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Chemistry of modern sediments in a hypersaline lagoon, north of Jeddah, Red Sea
Estuarine,
Coastal
Chemistry Hypersaline Red Sea
Mahmoud Department Alexandria, Received
and Shelf
(1987) 25,467-480
of Modern Lagoon,
Sediments in a North of Jeddah,
Kh. El-Sayed of Oceanography, Egypt
4 December
Keywords:
Science
Faculty
1986 and m revised
of Science,
form
Alexandria
13 May
University,
1987
salinity; lagoons; sediment; chemistry; diagenesis; Red Sea
Previous studies of modern peritidal sedimentary environments of the Red Sea, such as hypersaline lagoons and sea-marginal flats, have concentrated on its northern part, particularly in the Gulf of Aqaba. However, little is known about lagoon sediments in other localities along the Red Sea coastal stretches. This paper deals with the chemical characteristics of the sediments of a hypersaline (Ras Hatiba) lagoon, north of Jeddah, Saudi Arabia. The chemistry of hypersaline lagoon sediments is considerably changed following the modifications to the water chemistry by evaporation and precipitation. Ras Hatiba lagoon is a hypersaline elongated water body connected to the Red Sea by a narrow and shallow opening. The total area of the Lagoon is c. 30 km’. Coarse bioclastic sands are dominant in the lagoon and mostly surround lithified calcareous grounds. However, fine silt and clay sediments are present in separate patches. The sediments are rich in carbonates (average 785”,,) and organic carbon (average 7,3”,,), although they are negatively correlated. Calcium (average 25,1”,) and magnesium (average 10.8%0) show a similar distribution pattern in the lagoon sediments. Strontium (average 5.2%0) is positively correlated with calcium. Sodium and potassium are relatively highly concentrated in the sediments (average 118 ppm and 173 ppm, respectively). Magnesium and strontium are of prime importance in the process of mineralization and diagenesis. The sabkha formation surrounding the lagoon is of low carbonate and organic carbon content, compared with the lagoon sediments, whilst it is characterized by high magnesium, sodium and potassium concentrations. Ras Hatiba lagoon sediments and sabkha resemble those of the northern Red Sea in the Gulfs of Aqaba and Suez and the Arabian Gulf in their major sedimentological and chemical characteristics. Introduction Along the Red Sea coasts there are extensive areas intruded by coastal lagoons. Some of these lagoons are hypersaline, and most of them are surrounded by largely developed
sabkha. The coastal lagoons break the continuity of the Pleistocene and Recent reef complexes forming the coastal plains. Most of the lagoons are remnants of a much larger body of water and connected to the seaby narrow tidal channels. 467 027%7714,'87~040467+14$03.00/0
@ 1987 Academic Press I.imued
468
M.
Kh. El-Suyed
Rabaa (1980) critically reviewed the different hypotheses which explain the origin ofthe coastal lagoons of the Red Sea. The formation of most of these lagoons is considered to be collapse structures resulting from the selective solution of Miocene evaporite beds underlying the younger successions. These lagoons were also considered as erosional features formed in Post-Wiirm Wisconsin emergence, where new stream valleys were cut through the exposed reef formations; these valleys became lagoons. The prevailing meteorological, physical and chemical conditions in the coastal area of the Red Sea lead to the formation of hypersaline lagoons. In these lagoons, significant chemical compositional changes may result from the precipitation of some major and minor ions during the process of evaporation. Sediments and chemistry of hypersaline lagoons have been intensively studied along the coastal area of the Arabian Gulf (e.g. Butler, 1969; Kinsman, 1969; Purser, 1973; Gunatilake er al., 1984). Although the coastal lagoons, hypersaline lagoons and hypersaline sea-marginal flats have been intensively studied in the Gulf of Aqaba and in the triple junction of Sinai Peninsula with the Red Sea (e.g. Eckstein, 1970; Saas et al., 1972; Friedman, 1978; Gavish, 1980; Friedman & Foner, 1982 and recently by Friedman & Krumbein, 1985, where sabkhasand hypersaline pools in the Gulfs of Suez and Aqaba were treated in view of their sedimentology, biogeochemistry and ecology), the lagoonsand sabkhasof the Red Seaproper have received little attention. The Saudi coastalstretch along the Red Seaextends for about 1800km, and hasnumerous coastal lagoons, locally known as sharms;none of these lagoonshave been previously studied from a sedimentological point of view. In general, sea-marginal pools of the Red Seaoffer prime examplesof algal-mat formations with inferred precipitation of carbonates (Friedman & Krumbein, 1985). Ras Hatiba lagoon is a representative hypersaline lagoon from the Red Sea which has been selected for the present study and has been used in a previous hydrographical study (Meshal, 1986). These studies are contributions to a continuing program for the investigation of this area. The purpose of this study is to present somegeological results on the chemical characteristics of the sediments of an unexplored sea-marginal hypersaline lagoon along the eastern coast of the Red Sea, and to contribute to the understanding of the diagenetic processesin this area in relation to the chemical constituents. Geological
setting
RasHatiba lagoon is a cut through Pleistocene coral limestone in the Tihama coastalplain, north of Jeddah. It is bordered on the eastand south-eastern sidesby a flat-lying sabkha. Geological formations surround the lagoon further to the east and south-east. These formations largely consist of Quaternary trap basalts and Tertiary granite and granitegneisses.The coastal plain is covered by alluvium deposits in the eastern margin of the lagoon further to the sabkha, and resulted from the disintegration of the adjacent Tertiary formations. Study area
Ras Hatiba lagoon lies on the eastern coast of the Red Sea between latitudes 21”50’ and 21’57’N and longitudes 38”58’ and 39”01’E, about 70 km north of Jeddah, Saudi Arabia (Figure 1). The lagoon is an elongated water body extending for about 13 km along a NNW-SSE direction (Figure 1).
Chemistry
of lagoon
sediments
9 B
Jeddoh D %
zoo-
-ii
, 7.5 km
Figure
1. Study
,
area, Ras Hatiba
lagoon.
The lagoon comprises two basins, the northern (outer) basin and the southern (inner) basin. The northern basin is larger and deeper than the southern basin; it is eliptical in shape covering an area of about 20 km2, while the southern basin consists of a narrow channel about 7 km in length and has an average width of about 1.5 km (Figure 2). The rota1 area of the lagoon is about 30 km’. The lagoon is connected from its western side to the Red Sea by a narrow (about 700 m wide) and shallow tidal channel. The depth of this channel varies from 20 cm during the ebb tide to 50 cm during the flood tide (Meshal, 1986). At the northern extremity of the lagoon there was another opening to the Red Sea, which at present is blocked by accumulated sands and bioclastics forming a sand barrier (shoal). The maximum depth sounded in the lagoon is about 2.5 m in the central part of the northern basin. The depth decreases to about 1 .O m in all directions around the central deepest area. The periphery of the lagoon is a few centimeters deep. Because of its setting in an arid zone where precipitation is scarce (average 6 cm y ‘), and evaporation is intense (205 cm y- ‘), Ras Hatiba lagoon water is hypersaline (So/,, ranges from 51 to 113%0; Meshal, 1986). The oxygen in the lagoon water near the bottom averages 5.75 mg I- ‘, the pH ranges between 8.09 and 8.46, and the temperature (corrected) near the bottom is about 33.4 “C in June (Meshal, 1986). A supratidal sabkha surrounds the lagoon. Its sediments are coarse and cemented by evaporite minerals showing halite polygons on the surface. This sabkha bears the same characteristics as the Gavish sabkha (Gavish et al., 1985).
