Sulfur Isotope Geochemistry of Messinian Gypsum Deposits ...The studied Dir El-Baraqan area, Egypt,...

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Earth Science India, eISSN: 0974 – 8350 Vol. 5 (IV), October, 2012, pp. 122-138

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Sulfur Isotope Geochemistry of Messinian Gypsum Deposits from

the Northern Coast Egypt: Paleoenvironmental Implications

Ahmed Melegy1 and Ismael S. Ismael

2

1Department of Geological Sciences, National Research Centre, Dokki, Cairo, Egypt 2Faculty of Science, Suez Canal University, Suez, Egypt

Email: [email protected]

Abstract

Messinian gypsum deposits from Dir El-Baraqan area, Northern Coast, Egypt, were investigated for stable sulfur isotope, IR, optical microscope and SEM to differentiate features formed under substantial microbial influences as indicator of paleoenvironments. Gypsum deposits were classified into three zones: biolaminated gypsum, disordered selenite and swallow-tail selenitic crystals. Biolaminated gypsum is characterized by regular alternating dark and light laminae, which were formed due to the seasonal environmental changes in Dir El-Baraqan area. Stable sulfur isotope data showed that the gypsum deposits are characterized by δ34S values ranging from +19.0 ‰ to +27.5 ‰. In swallow-tail gypsum zones, the δ34S values are characterized by a narrow range (from +19.0 ‰ to +20.2 ‰) which is considered as the primary phase. In biolaminated zones, the δ34S values exhibit variation between the white laminae (δ34S = +23.55 ‰) and the dark laminae (δ34S = +27.5 ‰) which is considered as the secondary phase. The high δ34S values of dark laminae are considered probably due to the increasing activity of sulfate-reducing bacteria. Keywords: gypsum, sulfur isotope, sulfate reducing bacteria,

paleoenvironment

Introduction

Gypsum (CaSO4.2H2O) is one of the common sulfate mineral, forming in variety of environments including hydrothermal vents near mid ocean ridge, diagenetically altered marine sediments and evaporates. The classical gypsum deposits of the Miocene has been systematically studied (Aref, 2003; El Khoriby, 2005; Quevedo et al., 2005; Rosell and Anadón

2010). Gypsum in Dir El-Barakan area was selected for this investigation.

Sulfur Isotope Geochemistry of Messinian Gypsum Deposits from the Northern Coast Egypt: Paleoenvironmental

Implications:Melegy and Ismael

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Fig. 1: A simplified geological sketch map of Egypt showing study area. The studied Dir El-Baraqan area, Egypt, covers about 15 km2, is situated between

latitudes 29° 11` and 29° 13` N and longitudes 30° 27` and 30° 29` E (Fig.1). The sequence in the Dir El-Baraqan was measured from the exposed sections in the gypsum quarries (Fig.2), which lie about 60 km southeast of El-Alamein town, to the south of the Mediterranean coastal shoreline. The evaporites in Dir El-Baraqan area were described as coastal lagoon deposits (Youssef and Kamel, 1985). Structurally, the beds of Dir El-Baraqan area are characterized by ill stratification. Generally, they occur in lenticular form with large variations in thickness (El-Shazely, et al.,1976; Melegy, 1991) distinguished seven alternating layers of gypsum and clay indicating seven periods of dryness. Hassan (1977) studied the mineralogy and geochemistry of the evaporite deposits encountered in the coastal section, between west Alexandria and El-Alamein area. Melegy (1991) evaluated gypsum deposits from Dir El-Baraqan quarries as pure deposits of high grade to be used industrially for plaster production. Youssef and Kamel (1985) concluded that the Dir El-Baraqan evaporites subjected to the activities of blue green algae in an anaerobic condition. Youssef




(1988) also reported his observations on the depositional and diagenetic forms of some Neogene evaporites at Gebel El-Hagif and Dir El-Baraqan areas, northern Western Desert.

Fig. 2: A Geological section of the studied sequence in Dir El- Baraqan area.

