Taxonomy and palaeoceanographical significance of the genus Krithe (Ostracoda) in the Brazilian margin

The results of taxonomic, geographical distribution and palaeoecological studies of the genus Krithe from the Brazilian continental margin are presented. The following species and subspecies are recognized: Krithe reversa Bold, 1958; K. trinidadensis Bold, 1958; K. morkhoveni morkhoveni Bold, 1960; K. coimbrai sp. nov.; and K. gnoma sp. nov. Four species are left in open nomenclature. The occurrence of Krithe within the Brazilian continental margin is restricted to areas under the influence of cold waters and, considering their stratigraphic distribution in Cenozoic strata, they are here recognized as being useful as palaeoceanographic indicators. Of the above-mentioned species, only two occur within the Brazilian continental shelf; these are K. coimbrai and K. gnoma. These species are eurybathic and their occurrences in a shelf environment are restricted to areas under the influence of the Falkland Current. Considering their stratigraphic distribution within the Pelotas Basin, southern Brazil, and their association with other cold-water taxa, including species of foraminifera, it is here suggested that the Falkland Current was already operating in the Miocene.


INTRODUCTION
This paper presents a taxonomic and palaeocological study of Krithe reversa Bold, 1958, K. Trinidadensis Bold, 1958, K. m. rnorkhoveni Bold, 1960. and four species left in open nomenclature. Considering that their Recent geographical distribution is related to different oceanographic conditions within the Brazilian continental margin, it is possible to characterize their ecological setting. Their stratigraphic distribution and palaeoceanographical significance is also discussed.
A palaeoceanographical approach is undertaken as the occurrences of K. coimbrai and K. gnoma in the Miocene of the Pelotas Basin, as verified by the present workers, is used here as a parameter for palaeoceanographical analysis of the Falkland current. Also, a discussion concerning the establishment of the proto-NADW (North Atlantic Deep Water) is presented with reference to the geographical and stratigraphic distribution of the psychrospheric species K. reversa, K. trinidadensis and K. m. rnorkhoveni.

STUDY AREA
The Brazilian continental margin is located on the eastern border of the South American plate. It is subdivided into three physiographic provinces: shelf, slope and continental rise. Areas of the Brazilian continental margin, in terms of geographical classification, are here referred to as northern, eastern and southern (Fig. 1). The continental shelf slopes at an average of 0.1". The northern and eastern continental shelves extend from the coastline to isobaths of 40m and 70m. The southern continental shelf reaches from lO0m to 160m. The continental slope is inclined at an average of 4" to 5". The boundary between the slope and the continental rise occurs within the bathymetric interval of 2,800m to 3,600m. The continental rise extends to depths 4800-5400m (Chaves, 1983).
Four types of water masses influence the Brazilian continental margin: the Upper Water Mass, the Antarctic Intermediate Water, the NADW and the Antarctic Bottom Water. The last three comprise the psychrosphere layer, which affects the marine biota on the slope and continental rise (Kennett, 1982;Bearman, 1989).
The upper water masses are represented by the Atlantic Central Water and Sub-Antarctic Water, which are strongly affected by surface currents. The currents South Equatorial, Brazil and Guianas, mainly on the northern and eastern margins, control the movement of the Atlantic Central Water. As a consequence of these currents, the northern and eastern shelves are under the influence of warmer water than the southern shelf (Martins, 1984).
The Falkland Current, which carries Sub-Antarctic waters to the southern Brazilian continental shelf, is responsible for the introduction of colder water to this shelf area (Boltovskoy, 1981). The influence of this current on the Brazilian continental shelf is felt as far north as 22"S, in the Rio de Janeiro state (Boltovskoy, 1959;Vicalvi & Milliman, 1977;Carmo & Sanguinetti, 1995).
The Pelotas Basin is located on the southern Brazilian continental margin in an area that is also influenced by the Falkland Current. Most of the Pelotas Basin is located on the eastern part of the Rio Grande do Sul state, but it also extends north into part of the Santa Catarina state (Brazil) and south to Uruguay.
