 0 0 8 0 5 25 0

0 5 0 3 0 1 42 4 0 0 5 0 0 13 0 0 7 3 1 1 2 5 4 1 3 0 4 2 11 13 0

0 11 0 7 1 9 49 2 6 0 11 0 0 1 0 0 11 3 0 0 3 7 1 0 3 0 15 11 23 4 7

0 9 0 9 0 1 22 8 3 0 43 0 0 7 0 0 4 8 15 5 7 7 0 3 6 7 9 7 11 1 3

0 18 1 2 0 1 6 15 34 0 24 0 0 23 0 0 3 5 12 5 6 2 0 5 15 10 4 0 1 0 0

0 4 0 5 0 5 50 3 9 0 15 0 0 29 0 0 16 1 10 0 4 34 3 8 1 0 20 8 0 17 0

0 7 0 1 1 11 31 4 8 0 0 0 0 2 0 0 14 23 2 0 2 15 2 1 4 0 31 3 25 3 0

0 19 1 6 1 0 21 8 7 0 30 0 0 15 0 0 13 8 13 1 5 12 0 2 4 1 13 7 15 0 0

0 50 0 0 0 2 0 11 42 0 23 0 0 11 0 0 10 8 11 0 7 4 0 7 10 7 0 0 0 0 6

was determined as the mass difference between total organic carbon and total carbon values. 4. Results 4.1. Sedimentological data Sandy sediments in general predominate at 750 m and 1050 m depths. However, some exceptions occur in

the grain-size distribution, with higher mud contents at stations 50, 54, 64, 65 and 69. High mud content (up to 97%) is found at most of the stations located between 1350 and 1950 m depth. Calcium carbonate concentrations range between 24% and 50%, and relatively lower CaCO3 values are found at the shallowest stations (Table 1). The total organic carbon concentrations vary between 2.2 mg g− 1 and 18.1 mg g− 1. The lowest total organic carbon concentrations are observed in sandy

50

S.H. de Mello e Sousa et al. / Marine Micropaleontology 61 (2006) 40–57

sediments between 750 and 1050 m water depth, with an increase in the total organic carbon in muddy sediments at the shallowest and deepest stations (Table 1). 4.2. Abundance and diversity In the 63–125 μm size fraction the abundance of benthic foraminifera per 10 cc of sediment varies between 2248 and 19,392 individuals in the northern part of the basin and between 1384 and 38,016 specimens in the southern part of the study area. In general, the abundance of the benthic foraminifera in the fine size fraction was higher than that in the coarse fraction in both regions of the Campos Basin. Analyses of the >125 μm size fraction reveal abundances of benthic foraminifera per 10 cc of sediment varying between 315 and 6244 individuals in the northern part of the study area, and between 285 and 13,936 specimens in the southern part of the basin. Standard deviation values of the abundance of benthic foraminifera are higher at 750 m depth in both fractions (14,761 and 5230 for fine and coarse fractions, respectively). The lowest values of standard deviation (2669 for the fine fraction and 1189 for the coarse fraction) were observed in the deeper regions (between 1350 and 1950 m water depth). No correlation was found between benthic foraminiferal abundance calculated for 750 and 1950 m depth ranges and sedimentological parameters (mud, sand, organic carbon and carbonate contents) in either of the size fractions (R2 varying between 0.0545 and 0.2662 for the fine fraction and R2 varying between 0.004 and 0.2088 for the coarse fraction) (Table 2). Values H(S) for diversity in the 63–125 μm size fraction vary between 1.7 and 3.0 and between 1.6 and 3.5 in the northern and southern regions of the area investigated, respectively. The lowest diversities and equitabilities are found at 750 m water depth (Table 2). The calculated values of H(S) for diversity in the coarse fraction vary between 2.7 and 3.7 in the northern region of the Campos Basin. In the southern region the values for H(S) are slightly lower, varying between 2.6 and 3.7 (Table 2). In contrast to the fine size fraction, the highest diversities and equitabilities for the >125 μm size fraction occur in the shallowest part of the study area (between 750 and 1050 m water depth), with the lowest diversity and equitability values found in the deeper regions of the basin (Table 2). 4.3. Taxonomic composition Hyaline calcareous foraminifera are predominant at all of the stations: (a) 63–125 μm: calcareous-hyaline