The biochemical activity of associated bacteria in biolaminated gypsum formed by an

interplay between microbial and physical mechanisms causing precipitation and trapping of

iron (Orti et al., 2010). The type of mineral formed in marine evaporite deposits depends on

the bacterial action in geometric forms and physicochemical mechanisms (e.g. rates of Gypsum crystals form the surface; Tisato et al., 2012).

In the present study, we carried out stable sulfur isotope as well as infrared

spectroscopy (IR), optical microscope, scanning electron microscopic (SEM) investigations in order to recognize the depositional environment of formation of gypsum deposits in the Dir El-Baraqan area.

Geologic Setting

The sediments in the northern part of the Western Desert have attracted the attention of many workers. Abd-Alla (1991); Wali et al. (1994) recorded the presence of four ridges in the Mediterranean coastal strip separated by three longitudinal valleys. They added that these



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ridges are consist of siliceous limestone. In the valleys, El-Shazly et al. (1976) described a basal bed of clay, sometimes overlain by alternative layers of gypsum and marl. Said (1979) mentioned that the ridges consist of fossiliferous oolitic limestone with foraminifera, pelecypods and gastropods indicating marine origin. He considered them as bars cutting off lagoons in which gypsum was formed. Melegy (1991) studied the geology of the Pleistocene sediments of the Mediterranean coast, west of Abu Qir. Said (1962) mentioned that the Mediterranean Pleistocene sediments in the west of Alexandria consist of elongated ridges running parallel to the present coast and separated by depressions. Hilmy et al., (1976) studied the lithostratigraphy and petrology of Miocene and Post-Miocene section in the area between Burg El-Arab and El-Dabaa, north Western Desert. They reported that the upper unit of Pleistocene section named detrital oolitic limestone is intercalated by two subunits, the cardium limestone and the lagoonal deposits. Ali and West (1983) studied the geochemistry of the sediments of the depressions and flats between the ridges. Wali et al. (1994) described in detail the sedimentary and diagenetic features of the four ridges close to the Mediterranean.

Analytical Techniques

X-ray diffraction patterns were obtained for 12 samples using a Philips x-ray diffractometer type PW 1370. The patterns were run with Ni filter Cu radiation (Kα = 1.5418Ǻ ) at 40kv and 30mA current, at scanning speed of one 2θ per minute. Infra red spectra were obtained using (FTIR) spectrometer (BIORAD) FTS 6000. The KBr pressed pellet technique was used, with a spectral range

of 4000-400cm-1. Microscopic examinations of the samples were carried out by using Zise polarizing microscope. Some selected samples were analyzed using scanning electron microscope CamScan (IV. serie) with energy dispersive spectrometer eXL (Link Analytical Ltd.).

The sulfur isotopic analyses of gypsum (12 samples) were carried out at the Czech

Geological Survey, Czech Republic. Sulfur for stable isotope determinations was extracted by combustion with ESCHKA’s mixture (Chakrabarti, 1978) and converted to sulfur dioxide according to the method proposed by Yanagisawa and Sakai (1983). Isotopic composition of total sulfur was determined on a Finnigan MAT 251 mass spectrometer with reproducibility within 0.3 per mil based CDT scale. The results were expressed in the usual notation as per mil deviation of the 34S/32S ratio in the sample. They are expressed in 'delta notation' where:

δ34Ssample (‰) = [((34S/32S)sample/(

34S/32S)standard) - 1 ] × 1000 Sulfur isotope analyses are correlated to the isotope composition of sulfur from Canyon Diablo meteorite and the international sulfur isotope standard.

Results and Discussion

X- ray diffraction patterns of selected samples show that gypsum is the dominate mineral; in addition, calcite is detected in biolaminate samples. The black laminas contain more amount of calcite than in white laminas. Infrared spectra of the samples are shown in Fig. 3. In Mid-IR spectra, the strongest peaks were found at 1139 cm-1, which are υ3 antisymmetric stretch vibration modes of SO4 tetrahedra. The gypsum exhibits two (670, 600




