Biostratigraphy and Paleoecological Inferences Based on Oligocene Calcareous Nannofossils from the Ceará Rise (ODP Leg 154, Site 929A): Equatorial Atlantic Ocean Bioestratigrafia e Inferências Paleoecológicas com Base em Nanofósseis Calcários Oligocênicos da Elevação Ceará (ODP Leg 154, Site 929A): Oceano Atlântico Equatorial Anaiza

The Ceará Rise is an aseismic feature located in the Equatorial Atlantic Ocean, between the Amazon Cone, Demerara Abyssal Plain and the Ceará Abyssal Plain, which represents a key area for the understanding of deep ocean facies. In the oil and gas field, the identification of calcareous nannofossil-based biozones is important for the age modeling and comprehension of depositional evolution of the sedimentary strata. The study of Oligocene calcareous nannofossil-based succession at the Ceará Rise helped to detect paleoecological trends during the Oligocene. Using a polarized light optical microscope, forty-four species were observed, described and accounted in fifteen samples. The biostratigraphic model includes the standard biozones NP23, NP24 and NP25, as well as the recently proposed biozone NP26 Clausicoccus fenestratus. Among the species found, Sphenolithus ciperoensis, Sphenolithus distentus and Sphenolithus predistentus were the most important biostratigraphic markers for the section. The quantification of the nannofossils, enabled the division of the studied section into eight distinct intervals according to the fluctuation in paleoproxies values and diversity indices: Interval A (CP17: Rupelian): oligotrophic assemblage of warmer surface waters; Interval B (CP18: Rupelian): temperate assemblage of mesotrophic/eutrophic surface waters; Interval C (CP18 and CP19a: Rupelian/Chattian): oligotrophic assemblage of warmer surface waters; Interval D (CP19a and CP19b: Chattian): temperate assemblage of mesotrophic/eutrophic surface waters; Interval E (CP19b: Chattian): oligotrophic assemblage of warmer surface waters; Interval F (CP19b: Chattian): temperate assemblage of oligotrophic surface waters; Interval G (CP19b: Chattian): temperate assemblage of oligotrophic surface waters; and Interval H (CN1a: Chattian): temperate assemblage of eutrophic surface waters.


Introduction
The calcareous nannofossils are widely used in biostratigraphic and paleoecological researches. This group has a number of advantages for those studies, such as the ease of sample preparation and the small size of these organisms, which contributes to the preservation of their entire hard skeletal tissues, presenting wide geographical distribution and high abundance values (Schneidermann, 1973). Due to its high recovery, population density, rapid evolution and planktonic nature, also reinforces the applicability of these organisms for age modeling (Antunes, 1997). According to Sezen (2014), the Ocean Drilling Program (ODP) Site 929A (Leg 154) was sampled in an immersed area of Ceará Rise, in the Equatorial Atlantic Ocean. The Leg 154 was proposed to elaborate a model of chemical evolution of the Cenozoic deep water working near Ceará Abyssal Plain and Ceará Rise, as well as near the Amazon Mouth Basin (Sezen, 2014).
This geomorphological feature is located under heated surface waters, which provides a great framework for the execution of this study, also providing conditions to conduct studies based on carbonate microfossils. Previous researches were conducted in this region, with well-calibrated dating for comparison of results (Sezen, 2014;Blair et al., 2017;Boesiger et al., 2017;Browning et al., 2017;de Kaenel et al., 2017). Therefore, this study aims to identify and quantify the assemblage of calcareous nannofossils recovered in fifteen samples from an Oligocene section of the ODP Leg 154, Site 929A, in the Ceará Rise, proposing and age model and inferring paleoecological conditions for the studied section.

