Efectos postdepósito en la microestructura y composición de isotopos estables de foraminíferos planctónicos del Mioceno de la Cuenca de Pelotas, margen continental del sur de Brasil

Geise de Santana dos Anjos–Zerfass^1,*, Farid Chemale Jr.², and Candido A. V. Moura³

¹ BPA/PDEXP/CENPES/PETROBRAS, Av. Horácio Macedo, 950, prédio 20, 1120, 21941–915, Cidade Universitária, Ilha do Fundão, Rio de Janeiro, RJ, Brazil.

² Laboratório de Geologia isotópica da Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Porto Alegre, 91501–970, Brazil.

³ Universidade Federal do Pará, Centro de Geociências, Departamento de Geoquímica e Petrologia. Rua Augusto Corrêa n° 1, Guamá, 66075–900 – Belém, 8608, Brazil. *geise.zerfass@petrobras.com.br

ABSTRACT

An integrated study of planktic foraminiferal tests (Orbulina universa and Globigerinoides trilobus) imaging techniques and chemical/isotopic analyses has been carried out at the Miocene section of the Pelotas Basin (South Brazil) with the purpose of record and evaluate the effects of the diagenesis in its wall texture and isotopic composition. The characterization of the foraminiferal tests preservation prior performing isotopic analysis for paleoceanographic studies is essential to ensure the choice of suitable material for obtaining reliable data. Scanning electron microscopy, backscatter scanning electron microscopy, energy–dispersive X–ray spectrometry and stable isotope measurements were used to evaluate the post–depositional effects on the tests. It was possible to identify features of dissolution, neomorphism (recrystallization) and coating of autigenic minerals. The stable isotopic data define two compositional groups. The heavier and less scattered values characterize a preserved paleoenvironmental signal as the lighter and scattered ones indicate a signal derived from post–depositional alterations. Additionally, the characterization of the distinct types of diagenetic changes and their textural products provide a guideline for the evaluation of the diagenetic effects of deeply buried fossil foraminifera.

Key words: planktic foraminifera tests, diagenesis, stable isotopes, Miocene, Pelotas Basin, Brazil.

RESUMEN

Un estudio integrado de las características de textura y composición química e isotópica de foraminíferos planctónicos (Orbulina universa y Globigerinoides trilobus) del Mioceno de la Cuenca de Pelotas (sur de Brasil) fue llevado a cabo con el propósito de registrar y evaluar los efectos de la diagénesis. La caracterización del grado de preservación de los foraminíferos, previa a la obtención de análisis isotópicos para estudios paleoceanográficos, es esencial para asegurar una selección adecuada del material y así obtener datos fiables. Se empleó microscopía electrónica de barrido, imágenes con electrones retrodispersados, espectroscopía de rayos X de dispersión de energía y mediciones de isótopos estables para evaluar los efectos post–depósito en los caparazones. Esos estudios permitieron identificar características de disolución, recristalización y revestimiento de minerales autigénicos. Los datos de isótopos estables permitieron definir dos grupos de composición: los valores más pesados y menos dispersos caracterizan señales paleoambientales preservadas, mientras que los datos más dispersos y correspondientes a composiciones más ligeras indican una señal derivada de las alteraciones post–depósito. Además, la caracterización de los distintos tipos de cambios diagenéticos y sus productos texturales proporciona una guía para la evaluación de los efectos diagenéticos en los foraminíferos fósiles provenientes de secciones sedimentarias de gran espesor.

INTRODUCTION

The chemical and isotopic compositions of planktic foraminifera tests are widely used as paleoclimatic and paleoceanographic proxies. However, the susceptibility of foraminiferal tests to post–depositional processes, such as dissolution and precipitation of secondary phases, makes difficult to interpret the paleoenvironmental data (Lohmann, 1995; Brown and Elderfield, 1996; Shieh et al., 2002; Sadekov et al., 2010).

The more significant alterations caused by diagenesis in the marine organism tests are dissolution and neomorphism of aragonite to calcite, which cause changes in the trace element composition such as strontium and magnesium removal (Faure, 1986). Diagenesis acts to alter the paleoceanographic signal of the foraminifera calcite test, since the precipitation of secondary calcite and the dissolution of the tests tend to shift the oxygen isotopic composition towards a negative direction, resulting in warmer δ¹⁸O–derived paleotemperatures (Savin and Douglas, 1973; Killingley, 1983; Williams et al., 2005). The addition of secondary calcite presents a high potential of changing the Mg/Ca ratio, while the inorganic calcite has a higher proportion of magnesium than the biogenic calcite, causing erroneous paleotemperature estimates (Sexton et al., 2006).