470
M. Kh. El-Sa_vrd
Figure
2. Sampling
Materials Collection
locations,
Ras Hatiba
lagoon.
and methods of sediment samples
Bottom sediments were sampled at 25 stations in the lagoon during February 1984; in addition three short cores were collected from the sabkha (Figure 2). Surface sediment samples were retrieved using a small grab sampler. Core samples were obtained by introducing manually small PVC tubes into the sabkha deposits. Methods
The particle-size frequency distribution of the lagoon sediment sampleswas determined by the sieving (with l/2 v interval sieves) and pipette method according to Folk (1974). Chemical analysis was performed on the washed, dried and powdered lagoon and sabkha samples. Organic carbon was determined by a simple oxidation method following the technique of El-Wakeel and Riley (1957). A calcimeter was used to determine the total carbonates. Major and minor elements chiefly associatedwith carbonates and evaporites, such as Ca, Mg, Sr, Na and K, were determined in the acid-soluble fractions of the sediments. An acid mixture of HNO, and HClO, (1:l) was used for the digestion of sediments. Measurements of the element concentrations were carried out in duplicate using an Atomic Absorption Spectrophotometer IL 157. The necessary precautions were taken to prevent interference and contamination during the different stages of analysis. Internal standardswere introduced within the sediment matrix; the coefficient of variation was found to be: 3% Ca, 59,; Mg, 3% Sr, 59, Na and 496 K. Results Texture
and distribution
of the lagoon sediments
A general map presenting the sediment distribution in Ras Hatiba lagoon is shown in Figure 3. The southern extremity of the lagoon is covered by coarse sands (< - 1 p),
Chemistry
Figure
of lagoon
3. Sediment
sediments
distribution
map
another coarse sandy patch characterizes the western central side of the outer basin. These sandy facies surround the lithified calcareous ground which are mostly superficial cemented layers of fine carbonate deposits. Most of the outer lagoon basin is covered by relatively fine deposits. Medium and coarse sands blanket the tidal channel. A silt-clay patch spreads over the eastern part of the lagoon. The dominant sandy deposits in the lagoon are bioclastics with some detrital quartz and feldspars derived from the nearby coastal formations. The lagoon sediments are, therefore, of a mixed terrigenous and biogenous origin. Accumulation of coarse (< - 1 q), well-sorted sands, mostly biogenic, forms the sand bars near the lagoon outlets. In general, the relatively fine deposits tend to accumulate on the deepest area of the lagoon which characterizes the northern basin. Field observation on the sabkha sediments reveals that most of their constituents are coarse terrigenous deposits cemented by evaporites; halite polygons are frequently observed on the surface of this sabkha. Chemistry
of the lagoon
sediwents
Minimum, maximum, average concentrations and standard deviation of the elements Ca, Mg, Sr, Na and K as well as the total carbonates and organic carbon in Ras Hatiba sediments are listed in Table 1. Rich carbonate deposits (60-900,,) cover the entire lagoon basins apart from the tidal channel area which is covered by sediments of extremely high carbonate content (> 90”,,), with silt-clay patches of low carbonate content (30-60”,,; Figure 4). According to Durgaprasada Rao (1984, unpubl. report), carbonate minerals are dominant in the Ras Hatiba lagoon, their types and concentrations being relative to the proportion of the different sediment constituents.
472
M.
Kh.
El-Sqrd
Figure 4. Distribution
of total carbonates in the lagoon sediments.
Figure 5. Distribution
of organic carbon in the lagoon sediments.
The organic-carbon concentration in the lagoon sediments is generally high, ranging between 1 and 25”,,, with an average value of 7’ (, . However, most of the deposits have an organic-carbon content between 1 and lo”,, (Figure 5). The distribution of organic carbon
Chemistry
of lagoon
473
sediments
l
Figure
6. Relationship
TABLE
1. Results
between
of chemical
organic
analysis
Minimum concentration Total carbonate Organic carbon
Ca (“,,) Mg (‘%w) Sr (X0) Na @pm) K (w-n)
(‘I,,) (‘I,,)
carbon
of the lagoon Maximum concentration
35.5
98.5
0.78
25.3 38.9 38.0
11.8 2.1 1.6 56 30
and carbonate
19.3 308 518
sediments
contents.
(n = 25)
Average x
Standard deviation
78.5 7.3 25.1 10.8 9.2 108 173
16.7 7-l 9.8 9.2 5.9 56.9 179
in the sediments shows that high organic carbon content (> lo”,,) is associated with fine deposits which are low in carbonates. A negative correlation is revealed between organic carbon and carbonate contents of the lagoon sediments (Figure 6). The concentrations of calcium (average 25.1”,,) and magnesium (average 10.8%+) vary widely in the Ras Hatiba lagoon sediments (Table l), however their area1 distribution patterns show a general similarity (Figures 7 and 8, respectively). In general, high calcium concentrations (about 40”,,) characterize the coarse sediments; fine deposits are of low calcium and magnesium content (below 5%0). However, there is no strong correlation between calcium and magnesium in these sediments. On the other hand, a specimen of lithified (hard) ground is characterized by high magnesium content (510%0). Strontium (average concentration 5.2%) is randomly distributed in the lagoon sediments (Figure 9), however it follows more or less the same general pattern as the calcium distribution. Strontium is positively correlated with calcium (r = 0.58; Y = 0.35 X+ 0.31). The results of the calcium, magnesium and strontium analyses may serve in the interpretation of the mineralogical constituents of Ras Hatiba lagoon sediments which were determined by Durgaprasada Rao (1984, unpubl. report).
M.
Figure
Kh. El-Sa_vrd
7. Distribution
of calcium
in the lagoon sediments.
l-5 5-C ICI-20 20-30 >30
Figure
8. Distribution
of magnesium
in the lagoon
sediments
Chemistry
of lagoon
igure 9. Distribution
Figure
10. Distribution
sediments
of strontium
of sodium
in the lagoon
in the lagoon
sediments.
sediments.
Sodium and potassium are remarkably high in the lagoon sediments, their average concentrations are 118 and 173 ppm, respectively. The area1 distributions of these two elements are shown in Figures 10 and 11, respectively. There is also a positive correlation betweenNaandK(r=0.44; Y=1.4X+21.3).
Figure
11. Distribution
of potassium
in the lagoon
sediments.