In Brazil, the onshore part of the basin extends from around 34" to 28"s (Sanguinetti, 1979;Gomide, 1989;Dias et al., 1994) and in shelf areas it reaches around 26"s (Koutsoukos, 1982;Dias et al., 1994). The southern location of the Pelotas Basin and the stratigraphic range of its sediments (Cretaceous to Quaternary) give this basin special importance in palaeoceanographical studies of the Falkland Current.

MATERIAL
Of the 1210 samples analysed, 1030 were from on bottom sediments collected by the REMAC and GEOMAR oceano- graphic projects, and 180 were from offshore boreholes in the of bottom sediment samples, Krithe occurred in only 62. Pelotas Basin drilled by PETROBRAS-Petroleo Brasileiro SA. Analysis was also carried out on 179 cuttings samples of the Only 67 samples yielded specimens of Krithe. borehole 1-RSS-2, where Krithe occurred in four samples, and Most of the 1030 samples were collected out in the northern one cuttings sample from the borehole 1-SSC-3 (Fig. 1). and eastern margins. On the eastern margin this sampling was restricted to shelf areas, but deeper zones in the southern and TAXONOMY northern margins were sampled (Fig. 1). From the total number The supra-generic classification herein follows that of Moore Pitrat (1961) and Hartmann & Puri (1974). The nomenclature used to describe normal pore canals was proposed by Puri & Dickau (1969) and that for marginal pore canals was established by Morkhoven (1962). With respect to oceanographic projects, the following conventions are employed: R, REMAC; L, leg; and G, GEOMAR. Considering the specific frequency of carapace and/or valves, the following adjectives are used: rare (1-5 carapaces or valves), frequent (6-10) and abundant (more than 11). The type material referred to in this paper is held in the 'Museu de Paleontologia da Universidade Federal do Rio Grande do Sul' prefixed by MP-0-.  Remarks. The strong antero-dorsal concavity of the left valve of the female and its pocket-shaped anterior vestibulum makes this species easily identifiable. Krithe producta Brady, 1880(in Swain, 1967, despite being a young instar, can be identified as K. reversa by the distinctive antero-dorsal concavity of the left valve and the reversed hinge, so that the hinge bar is clearly in the left valve. The material from the Strait of Magellan (sampled 10c-d.    . South Atlantic, Upper Oligocene/Miocene (Benson & Peypouquet, 1983) to Recent (Carmo & Sanguinetti, 1995). Pacific, in the Middle Miocene to Quaternary Ayress, 1985, in Coles et al., 1994. Tertiary of Europe (Oertli, 1961;Deltel, 1964; Russo, 1964;Sissingh, 1972;PokornL, 1980;Ducasse et al., 1985), Central America (Bold, 1958(Bold, , 1960Steineck, 1981) and South America (Bold, 1966(Bold, , 1968. anterior. Internally, four adductor muscle scars are disposed in a vertical row, the single frontal is trefoil, there are three dorsal muscle scars and a mandibular scar is present antero-ventrally. The marginal zone follows the outline of the free margin and through it pass marginal and false marginal pore canals. Anterior and posterior vestibula are present; mushroom-shaped with wide neck anteriorly and posteriorly, tube-shaped. Eleven marginal pore canals extend from the distal portion of the anterior vestibulum and, internally, there are two distinct false marginal pore canals in the antero-dorsal position and one antero-ventral bifid (Fig. 2.1). Strong sexual dimorphism; males more elongate and dorsal margin less arched than in females. Remarks. This species differs from Krithe trinidadensis Bold, 1958 because of its less inflated outline, smaller size and anterior vestibulum lacking a long neck. It is similar to Krithe cf. K. glucialis Brady, Crosskey & Robertson, 1874(in Swain, 1971a), but it is here tentatively placed in synonymy because it is a young instar. Krithe sp. D12 Peypouquet (in Benson & Peypouquet, 1983: 818, fig. 7) has an outline and type of anterior vestibulum that is very close to K. coimbrai, but it is also tentatively placed in synonymy because of