The presence of overgrowths and/or neomorphic calcite on the micrometric scale is sometimes impossible to detect under binocular microscope (Sexton et al., 2006; Williams et al., 2007). Consequently, the foraminifera tests presenting this class of alteration preserve the microstructural features and, as a result, are frequently assumed to be well preserved. However, scanning electron microscopy constitutes an efficient method for appraising the diagenetic effects in materials presenting a systematic structure such as the biogenic carbonates, for which the good preservation of the morphological characteristics indicates sample integrity (McArthur, 1994). This technique is widely used in the evaluation of fossil foraminifera preservation (Adelseck, 1978; Sexton et al., 2006, Williams et al., 2007).

This paper presents an evaluation of the post–depositional effects on the wall texture and isotopic composition of two common species of planktic foraminifera, Globigerinoides trilobus and Orbulina universa, using scanning electron microscopy – SEM, stable isotope evaluation and energy dispersive X–ray spectrometry – EDS data and represents a case study on the effects of diagenesis in foraminiferal tests under great burial depths. Furthermore, we present an image guide to the differences between the various states of preservation.

GEOLOGICAL SETTING

The Pelotas Basin corresponds to the portion of the South American continental margin between 28°40'S and 34°S, limited to the north by the Florianópolis High, Brazil, and to the south by the Polonio High, Uruguay (Kowsmann et al., 1974; Rosa, 2007). The basin has an area of about 210,000 km², from which about 40,000 km² occurs as emerged area (Figure 1).

MATERIAL AND METHODS

A total of 66 samples were collected from five wells drilled in the Pelotas Basin, southernmost Brazil, four of which in the offshore segment of the basin and one in the onshore portion (Figure 1). Samples from drill–holes PEL–1, PEL–2, and PEL–3 are cutting, whereas those from 2–TG–96–RS and PEL–2A are from cores. Scanning electron microscopy analyses were carried out on foraminiferal tests from 57 samples, and isotopic analyses were performed in the tests recovered from 46 samples (Figure 2).

The samples were prepared following the conventional techniques for calcareous microfossils. Planktonic specimens from the >150 µm sieve fraction were hand–picked under a stereomicroscope Olympus–SZ1145. Well–preserved specimens were separated for isotopic analyses and then ultrasonically cleaned with ultrapure (Milli–Q) water in order to remove particles adhered to the tests.

Analyses by energy dispersive X–ray spectrometry (EDS), backscattered electron microscopy (BSEM) and scanning electron microscopy (SEM) imaging were carried out at the Electron Microscopy Center of the UFRGS with a Jeol JSM 5800 (BSEM and EDS) and Jeol JSM 6060 (SEM) microscopes with a power output of 10kV. Specimens intended for analysis were coated with an alloy of gold and palladium.

EDS mapping was performed in specimens presenting oxidized coatings in order to characterize the chemical composition and superficial distribution of the contaminant phases. SEM imaging was used to document the microstructural features of specimens of Globigerinoides trilobus and Orbulina universa. These were chosen for the microstructural analyses because they are widely used in paleoenvironmental and paleoceanographic reconstructions (e.g. Spero and Lea, 1993; Bemis et al., 1998, 2000; Sanyal et al., 2001; Zeebee et al., 2008) and also because they are abundant in the samples studied.

Carbon and oxygen isotope analyses were carried out in 54 monogeneric samples, of which 26 are from drill–hole PEL–1, 26 from drill–hole PEL–2 and two from the well PEL–2A. Isotopic analyses were performed at the Isotope Geology Laboratory of the Federal University of Pará, Brazil (Para–Iso). CO₂ gas was extracted from the foraminiferal tests (1 – 6 individuals) with 100% orthophosphoric acid at 70°C, using an on–line KIEL–III system coupled to the mass spectrometer Finnigan MAT 252. The results are reported in the conventional δ notation as per mil (‰) relative to the Chicago PDB standard. The analysis of the standard NBS–19, during the course of this study, has yield average δ¹³C and δ¹⁸O values of 2.0 ‰ and –2.13 ‰, respectively. The uncertainties of the isotope measurements are better than 0.07 % for carbon and 0.14 % for oxygen, based on replicated analysis of the NBS–19 carbonate standard.

RESULTS AND DISCUSSION

Scanning electron microscopy

The differentiation between distinct types of diagenetic changes is essential for the recognition of the processes that cause obliteration of the microfossiliferous record. In this context, the evaluation of microstructure preservation promotes the study of the nature of post–depositional processes. Indeed, is important to define the characteristic patterns of a species before being able to recognize the alterations caused by diagenesis.