Sediments of the supratidal zone ‘ sabkha ’ (samples Nos 18-20; Figure 2), show some peculiar characteristics compared with the lagoon sediments. Organic carbon is considerably lower (average concentration l-29,) in the sabkha than in the lagoon sediments, and the total carbonate content is also low (36.5-38.595). Extremely high magnesium concentrations characterize the sabkha deposits (32.3-47.2X). Sodium and potassium are also present high in concentrations in these deposits (about 140 and 500 ppm, respectively). A limited vertical variation in the concentrations of the different elements is observed in the three short cores (maximum retrieved samples depth is 18 cm). The calculated differences in concentrations from the top of the cores to the bottom is 1.2 ppm for Sr, 7%0 for Mg, 3”,, for Ca, 20 ppm for Na, and 50 ppm for K. High-Mg calcite, anhydrite and magnesite are the dominant minerals in the sabkha deposits (Durgaprasada Rao, 1984, unpubl. report).
Discussion
Ras Hatiba lagoon sedimentsconsist mostly of biogenous and terrigenous deposits, while the sabkha deposits are mostly of terrigenous origin. Sneh and Friedman (1985) reported that sea-marginal flat environments, including sabkhasin the Gulfs of Aqaba and Suez, in the Red Sea are intimately associated with terrigenous deposition from wadis. Phleger (1969) discussedthe process of the terrigenous contribution to the lagoon, he mentioned that sedimentary materials are introduced to the lagoons by tidal movements or by onshore winds transporting sand dunes. According to Evans and Bush (1969), wind transporting sand is important in desert regions such as the Arabian desert, where the growth of a wide sandy coastalplain is gradually reaching the lagoon areas.
Chemistry
of lagoon
sedinmts
477
The distribution of sediments in Ras Hatiba lagoon is random, except for the deep area which is characterized by the accumulation of fine deposits. The shallowness of this lagoon, the tide effect, and the water turbulence are the major factors controlling the sediment distribution and the formation of sand bars. Phleger (1969) pointed out that, because lagoons are generally very shallow, their sediments are readily exposed to resuspension by currents and wave action. This effect causes a resorting of the sediments, the coarsest fractions often being found at the bar inlet where the tidal currents are strong. In contrast, the finest fractions are deposited in the areas furthest away from the tidal inlets, or in basins where the water depth is greater than 5 m. However, in Ras Hatiba lagoon, the gradual decrease in grain size is not clear due to the shallowness of the whole basins. The sand bars are formed near the lagoon outlets by the action of tides. Similar bar formations are found in other lagoons in the Red Sea. A gravel bar, separating the Solar Lake from the Gulf of Aqaba, is built of well-rounded and well-sorted pebbles of igneous and metamorphic rocks with abundant molluscan shell fragments (Eckstein, 1970; Aharon L’t (Il., 1977). Ras Hatiba lagoon sediments are rich in organic carbon which is due to the formation of an algal-mat on the lagoon bottom, as previously mentioned by Friedman et al. (1985), or to a typical low-energy environment, a similar finding was reported by Saaset al. (1972) for the sediments in Ras Matarma lagoon, Gulf of Suez, Red Sea. The average concentration of organic carbon in Ras Hatiba lagoon (7”,,), is very much higher than those determined in the sediments of the Red Sea proper off Jeddah (average 0.38”,,; Behairy t’f (II., 1983), and in the sedimentsof the northern Red Sea(0.06-0.45”,,; Mohamed, 1949), as well as in the sediments of the Solar Lake (2.7--5.5”,,; Aharon et al., 1977) and the intertidal reef sediments of Al-Ghardaqa, Red Sea (average 0.32”,,; El-Sayed & Hosny, 1980). Carbonate content is generally high in the lagoon sediments of the Red Seaand in its normal or reefal sediments. The extent of mixing of the terrigenous materials may dilute the high concentration of carbonates. Mohamed (1949) reported that carbonate varies between 50 and 95”,, in the northern Red Sea sediments; a similar range (50-80”,,) was also reported for the Red Seasediments by Milliman et al. (1969). The average carbonate content in the Gulf of Suez is 73”,, (Mohamed, 1980). The mixed reefal-terrigenous sediments of Al-Ghardaqa are of relatively low carbonate content (average 38.5”,,; ElSayed & Hosny, 1980). Carbonate sediments of biogenic origin generally have a high strontium content, particularly in aragonitic mud. The average strontium content (5.2%0)in the studied lagoon sediments is in almost the same range as for other sediments from the Red Sea. SrO averages8.2%” in Al-Ghardaqa reefal sediments (El-Sayed, 1984), and ranges between 0.5 and 7.2%”in Aqaba reefal sediments(Friedman, 1968). Aqaba reefal sedimentsalsohave a high magnesium content (0.97-1.4%0; Friedman, 1968). Sodium and potassium contents are relatively low in the Red Seasediments compared with Ras Hatiba lagoon sediments. Na,O and K,O average O.O23O,, and O.Ol”,,, respectively, in Al-Ghardaqa sediments (El-Sayed, 1984). Calcium, magnesium and strontium are of prime interest in controlling the precipitation of carbonates and evaporites in the lagoon sediments and its surroundings. According to Durgaprasada Rao (1984, unpubl. report) high-Mg calcite, calcite, aragonite, dolomite, magnesite, quartz and feldspars are present in different proportions in Ras Hatiba lagoon sediments. Aragonite and Mg-calcite are the most common minerals in the shallow marine carbonates of the Red Sea (El-Sayed, 1984), while the deep-sea
sediments of the Red Sea are characterized by the presence of lithified aragonitic layers (Milliman, et al., 1969). Mohamed (1980) reported the presenceof dolomite in the shallow carbonate deposits of the Gulf of Suez. The mole”, MgCO, of calcite appearsto be much lower in RasHatiba lagoon sediments than in those of the Red Sea(Durgaprasada Rao, 1984, unpubl. report). The formation of dolomite in the lagoon sediments is debatable, but it seemsthat magnesiumis the essentialelement in its formation process.Aharon et al. (1977) suggested that the origin of the Solar Lake dolomite is not the associationof magnesiumwith organic matter as Friedman (1978) suggested, but the association of aragonite with Mg-rich hypersaline brines. On the other hand, the formation of dolomite by refluxing of hypersaline waters through carbonate sediments in similar environments was discussedby several authors. In this process, the seawater flows into, or over, the coastal sediments in hot, arid climates. Evaporation results in the precipitation of someminerals and thus the Mg2+/Ca2+ ratio in the water increases.The denseMg2+ -rich water transforms calcite or aragonite to dolomite (Adams & Rhodes, 1960). In the sabkha formation surrounding Ras Hatiba lagoon, high-Mg calcite, anhydrite and magnesite are the dominant minerals (Durgaprasada Rao, 1984, unpubl. report). It is evident therefore that aragonite and gypsum are absent in Ras Hatiba sabkha. The mineralogy of the studied sabkha in relation to the chemistry of its deposits resembles those of the recent sabkha along the coast of Sinai (Gavish, 1980); the formation of these minerals may be considered as in situ precipitation. Anhydrite indicates the replacement of gypsum in Ras Hatiba sabkha. Similar processeswere recorded in the Trucial coast sabkha (Kinsman, 1969), and in the northern Red Sea sabkha (Gavish, 1980), where the gypsum is not too extensive or distinguished. Pits dug below the surfacesof the supratidal sabkhasin the northern Gulf of Suez reveal that continuous layers of gypsum are absent, and that crusts of halite are only poorly developed (Sneh & Friedman, 1985). The main reason for the lack of gypsum accumulation in some zones of sabkha is probably the destruction of gypsum by sulphate-reducing bacteria (Gavish et al., 1985). Halite is found in RasHatiba sabkha;its occurrence is alsoreported in the northern Red Sea (Gavish, 1980) and in the Arabian Gulf (Kinsman, 1969; Bush, 1973). Conclusions Ras Hatiba lagoon sediments and sabkha have some characteristic features which resemblesimilar formations in the Gulfs of Suez and Aqaba aswell asin the Arabian Gulf. The chemistry of the hypersaline lagoon sediments is considerably changed following modification of the water chemistry by evaporation and precipitation. The shallownessof the lagoon and the excessive rate of evaporation, aswell asthe restricted connection to the sea,lead to the concentration of someelements. The formation of an algal-mat resulted in the high concentration of organic carbon in the bottom sediments of the lagoon. The concentration of someelements, such asmagnesium and strontium, and the Mg2+/Ca2+ ratio, are limiting factors in the processof mineralization and diagenesisin the lagoon and sabkha. Ras Hatiba sabkha follows the Gavish sabkha model, where aragonite is missing, and gypsum is absent under certain environmental conditions. The formation of dolomite most likely follows the mechanismssuggestedby Gavish et al. (1985); these mechanisms are reflux, evaporative pumping, and biological fixation and subsequent release of magnesium.
