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Zoogeography of Holocene Podocopina (Ostracoda) from the Brazilian Equatorial shelf
Marine Micropaleontology 37 (1999) 365–379 www.elsevier.com/locate/marmicro
Zoogeography of Holocene Podocopina (Ostracoda) from the Brazilian Equatorial shelf Joa˜o Carlos Coimbra a,b,Ł , Iraja´ Damiani Pinto a,b , Norma Luiza Wu¨rdig b , Dermeval Aparecido do Carmo a,c a
Departamento de Paleontologia e Estratigrafia, Instituto de Geocieˆncias, Universidade Federal do Rio Grande do Sul, Cx.P. 15001, CEP. 91501-970 Porto Alegre, RS, Brazil b CECLIMAR-Coastal, Limnological, and Marine Centre, Universidade Federal do Rio Grande do Sul, Av. Tramandaı´, 976, CEP. 95625-000 Imbe´, RS, Brazil c Departamento de Geologia Geral e Aplicada, Instituto de Geocie ˆ ncias, Universidade de Brası´lia, CEP. 70910-900 Brası´lia, DF, Brazil Received 31 July 1997; accepted 11 February 1999
Abstract This study reports on the Holocene ostracods of the Brazilian Equatorial shelf from French Guiana to near Natal, almost 1400 km in length. This part of the Western Atlantic Tropical Province, where the mouths of the Amazon and Para´ rivers separate two zoogeographical units, has distinct oceanographic and sedimentologic features. Three hundred and thirty nine samples were studied from the REMAC (legs 4, 5, 5A and 6) and the GEOMAR projects (legs I, II and III). The ostracod fauna includes 74 species, of which 37 have been previously described (mostly from other areas). The remaining 37 species are left in open nomenclature although 26 of them are probably new. Twelve species are confined to the northwest and 16 to the southeast of the mouths of the Amazon and Para´ rivers. The inner shelf of the northwestern region is a barren zone resulting from turbid freshwater discharged by the Amazon and Para´ rivers. Most of the studied species live at water depths of less than 100 m; 28 species inhabit waters shallower than 75 m, and 18 species live in waters shallower than 50 m. Although Brazilian ostracods have many genera and species in common with Holocene ostracods elsewhere in the tropical Atlantic, diversity differs greatly. For example, diversity on the Brazilian Equatorial shelf is lower than elsewhere in the Western Atlantic Tropical Province, but higher than in the Eastern Atlantic Tropical Province. Faunal similarities between these provinces are at the generic rather than species level; exceptions are few and consist of conservative Tethyan species that are cosmopolitan. 1999 Elsevier Science B.V. All rights reserved. Keywords: Holocene; Ostracoda; Brazil; marine; zoogeography
1. Introduction The study of Holocene ostracods from the studied area began with Brady (1880) who analyzed samples Ł Corresponding
author. Tel.: C51-3166384; Fax: C51-3191811; E-mail: [email protected]
collected by the famous H.M.S. Challenger Expedition towards the end of the last century. He described species assigned to the genera: Bairdia, Bythocypris, Cythere, Cytherella, Krithe and Macrocypris. Two species, Macrocypris similis and Cythere pyriformis, were collected at 1234 m depth on the continental margin of the Pernambuco State. Almost one cen-
0377-8398/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 8 3 9 8 ( 9 9 ) 0 0 0 2 5 - 0
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Fig. 1. Map of the Brazilian Equatorial Shelf indicating location of the 339 samples. Ð D with ostracods; C D barren.
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Fig. 2. Distribution of bottom sediments (after Kowsmann and Costa, 1979).
tury later, Pinto et al. (1978) recorded more than 50 genera in a preliminary study of ostracods collected along 7408 km of the Brazilian continental shelf. In the Brazilian Equatorial shelf they identified 42 genera and registered three indeterminate genera. These authors emphasized the absence of ostracods around
Marajo´ Island, in the Amazon and Para´ estuary, and explained it as the result of the mixing of freshwater from the rivers with seawater which lowered the salinity. Although the Brazilian Equatorial continental shelf comprises one of the most extensive areas of
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continental shelf in the world, few studies after Pinto et al. (1978) discussed the benthic Ostracoda from this region. The works at the species level began with Chukewiski and Purper (1985a,b) and includes Ornellas and Coimbra (1985), Coimbra and Ornellas (1986, 1987, 1989), Purper and Ornellas (1987a,b), Medeiros and Coimbra (1989), Do Carmo (1993), Do Carmo and Sanguinetti (1995), Ramos (1994, 1996), Coimbra et al. (1992, 1994, 1995) and Fauth and Coimbra (1998). 1.1. Study area The Brazilian Equatorial shelf is situated between the international boundary of Oiapoque River (Amapa´ State) and Sa˜o Roque Cape (Rio Grande do Norte State), including the coastal region of Para´, Maranha˜o, Piauı´ and Ceara´ states (Fig. 1). Nine samples only were collected a little to the south of this region, that is, south of the Sa˜o Roque Cape, all of them in abiotic conditions similar to the area of the Sa˜o Roque Cape. The water depths of the study area range to 200 m, except for 31 samples that have been collected in deeper waters, as follows: 15 (with ostracods) located to the southeast of the Amazon=Para´ estuary; and 16 (13 barren ones) located to the northwest of that estuary. The maximum shelf width is 330=350 km at the Amazon and Para´ rivers mouths (Martins and Coutinho, 1981). According to Kowsmann and Costa (1979) the extensive relict sands of the study area are better described as reworked sands, since they were susceptible to changes during and after the Holocene transgression.
Plate 1 Ostracods confined to the northwest of the mouth of the Amazon=Para´ rivers. 1. Cletocythereis sp. ð40.5. 2. Costa variabilicostata ssp. ð55.5. 3. Radimella ovata Van den Bold, 1988. ð48. 4. Eucytherura sp. 2. ð103. 5. Eucytherura sp. 1. ð104. 6. Kangarina ancyla Van den Bold, 1963. ð102.5. 7. Cytheropteron sp. ð71.5. 8. Pseudoceratina droogeri Van den Bold, 1965. ð52. 9. Phlyctocythere sp. 1. ð65. 10. Phlyctocythere sp. 2. ð71. 11. Bradleya sp. ð42.5. 12. Munseyella sp. ð90.5.
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Martins et al. (1975) recognized two physiographic provinces along Brazilian Equatorial shelf: (1) Orange Cape–Parnaı´ba River Delta; and (2) Parnaı´ba River Delta–Sa˜o Roque Cape (Fig. 2a–c). Martins et al.’s first province is dominated by terrigenous sediments in the inner shelf and reworked sandy sediments in the middle and outer shelves. Martins and Coutinho (1981) confirm the pelitic character of the fluvial terrigenous sediments found northwest of the mouth of the Amazon and Para´ rivers. According to Martins (1974), this muddy facies extends to the inner shelf (¾40 m depth). Kowsmann and Costa (1979) mapped a narrow sandy facies in this muddy facies, opposite the mouth of Amazon and Para´ rivers. Martins et al.’s second province, the Parnaı´ba River Delta–Sa˜o Roque Cape, consists of a narrower shelf where carbonate sediments predominate. Kowsmann and Costa (1979) have estimated grades of CaCO3 higher than 75% along this area. The carbonate sediments are constituted by sands and gravels of the red algae Lithothamnium and Lithophyllum or green algae Halimeda alone; facies with a predominance of foraminifer and mollusc grains are less common (Martins and Coutinho, 1981). The study area is strongly influenced by the Guianas Current which flows towards the northwest, with an average temperature of 27ºC, and salinity of 36‰ (Martins, 1984). Another strong influence is discharge from the Amazon River into the estuary and inner shelf along the Amapa´ coast. The river water discharge averages 180,000 m3 s 1 annually (Muller-Karger et al., 1988). The interaction of these two very different water masses produces particular biotic and abiotic characteristics in the region. These include lower salinity, cycles of sediment deposition and erosion resulting in instability of the sea bed, and relatively few living organisms (Die´gues, 1972; Kuehl et al., 1986).