cm-1), antisymmetric bending vibrations due to SO4 tetrahedra. The presence of water in the gypsum can be detected and characterized by peaks in spectral region near 3500 cm-1 and 1600 cm-1. In Mid-IR spectra, the peaks at 3405 cm-1 in gypsum is υ1 symmetric stretch vibration modes of water, and the peaks at 3548 cm-1 in gypsum is υ3 antisymmetric stretch vibration modes of water. There are two IR peaks for O-H bending vibration modes (1687, 1622 cm-1) in the gypsum spectrum. The presence of two peaks in gypsum indicates that there are two crystallographic distinct types of water. One type of water is linked with sulfate ions by hydrogen bonding, and the corresponding peak has lower frequency (also due to the hydrogen bonding) and high intensity. The other one is directly linked to calcium ions. The IR spectrum of calcium carbonate is quite characteristic with a very intense broad band at 1442 cm-1 and small intense band at 881cm-1.

Fig.3: IR spectra of studied gypsum samples- (w) biolaminated white sample (d) biolaminated dark sample, and (s) selenitic sample.

Lithological and Petrological Discussions

From a lithological point of view, the section from the quarry of Dir El-Baraqan consists of mainly gypsum. Fig. 2 shows that gypsum profile reaches a total thickness of 10m. The exposed gypsum deposit at the quarry face is characterized by biolaminated gypsum (Fig.4) at the base followed upward by disordered selenitic bed and swallow-tail selenite at the top of the section (Fig.5).



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Fig.4: Photograph of specimen showing the alteration of white and dark lamainae in biolaminated gypsum.

Fig. 5: Photograph of specimen showing upper selenite.




The Dir El-Baraqan section is divided into three zones from base to top as follows:

I. Zone- I (Biolaminated gypsum):

Zone- I is characterized by biolaminated gypsum with a thickness ranging from 2-4 m. The term “biolaminated” was used to encompass both microbial mats and stromatolites as illustrated by Gerdes and Krumbein, 1987; Freytet et al., 1996. The biolaminates are millimeter size, irregular, planar and wavy in nature, most of which have remains of algal origin (Youssef, 1988). They range in thickness from 200 µm to 2 mm of alternating gypsum and organic rich carbonate (Fig. 4). The lamination is very variable, and the relative proportion of gypsum and calcite ranges from almost pure carbonate to more than 80% gypsum. Usually the gypsum laminae are thicker than carbonate laminae. Youssef and Kamel (1985) believed that calcite was formed by action of anaerobic bacteria on calcium sulfate in the presence of organic matter (algae). The biolaminations consist of alternating dark (yellowish brown) and white laminae (Fig.6). Von et al., 1977; Kushnir, 1981; Warren, 1982 suggested that the gypsum laminae formed during period of high salinity, where the carbonate laminae represented period of bring dilution. Microscopically, the laminations are composed of a wavy alternation of layers with contrasting colors (Fig.6). The variation of this lamina may be explained by the differences in grain size and amount of organic matter.

Fig.6: Photomicrograph showing biolaminated gypsum with white and dark laminae (PPL).



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II. Zone II (Disorderd crystalline gypsum):

This zone is characterized by the twinned prisms (10-20 cm in length) overlying the

laminated type. Gypsum prisms have their crystal c-axis either at right angles or inclined to bedding. Gypsum is white to yellowish white, fine to medium crystalline, and moderately compact and attains a thickness of about 150 cm in the studied section. The selenitic crystals in this bed are without any laminations and arranged in different sizes and directions (Figs.7).

Fig. 7: Photomicrograph showing the crystallization of selenitic crystals in different direction around many centers (PPL).

III. Zone III (swallow-tail selenite):

This zone consists of the uppermost bed of the gypsum horizon at Dir El-Baraqan section. Its thickness reaches more than 6 m. It is made up of orderly rows of vertically standing twinned selenite crystals. The junctions between the two sides of the twin are Zig-Zag shape (Fig.8). The surface between the selenitic layers is marked by incorporated gypsarenite, calcareous and clayey materials reflecting the surface on which succeeding generations of crystals nucleated. The selenite crystals are twinned with perfect cleavage in one direction (010) and imperfect cleavage parallel to (100) and (111). The two imperfect cleavages intersect at an angle of 66 (Phillip and Griffen, 1981). This type of gypsum is called rhythemic primary gypsum (Ogniben, 1957). Regarding the crystallinity and macroscopic features of the gypsum, it varies from very coarse swallow-tail selenite (10 cm across) to fine grained gypsum. The coarse selenitic gypsum crystals are notably pure without any bacterial activity (Figs.9 & 15).The selenitic type in this zone is considered as the early precipitated phase due to the presence of the swallow-tail texture (Hardie and Eugster 1971; Schreiber et al., 1982).