Chemistry
of lagoon
sediments
Acknowledgements The author thanks Prof. A. K. A. Behairy, Faculty of Marine Science, King Abdulaziz University, Saudi Arabia for his encouragement and fruitful discussion during the progress of this work. The assistance of Dr N. V. N. Durgaprasada Rao is appreciated. Appreciation is extended to Mr A. Rifaat and Mr I. Gemii for assisting in sample collection and analysis. This work was financially supported from the research funds of King Abdulaziz University.
References Adams,
J. E. 8r Rhodes,
M. L. 1960 Dolomitization by seepage refluxion. A>nerico~ Associamn ofPetrol~~rrm, 1912-1920. Aharon, I’., Kolodny, Y. & Saas, E. 1977 Recent hot brine dolomitization in the ‘ Solar Lake ‘, Gulf of Elat, isotopic, chemical and mineralogical study. Journal of Geology 85,27-48. Behairy, A. K. A., Al-Kholy, A. A., Hashem, M. T. & El-Sayed, M. Kh. 1983 Preliminary study on the geology and fisheries of the coastal area betwen Jeddah and Yanbu. 3ourmz~ of the Faculty of Marim Science ofjreddah 2, l-47 (in arabic). Bush, I’. R. 1973 Some aspects of the diagenetic history of the sabkha in Abu Dhabi, Persian Gulf. In ?‘/I~, Persian Gulf(Purser, B. H., ed.). Springer-Verlag, Berlin, pp. 395-407. Butler, G. I’. 1969 Modern evaporite deposition and geochemistry of co-existing brines, the sabkha, Trucial coast, Arabian Gulf. 3mrrnul of Sedimentary Petrology 39,70-89. Eckstein, Y. 1970 Physicochemical limnology and geology of a meromictic pond on the Red Sea shore. Litwu~log~ arrd Oceanography 15, 363-372. Iwposiunz, Meszc~~ LTrziversidud, ll’acional Airrono~~za (Castanares, A. 8 Phleger, F. B.,edsj. pp. 155-170. Folk. R. L. 1974 Z’c,rrolog>a of Sedimentary Rocks. Hemphill Publishing Co., Austin, TX. I:riedman, G. M. 196X Geology and geochemistry of reefs, carbonate sediments and waters, Gulf of Ayaha, Krd Sea. Jortrml ~,f’S~di~~~~mary arld Petrolog-v 38,895-919. b’riedman, G. IM. 1978 Solar lake: A sea-marginal pond of the Red Sea (Gulf of Aqaba) in which algal mats generate carbonate particles and laminites. In Environmemal Biogeochemistr.v and Gromicrobiolog.v. 1 ‘(11. I, 7‘hc Aquuti~. Ewironnrent (Krumbein. W. E., ed. I. Ann Arbor Science Publishers, Ann Arbor, MI, pp. 227-735. Friedman, G. M. & Foner, H. A. 1982 pH and Eh changes in sea-marginal algal pools of the Red Sea and their effect on carbonate precipitation. 3021r12oI of Sedinwttary md Perr~~log~,~.\’ 52,41-46. Friedman, G. M. & Krumbein, W. E. 1985 Hypersaline ecosystems. Ecological Srtrdies, 1’01. 53. SpringerVerlag, Berlin, Heidelberg, New York. Friedman, G. M., Sneh, A. & Owen, R. W. 1985 The Ras Muhammad pool: implications for the Gavish Sabkha. In Hypcrsalim Ecosystems. Ecological Studies, C’ol. 53. (Friedman, G. M. & Krumbein, W E.. eds J. Springer-Verlag, Berlin, Heidelberg, New York, pp. 219-237. (;avi\h, E. 1980 Recent sabkhas marginal to the southern coasts of Sinai, Red Sea. In Hypersaitnr Brlnr,~ tttrd Ez~t~ptr~~t~ l:‘~Ir~in~~l~~~rrrrs. Drewlop,pr~~o~rs m Sedi,nelrrr,l~!~.\ 29. ~Nissenbaum, A., rd. I. Elsevier, Amsterdam, pp. 233-251. Gavish, E., Krumbein, W. E. & Halevy, J. 1985 Geomorphology, mineralogy andgroundwater geochemistry as factors of the hydrodynamic system of the Gavish sabkha. In Hypersaline Ecosystems, Ecological Srrrdies. I’&. 53. (Friedman, G. M. & Krumbein, W. E., eds). Springer-Verlag, Berlin, Heidelberg, New York, pp. 186-217. Gunatilaka, A., Saleh, A., Al-Temimi, A. & Nassar, N. 1984 Occurrence of subtidal dolomite in a hyprrsaline lagoon, Kuwait. Nature 311,450-452. Kinsman, D. J. J. 1969 Models of formation, sedimentary associations and diagnostic features of shallow water and supratidal evaporites. American Assoctarior~ of Petrolerm wd Geolists Bull&l 53, 830--X&~. Geologisrs
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Meshal, A. H. 1986 Hydrography of a hypersaline coastal lagoon in the Red Sea. Esr~~tiri~~~~. (,‘c~d\rdi crr/‘i 11Jurim Sricrr~~~~. Milliman, J. D., Ross, D. A. & Ku, I-Lung. 1969 Precipitation and lithification of deep-sea carbonates m the Red Sea. 3m1rm71of Srdimrnturv Petrologic 39, 724-736. Mohamed, A. F. 1949 The distribution of organic matter in sediments from the northern Red Sea. America?~ Journal of Science 241, 116-127. Mohamed, M. A. 1980 Distribution of carbonate content in the Recent bottom sediments of the Gulf of Suez, Red Sea. Proceeditrgs oj the Svrrrposiwn 011 Coastal axd Mm% Enz~ironnrents of thr Red Sea, KhLtrtoum -7. ALESCO,‘UNESCO,‘RSC, University of Khartoum, pp. 37-51. Phleger, F. B. 1969 A modern evaporite deposit in Mexico. Amrirmr Associatkm of Pctrolo,eum Gmlogists Bulletin 53, X24-829. Purser, B. H. 1973 The Persian Gulf. Springer-Verlag, Berlin. Rabaa, S. M. A. 1980 Geomorphological characteristics of the Red Sea coast with special emphasis on the formation of’ marsas ’ in the Sudan. Prosecdings of u Sywzposiurn ou Coastal and Marine Ewz~ironmwts of the Red &u, Khurtoum 2. ALESCO;UNESCO,‘RSC, University of Khartoum. Sass, E., Weiler, Y. & Katz, A. 1972 Recent sedimentation and oolite formation in the Ras Matarma lagoon, Gulf of Suez. In T/w Mediterrmrm Seu (Stanley, D. J., ed.). Dowden & Hutchinson & Ross Inc., Stroudsburg, PA, pp. 279-292. Sneh, A. & Friedman, G. M. 1985 Hypersaline sea-marginal flats of the Gulfs of Elat and Suez. In Hypersaline Ecosystems. Ecological Studies, vol. 53. (Friedman, G. M. & Krumbein, W. E., eds). Springer-Verlag, Berlin, Heidelberg, New York, pp. 103-135.