2. Material and methods The 339 samples on which this study is based were collected with a Van Veen grab during a series of cruises. One hundred and seventy seven samples were studied from the REMAC project (legs 4, 5, 5A and 6), coordinated by the Brazilian oil com-
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pany (PETROBRAS). The remaining 162 samples were collected by the hydrographic service of the Brazilian Navy (DHN). The material was provided as dried sediment and each sample had been sieved into three different size fractions: 0.250, 0.177 and 0.074 mm. Only the first two size fractions were picked totally for Ostracoda, while the last one was too fine and frequently barren. The specimens illustrated herein were drawn using a Leitz ‘camera lucida’.
3. Faunal analyses The sampling techniques used did not permit the distinction between living and sub-Recent ostracods, since only empty carapaces and valves were obtained. However, as has been discussed by authors such as Van Morkhoven (1972), Whatley (1988), Brouwers (1988), and Coimbra et al. (1995) the study of the total fauna, even in Recent material, presents some advantages over the study of living faunas: (1) It offers good ecological interpretations due to the great number of specimens available. (2) It eliminates the possibility of fluctuation in the composition of the fauna which results, for example, from seasonal variability. (3) It allows a more direct comparison between Recent and fossil assemblages.
Plate 2 Ostracods confined to the southeast of the mouth of the Amazon=Para´ rivers. 1. Neonesidea sp. ð34. 2. Gen. et sp. indet. 1. ð34. 3. Hermanites transoceanicus Teeter, 1975. ð50. 4. Puriana convoluta Teeter, 1975. ð65. 5. Orionina similis Van den Bold, 1963. ð48.5. 6. Cornucoquimba nana Ramos, 1996. ð79. 7. Hemicytherura bradyi (Puri, 1960). ð76.5. 8. Semicytherura sp. 1. ð69.5. 9. Loxoconcha (L.) tricornata (Krutak, 1971). ð53. 10. Keijia demissa Teeter, 1975. ð67. 11. Kotoracythere inconspicua (Brady, 1970). ð74. 12. Cornucoquimba sp. ð47.5. 13. Callistocythere cranekeyensis (Puri, 1960). ð83. 14. Radimella ex gr. confragosa (Edwards), 1944. ð47.5. 15. Auradilus sp. ð42.5. 16. Coquimba ornellasae Ramos, 1994. ð60.
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Whatley (1988) outlined the use of the ontogenetic composition of the populations to identify the effects of high- and low-energy post-mortem transport and sorting. This is very important when the zoogeographical and ecological analysis of Recent ostracods are based on empty carapaces and valves found in sediment samples, due to the problem of separate autochthonous and allochthonous components, since these small crustaceans grow by a series of molts through the process of ecdysis. This technique has been used with success by the first author in two previous studies (Coimbra et al., 1992, 1995). In the present work Whatley’s method has been taken in consideration only for determining the limits of bathymetrycal and geographical distributions of each species. Of the 339 samples examined 207 yielded ostracods and 132 were barren. Forty-five genera belonging to 17 families were identified. The families Trachyleberididae, Cytheruridae and Hemicytheridae represent 56% of the genera. Two genera are represented by a relatively high number of species: Paracytheridea (5 species) and Callistocythere (4 species); the great majority of other genera is represented by only one species. Of the 94 identified species, 74 were relatively abundant and are considered herein; the remaining 20 were very rare and will not be discussed in this paper. The great extent of the study area and relatively low number of samples do not permit high-resolution delimitation of biofacies and biotopes, but does allow the recognition of two zoogeographical units with distinct abiotic and faunal characteristics: one northwest and another southeast of the Amazon and Para´ rivers.
4. Endemicity and cosmopolitanism of the ostracod fauna The geographical distribution, diversity and abundance of the 74 species herein identified permit the subdivision of the Brazilian Equatorial shelf into two zoogeographical units divided by the mouths of the Amazon and Para´ rivers at ¾48ºW (Table 1). This boundary represents the southernmost limit of Radimella ovata Van den Bold, 1988, a typical species of the Caribbean region. Of the 58 species present in the northwestern
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Table 1 Zoogeographical distribution of the ostracods along the Brazilian Equatorial shelf
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zoogeographical unit, 12 were restricted to the unit in shallow, but infraneritic, depths (>40 m): Cletocythereis sp., Costa variabilicostata ssp., Radimella ovata, Eucytherura sp. 1, Eucytherura sp. 2, Kangarina ancyla, Cytheropteron sp., Pseudoceratina droogeri, Phlyctocythere sp. 1, Phlyctocythere sp. 2, except for Bradleya sp. and Munseyella sp., which were present in deeper waters (>100 m) (Plate 1). Bairdoppilata (B.) sp. 1 and Bairdoppilata (B.) sp. 2 are the most abundant and widely distributed in the northwestern unit, but are also present in the southeastern zoogeographical unit (Plate 3, 1–2). Keijcyoidea sp., Cytherella sp., Paracypris sp., Australoecia sp., Macrocyprina sp. and Radimella ovata are also very abundant in the northwestern unit. Some specimens are eroded and recovered with iron oxide or darkened by iron sulfide which indicate forms of a relict fauna, more common in the outer shelf. This phenomenon can be seen in other ostracod genera such as those recorded by Chukewiski and Purper (1985b), Coimbra and Ornellas (1986) and Medeiros and Coimbra (1989) for species of Puriana, Pseudoceratina and Kangarina, respectively. This fact corroborates previous sedimentologic studies carried out by several authors (Martins, 1974; Kowsmann and Costa, 1979; Martins and Coutinho, 1981) which showed the relict nature of great part of the sediments that now cover the Amazon shelf. Of the 62 species present southeast of the Amazon and Para´ rivers, 16 are restricted to this zoogeographical unit and are more abundant in the epineritic zone (<40 m) (Plate 2). In terms of abundance Paranesidea sp., Neonesidea longisetosa, Neonesidea sp., Puriana variabilis, Paracytheridea batei and Xestoleberis sp. 1 dominate the southeastern zoogeographical unit, followed by Macrocyprina sp., which is also abundant in the northwestern area (Plate 3, 3–8). Some species are widely distributed with a significant number of occurrences along all of the Brazilian Equatorial shelf. These species, with a strong preference for biodetritic sand, occur on the inner shelf of the southeastern zoogeographical unit and on the outer shelf of the northwestern zoogeographical unit. This is consistent with the inner shelf of the northwestern zoogeographical unit being dominated by terrigenous sediment and lower salinity due to the influence of Amazon river discharge. Macrocyprina
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sp. is the more typically cosmopolitan species in the study area, followed by Keijcyoidea sp., Paranesidea sp., Paracytheridea batei, and Neonesidea longisetosa (Plate 3, 9–13).