the inadequate illustrations of those workers. Occasionally, specimens with a punctate and smooth external surface are found within a single sample. This is interpreted as being related to the degree of erosion caused by natural post rnortem transport of these specimens. Of the illustrated males (Plate 1, figs 5-6). the specimen, which is not eroded ( fig. 5), is slightly punctate and the other more eroded specimen ( fig. 6) is smooth and probably allochthonous. Occurrence. Recent, southern Brazilian margin, this species is found from shelf areas to the continental rise. Miocene, Pelotas Basin, borehole 1-RSS-2, 31"19'3.6"S/5Oo59'1"W, 1641 m depth. Geographical and stratigraphic distribution. South Atlantic, Lower Miocene strata (dated by nannofossils, Gomide, 1989) to Recent (Carmo & Sanguinetti, 1995;Whatley et ul., 1998). The species is also known from onshore boreholes in the southern coastal plain (Sanguinetti, 1979;Gomide, 1989 Diagnosis. A small species of Krithe, subrectangular in lateral view, greatest height in postero-median area and two frontal muscle scars. Description. Subrectangular in lateral view, greatest high in postero-median area. Dorsal margin almost straight, ventral margin slightly concave in the antero-median area. Asymmetrical valves, left valve larger than right, overlap in dorsal area.

1A
Smooth external surface, hyaline, perforated by normal pore canals type 'C'. Rounded anterior and truncated at posterior area. Internally, four central muscle scars are disposed in a vertical row, the upper one is reniform and the others regularly elongate. Two frontal scars, the upper one in an 'L' shape and the other elongate. Three dorsal muscle scars. Mandibular scar mid-ventral. Narrow marginal zone with a constriction midanterior ventrally. Anterior and posterior vestibula are present, mushroom-shaped and tube-shaped, respectively; the anterior one has a wide neck. Ten marginal pore canals extend from the distal area of the anterior vestibulum. Within the anterior marginal zone, four false marginal pore canals are also present, two in the antero-ventral area and the other two in the anterodorsal area (Fig. 2.2). Strong sexual dimorphism; females swollen in the posterior area and greatest height in the postero-median area. Males greatest high in antero-median area. Dorsal and ventral margins almost straight and parallel. Remarks. Krithe gnoma has some similarity to K. producta Brady, 1880(in Kaesler & Lohmann, 1976, figs A, B) but it is easily distinguished by its greatest high in the postero-median area, two frontal scars and a mushroom-shaped anterior vestibulum. On the other hand the species identified as Krithe producta by Kaesler & Lohmann (1976) has its greatest high in the median area, a trefoil frontal scar and a pocket-shaped anterior vestibulum. Occurrence. Recent, southern Brazilian margin, this species is found from shelf to continental rise. ?Upper Eocene/Miocene, Pelotas basin within cuttings samples from the borehole 1-RSS-2 (31"19'3.6"S/50"59'1"W) at depths of 1971 m, 1881 m, 1641 m and 1221 m and borehole 1-SCS-3B (28"9'""S/47"29'6''W) at 570-585 m depth. Geographical and stratigraphic distribution. South Atlantic, ?Upper Eocene/Miocene (Pelotas Basin) dated by correlation with nannofossil biozones (Gomide, 1989) to Recent (Carmo & Sanguinetti, 1995;Whatley et al., 1998 Bold, 1958, K. trinidadensis Bold, 1958and K. m. morkhoveni Bold, 1960 have their first occurrences in the North Atlantic, Upper Paleocene, Middle Eocene and Middle Miocene, respectively . In the South Atlantic, K. trinidadensis and K. m. morkhoveni have their oldest occurrences in the Oligocene, with K. reversa appearing in the Upper Miocene (Benson & Peypouquet, 1983). K. coimbrai sp. nov. and K. gnoma sp. nov. have their oldest occurrences in the South Atlantic within boreholes in the Pelotas Basin (Fig. 3). K. coimbrai first occurrs in the Miocene, borehole 1-RSS-2, and K. gnoma in the ?Upper Eocene/Miocene, borehole 1-RRS-2, and in the Miocene, borehole 1-SSC-3B.