In terms of microstructural characteristics, Orbulina universa presents pore–like apertures scattered across the surface of the last chamber surface, which is densely perforated and commonly presents pores of two different sizes (Bolli and Saunders, 1985; Kennett and Srinivasan, 1983). As opposed to some authors who consider the bilobate forms of Orbulina as a separate species (Orbulina bilobata), we follow the taxonomic descriptions of Stainforth et al. (1975) and Bolli and Saunders (1985), both of whom consider these forms as a variation of Orbulina universa.

Globigerinoides trilobus subspecies are distinguished only by the morphology of the final chamber, which is larger then the other chambers combined in the subspecies trilobus, is slightly smaller when compared with trilobus in the subspecies immaturus and is sac–like in the subspecies sacculifer (Bolli and Saunders, 1985). On the other hand, considering the microstructural features of the wall, the three subspecies are identical. The microstructure of Globigerinoides trilobus corresponds to a densely perforated wall with a cancellate pattern and pores situated in well–developed pore pits separated by interpore elevations (Kennet and Srinivasan, 1983).

Signals of mechanical alterations were observed in specimens of both studied genera. The removal of parts of the outer surface of the test was observed mostly in Globigerinoides specimens, while breakage was more commonly observed in Orbulina specimens. Peeling can be an artifact of the sample preparation, and it acts to obliterate the original wall texture of the test by means of exposing the internal layers that present attenuated topography (Sexton et al., 2006). Breakage occurs due to the fragility of the tests under mechanical abrasion or as a consequence of the calcite infilling.

Three types of chemical alteration were observed in the tests collected at different levels of the studied drill–holes: (1) oxidized coating and/or pyrite infilling; (2) dissolution; and (3) calcite infilling / neomorphism of the biogenic calcite to an inorganic one.

Well–preserved specimens especially occurred between 1,044 and 1,314 m in drill–hole PEL–1 and between 2,142 and 2,448 m in drill–hole PEL–2. In the samples of the other studied wells, conversely, the occurrence of well–preserved tests was rare, as most of the specimens presented visible diagenetic alterations under stereo–microscopy.

With the increase of burial depth, the tests presented a rising degree of pervasive neomorphism and calcite infill, with overgrowths of calcite and obliteration of pores and apertures due to secondary carbonate precipitation. Tests presenting a reduction of pore sizes and calcite infilling and/or overgrowths were also observed in depths below 1,332 m in well PEL–1 and below 2,502 m in drill–hole PEL–2. SEM images revealed textural alterations in the wall texture of tests presenting neomorphism features, with increases in crystal size, given the aspect of a mosaic of crystals, as previous observed by Budd and Hiatt (1993) in benthic specimen tests. Figures 4 and 5 illustrate specimens presenting features of calcite infill and neomorphism with pore obstruction, secondary calcite infilling and complete obliteration of the wall texture.

The above–mentioned alteration features are strongly related to the chemical/isotopic changes; therefore, they are better discussed in association with the energy dispersive spectrometry and stable isotope data.

The initial stage of dissolution is characterized by relief degradation and slight enlargement of the pores. In a more advanced stage of dissolution presents fissures in the funnel walls, significant enlargement of the pores and flattened interpore areas predominantly near the sutures. The most advanced stage is characterized by areas presenting coalescence of pores due to destruction of the interpore areas and fissures through the remaining interpore areas.

Foraminifera tests exhibiting features of dissolution were observed in the samples in the well 2–TG–96–RS, at the higher levels of drill–hole PEL–1 (above 1,224 m) and in a few samples from the drill–holes PEL–3 and PEL–2A samples. Samples from the 2–TG–96–RS well presented indications of pervasive dissolution, whereas the dissolution features were moderate to subtle in the samples of the PEL–1 drill–hole. Specimens of Globigerinoides trilobus and Orbulina universa from the 2–TG–96–RS and PEL–3 drill–holes were significantly affected by dissolution (Figure 6). In the samples of drill–hole PEL–1, the dissolution features are less important and are represented by small fissures at the pore pit walls, while the dissolution features in drill–hole PEL–2 were not verified.

According to Collen and Burgess (1979), dissolution begins with the removal of the external layers of the test by the smoothing of the surface and, subsequently, the accentuation and widening of the sutural pores. This process is strongly related to the distribution of the pores, which constitute weak points. Consequently, finely perforated species such as Orbulina universa and Globigerinoides trilobus are intensely affected by dissolution.