5. Influence of depth and bottom sediments on the distribution of Ostracoda The 207 samples which yielded ostracods were classified according to their sedimentological characteristics and depth occurrence (Fig. 3a, b). The genus Bradleya, well known as living in deep water, occurs in the study area in only six samples from the continental slope at depths ranging from 172 to 560 m. On the other hand, the genus Munseyella, typical of shallow water, exceptionally occurs only in four samples deeper than 100 m (104, 109, 174 and 194 m), being accompanied by Bradleya sp. in two of them. Australoecia sp. in spite of being considered a genus typical of deep waters it only occurs at depths less than 100 m in this study. Northwest of the mouths of the Amazon and Para´ rivers the percentage of samples containing ostracods is very low at depths less than 50 m (Fig. 3b), because of the strong, continuous freshwater influx from the Amazon river (Die´gues, 1972). Species restricted to the northwestern zoogeographical unit are more abundant below 50 m depth perhaps due to the stratification and=or distance from river influx. The 16 species restricted to the southeastern zoogeographical unit are better adapted to the biodetritic sands that dominate the region’s shallow water depth (7 to 50 m), except Semicytherura sp. which prefers quartzose sand. Radimella ex gr. confragosa, Auradilus sp., Cornucoquimba sp. and Coquimba ornellasae are very abundant between 50 and 75 m depth. Abundance in this area, called ‘setor sul-oriental’ by Die´gues (1972), is attributed to low freshwater input, which only becomes significant during flood periods. This fact, along with the shallow shelf depths and warm, semi-arid climate reduces terrigenous sedimentation, and encourages carbonate production, especially from the delta of Parnaı´ba River (Piauı´ State) to Sa˜o Roque Cape (Rio Grande do Norte State) (Summerhayes et al., 1975; Melo et al., 1975; Milliman, 1977). The remaining 46 species present in both zoogeo-
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graphical units prefer shallow waters and biodetritic sands. As a result of the different sedimentary process taking place in the two zoogeographical units, the depth occurrence of these species tends to be greater in the northwestern zoogeographical unit.
6. Stratigraphic and palaeozoogeographic distribution Approximately one third of the ostracod species studied herein have their origin in the upper Tertiary (Table 2), the majority restricted to the West Atlantic, Caribbean and=or Gulf of Mexico. The cosmopolitan species Neomonoceratina m. mediterranea, Hermanites transoceanicus, Keijia demissa, Kotoracythere inconspicua, Kangarina abyssicola and Paracytheridea tschoppi, are typical of tropical shallow seas and have their origins in different biogeographical regions. The first four are of IndoPacific origin and the last two are of Tethyan origin, having experienced a long period of evolutionary stasis and geographical dispersion. In spite of being cosmopolitan, these species are normally a minor component in the ostracod faunas where they occur, a phenomenon also present in the Brazilian Equatorial shelf. The only exception is Paracytheridea tschoppi, which is highly abundant in the study area. Radimella ovata has a wide distribution in the
Plate 3 1–2: the most abundant species to the northwest of the mouth of the Amazon=Para´ rivers; 3–8: the most abundant species to the southeast of the mouth of the Amazon=Para´ rivers; 9–13: cosmopolitan ostracods on the Brazilian Equatorial shelf. 1. Bairdoppilata (B.) sp. 1. ð29. 2. Bairdoppilata (B.) sp. 2. ð33. 3. Paranesidea sp. ð32. 4. Neonesidea longisetosa (Brady, 1902). ð30. 5. Neonesidea sp. ð39.5. 6. Paracytheridea batei Purper and Ornellas, 1987a. ð64. 7. Xestoleberis sp. 1. ð70. 8. Puriana variabilis Chukewiski and Purper, 1985a. ð63. 9. Macrocyprina sp. ð26.5. 10. Keijcyoidea sp. ð46.5. 11. Paranesidea sp. ð31. 12. Paracytheridea batei Purper and Ornellas, 1987a. ð54. 13. Neonesidea longisetosa (Brady, 1902). ð25.5.
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Caribbean from the Upper Miocene to the Lower Pliocene, but is registered for the first time in Recent sediments in the northwestern zoogeographical unit, where it is very abundant. Orionina similis, Semicytherura sp. and Paracytheridea batei are rare as fossil occurring in the Upper Miocene and Pliocene of Trinidad, Neogene of Dominican Republic and Pliocene(?) of Cuba. As is true for Radimella ovata, they are reported for the Recent only in Brazil, where the first species is rare, the second common, and the third very abundant. Thus, ecological and zoogeographical information on such species with a fossil record can assist in palaeoenvironmental interpretation of Neogene and Quaternary marine sections from Western Atlantic basins.
7. Discussion and conclusions 7.1. Taxonomy Seventy-four species belonging to 45 genera (one undetermined) and 17 families were identified. The great majority of species and subspecies left in open nomenclature represent probably new species. Two species are provisionally assigned to the genus Semicytherura Wagner, 1957 based on the posterior internal lamella developed only in the males. Loxocorniculum Benson and Coleman, 1963 was considered subgenus of Loxoconcha Sars, 1866, as have done Bate et al. (1981) and Van den Bold (1988). In this study 16 genera are recorded for the first time in the study area: Keijcyoidea, Paranesidea, Gangamocytheridea, Neocaudites, Neomonoceratina, Keijia, Kotoracythere, Bairdoppilata, Cletocythereis, Neonesidea, Oculocytheropteron, Aversovalva, Australoecia, Eucytherura, Bythoceratina and Munseyella. The last six are new records for the Brazilian margin, whereas the other ten have been recorded in the Tamandare´ Bay, Pernambuco State by Coimbra et al. (1992). The subgenus Bairdoppilata (Glyptobairdia) was considered by those authors as the genus Glyptobairdia. Finally, the species identified by Coimbra et al. (1992) as Cletocythereis sp. is herein considered to represent a new species of the genus Cornucoquimba. Although Brady (1880) had described some species from the present study area, none of them are
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Fig. 3. Distribution of the 207 samples which yielded ostracods per: (a) bottom sediments and zoogeographical unit; (b) depth and zoogeographical unit. D1–D5 D bathymetrical intervals.
similar to the species identified in this paper. This is probably due to the fact that Brady’s samples were collected in deeper waters than those sampled for this study.