70%, agglutinated 20%, calcareous–porcelaneous 10%; (b) > 125 μm: calcareous-hyaline 60%, agglutinated 25%, calcareous–porcelaneous 15% (Tables 3 and 4). However, there are some significant differences when the species composition of each of the two fractions is compared. In the 63–125 μm size fraction, 142 benthic foraminifera species were identified. The species Globocassidulina subglobosa and Bolivina spp. were the most abundant species in nearly all the samples, with relative abundance of up 68% and 22%, respectively. The other species, such as Bolivina aenarienensis multicostata, Bolivina doniezi, Brizalina spp., Cymbaloporetta bradyi and Eponides weddellensis show relative abundance no higher than 16% (Table 3). In the >125 μm size fraction, 199 benthic foraminifera species were recognized. Although G. subglobosa is still abundant (relative abundance up to 44%), its relative abundances are < 20% at nearly half the stations. The species Bolivina spp., Brizalina spp., Cibicidoides kullenbergi, Cibicidoides wuellerstorfi, Uvigerina peregrina and Uvigerina proboscidea are present at almost all the stations, but their relative abundances are no higher than 19%. It should also be pointed out that the relative abundances of Rhizammina sp. may be overestimated as the tests of these individuals were frequently broken (Table 4). The most frequent species of benthic foraminifera in the combined > 63 μm size fraction are B. aenarienensis multicostata, B. doniezi, Bolivina spp., Brizalina spp., C. bradyi, E. weddellensis and G. subglobosa. 4.4. Distribution of benthic foraminiferal assemblages Statistical analyses were applied to 63–125 μm, >125 μm and the combined > 63 μm size fractions, and the results were similar for all the fractions. Thus, we have decided to present the results obtained in the statistical analyses applied to the combined > 63 μm size fraction. Q-mode cluster and CCA analyses show the dissimilarity between samples located on the middle slope (Group 1: 750 to 1050 m water depth) and most of the samples located in the deeper region of the basin (Group 2: 1350 to 1950 m water depth). Only the deeper stations 61, 62, 63, 81, 82, 83 , 85, 86 and 87, situated between 1350 and 1950 m water depth, were grouped together with the middle slope samples (Figs. 4 and 5). R-mode cluster and CCA analyses reveal two main assemblages (Groups A and B): Bolivina spp.(Group A) and G. subglobosa (Group B). The Bolivina spp. assemblage is composed of Angulogerina angulosa, Bolivina

S.H. de Mello e Sousa et al. / Marine Micropaleontology 61 (2006) 40–57

Fig. 4. Q-mode cluster analysis in the >63 μm fraction.

Fig. 5. Q-mode canonical correspondence analysis in the >63 μm fraction.

51

52

S.H. de Mello e Sousa et al. / Marine Micropaleontology 61 (2006) 40–57

spp., Bolivinella daggaziensis, Brizalina spp., Cibicides sp., Cibicidoides sp., C. bradyi, E. weddellensis, Hormosina spp., Laticarinina pauperata, Lenticulina cultrata, Miliolinella sp., Rhabdammina spp., Rhizammina sp. and Verneuilinulla sp. The most abundant species of this assemblage are Bolivina spp., Brizalina spp., C. bradyi and E. weddellensis (Figs. 6 and 7). This assemblage occurs preferentially at the deeper stations sampled (Group 2), characterized in general by muddy sediments, and relatively high organic carbon content, and total phosphate values measured in the water varying between 1.09 μmol kg− 1 and 2.3 μmol kg− 1 (see Table 1) (Fig. 8). The distribution of the species E. weddellensis, Hormosina spp., Miliolinella sp. and Verneuilinulla sp. is mainly determined by the carbonate and mud contents. Although the organic carbon content has a minor influence on the distribution of the species, Bolivina spp., Brizalina spp., Cibicides sp., L. cultrata and L. pauperata are apparently the most influenced by this parameter (Fig. 7). The G. subglobosa assemblage is composed of B. aenarienensis multicostata, Bolivina albatrossi, B. doniezi, Bolivina simplex, Bulimina aculeata, Cassidulina laevigata, Cassidulinoides parkerianus, C. kullen-