Dissolution constitutes one of the most important post–depositional processes affecting the planktic foraminifera. According to Williams et al. (2007), the dissolution of the tests can provide the carbonate for the precipitation of diagenetic calcite. During diagenesis, the primary calcite is dissolved, and it is substituted by secondary calcite precipitates in isotopic equilibrium with the pore fluids (Schrag et al., 1995). The dissolution of foraminifera tests occurs due to the interaction of the tests with the pore waters sub–saturated in calcium carbonate, as secondary calcite precipitation is supplied by pore water ions (Collen and Burgess 1979).

The morphology of the test also influences the resistance to the dissolution, since the species with more densely perforated walls and larger pores are more susceptible than the compact tests with thick walls (Hecht et al., 1975; Stainforth et al., 1975). Orbulina universa and Globigerinoides trilobus are included among the planktic foraminifera most affected by dissolution.

The occurrence of tests presenting dissolution features was observed in the most proximal wells (onshore, 2–TG–96–RS; shallow platform, PEL–3, PEL–2A and PEL–1). Furthermore, the mentioned alteration occurs in the shallower depths of the wells. In this position of the basin, it is reasonable to assume that the dissolution processes on the carbonate bioclasts may be driving meteoric water infiltration. According to Morad et al. (2000), widespread areas of the platform can be exposed by regressive events, and, as a result, there might be increases in the recharge zones and consequently in the input of meteoric water.

Dissolution could take place during the early diagenetic stage, as there was verified to be an important regressive event in the middle and late Miocene (Dias et al., 1994). Alternatively, this type of alteration could be produced during telodiagenesis. More recent regressive events reported to the early/late Pliocene (Dias et al., 1994) corroborate this assumption.

Energy dispersive spectrometry

Energy dispersive spectrometry analyses were carried out in specimens collected from 28 samples, of which 11 samples were from drill–hole PEL–1, 11 samples were from drill–hole PEL–2, four samples were from well PEL–2A and two samples were from PEL–3. Punctual analyses carried out in tests with visible signals of neomorphism in SEM had shown that their chemical compositions did not suffer alterations, at least for the levels of major elements (Figure 7).

Specimens exhibiting the oxidized coating and granular pyrite infilling and/or overgrowths were submitted to EDS mapping, revealing compositions with a high percentage of silica and subordinated aluminum, potassium and magnesium, similar to the composition of clay minerals. EDS maps documented the distribution of calcium, magnesium, aluminum, potassium, silica, iron and sulfur on the surfaces of specimens with oxidized coatings. In Figures 8 and 9, we can observe elemental distribution on the foraminifera test surface, with calcium restricted to the interpore areas while the other elements (Mg, Al, K, Si, Fe and S) appear to be concentrated in the pores.

Based on the compositions and habits of the contaminant phases, we can assume that they correspond to autigenic pyrite and the oxidized coating of glauconite. Autigenic pyrite is formed during sediment burial by the iron mineral reaction with H₂S, which is produced from the reduction of the SO4 dissolved in the seawater due to the action of anaerobic bacteria (Berner 1981, 1984). Sulfides derived from iron minerals are transformed into pyrite during eodiagenesis by the addition of elemental sulfur originating from bacterial activity (Berner 1981, 1984; Siesser and Roger, 2006). As a result, pyrite formation is also controlled by the amount of detritic iron minerals, which, in the studied sedimentary succession, are composed almost exclusively of the shales that are represented by clay minerals.

According to Odin and Fullagar (1988), glauconite also occurs as coatings covering carbonate bioclasts. Moreover, Closs (1970) reported the occurrence of glauconitized foraminifera associated with glauconitic pellets in the Miocene deposits of the basin. Subsequently, based on the composition, we can presume that the clay mineral coatings observed in the studied samples may be the result of the oxidation of the glauconitic films that cover the tests.

Additionally, the Miocene section of the Pelotas Basin represents a record of a regressive episode, which is marked by disconformities and turbidite deposition (Fontana, 1996). Thus, these deposits are punctuated of discontinuities of regional extension as observed by Fontana (1996) and Anjos–Zerfass (2009), that may have promoted pathways to meteoric water percolation and, consequently, the oxidation of the clay mineral films.

Stable isotope data

To perform carbon and oxygen isotope analyses, well–preserved specimens from drill–holes PEL–1, PEL–2 and PEL–2A were selected to compose monogeneric samples.

Specimens of samples proceeding from wells 2–TG–96–RS and PEL–3 were not collected for isotopic analyses since all the foraminifera tests in these samples presented signs of dissolution, such as deepening of the sutures, superficial corrosion and widening of the opening, as well as calcite infill, found in some samples of the PEL–3 drill–hole. Oxygen and carbon isotope data of the foraminiferal tests are presented in Table 1.