7.2. Zoogeography and ecology Analysis of 339 bottom samples showed that 132 were barren of ostracods. These samples mainly oc-
J.C. Coimbra et al. / Marine Micropaleontology 37 (1999) 365–379 Table 2 Stratigraphical distribution of the ostracod species with well defined fossil record
cur near the mouth of the Amazon and Para´ rivers and on the inner shelf of the northwestern zoogeographical unit. The reason for the absence of ostracods near the river mouth probably relates to the associated turbulence, and salinity variations while the inner shelf in the northwestern zoogeographical unit is muddy and of low salinity. Twenty-eight species are restricted either to the northwestern or southeastern zoogeographical units: the 12 species restricted to the northwest are more abundant in the infraneritic zone (>40 m), while restricted to the southeast are the epineritic taxa (<40 m). The most abundant species in the northwest are Bairdoppilata (B.) sp. 1 and Bairdoppilata (B.) sp. 2, which are also present in the southeast, where the most abundant species are Paranesidea sp., Neonesidea longisetosa, Neonesidea sp., Puri-
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ana variabilis, Paracytheridea batei and Xestoleberis sp. 1. The more typically cosmopolitan species in the study area Macrocyprina sp., followed by Keijcyoidea sp., Paranesidea sp., Paracytheridea batei, and Neonesidea longisetosa. The results of this study extend the geographical distribution of Cativella, Cytherura, Henryhowella, Hulingsina, Orionina, Phlyctocythere, Puriana and Semicytherura to the northwestern zoogeographical unit. This probably relates to the greatly increased number of samples made available by GEOMAR I, II and III expeditions. Furthermore, Cletocythereis sp. is recorded in the Atlantic Ocean for the first time. Witte (1993) questionably recorded one species of this genus off the coast of Senegal, West Africa. The bathymetric preferences of the species identified in this analysis could be divided into six groups and four subgroups, with approximately 60% from depths shallower than 75 m. Munseyella sp. and Bradleya sp. are restricted to the northwestern area, were they are the only species occurring in waters deeper than 100 and 170 m, respectively. Australoecia sp., a rare species considered typical of very deep waters, only occurs in waters shallower than 100 m in the study area. The relationship between the distribution of species and bottom sediments shows that most species prefer biodetritic sands. Exceptions are: Orionina similis, Puriana variabilis, Pellucistoma ex gr. magniventra, Callistocythere fossulata, Callistocythere ornata, Callistocythere laminata, Hulingsina sp. and Semicytherura sp., which are more common in quartzose sands and Neomonoceratina m. mediterranea, which is restricted to sandy mud. 7.3. Stratigraphic distribution and palaeozoogeography Most of the genera recorded in this study are known as fossils and a third have their origin in the late Tertiary. Four cosmopolitan species originated in the Indo-Pacific and two in Tethyan areas; all others are restricted to the Western Atlantic, Caribbean and=or Gulf of Mexico. Comparing the diversity of the ostracod fauna of the Western Atlantic, represented by this study of the Brazilian Equatorial shelf, with the ostracod faunas of the Indo-Pacific and Western Pacific,
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shows that the diversity in the later two provinces are much higher, despite all three being of tropical shallow water. Another observation is that species diversity in the tropical shelf of the Western Atlantic is greater than in the Eastern Atlantic. Van Harten and Witte (1999) attribute differences in diversity between Eastern Atlantic, Indo-Pacific, and Western Pacific to allopatric speciation controlled by sea level oscillations in the Neogene and Quaternary. The Indo-Pacific and the Western Pacific are favored by highly diverse and isolated coasts as well as several archipelagos, while the Eastern Atlantic is essentially a continuous continental margin with a comparatively uniform coastline. If this phenomenon relating area and faunistic diversity really controlled the speciation of ostracod faunas in tropical shallow seas, this may explain the lower diversity of the Western Atlantic in relation to Indo-Pacific and Western Pacific.
Acknowledgements The present study largely benefits from the surface samples provided by Petro´leo Brasileiro S.A. and Diretoria de Hidrografia e Navegac¸a˜o da Marinha do Brasil. During an early stage of the project, R.C. Whatley (University of Wales) and Y.T. Sanguinetti (Universidade Federal do Rio Grande do Sul) made valuable suggestions. A. Moguilevsky (University of Wales) and two anonymous reviewers contributed thoughtful comments on the draft manuscript and assisted with the English. We are grateful to Marcos Jose´ Michelli and E´dson Souza de Arau´jo by typewriting the manuscript and improving drawings and charts. Dra. Ivone Purper (Universidade Federal do Rio Grande do Sul) is thanked for assistance with the preparation of the figure one. The authors were funded by the FAPERGS (Fundac¸a˜o de Amparo a` Pesquisa do Estado do Rio Grande do Sul) and by the CNPq (Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico).
References Bate, R.H., Whittaker, J.E., Mayes, C.A., 1981. Marine Ostracoda of the Galapagos Islands and Ecuador. Zool. J. Linn. Soc.
London 73 (1), 1–79. Benson, R.H., Coleman, G.L., 1963. Recent marine ostracodes from the eastern Gulf of Mexico. Univ. Kansas Paleontol. Contrib. 31 (Arthropoda: 2), 1–52. Brady, G.S., 1880. Report on the Ostracoda dredged by H.M.S. Challenger during the years 1873–1876. Report on the Scientific results of the Voyage of H.M.S. Challenger during the years 1873–1876. Zoology 1 (3), 1–184. Brady, G.S., 1902. On new or imperfectly-known Ostracoda, chiefly from a collection in the Zoological Museum of Copenhagen. Trans. Zool. Soc. London 16 (4), 179–210. Brouwers, E.M., 1988. Sediment transport detected from the analysis of ostracod population structure: an example from the Alaskan continental shelf. In: De Deckker, P., Colin, J.P., Peypouquet, J.P. (Eds.), Ostracoda in the Earth Sciences. Elsevier, Amsterdam, pp. 231–244. Chukewiski, K., Purper, I., 1985a. A new ostracode Puriana variabilis Chukewiski et Purper, sp. nov. from Brazilian coast. 2º Simpo´sio de Geologia da Amazoˆnia, Bele´m, An. SBG 2, 312–326. Chukewiski, K., Purper, I., 1985b. Occurrence of Puriana (Ostracoda) in the Brazilian continental shelf: its ecology, geographical and stratigraphical distribution. 2º Simpo´sio de Geologia da Amazoˆnia, Bele´m, SBG 2, 327–339. Coimbra, J.C., Ornellas, L.P., 1986. The subfamily Orionininae Puri, 1973 (Ostracoda, Hemicytheridae) in the Brazilian continental shelf. Part I. General considerations and systematic of the genus Orionina Puri, 1953. 34º Congr. Brasileiro de Geologia, Goiaˆnia, An. SBG 1, 539–555. Coimbra, J.C., Ornellas, L.P., 1987. The subfamily Orionininae Puri, 1973 (Ostracoda, Hemicytheridae) in the Brazilian continental shelf. Part II. Caudites Coryell and Fields, 1937. Pesquisas 19, 55–79. Coimbra, J.C., Ornellas, L.P., 1989. Distribution and ecology of Sub-Recent Orionininae (Ostracoda) in the Brazilian continental shelf. Rev. Bras. Geosci. 19 (2), 177–186. Coimbra, J.C., Ramos, M.I.F., Sanguinetti, Y.T., 1992. SubRecent ostracodes of the Tamandare´ Bay, Northeastern Brazil — A preliminary report on biofacies. Pesquisas 19 (1), 94– 105. Coimbra, J.C., Carren˜o, A.L., Ferron, F.A., 1994. Holocene Podocopida Ostracoda from Sepetiba Bay, Brasil — Some dominant taxa. Pesquisas 21 (2), 90–99. Coimbra, J.C., Sanguinetti, Y.T., Bittencourt-Calcagno, V.M., 1995. Taxonomy and distribution patterns of Recent species of Callistocythere Ruggieri, 1953 (Ostracoda) from the Brazilian continental shelf. Rev. Esp. Micropaleontol. 27 (3), 117–136. Die´gues, F.M.F., 1972. Introduc¸a˜o a` oceanografia do estua´rio amazoˆnico. 26º Congr. Brasileiro de Geologia, Bele´m, An. SBG, 1–318. Do Carmo, D.A., 1993. Taxonomia e ecologia do geˆnero Krithe (Ostracoda) na margem continental brasileira, com eˆnfase na variabilidade do vestı´bulo de Krithe leonidesae sp. nov. M.Sc. Thesis, Univ. Federal do Rio Grande do Sul, 59 pp. Do Carmo, D.A., Sanguinetti, Y.T., 1995. Krithe occurrence on the Brazilian continental margin: An ecological approach.