bergi, C. wuellerstorfi, Epistominella exigua, G. subglobosa, Haplophragmoides sp., Karrerulina conversa, Miliolinella subrotunda, Oridorsalis umbonatus, U. peregrina and U. proboscidea. The most abundant species of this assemblage are B. aenarienensis multicostata, B. albatrossi, B. doniezi, C. laevigata and G. subglobosa (Figs. 6 and 7). This assemblage is present in the shallower regions of the basin (between 750 and 1050 m water depth), with sandy sediments and highly variable organic carbon content (between 2.2 mg g− 1 and 18.1 mg g− 1), also characterized by total phosphate values varying between 1.27 μmol kg− 1 and 2.29 μmol kg− 1 (see Table 1) (Fig. 8). The species E. exigua, C. kullenbergi and U. proboscidea are those most influenced by the sand content in the sediment (Fig. 7). Fig. 9 shows the vertical distribution of selected species in the combined> 63 μm size fraction. It is evident that the increase in the frequencies of B. aenarienensis multicostata, B. doniezi, B. simplex, E. exigua and G. subglobosa, species characteristic of the region under AAIW conditions, is often accompanied by a decrease in the frequencies of Bolivina spp., B. daggaziensis, Brizalina spp., C. bradyi, E. weddellensis

Fig. 6. R-mode cluster analysis in the >63 μm fraction (species abbreviations: Aa = Angulogerina angulosa; Bam = Bolivina aenarienensis multicostata; Ba = Bolivina albatrossi; Bd = Bolivina doniezi; Bs = Bolivina simplex; Bsp = Bolivina spp.; Bolivinella daggaziensis; Brs = Brizalina spp.; Bu = Bulimina aculeata; Cl = Cassidulina laevigata; Cp = Cassidulinoides parkerianus; Cs = Cibicides sp.; Ck = Cibicidoides kullenbergi; Cw = Cibicidoides wuellerstorfi; Ci = Cibicidoides sp.; Cb = Cymbaloporetta bradyi; Ee = Epistominella exigua; Ew = Eponides weddellensis; Gs = Globocassidulina subglobosa; Hs = Haplophragmoides sp.; Hos = Hormosina spp., Kc = Karrerulina conversa; Lp = Laticarinina pauperata; Lc = Lenticulina cultrata; Ms = Miliolinella subrotunda; Msp = Miliolinella sp.; Ou = Oridorsalis umbonatus; Rha = Rhabdammina spp.; Rhi = Rhizammina sp.; Ss = Sigmoilopsis schlumbergeri; Up = Uvigerina peregrina; Upr = Uvigerina proboscidea; Vs = Verneuilinulla sp.).

S.H. de Mello e Sousa et al. / Marine Micropaleontology 61 (2006) 40–57

53

Fig. 7. R-mode canonical correspondence analysis in the> 63 μm fraction (species abbreviations: Aa =Angulogerina angulosa; Bam =Bolivina aenarienensis multicostata; Ba=Bolivina albatrossi; Bd =Bolivina doniezi; Bs =Bolivina simplex; Bsp =Bolivina spp.; Bolivinella daggaziensis; Brs =Brizalina spp.; Bu=Bulimina aculeata; Cl=Cassidulina laevigata; Cp=Cassidulinoides parkerianus; Cs=Cibicides sp.; Ck =Cibicidoides kullenbergi; Cw=Cibicidoides wuellerstorfi; Ci=Cibicidoides sp.; Cb =Cymbaloporetta bradyi; Ee=Epistominella exigua; Ew=Eponides weddellensis; Gs =Globocassidulina subglobosa; Hs =Haplophragmoides sp.; Hos=Hormosina spp., Kc =Karrerulina conversa; Lp=Laticarinina pauperata; Lc=Lenticulina cultrata; Ms=Miliolinella subrotunda; Msp=Miliolinella sp.; Ou =Oridorsalis umbonatus; Rha =Rhabdammina spp.; Rhi=Rhizammina sp.; Ss=Sigmoilopsis schlumbergeri; Up =Uvigerina peregrina; Upr=Uvigerina proboscidea; Vs =Verneuilinulla sp.).

and Verneuilinulla sp., which occur under the dominance of the NADW. 5. Discussion Although no correlation was found between the number of individuals and the sedimentological parameters on the middle slope (between 750 and 1050 m water depth), the highest standard deviation values of the number of foraminifera specimens (abundance varying between 315 and 38,016 individuals per 10 cc) may be explained by the predominance of sandy sediments at these depths and the high variability of the organic carbon content in the sediment. Conversely, the lowest values of standard deviation in the fine and coarse fractions (abundance varying between 285 and 25,536 specimens per 10 cc) occur in the sector between the 1350 and 1950 m isobaths, characterized by muddy sediments and higher organic carbon content (values between 6.4 mg g− 1 and 16.4 4 mg g− 1). Diversity values of the faunas are highly homogenous in the study area and are in general greater than the

