Radiolarians and Diatoms from the mid-Cretaceous Successions of the Sergipe Basin, Northeastern Brazil: palaeoceanographic Assessment

Radiolarians and diatoms are documented for the first time from the mid-Cretaceous succession of the Sergipe Basin, a passive marginal basin in northeastern Brazil. Prevailing palaeoceanographic conditions are inferred for the episodes of siliceous radiolarian and diatom biomineralization/preservation. Radiolarian faunas are first recorded in the middle to upper Albian, from scattered occurrences, and subsequently throughout most of the Cenomanian-Turonian succession. Spumellarian forms are dominant in all the sections. Nassellarian forms seem to have thrived in relatively deep-water environments, in middle neritic to upper bathyal pelagic biotopes, and have been recovered from upper Albian and uppermost Cenomanian to middle Turonian sediments. Diatom frustules are only recorded from upper Cenomanian and lower Turonian deposits. These seem to have been more abundant in shallower neritic environments. The onset of the radiolarian assemblages in middle-late Albian times (with waning low-oxygen pelagic conditions) is thought to be a response to better developed oceanic circulation patterns and to a water mass saturated in dissolved silica, perhaps generated by deep-sea volcanic processes in the formation of early oceanic crust and the mid-oceanic ridge in the northern South Atlantic. On the other hand, the record of radiolarian and diatom tests throughout the Cenomanian-Turonian succession is commonly associated with dysaerobic to quasi-anaerobic bottom conditions. This is not only in keeping with high epipelagic primary productivity in well-oxygenated surface waters and that the sea water apparently contained a high level of dissolved silica, but also suggests that the bottom and interstitial waters were enriched in carbon dioxide, had a low pH and slightly negative redox-potential (Eh). The overall conditions would have favoured the biomineralization and post-mortem preservation of siliceous organisms increasing, therefore, the radiolaria+diatom/foraminifera ratio in the sediments, which supports the conclusions of several previous authors.


INTRODUCTION
Radiolarians and diatoms are recorded for the first time from the mid-Cretaceous (middle Albian to Turonian) succession of the Sergipe Basin, a passive marginal basin in northeastern Brazil (Figs. 1,2). Their patterns of distribution and diversity demonstrate a close correspondence with the palaeobathymetry and overall palaeoceanographic conditions (depth-related in part) prevailing during the time on the northwestern margins of the South Atlantic.
The systematics of the radiolarian microfauna studied is documented. Because of the great uncertainty regarding the systematic positions of diatom frustules recovered during this study, a taxonomic listing of species is not attempted. Rather, the microflora is briefly discussed in general informal terms (and arranged in morphotypes). The specimens may be preserved as unaltered siliceous tests and either pyritized (late Aptian radiolarians) or calcified tests, where the original amorphous silica forming the skeleton has been replaced.

Localities and Well-Sections
All assemblages examined during this study come from mid-Cretaceous outcrops and oil well-sections in the Sergipe Basin. The sites from which Albian to Turonian radiolarian/ diatom bearing sediments have been recovered are listed in the Appendix (UTM coordinates, local references, lithostratigraphic units) and their location can be found in Fig. I . All studied Cenomanian-Turonian outcrop sections correspond to localities that were named and documented in full by Bengtson (1983, p. 63-71), to whom the reader is referred for complete locality descriptions. The toponyms and respective locality numbers used here follow the same references given in the above work. Inferred palaeobathymetries of the studied sections were based on the interpretation of the patterns of paleoenvironmental distribution of foraminifera1 palaeocommunities and lithofacies and have been documented elsewhere (Mello er ul., 1989;Koutsoukos & Hart, in press;Koutsoukos et ul., in presse.g. see Fig. 3). The Cenomanian-Coniacian succession (Cotinguiba Formation) has been studied in detail by Bengtson ( I 983) and Berthou & Bengtson ( 1 988), who established an ammonite biostratigraphic zonation and investigated the potential of  Ojeda &Fugita, 1976 andBengtson, 1983). Tectonic evolutionary phases after Asmus & Baisch (1983). Abbreviations refer to lithostratigraphic ugits. microfacies analysis for stratigraphic correlation, respectively. An extensive survey and taxonomic revision has been recently carried out by the present first author on the foraminiferal microfauna recovered from the late Aptian-Maastrichtian succession. The dating of sections is mostly based on an integrated biostratigraphical scheme, currently in preparation, based on foraminifera and ammonites.