J.C. Coimbra et al. / Marine Micropaleontology 37 (1999) 365–379 In: Rı´ha, J. (Ed.), Ostracoda and Biostratigraphy. Balkema, Rotterdam, pp. 407–412. Fauth, G., Coimbra, J.C., 1998. Zoogeography of the ostracode genera Auradilus and Radimella on the Brazilian continental shelf. Neues Jahrb. Geol. Palao¨ntol. 8, 463–474. Kowsmann, R.O., Costa, M.P.A., 1979. Sedimentac¸a˜o quaterna´ria da margem continental brasileira e das a´ guas oceaˆnicas adjacentes. In: Projeto REMAC — Reconhecimento Global da ´ S, Margem Continental Brasileira. Rio de Janeiro, PETROBRA CENPES, DINTEP, 8, 1–55. Kuehl, S.A., DeMaster, D.J., Nittrouer, C.A., 1986. Nature of sediment accumulation on the Amazon continental shelf. Cont. Shelf Res. 6, 209–225. Martins, I.R., 1984. Aspectos da oceanografia fı´sica do Atlaˆntico Sul. Pesquisas 16, 76–90. Martins, L.R., 1974. Sedimentologia da margem continental amazoˆnica. Thesis, Univ. Federal do Rio Grande do Sul, 104 pp. Martins, L.R., Coutinho, P.N., 1981. The Brazilian continental margin. Earth-Sci. Rev. 17, 87–107. Martins, L.R., Urien, C.M., Butler, L.W., Martins, I.R., 1975. Morfologia e sedimentos da plataforma continental atlaˆntica sul-america entre Cabo Orange e Chuı´ (Brasil). An. Hidrogr. 23, 83–109. Medeiros, M.A., Coimbra, J.C., 1989. Sub-Recent distribution of Kangarina Coryell and Fields, 1937 (Crustacea; Ostracoda) in the Northern Brazilian Coast. 11º Congr. Brasileiro de Paleontologia, Curitiba, An. SBP 1, 479–491, Melo, U., Summerhayes, C.P., Ellis, J.P., 1975. Upper continental margin sedimentation off Brazil. Contrib. Sedimentol. 4, 78– 116. Milliman, J.D., 1977. Relict magnesian calcite oolite and subsidence of the Amazon shelf: reply. Sedimentology 24, 149– 151. Muller-Karger, F.E., McClain, C.T.R., Richardson, P.L., 1988. The disposal of the Amazon’s water. Nature 333, 56–59. Ornellas, L.P., Coimbra, J.C., 1985. Occurrence of Pseudoceratina droogeri van den Bold, 1965 in the Northern Brazilian Continental Shelf. 2º Simpo´sio de Geologia da Amazonia, Bele´m, An. SBG 1, 396–407. Pinto, I.D., Ornellas, L.P., Purper, I., Kotzian, S.C.B., Sanguinetti, Y.T., 1978. Recent ostracodes along 7,408km of the
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Brazilian coast (33º450 S to 04º250 N). Pesquisas 9, 109–120. Puri, H.S., 1960. Recent Ostracoda from the west coast of Florida. Trans. Gulf Coast Assoc. Geol. Soc. 10, 107–149. Purper, I., Ornellas, L.P., 1987a. Paracytheridea tschoppi van den Bold, 1946 and its allied species, Paracytheridea batei Purper et Ornellas, sp. nov. in the Brazilian continental shelf. 10º Congr. Brasileiro de Paleontologia, Rio de Janeiro, An. SBP 1, 747–759. Purper, I., Ornellas, L.P., 1987b. The genus Paracytheridea (Ostracoda) in the Northern=Northwestern Brazilian continental shelf. Pesquisas 20, 103–124. Ramos, M.I.F., 1994. The ostracod genus Coquimba Ohmert, 1968 from the Brazilian continental shelf. Rev. Esp. Micropaleontol. 26 (3), 65–82. Ramos, M.I.F., 1996. Taxonomy and zoogeography of the ostracod genera Nanocoquimba and Cornucoquimba Ohmert, 1968 from Recent sediments on the Brazilian continental shelf. Rev. Esp. Micropaleontol. 28 (1), 105–128. Summerhayes, C.P., Coutinho, P.N., Franc¸a, A.M.C., Ellis, J.P., 1975. Upper continental margin sedimentation off Brazil: Part III — Salvador to Fortaleza, northeastern Brazil. Contrib. Sedimentol. 4, 44–78. Van den Bold, W.A., 1988. Neogene paleontology in the northern Dominican Republic 7. The Subclass Ostracoda (Arthropoda: Crustacea). Bull. Am. Paleontol. 94 (329), 1–105. Van Harten, D., Witte, L.J., 1999. Centres of shallow marine origin and sea-level changes. Paleobiology (in press). Van Morkhoven, F.P.V.M., 1972. Bathymetry of Recent marine Ostracoda in the Northwest Gulf of Mexico. Trans. Gulf Coast Assoc. Geol. Soc. 22, 241–252. Wagner, C.W., 1957. Sur les ostracodes du Quaternaire–Re´cent des Pays-Bas et leur utilisation dans l’e´tude ge´ologique des de´poˆts Holoce`nes. Ph.D. Thesis, Univ. Paris, 259 pp. Whatley, R.C., 1988. Population structure of ostracods: some general principles for the recognition of palaeoenvironments. In: De Deckker, P., Colin, J.P., Peypouquet, J.P. (eds.). Ostracoda in the Earth Sciences. Elsevier, Amsterdam, pp. 245– 256. Witte, L.J., 1993. Taxonomy and origin of Modern West African shallow marine Ostracoda. Ph.D. Thesis, Vrije Univ. Amsterdam, Amsterdam, 201 pp.
Zoogeography of Holocene Podocopina (Ostracoda) from the Brazilian Equatorial shelf Joa˜o Carlos Coimbra a,b,Ł , Iraja´ Damiani Pinto a,b , Norma Luiza Wu¨rdig b , Dermeval Aparecido do Carmo a,c a
Departamento de Paleontologia e Estratigrafia, Instituto de Geocieˆncias, Universidade Federal do Rio Grande do Sul, Cx.P. 15001, CEP. 91501-970 Porto Alegre, RS, Brazil b CECLIMAR-Coastal, Limnological, and Marine Centre, Universidade Federal do Rio Grande do Sul, Av. Tramandaı´, 976, CEP. 95625-000 Imbe´, RS, Brazil c Departamento de Geologia Geral e Aplicada, Instituto de Geocie ˆ ncias, Universidade de Brası´lia, CEP. 70910-900 Brası´lia, DF, Brazil Received 31 July 1997; accepted 11 February 1999
Abstract This study reports on the Holocene ostracods of the Brazilian Equatorial shelf from French Guiana to near Natal, almost 1400 km in length. This part of the Western Atlantic Tropical Province, where the mouths of the Amazon and Para´ rivers separate two zoogeographical units, has distinct oceanographic and sedimentologic features. Three hundred and thirty nine samples were studied from the REMAC (legs 4, 5, 5A and 6) and the GEOMAR projects (legs I, II and III). The ostracod fauna includes 74 species, of which 37 have been previously described (mostly from other areas). The remaining 37 species are left in open nomenclature although 26 of them are probably new. Twelve species are confined to the northwest and 16 to the southeast of the mouths of the Amazon and Para´ rivers. The inner shelf of the northwestern region is a barren zone resulting from turbid freshwater discharged by the Amazon and Para´ rivers. Most of the studied species live at water depths of less than 100 m; 28 species inhabit waters shallower than 75 m, and 18 species live in waters shallower than 50 m. Although Brazilian ostracods have many genera and species in common with Holocene ostracods elsewhere in the tropical Atlantic, diversity differs greatly. For example, diversity on the Brazilian Equatorial shelf is lower than elsewhere in the Western Atlantic Tropical Province, but higher than in the Eastern Atlantic Tropical Province. Faunal similarities between these provinces are at the generic rather than species level; exceptions are few and consist of conservative Tethyan species that are cosmopolitan. 1999 Elsevier Science B.V. All rights reserved. Keywords: Holocene; Ostracoda; Brazil; marine; zoogeography
1. Introduction The study of Holocene ostracods from the studied area began with Brady (1880) who analyzed samples Ł Corresponding
author. Tel.: C51-3166384; Fax: C51-3191811; E-mail: [email protected]
collected by the famous H.M.S. Challenger Expedition towards the end of the last century. He described species assigned to the genera: Bairdia, Bythocypris, Cythere, Cytherella, Krithe and Macrocypris. Two species, Macrocypris similis and Cythere pyriformis, were collected at 1234 m depth on the continental margin of the Pernambuco State. Almost one cen-
0377-8398/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 8 3 9 8 ( 9 9 ) 0 0 0 2 5 - 0
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Fig. 1. Map of the Brazilian Equatorial Shelf indicating location of the 339 samples. Ð D with ostracods; C D barren.