Geological Setting
The Ceará Rise is an aseismic zone located in the Equatorial Atlantic Ocean with approximately 3000 meters deep, limited by the Ceará Abyssal Plain to the Southeast and the Amazon Mouth Basin to the Northwest. This feature is morphologically defined as an underwater topographic rise, located in the oceanic crust region (Curry et al., 1995).
According to Kumar & Embley (1977), the igneous basement of the Ceará Rise was formed around 80 my, during the Campanian, when the Mid-Atlantic Ridge, situated went through a period of intense volcanism. During the spreading of the ocean floor, the volcanic extrusion was divided into two distinct segments: the Ceará Rise was formed in the western side and the Sierra Leone Rise was formed in the eastern side ( Figure 1).
The site 929A is located at the coordinates 5º58.573'N and 43º44.396'W, beneath warm surface waters of about 27ºC on the northwest flank of the Ceará Rise, under 4397 meters deep (Curry et al., 1995;Sezen, 2014). The section shows three stratigraphic units and the studied interval (unit III, illustrated in association with the paleoecological indices in Figure 6) consists mostly of clayey nanno-foram chalks (Curry et al., 1995).

Material and Methods
Fifteen samples were prepared according to the method described on Wanderley (2004). A polarized light optical microscope (Zeiss), with 1000x to 1600x total magnification, was used for the micropaleontological analysis. The identified taxa were classified based on the taxonomy described in Perch-Nielsen (1985), de Kaenel & Villa (1996), Bown (1998)  , through four complete traverses of the coverslip (with polarized and natural lights). Ten fixed fields of view (FOV), without overlapping, were counted and relative abundances (RA%) were calculated for each sampled depth as described in Bown & Young (1998).

Results and Discussion
Forty-four species and morphotypes were identified and illustrated (Figures 2, 3 and 4). The photomicrographs have been ordered according to the informal taxonomic groups described in Young et al. (2017).

Biostratigraphy
The reference biozonations used in this study were Martini (1971), Okada & Bukry (1980), and Agnini et al. (2014). The biozonation of Martini (1971) is based on the first and last occurrences of calcareous nannofossils from cores drilled by the Deep Sea Drilling Program (DSDP) in many latitudes (Bukry, 1973). This biozonation is represented, in this research, by the zones NP23, NP24, NP25, and NP26. Biozones of Okada & Bukry (1980) were based on data from the ODP, recovered in the Equatorial Atlantic Ocean, and, in this study, it's covered by the zones CP17, CP18, CP19a, CP19b, and CN1a. The biozonation of Agnini et al. (2014) is based on drilling sequences in deep sea areas at low and medium latitudes, as well as marine sections of the ancient Tethys Sea. This zonation is characterized, in this analysis, by the zones CNO3, CNO4, CNO5, and CNO6.
A new biozone was proposed by de , referred as NP26, which is positioned at the top of Oligocene, between the top of NP25 biozone and the base of NN1 biozone. The NP26 was first studied by Martini (1971) and then amended by de Kaenel et al. (2017) so its lower limit has shifted to newer ages. Okada & Bukry (1980) revealed a "NP26" informal zone, corresponding to CN1 -Cyclicargolithus abisectus. This biozonation uses the higher occurrence of Sphenolithus ciperoensis (24.215 million years) as a consistent biohorizon for the base of this zone, as the the top is marked by the lower occurrence of Discoaster druggi (10-15 μm) in 23.155 million years .
The studied section shows its base in Lower Oligocene -Rupelian (NP23 biozone) with average depth of 461 meters below sea floor. The top of the range is located in Upper Oligocene -Chattian (biozone NP26) with average depth of 335 meters below sea floor. The biozones in this work are in accordance with previous zonations of Curry et al. (1995) and Sezen (2014), for the same Leg, differing only in the detection of the recent described NP26 and the biozonation of Agnini et al. (2014) for the Site. All the detected zones and respective depths and ages were plotted with the nannofossils stratigraphic ranges in Figure 5.