Lighter isotopic compositions (F2 group) occurred in the interval where neomorphism and calcite infill occur in a pervasive form, from between 1,404 and 1,638 m in drill–hole PEL–1 and 2,898 and 3,114 m depths of drill–hole PEL–2; these coincide with the intervals where the specimens presented the highest levels of alteration of the microstructure. In these intervals, the tests presented neomorphism of the wall, and eventually, these tests had secondary calcite crystals inside their chambers (Figures 4.3a and 4.5a).

According to Schrag et al. (1995), the values of δ¹⁸O usually decline with increases in burial depth. However, in the studied drill–holes, the δ¹⁸O values oscillate between –1.42 ‰ and 0.57 ‰ in drill–hole PEL–1 and between 3.54‰ and 0.89‰ in drill–hole PEL–2 (Figure 11). The mimetic effect in δ¹³C and δ¹⁸O curves for depths greater than 1,400 m for PEL–1 and at depths greater than 2,862 m with relation to PEL–2 may represents an artifact produced by diagenesis, modulating both isotopic signals.

As previously verified (Sexton et al., 2006; Veizer et al., 1999; Scholle and Arthur, 1980), δ¹³C is less susceptible to diagenetic alteration when compared with δ¹⁸O. Likewise, the isotopic data from drill–holes PEL–1 and PEL–2 show narrower ranges for the carbon data. The shift of the isotopic ratios in drill–hole PEL–1 is 3.97 ‰ for carbon and 6.94‰ for oxygen, while for drill–hole PEL–2 it is 6.48 ‰ and 9.77 ‰ for carbon and oxygen, respectively.

Neomorphism (recrystallization) during early diagenesis acts to increase the δ¹⁸O values (Killingley, 1983; Williams et al., 2005). Conversely, in the interval where the occurrence of neomorphism was observed, the δ¹⁸O values are negative. This can indicate that this trend of lighter isotopic ratios would be related to the non–marine carbonate interaction. It is plausible that the shift of the isotopic compositions in the direction of negative values in the studied samples derive from the dissolution and re–precipitation of carbonate, since non–marine carbonates are enriched in ¹⁶O and ¹²C (Friedman, 1998).

According to Scholle and Arthur (1980), the degree of isotopic composition alterations in the pelagic carbonate depends on the level of diagenetic changes. The oxidation of the organic matter after the burial of the sediments promotes the enrichment in ¹²C of the dissolved carbon, which can contribute to the formation of isotopically lighter cements. As a result, the secondary calcite formed inside and on the foraminifera tests of the deeper intervals in drill–hole PEL–1 could be a factor contributing to the lighter isotopic composition observed in the analyses. These crystals are exchange products of the interaction between the tests and the water with organic matter dissolved (Shieh et al., 2002). This implies a strong influence on the isotopic composition, and, consequently, indicates that the data obtained from these samples should not be used for paleoenvironmental interpretation.

CONCLUSIONS

The combination of imaging techniques and chemical/isotopic analyses allow the identification of post–depositional alterations in foraminifera tests. On this basis, it is possible to select adequate material with which to attain isotopic data aimed at paleoenvironmental characterization. To perform this study, two species were chosen (Orbulina universa and Globigerinoides trilobus). The selected specimens were investigated using scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and stable isotope analyses.

By means of SEM imaging, it was possible to identify the alterations of the original wall texture features as dissolution, neomorphism (recrystallization) and coating of autigenic minerals. The application of the EDS technique allowed us to recognize the distribution of Ca, Mg, Al, Si, Fe and S with the texturally well–preserved tests presenting unaltered calcitic composition, whereas those with textural alterations exhibited variable contributions of Mg, Si, Al, Fe and S.

The application of these combined techniques in Miocene foraminifera from the Pelotas basin unequivocally indicate the material suitable for providing accurate paleoenvironmental information, as in many cases the alterations carried out by diagenesis are detected only by means of the analyses of textural patterns at the micrometric scale and/or isotopic analyses.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge Petrobras (Petróleo Brasileiro S.A.) and CPRM (Geological Survey of Brazil) for providing the drill–hole samples. We are indebted to Victor Pereira (UFRGS) for his suggestions and to Francisco Vega (UNAM) for the linguistic revision. The authors are very grateful to Alcides Nobrega Sial and Valesca Maria Portilla Eilert who reviewed the manuscript and made helpful comments and suggestions. G. S. Anjos–Zerfass thanks the Brazilian National Petroleum Agency (ANP) for the grant.

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