Sample Preparation and Deposition of Types
The studied assemblages were collected as a result of conventional preparation of samples for foraminiferal analysis. Laboratory sample preparation was mostly carried out in the 'Setor d e Bioestratigrafia e Paleoecologia' of the PETROBRAS' Research Centre (CENPES), in Rio de Janeiro, Brazil. All the indurated samples were crushed prior to processing. Representative cuts of 60 grams (ditch-cuttings) and I20 grams of sediment (cores and outcrops) were taken from each sample and then immersed into a solution of concentrated hydrogen peroxide, and allowed to sit for about 6-8 hours in a fume cupboard until break down. The disaggregated sediment was then washed through one finemesh sieve of 63pm to eliminate the clay. The residues were allowed to dry and were, subsequently, dry-sieved with three screens of 125pm, 250pm and 500pm mesh. All size fractions were examined and the microfossils (foraminifera, ostracods, radiolarians, micromolluscs) were picked out on a gridded tray and collected into one-hole slides, until a minimum representative count of approximately 300 specimens was reached.
Samples from a complementary locality (Pati 1) were processed at the laboratories of the Department of Geological Sciences of Polytechnic South West, Plymouth, in a standard manner as follows. Samples were broken down into small pieces using a mortar and pestle. Crushed samples were then dried and soaked in 'White Spirit' overnight. Excess solvent was decanted off and distilled water added (lh) until breakdown. Normal washing procedures and picking followed (as above).
The specimens selected to be photographically recorded were mounted on standard copper stubs with double-sided tape, shadowcasted with approximately 13 A" coating of gold, and observed on a JEOL (JSM -T20) Scanning Electron Microscope, operated at 20Kv. Elemental analysis of test composition were carried out under a JEOL (JSM -35C) Scanning Electron Microscope with a Link System X-Ray Spectrometer (860B Series), operated at 25Kv. All figured specimens will be deposited and registered in the micropalaeontological collections of the PETROBRAS ' Research Centre (CENPES).

GEOLOGICAL SETTING
The basin is the southern part of the Sergipe-Alagoas sedimentary complex, a structural1 y-elongated marginal basin located in northeastern Brazil (Fig. 1). It is closely related to the rifting and drifting phases of the South American and African continents in the Late Mesozoic (e.g., Asmus & Baisch, 1983). The understanding of the biostratigraphical and palaeobiogeographical characteristics of its Cretaceous stratigraphic sequence is, therefore, of foremost significance for studies relating to the early geological history and subsequent palaeoceanographic evolution of the northern South Atlantic Ocean.
Three maxima in oxygen depletion (dysaerobic/anaerobic conditions) are noticed in the succession, from middle shelf to upper slope environments: 1) in the late Aptian-earliest Albian (see Fig. 3); 2) in the early Cenomanian; and 3) in the latest Cenomanian-earliest Turonian (Koutsoukos et al., in prep. ). Waning low-oxygen pelagic conditions (dysaerobic to aerobic) are apparent in the upper Albian succession of Sergipe (Koutsoukos et al., in press; Fig. 3), a probable consequence of less restricted oceanic exchange between the South and North Atlantic, with the establishment of more open oceanic circulation patterns. During early to middle Cenomanian times, lowering sea-level and more humid climate (increased continental runoff) coupled with at least two NW-SE transpressional pulses along the equatorial margin (R.P. de Azevedo, 1988, andpersonal communication, May 1989), could have further restricted the oceanic circulation patterns and contributed to the build-up of an oxygen-minimum zone of oceanic proportions.
A latest Cenomanian sea level rise is noticeable in Sergipe by the occurrence of upper slope thin laminated pelitic sediments, represented by the only known exposure of the Aracaju Member (locality Pati 2), in a carbonate-dominated sedimentary cycle (Mello et al., op.cit.). There is evidence of an expansion of the oxygen minimum zone over the continental shelf during the latest Cenomanian-earliest Turonian times, sporadically affecting the upper epipelagic layers, and very probably associated with this sea level maximum (Mello et al., op.cit. ). This sea level rise in the latest Cenomanian is of world-wide significance (see Haq et al., 1987), as well as the rise and expansion of the oxygen minimum zone (Arthur et.al., 1987;Schlanger et al., 1987), the so-called "Oceanic Anoxic Event 2" (OAE) of Schlanger & Jenkyns (1976), Arthur & Schlanger (1979) and Jarvis et al. (1988).
As can be seen, few studies have been published on the taxonomy and distribution of Cretaceous radiolarian taxa, and, even fewer on Cretaceous diatomaceous floras. This situation is made worse by the incomplete understanding of their biostratigraphical value and very rare preservation in the stratigraphical record, which usually places this less conspicuous microfossil group in a secondary level during conventional micropalaeontological studies. However, it is exactly this "rare" and ''limited'' occurrence in the stratigraphical record that makes the group a primary tool for palaeoecological and palaeoceanographical studies. The onset of radiolarians in middle Albian times and the drastic changes (in abundance and morphotypic composition) of the radiolarian and diatom populations in the uppermost Cenomanianlowermost Turonian of Sergipe are both related to the major palaeoceanographic events which affectedmost of thenorthem South Atlantic.