Coastal
Chemistry Hypersaline Red Sea
Mahmoud Department Alexandria, Received
and Shelf
(1987) 25,467-480
of Modern Lagoon,
Sediments in a North of Jeddah,
Kh. El-Sayed of Oceanography, Egypt
4 December
Keywords:
Science
Faculty
1986 and m revised
of Science,
form
Alexandria
13 May
University,
1987
salinity; lagoons; sediment; chemistry; diagenesis; Red Sea
Previous studies of modern peritidal sedimentary environments of the Red Sea, such as hypersaline lagoons and sea-marginal flats, have concentrated on its northern part, particularly in the Gulf of Aqaba. However, little is known about lagoon sediments in other localities along the Red Sea coastal stretches. This paper deals with the chemical characteristics of the sediments of a hypersaline (Ras Hatiba) lagoon, north of Jeddah, Saudi Arabia. The chemistry of hypersaline lagoon sediments is considerably changed following the modifications to the water chemistry by evaporation and precipitation. Ras Hatiba lagoon is a hypersaline elongated water body connected to the Red Sea by a narrow and shallow opening. The total area of the Lagoon is c. 30 km’. Coarse bioclastic sands are dominant in the lagoon and mostly surround lithified calcareous grounds. However, fine silt and clay sediments are present in separate patches. The sediments are rich in carbonates (average 785”,,) and organic carbon (average 7,3”,,), although they are negatively correlated. Calcium (average 25,1”,) and magnesium (average 10.8%0) show a similar distribution pattern in the lagoon sediments. Strontium (average 5.2%0) is positively correlated with calcium. Sodium and potassium are relatively highly concentrated in the sediments (average 118 ppm and 173 ppm, respectively). Magnesium and strontium are of prime importance in the process of mineralization and diagenesis. The sabkha formation surrounding the lagoon is of low carbonate and organic carbon content, compared with the lagoon sediments, whilst it is characterized by high magnesium, sodium and potassium concentrations. Ras Hatiba lagoon sediments and sabkha resemble those of the northern Red Sea in the Gulfs of Aqaba and Suez and the Arabian Gulf in their major sedimentological and chemical characteristics. Introduction Along the Red Sea coasts there are extensive areas intruded by coastal lagoons. Some of these lagoons are hypersaline, and most of them are surrounded by largely developed
sabkha. The coastal lagoons break the continuity of the Pleistocene and Recent reef complexes forming the coastal plains. Most of the lagoons are remnants of a much larger body of water and connected to the seaby narrow tidal channels. 467 027%7714,'87~040467+14$03.00/0
@ 1987 Academic Press I.imued
468
M.
Kh. El-Suyed
Rabaa (1980) critically reviewed the different hypotheses which explain the origin ofthe coastal lagoons of the Red Sea. The formation of most of these lagoons is considered to be collapse structures resulting from the selective solution of Miocene evaporite beds underlying the younger successions. These lagoons were also considered as erosional features formed in Post-Wiirm Wisconsin emergence, where new stream valleys were cut through the exposed reef formations; these valleys became lagoons. The prevailing meteorological, physical and chemical conditions in the coastal area of the Red Sea lead to the formation of hypersaline lagoons. In these lagoons, significant chemical compositional changes may result from the precipitation of some major and minor ions during the process of evaporation. Sediments and chemistry of hypersaline lagoons have been intensively studied along the coastal area of the Arabian Gulf (e.g. Butler, 1969; Kinsman, 1969; Purser, 1973; Gunatilake er al., 1984). Although the coastal lagoons, hypersaline lagoons and hypersaline sea-marginal flats have been intensively studied in the Gulf of Aqaba and in the triple junction of Sinai Peninsula with the Red Sea (e.g. Eckstein, 1970; Saas et al., 1972; Friedman, 1978; Gavish, 1980; Friedman & Foner, 1982 and recently by Friedman & Krumbein, 1985, where sabkhasand hypersaline pools in the Gulfs of Suez and Aqaba were treated in view of their sedimentology, biogeochemistry and ecology), the lagoonsand sabkhasof the Red Seaproper have received little attention. The Saudi coastalstretch along the Red Seaextends for about 1800km, and hasnumerous coastal lagoons, locally known as sharms;none of these lagoonshave been previously studied from a sedimentological point of view. In general, sea-marginal pools of the Red Seaoffer prime examplesof algal-mat formations with inferred precipitation of carbonates (Friedman & Krumbein, 1985). Ras Hatiba lagoon is a representative hypersaline lagoon from the Red Sea which has been selected for the present study and has been used in a previous hydrographical study (Meshal, 1986). These studies are contributions to a continuing program for the investigation of this area. The purpose of this study is to present somegeological results on the chemical characteristics of the sediments of an unexplored sea-marginal hypersaline lagoon along the eastern coast of the Red Sea, and to contribute to the understanding of the diagenetic processesin this area in relation to the chemical constituents. Geological
setting
RasHatiba lagoon is a cut through Pleistocene coral limestone in the Tihama coastalplain, north of Jeddah. It is bordered on the eastand south-eastern sidesby a flat-lying sabkha. Geological formations surround the lagoon further to the east and south-east. These formations largely consist of Quaternary trap basalts and Tertiary granite and granitegneisses.The coastal plain is covered by alluvium deposits in the eastern margin of the lagoon further to the sabkha, and resulted from the disintegration of the adjacent Tertiary formations. Study area
Ras Hatiba lagoon lies on the eastern coast of the Red Sea between latitudes 21”50’ and 21’57’N and longitudes 38”58’ and 39”01’E, about 70 km north of Jeddah, Saudi Arabia (Figure 1). The lagoon is an elongated water body extending for about 13 km along a NNW-SSE direction (Figure 1).