Sulfur Isotope Composition

Table-1 shows the sulfur isotope data along the profile of Dir El-Baraqan area. The

δ34S composition shows that the gypsum deposit in the three zones of Dir El-Baraqan vary

from +19.0 ‰ to +27.5 ‰ (CDT). In biolaminated gypsum (Zone I), sulfur isotope values range between +23.5 for white laminae and +27.5 for dark laminae. The enriched values in Zone I signify the activity of bacterial sulfate reduction in a reducing environment during deposition causing the deviation of the δ-values (Nielsen 1979; Longinelli and Flora, 2007). In Zone III and Zone II, the δ34S compositions are +20 ‰ and +19 ‰ for swallow tail selenite and disordered selenite, respectively.

The results of stable sulfur isotope indicate that the bacterial reduction of sulfate to

sulfide yields an enrichment of δ34S values of H2S that reflecting the degree of reduction (Nielsen, 1979; Stemmerik et al.,1988). Sulfur isotopic data strongly suggest that sulfide was produced by reduction of sulfate having a δ34S values greater in dark laminae than in white laminae. Higher δ34S values in this zone are interpreted as a result of S mineralization. The mechanism of preferential removal of the more rapidly mineralized light 32S is identical to that evoked for S mineralization.

Fig.8: Photomicrograph showing the two parts of the twin in swallow- tail selenite are Zig-Zag shape (C.N.).



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Fig.9: Photomicrograph showing coarse selenitic gypsum crystals, are notably pure without

any bacterial activity (C.N.).

Table-1: S-isotope data along the profile of Dir El-Baraqan area.

Zone (from bottom to

top)

Kind of gypsum deposit

Description S-isotope data (units of δ34S is in

‰)

III

Swallow-tail selenite

Small selenitic crystals

+ 20.2

Medium selenitic crystals

+ 20

Large selenitic crystals

+ 20.1

II Disordered selenite selenitic disordered crystals

+ 19

I

Biolaminated gypsum

Dark Lamina + 27.5

White Lamina + 23.5

Depositional Environments of Dir El-Baraqan Gypsum Deposit

During the Upper Miocene (Messinian) the Mediterranean was partially cut off from

the world Ocean (Nielsen, 1979; Bellanca and Neri, 1986). The Dir El-Baraqan basin is as restricted as to inflow, it may readily suffer either flooding or drawdown. For a water body to become sufficiently saline for the formation of evaporites, there must be a high rate of evaporation. During phases of rapid evaporation, salinity rises causing an increase in




alkalinity and the solubility of oxygen in the brine decreases (Hite, 1970; Kinsman et al., 1973). Jorgensen (1983) mentioned that the combination of the main requirements such as reducing condition, sulfate ions, and suitable energy sources largely determine the growth and metabolism of reducing bacteria in marine environments. The sulfate is reduced to sulfide or is incorporated into organo-sulfur compounds (Fritz and Bachofen, 1998; Vogel et

al., 2010; Barbieri et al., 2006). Sulfate-reducing bacteria have been grouped in two genera, Desulfovibrio desulfotomaculum (Buchanan and Gibbons, 1974). Krikland and Evans (1980) mentioned that Desulfovibrio desulfateicans destroy the calcium sulfate to sulfide via a reaction similar to:

CH4 + SO4

2- + 2H+ = H2S + CO2 + 2H2O

Microscopic view of the dark laminae shows that the gypsum crystals decrease in size towards dark laminae probably due to Desulfovibrio action (Fig.10). The major role of microbes as geochemical agents may be as mineralizers of organic matter (Vogel et al., 2010; Barbieri et al., 2006; Canfield et al., 2004). In the dark laminae, the high supply of organic matter commonly leads to oxygen deficiency and increases bacterial sulfate reduction causing in the mineralization of organic matter (Dawson, 1966; Jorgensen, 1983). The numbers of bacteria (or rather of colony- forming dark laminae units) due to the presence of sulfate- reducing bacteria are attached to the gypsum crystals. Microscopic investigation shows clearly some characteristic differences of the activity of sulfate-reducing bacteria causing penetration in gypsum crystals (Fig.11).