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Fig. 2. Distribution of bottom sediments (after Kowsmann and Costa, 1979).
tury later, Pinto et al. (1978) recorded more than 50 genera in a preliminary study of ostracods collected along 7408 km of the Brazilian continental shelf. In the Brazilian Equatorial shelf they identified 42 genera and registered three indeterminate genera. These authors emphasized the absence of ostracods around
Marajo´ Island, in the Amazon and Para´ estuary, and explained it as the result of the mixing of freshwater from the rivers with seawater which lowered the salinity. Although the Brazilian Equatorial continental shelf comprises one of the most extensive areas of
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continental shelf in the world, few studies after Pinto et al. (1978) discussed the benthic Ostracoda from this region. The works at the species level began with Chukewiski and Purper (1985a,b) and includes Ornellas and Coimbra (1985), Coimbra and Ornellas (1986, 1987, 1989), Purper and Ornellas (1987a,b), Medeiros and Coimbra (1989), Do Carmo (1993), Do Carmo and Sanguinetti (1995), Ramos (1994, 1996), Coimbra et al. (1992, 1994, 1995) and Fauth and Coimbra (1998). 1.1. Study area The Brazilian Equatorial shelf is situated between the international boundary of Oiapoque River (Amapa´ State) and Sa˜o Roque Cape (Rio Grande do Norte State), including the coastal region of Para´, Maranha˜o, Piauı´ and Ceara´ states (Fig. 1). Nine samples only were collected a little to the south of this region, that is, south of the Sa˜o Roque Cape, all of them in abiotic conditions similar to the area of the Sa˜o Roque Cape. The water depths of the study area range to 200 m, except for 31 samples that have been collected in deeper waters, as follows: 15 (with ostracods) located to the southeast of the Amazon=Para´ estuary; and 16 (13 barren ones) located to the northwest of that estuary. The maximum shelf width is 330=350 km at the Amazon and Para´ rivers mouths (Martins and Coutinho, 1981). According to Kowsmann and Costa (1979) the extensive relict sands of the study area are better described as reworked sands, since they were susceptible to changes during and after the Holocene transgression.
Plate 1 Ostracods confined to the northwest of the mouth of the Amazon=Para´ rivers. 1. Cletocythereis sp. ð40.5. 2. Costa variabilicostata ssp. ð55.5. 3. Radimella ovata Van den Bold, 1988. ð48. 4. Eucytherura sp. 2. ð103. 5. Eucytherura sp. 1. ð104. 6. Kangarina ancyla Van den Bold, 1963. ð102.5. 7. Cytheropteron sp. ð71.5. 8. Pseudoceratina droogeri Van den Bold, 1965. ð52. 9. Phlyctocythere sp. 1. ð65. 10. Phlyctocythere sp. 2. ð71. 11. Bradleya sp. ð42.5. 12. Munseyella sp. ð90.5.
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Martins et al. (1975) recognized two physiographic provinces along Brazilian Equatorial shelf: (1) Orange Cape–Parnaı´ba River Delta; and (2) Parnaı´ba River Delta–Sa˜o Roque Cape (Fig. 2a–c). Martins et al.’s first province is dominated by terrigenous sediments in the inner shelf and reworked sandy sediments in the middle and outer shelves. Martins and Coutinho (1981) confirm the pelitic character of the fluvial terrigenous sediments found northwest of the mouth of the Amazon and Para´ rivers. According to Martins (1974), this muddy facies extends to the inner shelf (¾40 m depth). Kowsmann and Costa (1979) mapped a narrow sandy facies in this muddy facies, opposite the mouth of Amazon and Para´ rivers. Martins et al.’s second province, the Parnaı´ba River Delta–Sa˜o Roque Cape, consists of a narrower shelf where carbonate sediments predominate. Kowsmann and Costa (1979) have estimated grades of CaCO3 higher than 75% along this area. The carbonate sediments are constituted by sands and gravels of the red algae Lithothamnium and Lithophyllum or green algae Halimeda alone; facies with a predominance of foraminifer and mollusc grains are less common (Martins and Coutinho, 1981). The study area is strongly influenced by the Guianas Current which flows towards the northwest, with an average temperature of 27ºC, and salinity of 36‰ (Martins, 1984). Another strong influence is discharge from the Amazon River into the estuary and inner shelf along the Amapa´ coast. The river water discharge averages 180,000 m3 s 1 annually (Muller-Karger et al., 1988). The interaction of these two very different water masses produces particular biotic and abiotic characteristics in the region. These include lower salinity, cycles of sediment deposition and erosion resulting in instability of the sea bed, and relatively few living organisms (Die´gues, 1972; Kuehl et al., 1986).
2. Material and methods The 339 samples on which this study is based were collected with a Van Veen grab during a series of cruises. One hundred and seventy seven samples were studied from the REMAC project (legs 4, 5, 5A and 6), coordinated by the Brazilian oil com-
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pany (PETROBRAS). The remaining 162 samples were collected by the hydrographic service of the Brazilian Navy (DHN). The material was provided as dried sediment and each sample had been sieved into three different size fractions: 0.250, 0.177 and 0.074 mm. Only the first two size fractions were picked totally for Ostracoda, while the last one was too fine and frequently barren. The specimens illustrated herein were drawn using a Leitz ‘camera lucida’.
3. Faunal analyses The sampling techniques used did not permit the distinction between living and sub-Recent ostracods, since only empty carapaces and valves were obtained. However, as has been discussed by authors such as Van Morkhoven (1972), Whatley (1988), Brouwers (1988), and Coimbra et al. (1995) the study of the total fauna, even in Recent material, presents some advantages over the study of living faunas: (1) It offers good ecological interpretations due to the great number of specimens available. (2) It eliminates the possibility of fluctuation in the composition of the fauna which results, for example, from seasonal variability. (3) It allows a more direct comparison between Recent and fossil assemblages.
Plate 2 Ostracods confined to the southeast of the mouth of the Amazon=Para´ rivers. 1. Neonesidea sp. ð34. 2. Gen. et sp. indet. 1. ð34. 3. Hermanites transoceanicus Teeter, 1975. ð50. 4. Puriana convoluta Teeter, 1975. ð65. 5. Orionina similis Van den Bold, 1963. ð48.5. 6. Cornucoquimba nana Ramos, 1996. ð79. 7. Hemicytherura bradyi (Puri, 1960). ð76.5. 8. Semicytherura sp. 1. ð69.5. 9. Loxoconcha (L.) tricornata (Krutak, 1971). ð53. 10. Keijia demissa Teeter, 1975. ð67. 11. Kotoracythere inconspicua (Brady, 1970). ð74. 12. Cornucoquimba sp. ð47.5. 13. Callistocythere cranekeyensis (Puri, 1960). ð83. 14. Radimella ex gr. confragosa (Edwards), 1944. ð47.5. 15. Auradilus sp. ð42.5. 16. Coquimba ornellasae Ramos, 1994. ð60.