Chemistry
of lagoon
sediments
9 B
Jeddoh D %
zoo-
-ii
, 7.5 km
Figure
1. Study
,
area, Ras Hatiba
lagoon.
The lagoon comprises two basins, the northern (outer) basin and the southern (inner) basin. The northern basin is larger and deeper than the southern basin; it is eliptical in shape covering an area of about 20 km2, while the southern basin consists of a narrow channel about 7 km in length and has an average width of about 1.5 km (Figure 2). The rota1 area of the lagoon is about 30 km’. The lagoon is connected from its western side to the Red Sea by a narrow (about 700 m wide) and shallow tidal channel. The depth of this channel varies from 20 cm during the ebb tide to 50 cm during the flood tide (Meshal, 1986). At the northern extremity of the lagoon there was another opening to the Red Sea, which at present is blocked by accumulated sands and bioclastics forming a sand barrier (shoal). The maximum depth sounded in the lagoon is about 2.5 m in the central part of the northern basin. The depth decreases to about 1 .O m in all directions around the central deepest area. The periphery of the lagoon is a few centimeters deep. Because of its setting in an arid zone where precipitation is scarce (average 6 cm y ‘), and evaporation is intense (205 cm y- ‘), Ras Hatiba lagoon water is hypersaline (So/,, ranges from 51 to 113%0; Meshal, 1986). The oxygen in the lagoon water near the bottom averages 5.75 mg I- ‘, the pH ranges between 8.09 and 8.46, and the temperature (corrected) near the bottom is about 33.4 “C in June (Meshal, 1986). A supratidal sabkha surrounds the lagoon. Its sediments are coarse and cemented by evaporite minerals showing halite polygons on the surface. This sabkha bears the same characteristics as the Gavish sabkha (Gavish et al., 1985).
470
M. Kh. El-Sa_vrd
Figure
2. Sampling
Materials Collection
locations,
Ras Hatiba
lagoon.
and methods of sediment samples
Bottom sediments were sampled at 25 stations in the lagoon during February 1984; in addition three short cores were collected from the sabkha (Figure 2). Surface sediment samples were retrieved using a small grab sampler. Core samples were obtained by introducing manually small PVC tubes into the sabkha deposits. Methods
The particle-size frequency distribution of the lagoon sediment sampleswas determined by the sieving (with l/2 v interval sieves) and pipette method according to Folk (1974). Chemical analysis was performed on the washed, dried and powdered lagoon and sabkha samples. Organic carbon was determined by a simple oxidation method following the technique of El-Wakeel and Riley (1957). A calcimeter was used to determine the total carbonates. Major and minor elements chiefly associatedwith carbonates and evaporites, such as Ca, Mg, Sr, Na and K, were determined in the acid-soluble fractions of the sediments. An acid mixture of HNO, and HClO, (1:l) was used for the digestion of sediments. Measurements of the element concentrations were carried out in duplicate using an Atomic Absorption Spectrophotometer IL 157. The necessary precautions were taken to prevent interference and contamination during the different stages of analysis. Internal standardswere introduced within the sediment matrix; the coefficient of variation was found to be: 3% Ca, 59,; Mg, 3% Sr, 59, Na and 496 K. Results Texture
and distribution
of the lagoon sediments
A general map presenting the sediment distribution in Ras Hatiba lagoon is shown in Figure 3. The southern extremity of the lagoon is covered by coarse sands (< - 1 p),
Chemistry
Figure
of lagoon
3. Sediment
sediments
distribution
map
another coarse sandy patch characterizes the western central side of the outer basin. These sandy facies surround the lithified calcareous ground which are mostly superficial cemented layers of fine carbonate deposits. Most of the outer lagoon basin is covered by relatively fine deposits. Medium and coarse sands blanket the tidal channel. A silt-clay patch spreads over the eastern part of the lagoon. The dominant sandy deposits in the lagoon are bioclastics with some detrital quartz and feldspars derived from the nearby coastal formations. The lagoon sediments are, therefore, of a mixed terrigenous and biogenous origin. Accumulation of coarse (< - 1 q), well-sorted sands, mostly biogenic, forms the sand bars near the lagoon outlets. In general, the relatively fine deposits tend to accumulate on the deepest area of the lagoon which characterizes the northern basin. Field observation on the sabkha sediments reveals that most of their constituents are coarse terrigenous deposits cemented by evaporites; halite polygons are frequently observed on the surface of this sabkha. Chemistry
of the lagoon
sediwents
Minimum, maximum, average concentrations and standard deviation of the elements Ca, Mg, Sr, Na and K as well as the total carbonates and organic carbon in Ras Hatiba sediments are listed in Table 1. Rich carbonate deposits (60-900,,) cover the entire lagoon basins apart from the tidal channel area which is covered by sediments of extremely high carbonate content (> 90”,,), with silt-clay patches of low carbonate content (30-60”,,; Figure 4). According to Durgaprasada Rao (1984, unpubl. report), carbonate minerals are dominant in the Ras Hatiba lagoon, their types and concentrations being relative to the proportion of the different sediment constituents.
472
M.
Kh.
El-Sqrd
Figure 4. Distribution
of total carbonates in the lagoon sediments.
Figure 5. Distribution
of organic carbon in the lagoon sediments.
The organic-carbon concentration in the lagoon sediments is generally high, ranging between 1 and 25”,,, with an average value of 7’ (, . However, most of the deposits have an organic-carbon content between 1 and lo”,, (Figure 5). The distribution of organic carbon
Chemistry
of lagoon
473
sediments
l
Figure
6. Relationship
TABLE
1. Results
between
of chemical
organic
analysis
Minimum concentration Total carbonate Organic carbon
Ca (“,,) Mg (‘%w) Sr (X0) Na @pm) K (w-n)
(‘I,,) (‘I,,)
carbon
of the lagoon Maximum concentration
35.5
98.5
0.78
25.3 38.9 38.0
11.8 2.1 1.6 56 30
and carbonate
19.3 308 518
sediments
contents.
(n = 25)
Average x
Standard deviation
78.5 7.3 25.1 10.8 9.2 108 173
16.7 7-l 9.8 9.2 5.9 56.9 179
in the sediments shows that high organic carbon content (> lo”,,) is associated with fine deposits which are low in carbonates. A negative correlation is revealed between organic carbon and carbonate contents of the lagoon sediments (Figure 6). The concentrations of calcium (average 25.1”,,) and magnesium (average 10.8%+) vary widely in the Ras Hatiba lagoon sediments (Table l), however their area1 distribution patterns show a general similarity (Figures 7 and 8, respectively). In general, high calcium concentrations (about 40”,,) characterize the coarse sediments; fine deposits are of low calcium and magnesium content (below 5%0). However, there is no strong correlation between calcium and magnesium in these sediments. On the other hand, a specimen of lithified (hard) ground is characterized by high magnesium content (510%0). Strontium (average concentration 5.2%) is randomly distributed in the lagoon sediments (Figure 9), however it follows more or less the same general pattern as the calcium distribution. Strontium is positively correlated with calcium (r = 0.58; Y = 0.35 X+ 0.31). The results of the calcium, magnesium and strontium analyses may serve in the interpretation of the mineralogical constituents of Ras Hatiba lagoon sediments which were determined by Durgaprasada Rao (1984, unpubl. report).