Diatoms
Diatom frustules are only recorded from uppermost Cenomanian (locality Pati 2) and 1owermostTuronian (locality Pati 1; well 1-US-1-SE: 336-351m) deposits (Figs 4 and 5). Six morphotypes are present in the assemblages (illustrated in P1. 4 , Figs 1-14). An outcrop of the earliest Turonian age (locality Pati l), thought to have been deposited in a probable middle-outer shelf environment, yield an abundant diatom assemblage of discoid forms (morphotypes A, B and C), all represented by calcareous frustules, a result of preservational processes similar to the ones that affected the radiolarian assemblages. The record of diatom frustules in the uppermost Cenomanian-lowermost Turonian sections attests to the particular palaeoceanographic conditions at the time (see below).

PALAEOECOLOGICAL IMPLICATIONS Preservation of Siliceous Tests
In most recent sediments radiolarians and diatoms are absent through post-mortem dissolution of their tests. They are composed of relatively unstable amorphous silica (opaline biogenic silica) and are readily dissolved in normal sea water which is usually undersaturated with respect tonatural opaline silica (Berner,197 1). Furthermore, during laboratory experiments carried out by the first author, extant diatom frustules have been shown to have higher fragility and susceptability to dissolution than radiolarian tests. Radiolarians are nowadays relatively common, however, only in certain areas of vigorous upwelling, such as the peri-equatorial Pacific and part of the northwestern African slope (e.g., Lisitzin, 1972;Diester-Haass, 1978).
In the outcrop sections of the Cotinguiba Formation chert nodules are commonly found, presumably formed from diagenetic dissolution andreplacement of the unstable opaline tests ofradiolarians anddiatoms by calcedony, with subsequent formation of chert at certain horizons where the remobilised silica was concentrated. Depleted oxygen concentrations occurred sporadically in the bottom waters, as suggested by the usual occurrence of thin laminated dark grey marls and calcareous black shales with high organic carbon contents (Mello, 1988;Koutsoukos et af., in prep. ). Enhanced primary productivity in the surface waters coupled with epi-and mesopelagic water masses saturated in silica (perhaps primarily contributed by deep-sea volcanismcf. Berthou & Bengtson, 1988), low pH and slightly negative redox-potential (Eh) of the bottom and interstitial waters, enriched in carbon dioxide, would have greatly favoured the biomineralization and postmortem preservation of these fragile siliceous organisms in the sediments.

Palaeoceanograph y
The development and distribution patterns of siliceous assemblages in the mid-Cretaceous succession of Sergipe is closely related to major palaeoceanographic changes that occurred in the northern South Atlantic at that time. An idealised palaeoceanographic model integrating radiolarian and diatom assemblages is proposed for the Cenomanian-Turonian (see Fig. 5). Spumellarian radiolarians are dominant throughout all the sequence. Nassellarian forms are recorded from upper Albian and uppermost Cenomanian to middle Turonian deposits and seem to have thrived in relatively deep-water environments. These radiolarian assemblages apparently suggest open-water conditions with a peak in abundance and diversity in middleouter shelf or greater depths (i,e., in excess of 30/50 meters - Fig. 5). On the other hand, diatom frustules appear to have been more abundant in shallower neritic environments and have only been recorded from uppermost Cenomanian and lowermost Turonian deposits ( Fig. 4 and 5).
The apparently restricted record of radiolarian-rich beds, generally limited to certain time-intervals and palaeogeographic settings. caused some workers to refer to them as 'The Radiolarian Event' (Jenkyns & Winterer, 1982), that is to say, times in the geological record when siliceous biogenic accumulations was favoured and usually associated with regional palaeoceanographic events (Jenkyns & Winterer, op.cit.;Baumgartner, 1984;Miskell et ul., 1985). Similar features have been reported for sediments from many areas of the world, deposited during OAE's (Kuhnt et ul., 1986;Schlangeretul., 1987) andduringolderupwellingevents (e.g. Mazagan Escarpment, offshore Central Morocco: Leckie,198 1 ; Monterey Formation, California: Katz & Elrod, 1983: Rayda Formation, Arabian Peninsula: Connally & Scott, 1985. Upwelled nutrient-rich deep waters would promote high primary productivity in surface waters and favour a biogenic siliceousrecord (Jenkyns & Winterer, op. cit.; Leckie, op. cit.). However, the episodes of siliceous radiolarian and diatom biomineralization/preservation in Sergipe are not merely interpreted as indications of coastal upwelling events, as the palaeoceanographic evidence suggests restricted physiography at the deep-oceanic basic for most of the mid-Cretacious with salinity-stratified water masses (Mello rt a / , , in press; Koutsoukos etul., in prep). High primary productivity in the epipelagic layers is most likely to have been a response to high nutrient levels (eutrophic conditions), perhaps largely controlled by increased continental runoff. Increa4ing sealevels and improved deep circulation (such as in the latest Cenomanian-earliest Turonian) may also have significantly contributed to overturn nutrient-rich oxygen-depleted bottom waters and, consequently, increase surface-water productivity (cf. Arthur & Schlanger, 1979).
The earlieronset of the radiolarian assemblages in mid-late Albian times (with waning low-oxygen pelagic conditions), through scattered occurrences, is inferred to be a response to better developed oceanic circulation patterns and to a water mass with high levels of dissolved silica, perhaps originated . BUILD -UP OF AN 0. m z.