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Whatley (1988) outlined the use of the ontogenetic composition of the populations to identify the effects of high- and low-energy post-mortem transport and sorting. This is very important when the zoogeographical and ecological analysis of Recent ostracods are based on empty carapaces and valves found in sediment samples, due to the problem of separate autochthonous and allochthonous components, since these small crustaceans grow by a series of molts through the process of ecdysis. This technique has been used with success by the first author in two previous studies (Coimbra et al., 1992, 1995). In the present work Whatley’s method has been taken in consideration only for determining the limits of bathymetrycal and geographical distributions of each species. Of the 339 samples examined 207 yielded ostracods and 132 were barren. Forty-five genera belonging to 17 families were identified. The families Trachyleberididae, Cytheruridae and Hemicytheridae represent 56% of the genera. Two genera are represented by a relatively high number of species: Paracytheridea (5 species) and Callistocythere (4 species); the great majority of other genera is represented by only one species. Of the 94 identified species, 74 were relatively abundant and are considered herein; the remaining 20 were very rare and will not be discussed in this paper. The great extent of the study area and relatively low number of samples do not permit high-resolution delimitation of biofacies and biotopes, but does allow the recognition of two zoogeographical units with distinct abiotic and faunal characteristics: one northwest and another southeast of the Amazon and Para´ rivers.
4. Endemicity and cosmopolitanism of the ostracod fauna The geographical distribution, diversity and abundance of the 74 species herein identified permit the subdivision of the Brazilian Equatorial shelf into two zoogeographical units divided by the mouths of the Amazon and Para´ rivers at ¾48ºW (Table 1). This boundary represents the southernmost limit of Radimella ovata Van den Bold, 1988, a typical species of the Caribbean region. Of the 58 species present in the northwestern
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Table 1 Zoogeographical distribution of the ostracods along the Brazilian Equatorial shelf
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zoogeographical unit, 12 were restricted to the unit in shallow, but infraneritic, depths (>40 m): Cletocythereis sp., Costa variabilicostata ssp., Radimella ovata, Eucytherura sp. 1, Eucytherura sp. 2, Kangarina ancyla, Cytheropteron sp., Pseudoceratina droogeri, Phlyctocythere sp. 1, Phlyctocythere sp. 2, except for Bradleya sp. and Munseyella sp., which were present in deeper waters (>100 m) (Plate 1). Bairdoppilata (B.) sp. 1 and Bairdoppilata (B.) sp. 2 are the most abundant and widely distributed in the northwestern unit, but are also present in the southeastern zoogeographical unit (Plate 3, 1–2). Keijcyoidea sp., Cytherella sp., Paracypris sp., Australoecia sp., Macrocyprina sp. and Radimella ovata are also very abundant in the northwestern unit. Some specimens are eroded and recovered with iron oxide or darkened by iron sulfide which indicate forms of a relict fauna, more common in the outer shelf. This phenomenon can be seen in other ostracod genera such as those recorded by Chukewiski and Purper (1985b), Coimbra and Ornellas (1986) and Medeiros and Coimbra (1989) for species of Puriana, Pseudoceratina and Kangarina, respectively. This fact corroborates previous sedimentologic studies carried out by several authors (Martins, 1974; Kowsmann and Costa, 1979; Martins and Coutinho, 1981) which showed the relict nature of great part of the sediments that now cover the Amazon shelf. Of the 62 species present southeast of the Amazon and Para´ rivers, 16 are restricted to this zoogeographical unit and are more abundant in the epineritic zone (<40 m) (Plate 2). In terms of abundance Paranesidea sp., Neonesidea longisetosa, Neonesidea sp., Puriana variabilis, Paracytheridea batei and Xestoleberis sp. 1 dominate the southeastern zoogeographical unit, followed by Macrocyprina sp., which is also abundant in the northwestern area (Plate 3, 3–8). Some species are widely distributed with a significant number of occurrences along all of the Brazilian Equatorial shelf. These species, with a strong preference for biodetritic sand, occur on the inner shelf of the southeastern zoogeographical unit and on the outer shelf of the northwestern zoogeographical unit. This is consistent with the inner shelf of the northwestern zoogeographical unit being dominated by terrigenous sediment and lower salinity due to the influence of Amazon river discharge. Macrocyprina
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sp. is the more typically cosmopolitan species in the study area, followed by Keijcyoidea sp., Paranesidea sp., Paracytheridea batei, and Neonesidea longisetosa (Plate 3, 9–13).
5. Influence of depth and bottom sediments on the distribution of Ostracoda The 207 samples which yielded ostracods were classified according to their sedimentological characteristics and depth occurrence (Fig. 3a, b). The genus Bradleya, well known as living in deep water, occurs in the study area in only six samples from the continental slope at depths ranging from 172 to 560 m. On the other hand, the genus Munseyella, typical of shallow water, exceptionally occurs only in four samples deeper than 100 m (104, 109, 174 and 194 m), being accompanied by Bradleya sp. in two of them. Australoecia sp. in spite of being considered a genus typical of deep waters it only occurs at depths less than 100 m in this study. Northwest of the mouths of the Amazon and Para´ rivers the percentage of samples containing ostracods is very low at depths less than 50 m (Fig. 3b), because of the strong, continuous freshwater influx from the Amazon river (Die´gues, 1972). Species restricted to the northwestern zoogeographical unit are more abundant below 50 m depth perhaps due to the stratification and=or distance from river influx. The 16 species restricted to the southeastern zoogeographical unit are better adapted to the biodetritic sands that dominate the region’s shallow water depth (7 to 50 m), except Semicytherura sp. which prefers quartzose sand. Radimella ex gr. confragosa, Auradilus sp., Cornucoquimba sp. and Coquimba ornellasae are very abundant between 50 and 75 m depth. Abundance in this area, called ‘setor sul-oriental’ by Die´gues (1972), is attributed to low freshwater input, which only becomes significant during flood periods. This fact, along with the shallow shelf depths and warm, semi-arid climate reduces terrigenous sedimentation, and encourages carbonate production, especially from the delta of Parnaı´ba River (Piauı´ State) to Sa˜o Roque Cape (Rio Grande do Norte State) (Summerhayes et al., 1975; Melo et al., 1975; Milliman, 1977). The remaining 46 species present in both zoogeo-
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graphical units prefer shallow waters and biodetritic sands. As a result of the different sedimentary process taking place in the two zoogeographical units, the depth occurrence of these species tends to be greater in the northwestern zoogeographical unit.
6. Stratigraphic and palaeozoogeographic distribution Approximately one third of the ostracod species studied herein have their origin in the upper Tertiary (Table 2), the majority restricted to the West Atlantic, Caribbean and=or Gulf of Mexico. The cosmopolitan species Neomonoceratina m. mediterranea, Hermanites transoceanicus, Keijia demissa, Kotoracythere inconspicua, Kangarina abyssicola and Paracytheridea tschoppi, are typical of tropical shallow seas and have their origins in different biogeographical regions. The first four are of IndoPacific origin and the last two are of Tethyan origin, having experienced a long period of evolutionary stasis and geographical dispersion. In spite of being cosmopolitan, these species are normally a minor component in the ostracod faunas where they occur, a phenomenon also present in the Brazilian Equatorial shelf. The only exception is Paracytheridea tschoppi, which is highly abundant in the study area. Radimella ovata has a wide distribution in the
Plate 3 1–2: the most abundant species to the northwest of the mouth of the Amazon=Para´ rivers; 3–8: the most abundant species to the southeast of the mouth of the Amazon=Para´ rivers; 9–13: cosmopolitan ostracods on the Brazilian Equatorial shelf. 1. Bairdoppilata (B.) sp. 1. ð29. 2. Bairdoppilata (B.) sp. 2. ð33. 3. Paranesidea sp. ð32. 4. Neonesidea longisetosa (Brady, 1902). ð30. 5. Neonesidea sp. ð39.5. 6. Paracytheridea batei Purper and Ornellas, 1987a. ð64. 7. Xestoleberis sp. 1. ð70. 8. Puriana variabilis Chukewiski and Purper, 1985a. ð63. 9. Macrocyprina sp. ð26.5. 10. Keijcyoidea sp. ð46.5. 11. Paranesidea sp. ð31. 12. Paracytheridea batei Purper and Ornellas, 1987a. ð54. 13. Neonesidea longisetosa (Brady, 1902). ð25.5.