The occurrence of K. gnoma within cuttings samples at 1971 and 1881 m in the borehole 1-RSS-2 is dated as ?Upper Eocene by correlation with nannofossil biozones (Gomide, 1989). This interval is suggested as being deposited in an upper bathyal palaeoenvironment (Koutsoukos, 1982). An Eocene age for K. gnoma is questionable because it comes from cuttings samples and the rare specimens are worn and broken, possibly suggesting that they were caved from higher levels. This hypothesis of downhole contamination is also supported by the absence of this species in the Oligocene levels of the borehole 1-RSS-2. Also within the borehole 1-RSS-2, K. gnoma and K. coimbrai occur at 1641 m depth, at an interval dated as Early Miocene (Gomide, 1989). K. gnoma also occurs at 1221m depth in a interval dated as Middle Miocene (Gomide, 1989). These two horizons are considered to have been deposited in the upper/ middle bathyal zone. The occurrence of K. gnoma in the borehole 1-SSC-3B is at a depth of 585-570m. This interval is suggested as a shelf palaeoenvironment and dated as Late Miocene (Koutsoukos, 1982;Gomide, 1989).
K. coimbrai, in onshore boreholes of the Pelotas Basin, occurs in intervals dated as Early and Late Miocene (Sanguinetti, 1979;Gomide, 1989). These occurrences of K. coimbrai are herein interpreted as middle to outer shelf palaeoenvironments based on the Recent shelf occurrence of this species.
This palaeoenvironmental analysis does not indicate that the distribution is the result of upwelling. By analysing the pattern of the faunal succession in these boreholes, two different associations are recognized: (1) Inner shelf warm water; and (2) middle to outer shelf cold water. The inner shelf association is indicated by the presence of the foraminifera Marginopora vertebralis, Amphistegina lessonii and the ostracod Orionina vaughani. The other association, represented by K. coimbrai, K. gnoma, Callistocythere litoralensis, Cytheretta punctata and Henryhowella indicates relatively colder water and a palaeobathymetry of middle and outer shelf. It would be expected that these two distinct associations would be mixed if aupwelling was the controlling influence.

PALAEOECOLOGY
The occurrences of K. reversa Bold, 1958 andK. trinidadensis Bold, 1958 are restricted to the bathyal zone of the northern and southern Brazilian margin, while Krithe sp. 3 and K. sp. 4 also occur in the bathyal zone, but restricted to the northern Brazilian margin. K. coimbrai sp. nov., K. gnoma sp. nov., K. m. morkhoveni Bold, 1960, K. sp. 1 and K. sp. 2 are restricted to the southern Brazilian margin. K. coimbrai and K. gnoma are eurybathic species; the others species show a preference for deepwater conditions.
Krithe reversa, K. trinidadensis and K. m. morkhoveni all have a wide geographical distribution. K. reversa and K . m. morkhoveni occur in the Atlantic and Pacific oceans (Coles et al., 1994;Carmo & Sanguinetti, 1995;Ayress, 1985, in Coles et al., 1994Swain, 1967Swain, , 1971a; and K. trinidadensis is found in the Atlantic, Indian and Pacific oceans Carmo & Sanguinetti, 1995;Swain, 1971a,b). Despite their wide geographical distributions, these species are always restricted to the colder and deeper areas of the oceans.
K. coimbrai and K. gnoma occur in a bathymetric range from shelf to continental slope in the southern Brazilian margin. Considering the occurrences of K. sp.1 and K. sp.2 (in Whatley et al., 1998), synonyms of K. coimbrai and K . gnorna, respectively, it is possible to extend the occurrences of these species into the Argentinean continental margin and also to confirm their eurybathyal habit. The shelf occurrences of these two species are restricted to the southern Brazilian margin and further south into areas under the influence of the Falkland Current (Carmo & Sanguinetti, 1995;Whatley et al., 1998).