M.
Figure
Kh. El-Sa_vrd
7. Distribution
of calcium
in the lagoon sediments.
l-5 5-C ICI-20 20-30 >30
Figure
8. Distribution
of magnesium
in the lagoon
sediments
Chemistry
of lagoon
igure 9. Distribution
Figure
10. Distribution
sediments
of strontium
of sodium
in the lagoon
in the lagoon
sediments.
sediments.
Sodium and potassium are remarkably high in the lagoon sediments, their average concentrations are 118 and 173 ppm, respectively. The area1 distributions of these two elements are shown in Figures 10 and 11, respectively. There is also a positive correlation betweenNaandK(r=0.44; Y=1.4X+21.3).
Figure
11. Distribution
of potassium
in the lagoon
sediments.
Sediments of the supratidal zone ‘ sabkha ’ (samples Nos 18-20; Figure 2), show some peculiar characteristics compared with the lagoon sediments. Organic carbon is considerably lower (average concentration l-29,) in the sabkha than in the lagoon sediments, and the total carbonate content is also low (36.5-38.595). Extremely high magnesium concentrations characterize the sabkha deposits (32.3-47.2X). Sodium and potassium are also present high in concentrations in these deposits (about 140 and 500 ppm, respectively). A limited vertical variation in the concentrations of the different elements is observed in the three short cores (maximum retrieved samples depth is 18 cm). The calculated differences in concentrations from the top of the cores to the bottom is 1.2 ppm for Sr, 7%0 for Mg, 3”,, for Ca, 20 ppm for Na, and 50 ppm for K. High-Mg calcite, anhydrite and magnesite are the dominant minerals in the sabkha deposits (Durgaprasada Rao, 1984, unpubl. report).
Discussion
Ras Hatiba lagoon sedimentsconsist mostly of biogenous and terrigenous deposits, while the sabkha deposits are mostly of terrigenous origin. Sneh and Friedman (1985) reported that sea-marginal flat environments, including sabkhasin the Gulfs of Aqaba and Suez, in the Red Sea are intimately associated with terrigenous deposition from wadis. Phleger (1969) discussedthe process of the terrigenous contribution to the lagoon, he mentioned that sedimentary materials are introduced to the lagoons by tidal movements or by onshore winds transporting sand dunes. According to Evans and Bush (1969), wind transporting sand is important in desert regions such as the Arabian desert, where the growth of a wide sandy coastalplain is gradually reaching the lagoon areas.
Chemistry
of lagoon
sedinmts
477
The distribution of sediments in Ras Hatiba lagoon is random, except for the deep area which is characterized by the accumulation of fine deposits. The shallowness of this lagoon, the tide effect, and the water turbulence are the major factors controlling the sediment distribution and the formation of sand bars. Phleger (1969) pointed out that, because lagoons are generally very shallow, their sediments are readily exposed to resuspension by currents and wave action. This effect causes a resorting of the sediments, the coarsest fractions often being found at the bar inlet where the tidal currents are strong. In contrast, the finest fractions are deposited in the areas furthest away from the tidal inlets, or in basins where the water depth is greater than 5 m. However, in Ras Hatiba lagoon, the gradual decrease in grain size is not clear due to the shallowness of the whole basins. The sand bars are formed near the lagoon outlets by the action of tides. Similar bar formations are found in other lagoons in the Red Sea. A gravel bar, separating the Solar Lake from the Gulf of Aqaba, is built of well-rounded and well-sorted pebbles of igneous and metamorphic rocks with abundant molluscan shell fragments (Eckstein, 1970; Aharon L’t (Il., 1977). Ras Hatiba lagoon sediments are rich in organic carbon which is due to the formation of an algal-mat on the lagoon bottom, as previously mentioned by Friedman et al. (1985), or to a typical low-energy environment, a similar finding was reported by Saaset al. (1972) for the sediments in Ras Matarma lagoon, Gulf of Suez, Red Sea. The average concentration of organic carbon in Ras Hatiba lagoon (7”,,), is very much higher than those determined in the sediments of the Red Sea proper off Jeddah (average 0.38”,,; Behairy t’f (II., 1983), and in the sedimentsof the northern Red Sea(0.06-0.45”,,; Mohamed, 1949), as well as in the sediments of the Solar Lake (2.7--5.5”,,; Aharon et al., 1977) and the intertidal reef sediments of Al-Ghardaqa, Red Sea (average 0.32”,,; El-Sayed & Hosny, 1980). Carbonate content is generally high in the lagoon sediments of the Red Seaand in its normal or reefal sediments. The extent of mixing of the terrigenous materials may dilute the high concentration of carbonates. Mohamed (1949) reported that carbonate varies between 50 and 95”,, in the northern Red Sea sediments; a similar range (50-80”,,) was also reported for the Red Seasediments by Milliman et al. (1969). The average carbonate content in the Gulf of Suez is 73”,, (Mohamed, 1980). The mixed reefal-terrigenous sediments of Al-Ghardaqa are of relatively low carbonate content (average 38.5”,,; ElSayed & Hosny, 1980). Carbonate sediments of biogenic origin generally have a high strontium content, particularly in aragonitic mud. The average strontium content (5.2%0)in the studied lagoon sediments is in almost the same range as for other sediments from the Red Sea. SrO averages8.2%” in Al-Ghardaqa reefal sediments (El-Sayed, 1984), and ranges between 0.5 and 7.2%”in Aqaba reefal sediments(Friedman, 1968). Aqaba reefal sedimentsalsohave a high magnesium content (0.97-1.4%0; Friedman, 1968). Sodium and potassium contents are relatively low in the Red Seasediments compared with Ras Hatiba lagoon sediments. Na,O and K,O average O.O23O,, and O.Ol”,,, respectively, in Al-Ghardaqa sediments (El-Sayed, 1984). Calcium, magnesium and strontium are of prime interest in controlling the precipitation of carbonates and evaporites in the lagoon sediments and its surroundings. According to Durgaprasada Rao (1984, unpubl. report) high-Mg calcite, calcite, aragonite, dolomite, magnesite, quartz and feldspars are present in different proportions in Ras Hatiba lagoon sediments. Aragonite and Mg-calcite are the most common minerals in the shallow marine carbonates of the Red Sea (El-Sayed, 1984), while the deep-sea