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Caribbean from the Upper Miocene to the Lower Pliocene, but is registered for the first time in Recent sediments in the northwestern zoogeographical unit, where it is very abundant. Orionina similis, Semicytherura sp. and Paracytheridea batei are rare as fossil occurring in the Upper Miocene and Pliocene of Trinidad, Neogene of Dominican Republic and Pliocene(?) of Cuba. As is true for Radimella ovata, they are reported for the Recent only in Brazil, where the first species is rare, the second common, and the third very abundant. Thus, ecological and zoogeographical information on such species with a fossil record can assist in palaeoenvironmental interpretation of Neogene and Quaternary marine sections from Western Atlantic basins.
7. Discussion and conclusions 7.1. Taxonomy Seventy-four species belonging to 45 genera (one undetermined) and 17 families were identified. The great majority of species and subspecies left in open nomenclature represent probably new species. Two species are provisionally assigned to the genus Semicytherura Wagner, 1957 based on the posterior internal lamella developed only in the males. Loxocorniculum Benson and Coleman, 1963 was considered subgenus of Loxoconcha Sars, 1866, as have done Bate et al. (1981) and Van den Bold (1988). In this study 16 genera are recorded for the first time in the study area: Keijcyoidea, Paranesidea, Gangamocytheridea, Neocaudites, Neomonoceratina, Keijia, Kotoracythere, Bairdoppilata, Cletocythereis, Neonesidea, Oculocytheropteron, Aversovalva, Australoecia, Eucytherura, Bythoceratina and Munseyella. The last six are new records for the Brazilian margin, whereas the other ten have been recorded in the Tamandare´ Bay, Pernambuco State by Coimbra et al. (1992). The subgenus Bairdoppilata (Glyptobairdia) was considered by those authors as the genus Glyptobairdia. Finally, the species identified by Coimbra et al. (1992) as Cletocythereis sp. is herein considered to represent a new species of the genus Cornucoquimba. Although Brady (1880) had described some species from the present study area, none of them are
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Fig. 3. Distribution of the 207 samples which yielded ostracods per: (a) bottom sediments and zoogeographical unit; (b) depth and zoogeographical unit. D1–D5 D bathymetrical intervals.
similar to the species identified in this paper. This is probably due to the fact that Brady’s samples were collected in deeper waters than those sampled for this study.
7.2. Zoogeography and ecology Analysis of 339 bottom samples showed that 132 were barren of ostracods. These samples mainly oc-
J.C. Coimbra et al. / Marine Micropaleontology 37 (1999) 365–379 Table 2 Stratigraphical distribution of the ostracod species with well defined fossil record
cur near the mouth of the Amazon and Para´ rivers and on the inner shelf of the northwestern zoogeographical unit. The reason for the absence of ostracods near the river mouth probably relates to the associated turbulence, and salinity variations while the inner shelf in the northwestern zoogeographical unit is muddy and of low salinity. Twenty-eight species are restricted either to the northwestern or southeastern zoogeographical units: the 12 species restricted to the northwest are more abundant in the infraneritic zone (>40 m), while restricted to the southeast are the epineritic taxa (<40 m). The most abundant species in the northwest are Bairdoppilata (B.) sp. 1 and Bairdoppilata (B.) sp. 2, which are also present in the southeast, where the most abundant species are Paranesidea sp., Neonesidea longisetosa, Neonesidea sp., Puri-
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ana variabilis, Paracytheridea batei and Xestoleberis sp. 1. The more typically cosmopolitan species in the study area Macrocyprina sp., followed by Keijcyoidea sp., Paranesidea sp., Paracytheridea batei, and Neonesidea longisetosa. The results of this study extend the geographical distribution of Cativella, Cytherura, Henryhowella, Hulingsina, Orionina, Phlyctocythere, Puriana and Semicytherura to the northwestern zoogeographical unit. This probably relates to the greatly increased number of samples made available by GEOMAR I, II and III expeditions. Furthermore, Cletocythereis sp. is recorded in the Atlantic Ocean for the first time. Witte (1993) questionably recorded one species of this genus off the coast of Senegal, West Africa. The bathymetric preferences of the species identified in this analysis could be divided into six groups and four subgroups, with approximately 60% from depths shallower than 75 m. Munseyella sp. and Bradleya sp. are restricted to the northwestern area, were they are the only species occurring in waters deeper than 100 and 170 m, respectively. Australoecia sp., a rare species considered typical of very deep waters, only occurs in waters shallower than 100 m in the study area. The relationship between the distribution of species and bottom sediments shows that most species prefer biodetritic sands. Exceptions are: Orionina similis, Puriana variabilis, Pellucistoma ex gr. magniventra, Callistocythere fossulata, Callistocythere ornata, Callistocythere laminata, Hulingsina sp. and Semicytherura sp., which are more common in quartzose sands and Neomonoceratina m. mediterranea, which is restricted to sandy mud. 7.3. Stratigraphic distribution and palaeozoogeography Most of the genera recorded in this study are known as fossils and a third have their origin in the late Tertiary. Four cosmopolitan species originated in the Indo-Pacific and two in Tethyan areas; all others are restricted to the Western Atlantic, Caribbean and=or Gulf of Mexico. Comparing the diversity of the ostracod fauna of the Western Atlantic, represented by this study of the Brazilian Equatorial shelf, with the ostracod faunas of the Indo-Pacific and Western Pacific,
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shows that the diversity in the later two provinces are much higher, despite all three being of tropical shallow water. Another observation is that species diversity in the tropical shelf of the Western Atlantic is greater than in the Eastern Atlantic. Van Harten and Witte (1999) attribute differences in diversity between Eastern Atlantic, Indo-Pacific, and Western Pacific to allopatric speciation controlled by sea level oscillations in the Neogene and Quaternary. The Indo-Pacific and the Western Pacific are favored by highly diverse and isolated coasts as well as several archipelagos, while the Eastern Atlantic is essentially a continuous continental margin with a comparatively uniform coastline. If this phenomenon relating area and faunistic diversity really controlled the speciation of ostracod faunas in tropical shallow seas, this may explain the lower diversity of the Western Atlantic in relation to Indo-Pacific and Western Pacific.
Acknowledgements The present study largely benefits from the surface samples provided by Petro´leo Brasileiro S.A. and Diretoria de Hidrografia e Navegac¸a˜o da Marinha do Brasil. During an early stage of the project, R.C. Whatley (University of Wales) and Y.T. Sanguinetti (Universidade Federal do Rio Grande do Sul) made valuable suggestions. A. Moguilevsky (University of Wales) and two anonymous reviewers contributed thoughtful comments on the draft manuscript and assisted with the English. We are grateful to Marcos Jose´ Michelli and E´dson Souza de Arau´jo by typewriting the manuscript and improving drawings and charts. Dra. Ivone Purper (Universidade Federal do Rio Grande do Sul) is thanked for assistance with the preparation of the figure one. The authors were funded by the FAPERGS (Fundac¸a˜o de Amparo a` Pesquisa do Estado do Rio Grande do Sul) and by the CNPq (Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico).
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