SPUMELLARIANS OF OCEANIC PROPORTIONS
by deep-sea volcanic processes in the formation of early oceanic crust and the mid-oceanic ridge. On the other hand, the record of radiolarians and diatoms throughout the Cenomanian-Turonian succession (Figs. 4and5) is commonly associated with dysaerobic to quasi-anaerobic bottom conditions. The higher abundance and diversity are observed in the upper Cenomanian-lower Turonian deposits (Fig. 4). These biotic patterns are recognised in coeval sediments of most marine basins (e.g. Jenkyns & Winterer, 1982;Thurow et al., 1982;Kuhnt et al., 1986;Arthur et al., 1987) and have been associated with high-sea level, increased productivity and the onset of intensified bottom oxygen depletion over the shelf during latest Cenomanian-earliest Turonian times.

CONCLUDING REMARKS
Radiolarian faunas are first recorded in the middle-late Albian (Riachuelo Formation), from scattered occurrences, and subsequently throughout most of the Cenomanian-Turonian succession (Continguiba Formation). Spumellarian forms [mostly represented by Arachnosphaera (?), Crucella, Orhiculiforma and Phaseliforrna species] are dominant in all the sections. Nassellarian forms seem to have thrived in relatively deep-water environments, in middle neritic to upper bathyal pelagic biotopes, and have been recorded from upper Albian and uppermost Cenomanian to lower-upper Turonian sediments. Dictyornitra ex gr. rnulticostata, Gongylothorax sp. A andLithostrohus spp. are the most common morphotypes of Nassellariina recovered during this study. Diatom frustules are only recorded from uppermost Cenomanian and lowermost Turonian deposits. These seem to have been more abundant in shallower neritic environments.
The development of siliceous assemblages in the mid-Cretaceous succession of Sergipe is closely related with major palaeoceanographic changes that occurred in the northern South Atlantic at that time. The onset of theradiolarian assemblages in middle-late Albian times is thought to be a response to better developed oceanic circulation patterns and to a water mass with high levels of dissolved silica, perhaps generated by deep-sea volcanic processes in the formation of early oceanic crust and the mid-oceanic ridge. On the other hand, the record of radiolarians and diatoms throughout the Cenomanian-Turonian succession is not only in keeping with high epipelagic primary productivity in well-oxygenated surface waters, saturated in dissolved silica, but also suggests that the bottom and interstitial waters were enriched in carbon dioxide, had a low pH and a slightly negative redox-potential (Eh). The overall conditions would have favoured the biomineralization and post-mortem preservation of siliceous organisms increasing, therefore, the radiolaria+diatom/ foraminifera ratio in the sediments, which supports the conclusions of several previous authors.
The low-diversity radiolarians from Sergipe reveal taxonomic and morphotypic affinities with coeval assemblages reported from the southeastern Brazilian margin (Campos Basin), Caribbean area (Puerto Rico), western interior region of North America (Colorado, Kansas, Wyoming), eastern Atlantic regions (NW Africa), northern Australia (Queensland), western Canada sedimentary basin (Alberta), and northeastern Pacific margin (California Coast Ranges). The evidence appears to suggest wide palaeobiogeographic distribution patterns of these radiolarian taxa at low and high latitudes during the mid-Cretaceous.

TAXONOMIC NOTES
Phylum Protozoa Subphylum Sarcodina Class Actinopodea Subclass Radiolaria Entries and taxonomic remarks for each radiolarian species are brief. References are restricted to the original description of the species and to relevant references used in this study. Furthermore, because of the fragmentation of tests and common recrystallisation, no decision could be made on specific attribution of several morphotypes. Such forms were either placed in single species-groups, each regarded with broad morphological variability, or placed in open nomencla'ture.

Explanation of Plate 1
All illustrations are scanning electron photomicrographs. Scale bars = IOOpn. Remarks. The species is distinguished by its large (c. 480 pm in maximum length) test with the presence of three to four irregularly developed arms. Range. Latest Cenomanian. Occurrence. Locality Pati 1 (rare occurrence).