The close ecological relationship between the Recent shelf distribution of K. coimbrai and K. gnoma in areas under the influence of the Falkland Current in the Brazilian continental margin, make them excellent palaeoceanographical indicators. It is possible, therefore, to infer relatively cold palaeoenvironmental conditions for those levels where these two species are found in the Neogene of the Pelotas Basin (Brazil). The occurrence of these species in association with C. litoralensis (Rossi de Garcia, 1966), C. punctata Sanguinetti, 1979 and specimens of Henryhowella with ocular tubercles (Sanguinetti, 1979) support the present hypothesis of a palaeobathymetry from middle to outer shelf under relatively cold water conditions.
Callistocythere litoralensis is a species characteristic of shelf environments, ranging from 19 to 165m on the Brazilian continental shelf, but it is more frequently found at depths below 80 m. Considering the shelf distribution of C. litoralensis in this area, its shallowest occurrence is registered farther south in Rio Grande do Sul State (Coimbra et al., 1995), where the influence of the Falkland Current is stronger. This same pattern of shelf distribution is assigned to K. gnoma and K. coimbrai (Carmo & Sanguinetti, 1995).
Henryhowella is very common in cold shelf environments (Cronin, 1996). This hypothesis is corroborated by Recent occurrences of species of Henryhowella with ocular tubercles restricted to the southern Brazilian continental shelf (personal communication M. I. Ramos, 1996) in areas under the influence of the Falkland Current.
Considering this palaeoecological setting, it is possible to suggest that the Falkland current was already established in the Miocene (Fig. 4). It is possible, therefore, to corroborate the palaeomagnetic data that indicates the opening of the Drake Passage in the Late Oligocene (Barker & Burrell, 1977: Kennett, 1982. The opening of the Drake Passage is the important tectonic event that allowed the establishment of the Falkland Current and the circumpolar pattern of ocean circulation (Boltovskoy, 1979(Boltovskoy, , 1980.

DISCUSSION
The dating of the establishment of the Falkland Current, as well as inferences about the palaeoecology of the Neogene strata within the Pelotas Basin, are polemic subjects. The Falkland Current penetrates into the Brazilian continental margin through the complex Subtropical/Sub-Antarctic Convergence (Boltovskoy, 1979). Within the Subtropical/Sub-Antarctic Convergence, the warm waters of the Brazil Current, which flows from the north, interact with cold waters flowing from the southwest (Falkland Current) (Boltovskoy, 1979;Robinson & Guymer, 1996). In this convergence, the greater part of the waters from the Falkland Current (higher density) sinks beneath waters from the Brazil Current and reaches north to latitudes around 22"s within the Brazilian continental shelf. For oceanographic details of the interaction between the Falkland and the Brazil Currents, see Boltovskoy (1959) and Boltovskoy (1 98 1).
Earlier studies, developed with foraminifera, indicated the establishment of the Falkland Current during the post-Miocene (Boltovskoy, 1979;Boltovskoy, 1980). Benthonic foraminifera, such as A . lessonii and M . vertebralis in Miocene strata, were used to infer a warm water palaeoenvironment in southern South America (Closs, 1970;Boltovskoy, 1979).
These foraminifera are found with Orionina, an ostracod genus which is also characteristic of inner shelf areas (Sanguinetti, 1979;Coimbra & Ornellas, 1989). It is therefore possible to extrapolate the Recent ecological setting of A . lessonii, M . vertebralis and Orionina to their Miocene occurrences in the Pelotas Basin. It is here suggested that the warm water conditions of an inner shelf palaeoenvironment be applied to those strata where this association is found.
The establishment of the post-Miocene Falkland Current was based on the thermophylic species described here, in addition to the distribution of foraminifera characteristic of Argentinean and Brazilian subprovinces (Boltovskoy, 1979(Boltovskoy, , 1980. However considering the restriction of the benthonic thermophylic taxa to the inner shelf, it is suggested that this was a result of the stronger influence of the Brazil Current in the southern areas of the South America during the Miocene rather than due to the absence of the Falkland Current during this time.