values obtained for the dead specimens by Schmiedl et al. (1997) on the middle and lower continental slope (from 700 to 1900 m water depth) in the eastern South Atlantic. Additionally, the total organic carbon content of the sediment is lower on the slope of the Campos Basin than in the eastern South Atlantic by generally > 3% (Schmiedl et al., 1997). According to these authors, high diversity values, both in the thanatocoenoses and biocoenoses, were observed in oligotrophic environments, low diversity values occurring in the highproductivity areas of the Southwest African shelf. Thus, all these facts seem to reflect the prevailing oligotrophic conditions on the slope of the southeastern Brazilian continental margin. The fact that the stations 61, 62, 63, 81, 82, 83 , 85, 86 and 87, situated between 1350 and 1950 m water depth, were grouped together with the shallower stations may be explained by the presence of the Itapemirim and São Tomé canyons (see Fig. 1), which can lead to the occurrence of downslope sediment movements. However, no reworked foraminiferal tests were recognized, probably due to the paucity of taphonomic data.

54

S.H. de Mello e Sousa et al. / Marine Micropaleontology 61 (2006) 40–57

Fig. 8. Distribution map showing the benthic foraminiferal assemblages in the study area.

Although we recognize occurrence of this downslope transport it is evident that the distribution of the foraminiferal assemblages in this area is greatly influenced by factors that vary with water depth, and consequently by water mass dynamics. The assemblage on the middle slope (750–1050 m) is G. subglobosa which is characterized, among others, by the presence of C. wuellerstorfi, E. exigua and G. subglobosa species which seem to prefer an epifaunal or shallow infaunal microhabitat characterized by low organic matter fluxes and high oxygen content (Lutze and Coulbourn, 1984; Corliss, 1985). Schmiedl et al. (1997) also concluded that G. subglobosa seems to be adapted to environments characterized by enhanced bottom current velocities. This inference is supported by the presence of a narrow core of the IWBC (velocity >0.30 m s− 1) at 800 m water depth on the southeastern continental Brazilian margin, which probably promotes re-suspension of bottom sediments and hinders retention of organic matter. The lowest values of organic carbon content (2.2 mg g− 1, 4.9 mg g− 1 and 5.3 mg g− 1), observed at some stations in this sector of the continental margin (stations 53, 79 and 80), fit this scenario. The relationship between the

sand content and the frequency of C. wuellerstorfi and E. exigua, as observed in the correspondence analysis, seems to corroborate the occurrence of relatively strong bottom currents on the middle slope, which contribute to the reworking of the bottom sediment, promoting the formation of the sandy deposits (see Table 1). The abundance in the G. subglobosa assemblage of various species of Bolivina (B. albatrossi, B. aenarienensis multicostata, B. doniezi), B. aculeata, C. laevigata and U. peregrina, considered epi- or shallow infaunal, deposit feeder species, and proxies of food availability (see Jones and Charnock, 1985; Corliss, 1985; Corliss and Chen, 1988; Mackensen and Douglas, 1989; Debenay and Redois, 1997; Schmiedl et al., 1997, for further references) seem to attest the input of nutrients to the middle slope of the Campos basin. Further, the presence of E. exigua, regarded as the characteristic species for areas with seasonally deposited phytodetritus in the North Atlantic Ocean and considered an opportunist species (r-strategist), able to grow and reproduce rapidly in the presence of phytodetritus (Gooday, 1993; Smart et al., 1994), may reflect that seasonally fluctuating organic fluxes to the sector of the continental Brazilian margin situated between 750 and 1050 m water depth. The input of organic material to the slope of the Brazilian margin is still uncertain. Food supply may be brought to the sea floor by the sinking of the primary productivity and also by the action of turbidite currents or/ and boundary currents. Although these currents promote the re-suspension of mud, they can be rich in nutrients, which are sometimes rapidly utilized by the benthic microfauna. The bacterial activity may also play a role in the organic material present in the substrate. There are no available data concerning this for the Brazilian continental slope, but studies carried out by Sumida et al. (2005) show an increase in microbial biomass in the southeastern Brazilian continental shelf areas characterized by upwelling events. As with the G. subglobosa assemblage, an assemblage that exhibits species with different characteristics is to be observed on the lower slope (between 1350 m and 1950 m depth). The Bolivina spp. assemblage is composed of species which are considered by some authors (e.g., Corliss, 1985; Jones and Charnock, 1985; Corliss and Chen, 1988) to be deposit feeders, preferring to live epi- or infaunally (e.g., A. angulosa, Bolivina spp., Brizalina spp., E. weddellensis, L. cultrata, Hormosina sp. and Verneuilinulla spp.), as well as species that are interpreted as suspension feeders (e.g. Rhabdammina spp., Rhizammina sp.) (cf. Koutsoukos and Hart, 1990; Murray, 1991).