Fig.10: Photomicrograph showing dark laminae where the gypsum crystals decrease in size (C.N.).



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The biochemical activity of associated bacteria in biolaminated gypsum is important for precipitation and trapping of iron.

The abundance of stromatolite reefs suggests tidal inlet deposits and leached ferrous

iron from country rocks. Sulfate may have been reduced by ferrous iron to sulfide and ferric iron according to a reaction similar to Ripley and Ohmoto (1977):

8 Fe+2 + SO4

2- + 10 H+ = 8 Fe+3 + H2S + 4 H2O In the dark laminae, iron reduction was strongly influenced by bacteria (Scorensen, 1982; Pastorelli et al., 2001). Electron microprob investigation of the dark laminae indicate that they contain mainly ferrous iron which reveals that this zone is strongly influenced by bacteria and re-reduction process of Fe+3. Many crystals in buried mats have invaded by bacteria causing partly or complete dissolution of gypsum crystals (Figs. 12-14).

Fig.11: Photomicrograph showing the bacterial activity causing penetration of gypsum crystal from the center and the edges (C.N.).

In general, microscopic examination of biolaminated gypsum revealed that the activity of sulfate-reducing bacteria probably formed a regularly alternating dark and light laminae explained as seasonal alterations.




Fig.12: Scanning electron micrograph showing the activity of sulfate reducing bacteria causing dissolution of gypsum crystals.

Fig.13: Scanning electron micrograph showing high activity of sulfate reducing bacteria (in the form of colonies) causing dissolution of gypsum crystals.



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Fig.14: Scanning electron micrograph showing the gypsum crystals without bacteria action. The disordered arrangement of selenitic crystals in Zone II is due to the crystallization

of selenitic crystals at different sizes arranged around many centers of crystallization. Regarding the origin of similar twinned and split gypsum crystals, Schreiber et al., (1982) suggested that they were deposited in shallow lagoons which remain continuously concentrated into gypsum precipitation range.

Next upward in sequence of the gypsum profile in Dir El-Baraqan, the gypsum

crystals increase in size, the maximum size of the swallow-tail selenite crystals is about 15cm. The evolution of selenite crystal size is related to the changes in temperature, pressure and salinity. Largest crystals of swallow-tail selenite gypsum probably grow in low temperature with free growing in aqueous hypersaline environments.

Conclusions

Microscopic examinations revealed that biolaminated gypsum are characterized by

regularly alternating dark and light laminas probably reflecting seasonal changes in Dir El-Baraqan area.

The high supply of organic matter commonly leads to oxygen deficiency and

increases bacterial sulfate reduction causing in the mineralization of organic matter. The numbers of bacteria (or rather of colony - forming dark laminae units) due to the presence of sulfate- reducing bacteria are attached to the gypsum crystals.




In swallow-tail gypsum zones, the δ34S values are characterized by a narrow range that is considered as the early precipitated phase. The highest δ34S values recorded in biolaminated gypsum are probably due to the increasing activity of sulfate-reducing bacteria reflecting prevalence of reducing environment. Electron microprobe analysis indicated that the dark lamina in biolaminated zone contains mainly ferrous iron which reveals that this zone is strongly influenced by bacteria and re-reduction process of Fe+3. Higher δ34S values in the biolaminated zone are most likely due to the mechanisms of preferential release of the more rapidly mineralized light isotope 32S.

Acknowledgment: The authors would like to express their deepest gratitude to Dr. F. Buzek (Czech Geological

Survey, Prague) for sulfur isotopic analyses of gypsum samples.

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Sulfur Isotope Geochemistry of Messinian Gypsum Deposits ...The studied Dir El-Baraqan area, Egypt,...

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