sediments of the Red Sea are characterized by the presence of lithified aragonitic layers (Milliman, et al., 1969). Mohamed (1980) reported the presenceof dolomite in the shallow carbonate deposits of the Gulf of Suez. The mole”, MgCO, of calcite appearsto be much lower in RasHatiba lagoon sediments than in those of the Red Sea(Durgaprasada Rao, 1984, unpubl. report). The formation of dolomite in the lagoon sediments is debatable, but it seemsthat magnesiumis the essentialelement in its formation process.Aharon et al. (1977) suggested that the origin of the Solar Lake dolomite is not the associationof magnesiumwith organic matter as Friedman (1978) suggested, but the association of aragonite with Mg-rich hypersaline brines. On the other hand, the formation of dolomite by refluxing of hypersaline waters through carbonate sediments in similar environments was discussedby several authors. In this process, the seawater flows into, or over, the coastal sediments in hot, arid climates. Evaporation results in the precipitation of someminerals and thus the Mg2+/Ca2+ ratio in the water increases.The denseMg2+ -rich water transforms calcite or aragonite to dolomite (Adams & Rhodes, 1960). In the sabkha formation surrounding Ras Hatiba lagoon, high-Mg calcite, anhydrite and magnesite are the dominant minerals (Durgaprasada Rao, 1984, unpubl. report). It is evident therefore that aragonite and gypsum are absent in Ras Hatiba sabkha. The mineralogy of the studied sabkha in relation to the chemistry of its deposits resembles those of the recent sabkha along the coast of Sinai (Gavish, 1980); the formation of these minerals may be considered as in situ precipitation. Anhydrite indicates the replacement of gypsum in Ras Hatiba sabkha. Similar processeswere recorded in the Trucial coast sabkha (Kinsman, 1969), and in the northern Red Sea sabkha (Gavish, 1980), where the gypsum is not too extensive or distinguished. Pits dug below the surfacesof the supratidal sabkhasin the northern Gulf of Suez reveal that continuous layers of gypsum are absent, and that crusts of halite are only poorly developed (Sneh & Friedman, 1985). The main reason for the lack of gypsum accumulation in some zones of sabkha is probably the destruction of gypsum by sulphate-reducing bacteria (Gavish et al., 1985). Halite is found in RasHatiba sabkha;its occurrence is alsoreported in the northern Red Sea (Gavish, 1980) and in the Arabian Gulf (Kinsman, 1969; Bush, 1973). Conclusions Ras Hatiba lagoon sediments and sabkha have some characteristic features which resemblesimilar formations in the Gulfs of Suez and Aqaba aswell asin the Arabian Gulf. The chemistry of the hypersaline lagoon sediments is considerably changed following modification of the water chemistry by evaporation and precipitation. The shallownessof the lagoon and the excessive rate of evaporation, aswell asthe restricted connection to the sea,lead to the concentration of someelements. The formation of an algal-mat resulted in the high concentration of organic carbon in the bottom sediments of the lagoon. The concentration of someelements, such asmagnesium and strontium, and the Mg2+/Ca2+ ratio, are limiting factors in the processof mineralization and diagenesisin the lagoon and sabkha. Ras Hatiba sabkha follows the Gavish sabkha model, where aragonite is missing, and gypsum is absent under certain environmental conditions. The formation of dolomite most likely follows the mechanismssuggestedby Gavish et al. (1985); these mechanisms are reflux, evaporative pumping, and biological fixation and subsequent release of magnesium.
Chemistry
of lagoon
sediments
Acknowledgements The author thanks Prof. A. K. A. Behairy, Faculty of Marine Science, King Abdulaziz University, Saudi Arabia for his encouragement and fruitful discussion during the progress of this work. The assistance of Dr N. V. N. Durgaprasada Rao is appreciated. Appreciation is extended to Mr A. Rifaat and Mr I. Gemii for assisting in sample collection and analysis. This work was financially supported from the research funds of King Abdulaziz University.
References Adams,
J. E. 8r Rhodes,
M. L. 1960 Dolomitization by seepage refluxion. A>nerico~ Associamn ofPetrol~~rrm, 1912-1920. Aharon, I’., Kolodny, Y. & Saas, E. 1977 Recent hot brine dolomitization in the ‘ Solar Lake ‘, Gulf of Elat, isotopic, chemical and mineralogical study. Journal of Geology 85,27-48. Behairy, A. K. A., Al-Kholy, A. A., Hashem, M. T. & El-Sayed, M. Kh. 1983 Preliminary study on the geology and fisheries of the coastal area betwen Jeddah and Yanbu. 3ourmz~ of the Faculty of Marim Science ofjreddah 2, l-47 (in arabic). Bush, I’. R. 1973 Some aspects of the diagenetic history of the sabkha in Abu Dhabi, Persian Gulf. In ?‘/I~, Persian Gulf(Purser, B. H., ed.). Springer-Verlag, Berlin, pp. 395-407. Butler, G. I’. 1969 Modern evaporite deposition and geochemistry of co-existing brines, the sabkha, Trucial coast, Arabian Gulf. 3mrrnul of Sedimentary Petrology 39,70-89. Eckstein, Y. 1970 Physicochemical limnology and geology of a meromictic pond on the Red Sea shore. Litwu~log~ arrd Oceanography 15, 363-372. I
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Meshal, A. H. 1986 Hydrography of a hypersaline coastal lagoon in the Red Sea. Esr~~tiri~~~~. (,‘c~d\rdi crr/‘i 11Jurim Sricrr~~~~. Milliman, J. D., Ross, D. A. & Ku, I-Lung. 1969 Precipitation and lithification of deep-sea carbonates m the Red Sea. 3m1rm71of Srdimrnturv Petrologic 39, 724-736. Mohamed, A. F. 1949 The distribution of organic matter in sediments from the northern Red Sea. America?~ Journal of Science 241, 116-127. Mohamed, M. A. 1980 Distribution of carbonate content in the Recent bottom sediments of the Gulf of Suez, Red Sea. Proceeditrgs oj the Svrrrposiwn 011 Coastal axd Mm% Enz~ironnrents of thr Red Sea, KhLtrtoum -7. ALESCO,‘UNESCO,‘RSC, University of Khartoum, pp. 37-51. Phleger, F. B. 1969 A modern evaporite deposit in Mexico. Amrirmr Associatkm of Pctrolo,eum Gmlogists Bulletin 53, X24-829. Purser, B. H. 1973 The Persian Gulf. Springer-Verlag, Berlin. Rabaa, S. M. A. 1980 Geomorphological characteristics of the Red Sea coast with special emphasis on the formation of’ marsas ’ in the Sudan. Prosecdings of u Sywzposiurn ou Coastal and Marine Ewz~ironmwts of the Red &u, Khurtoum 2. ALESCO;UNESCO,‘RSC, University of Khartoum. Sass, E., Weiler, Y. & Katz, A. 1972 Recent sedimentation and oolite formation in the Ras Matarma lagoon, Gulf of Suez. In T/w Mediterrmrm Seu (Stanley, D. J., ed.). Dowden & Hutchinson & Ross Inc., Stroudsburg, PA, pp. 279-292. Sneh, A. & Friedman, G. M. 1985 Hypersaline sea-marginal flats of the Gulfs of Elat and Suez. In Hypersaline Ecosystems. Ecological Studies, vol. 53. (Friedman, G. M. & Krumbein, W. E., eds). Springer-Verlag, Berlin, Heidelberg, New York, pp. 103-135.