S.H. de Mello e Sousa et al. / Marine Micropaleontology 61 (2006) 40–57

55

Fig. 9. Water masses and frequencies of selected benthic foraminiferal species (>63 μm fraction) from the study area.

The common occurrence of Rhizammina spp., considered to be epifaunal suspension-feeders, in the Bolivina spp. assemblage, might also reflect a stable substrate with little reworking by deposit feeders in deeper areas of the basin, probably due to a decrease in the bottom current velocities (Fig. 3) which promotes a retention of the organic matter in the sediment and is reflected in a relative increase in the mud and organic carbon contents of the sediment (see Table 1). A positive correlation can be observed between the abundance of the species Bolivina spp. and Brizalina spp., present in the Bolivina spp. assemblage, with the organic carbon content of the sediment (see Fig. 7), confirming these species as food supply markers in this specific case. Conversely, no correlation was found between TOC and all the Bolivina species observed in the G. subglobosa assemblage. One reason that might explain this fact would be the relatively high instability of the substrate on the middle slope of the basin, due to the presence of vigorous bottom currents. The relatively low nutrient (e.g., total phosphate content, see Table 1), high dissolved-oxygen content of the North Atlantic Deep Water (see Tsuchiya et al., 1994

for reference) and foraminiferal community structure attest oligotrophic conditions in the deeper regions of the study area. In contrast to these regions, although relatively high total phosphate contents are recognized in the bottom waters of the shallower regions of the basin (between 750 and 1050 m water depth) (see Table 1), the presence of strong bottom currents (see Fig. 3) seems to enhance the oligotrophic conditions of the environment, by the establishment of the instability at the benthic/pelagic boundary and the re-suspension of the particulate organic carbon. As shown by the abundance of the species U. peregrina, reported in normally oxygenated waters (Quinterno and Gardner, 1987), the dissolved-oxygen content in the bottom waters seems not to be a restrictive factor for the microfaunal establishment on the middle slope of the Campos Basin. 6. Conclusions The distribution of benthic foraminifera in the Campos Basin seems to be determined chiefly by two environmental factors, both mainly controlled by the

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Antarctic Intermediate Water and North Atlantic Deep Water: a) Food supply: The benthic foraminiferal fauna is characterized by high diversity values and many species are well adapted to high oxygen levels and high heterogeneity of food supply. The lower continental slope (between 1600 and 1900 m water depth) is inhabited by a large number of preferentially epifaunal and suspension species that are adapted to oligotrophic conditions and relatively high dissolved oxygen levels in the bottom waters (e.g., E. weddellensis, Rhabdammina spp. and Rhizammina sp.). Additionally, the presence on the middle and lower slope of a large number of preferentially infaunal and deposit feeder species (e.g., Bolivina, Bulimina, Cassidulina and Uvigerina), adapted to high organic matter fluxes, might reflect local changes in the trophic structure of the environment. The oxygen content of the bottom waters does not seem to be a restrictive factor determining the distribution of the foraminifera in the study area. b) Energy state (stability) at the benthic/pelagic boundary and the grain size of the substrate. The middle continental slope (between 750 and 1050 m water depth) is characterized by G. subglobosa, adapted to strong bottom currents and sandy sediments, differently from the lower continental slope, which may exhibit a more stable oceanic bottom. Acknowledgments We would like to thank Rodolfo Pinheiro da Rocha Paranhos (Universidade Federal do Rio de Janeiro) for the chemical analyses, Mr. Marcelo Rodrigues for his help with the figures, and to PETROBRAS S/A for their permission to publish this manuscript. The authors also thank Bruce W. Hayward and Johann Hohenegger for the revision of the manuscript. We also thank Jean-Pierre Debenay for valuable comments and criticism. This paper is a contribution to the PETROBRAS multidisciplinary environmental research project called the “Campos Basin Deep-sea Environmental Program”. References Barbosa, V.P., 2002. Sistemática, bioestratigrafia e paleoceanografia de foraminíferos do Quaternário do talude continental das bacias de Santos e Campos. Ph.D. Thesis, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brasil, 455p. Bernhard, J.M., Sen Gupta, B.K., Borne, P.F., 1997. Benthic foraminiferal proxy to estimate dysoxic bottom-water oxygen con-

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