Paleoecological Inferences
The calcareous nannofossil population studied at Site 929A was divided into eight intervals described according to the comparison of the RA% of the paleoproxies and diversity indices ( Figure 6) (Shannon, 1948;McNaughton & Wolf, 1970;Gotelli & Colwell, 2001;Whittaker et al., 2001). Discoaster spp. and Sphenolithus spp. are generally considered as tropical warm and oligotrophic taxa (Gibbs et al., 2004;Villa et al., 2008). Helicosphaera spp. are adapted to warm coastal and platform environments, with high nutrient avail-ability, being limited to low latitudes and shallow seas (Wei & Wise, 1990). Cyclicargolithus spp. does not show preferential temperature (Persico & Villa, 2004), but according to Wei & Wise (1990), this group is better suited to temperate environments and medium latitudes. R. bisecta is adapted to low and medium latitudes, preferring temperate waters (Wei & Wise, 1990). However, results from Haq & Lohmann (1976) characterized the species as high latitudes taxa.
Reticulofenestra daviesii and C. pelagicus are adapted to eutrophic cold waters from high to medium latitudes, evidenced by the increase in the number of individuals during the Oi-1 (Oxygen Isotope Event) cooling event in the Oligocene, and Reticulofenestra spp. ≤ 4 μm could indicate heated meso-eutrophic waters (Persico & Villa, 2004).
This comparative dataset enabled the identification of eight intervals of paleoecological trends, as described below in chronological order: Interval A: occurs in samples 49-1 and 50-1 (CP17: Rupelian), showing a typical assemblage of warmer oligotrophic surface waters, composed of subtropical species, associated with the increase of richness, number of individuals, equitability and Shannon diversity and decrease of dominance, suggesting favorable conditions to nannoplankton development.
Interval B: occurs between the samples 46-1 and 49-1 (CP18: Rupelian), showing a typical assemblage of temperate mesotrophic/eutrophic surface waters, composed of subtropical latitude species, associated with the increase of dominance and decrease of richness, number of individuals, equitability and Shannon diversity, suggesting some influence of terrigenous input.
Interval C: occurs between the samples 44-1 and 46-1 (CP18 and CP19a: Rupelian/Chattian), showing a typical assemblage of warmer oligotrophic surface waters, composed of tropical latitude species, associated with the increase of richness, number of individuals, equitability and Shannon diversity values and decrease of dominance, suggesting favorable conditions to nannoplankton development.
Interval D: occurs between the samples 44-5 and 44-1 (CP19a and CP19b: Chattian), showing a typical assemblage of temperate mesotrophic/eutrophic surface waters, composed of subtropical latitude species, associated with the increase of the number of individuals, equitability and Shannon diversity, associated with low decrease of richness and dominance, showing week evidence of eutrophic opportunism.
Interval E: occurs between the samples 40-1 and 41-5 (CP19b: Chattian), showing a typical assemblage of warmer oligotrophic surface waters, composed of tropical species, associated with the increase of richness, number of individuals and dominance, and decrease of equitability and Shannon diversity, which could indicate an abrupt input of warmer surface waters, with strong indicators of increased nutrient levels in the lower portion of the interval, and stabilization of paleoceanographic conditions in the upper portion.
Interval F: occurs in the samples 39-1 and 40-1 (CP19b: Chattian), showing a typical assemblage of more temperate oligotrophic surface waters, composed of subtropical latitude species, associated with the increase of dominance and decrease of richness, number of individuals, equitability and Shannon diversity, suggesting a possible transitional environment between warm and cold surface waters, with gradual increase in eutrophic indicators, pointing out a possible establishment of opportunistic populations.
Interval G: occurs between the samples 37-1 and 39-1 (CP19b: Chattian), showing a typical assemblage of temperate oligotrophic surface waters, composed of typical subtropical latitude species, associated with the increase of richness, equitability and Shannon diversity, and decrease of the number of individuals and dominance, suggesting a transitional surface water environment with possible fluctuation and mixing between higher trophic levels at the base and top of the interval with the central portion with decreased nutrient levels.
Interval H: occurs in samples 36-1 and 37-1 (CN1a: Chattian), showing a typical assemblage of temperate eutrophic surface waters, composed of subtropical species, associated with the increase of richness, number of individuals, equitability and Shannon diversity, and decrease of dominance, suggesting a possible colder surface water environment and increased nutrient levels due terrigenous input or increased influence of deep thermohaline water masses (upwelling).
Finally, we suggest the comparison between the quantification of calcareous nannofossils in higher sample resolutions, with the comparative quantification of dissolution levels of coccoliths and nannoliths, for future researches, in order to identify intervals of greater interference of diagenetic effects on the paleoecological signal.