On the other hand, considering the occurrences of K. coimbrai and K. gnoma with the ostracods C. litoralensis, C. punctata and Henryhowella in the Miocene of the Pelotas Basin, it is possible to suggest a palaeoenvironment of relatively cold waters in the middle and outer shelves under the influence of the Falkland Current. This corroborates the hypothesis proposed here that the Falkland Current was already established in the Early Miocene.
The suggestion that the middle and outer shelves were under the influence of the Falkland Current on the southern Brazilian margin during the Early Miocene does not disagree completely with palaeoecological inferences based on thermophylic species, but it does limit the warm water conditions to the inner shelf areas. Boltovskoy (1979) and Hoddel & Kennett (1985) have suggested that the Miocene occurrences of warm water faunas in southern South America are probably the result of a stronger influence by the Brazil Current during this time. This stronger influence is supposed to be related to the relatively warmer climate during the Miocene, as indicated by foraminifera (Bertels, 1975) and by nannofossils (Haq, 1980). This climatic setting may be used to explain the extension of thermophylic faunas southwards during the Miocene rather than removing the influence of the Falkland Current.
Another approach, using the geographical and stratigraphic distribution of Krithe species in the Atlantic, Pacific and Indian oceans, can also be considered. The Recent wide geographical distribution of K. reversa, K. trinidadensis and K. m. morkhoveni in the deep sea may be interpreted as being a consequence of their psychrospheric habit. Thus it would be related to the dynamic connection, at psychrospheric levels, between the Atlantic, Pacific and Indian oceans, maintained by the influence of the NADW and Antaractic Bottom Water.
In the North Atlantic the oldest occurrences of K. m. morkhoveni and K. trinidadensis are recorded in Palaeocene and Eocene strata, while, in the South Atlantic, their oldest occurrences are in the Oligocene. These younger appearances in the South Atlantic, compared with the North Atlantic, could be the result of a migration southwards through the NADW mass. This assumption corroborates the hypothesis of the polar or subpolar origin of oceanic deep water in the Oligocene (Hay, 1988), as well as confirming the Oligocene dating of the establishment of the proto-NADW (Benson et al., 1985).

CONCLUSIONS
Nine species belonging to the genus Krithe have been identified within the Brazilian continental margin: K. reversa Bold, 1958, K. trinidadensis Bold, 1958, K. m. morkhoveni Bold, 1960 coimbrai sp. nov. and K. gnoma sp. nov., and the four species that have been left in open nomenclature. It is concluded that these species are cryophilic.
Krithe reversa, K. trinidadensis and K. m. morkhoveni show a preference for psychrospheric conditions and for this reason they have a wide distribution within the largest oceanic basins.
Krithe coimbrai and K. gnoma are restricted to the southern Brazilian margin. They are eurybathyal, ranging from the shelf to the continental rise. Their shelf occurrences on the Brazilian continental margin are restricted to zones under the influence of the Falkland Current. This peculiar distribution pattern characterizes them as being oceanographic indicators of the Falkland Current.
Krithe coimbrai and K. gnoma can tolerate temperatures up to 15"C, whereas the other species occur only in colder waters and preferably in deeper zones than those present in shelf areas.
Krithe coimbrai ranges from the Miocene to Recent and K. gnoma from ?Upper Eocene/Miocene to Recent. Their occurrences in the Miocene levels of the Pelotas Basin suggest a palaeobathymetry of middle to outer shelf under relatively cold water conditions (below 15°C). This palaeoenvironmental interpretation is supported by the occurrence of foraminiferid species such as C. crassa, C. curvata, T. calva, C. bertheloti, M . secans, N. atlantica, P. ,faveolata, Q. larnarckiana, T. candeiana, Q. seminulum, and ostracods such as C. litoralensis, C. punctata and Henryhowella, which occurr in the Miocene of the Pelotas Basin.
The Recent shelf occurrences of K. coimbrai and K. gnoma, allied to the association mentioned above, are used to suggest that the Falkland Current was already established in the Miocene. It is therefore possible to corroborate the hypothesis, based on the palaeomagnetic data, which indicates an opening of the Drake Passage in the Late Oligocene.  Kennett (1982)