K.A.U. Mar. Sci.. Vol. I,pp. 41-51 (1410A.H./199<IAD.

Organic Matter and Carbonate Contents of BottomSediments of Shanaab Bay, Sudanese Red Sea

OSMAN M. FARAH, SAYED M. ALl, ISMAILH. RAHAMA AND ABDALLA G. ELNAIEM

Red Sea Fisheries Research Section*Port Sudan, Sudan.

ABSTRACf. The distribution patterns of total carbonate and organic matterin the sediments of the ShanaabBay, Red Sea, has been studied. Carbonatecontent ranges between 2% and 98% with an average of 62%. Carbonate ismainly contributed by skeletal fragments and is biogenic in nature. Physico-chemical precipitation of calcium carbonate might have added some carbo-nate to the sediments. Dilution by non-calcareous terrigenous material,shore erosion by waves and hydrographic conditions control the distribu-tion of carbonate contents in the bay.

Organic matter contents vary from 1.180;0 to 5.88% with an average of2.760;0. Organic matter is largely derived from the sources within the bay.Nature of the sediments and hydrographic parameters control regional var-iations in organic matter contents in the bay.

Introduction

The Red Sea is a site for extensive precipitation of calcium carbonate (Milliman1977). Unlike the carbonates of the other seas, the deep Red Sea carbonates arecomposed of inorganically precipitated high-magnesian calcite with appreciableamounts of organically derived low-magnesian calcite and small amounts of organi-cally and inorganically precipitated aragonite (Friedman 1965, Gevirtz and Fried-man 1966, Milliman et al. 1969). Most of the studies conducted on the Red Sea shal-

'resent Address: P.O. Box 730, Port Sudan. Sudan

42 Osman M. Farah eta/.

low water sediments were mainly confined to the Egyptian and Saudi Arabian coasts(Mohamed 1940, Shukri and Higazy 1944a, b, Said 1951, Behairy 1980, Behairy andJaubert 1983, Durgaprasada Roa and Behairy 1984,1986, Montaggioni et aI. 1986).Comparatively, few studies have been done on the Sudanese Red Sea shallow watersediments (Aboul-Basher 1980, Braithwaite .1982, Farah 1982, Schroeder and Dirar1983, Schroeder et aI. 1984). Much work is still needed on the Red Sea shallow waterregions to facilitate better understanding of the origin and factors controlling the dis-tribution of carbonate sediments. These studies include total carbonate content, tex-ture, constituent composition, mineralogy and chemical composition of the sedi-ments.

In order to accomplish the above task, a ;study has been undertaken on the dis-tribution patterns of total carbonate and organic carbon in the Shanaab bay of theSudanese Red Sea. Information about carbonate content is highly desirable becausethe carbonate content of the sediment can be correlated to the productivity of theoverlying waters (Arrthenius 1963). Carbonate content among other parameters is agood indicator of environmental character. Carbonate sediments are important topetroleum geologists because they contain stratigraphic traps for petroleum (Swin-chatt 1965). Although organic carbon is rarely a major constituent in bottom sedi-ments of near-shore and estuarine zone, information concerning the abundance anddistribution of organic carbon in marine sediments is highly desirable by oilgeologists and marine biologists. Organic carbon is often a good index of the envi-ronment in which the sediments were deposited. Organic carbon isotope ratioswhere used to estimate marine versus terrestial input of organic matter in marinesediments (Hunt 1966). Further, organic carbon has been cited as an index to thelevel of pollution (Mencher et aI. 1968, Folger 1972, Hobbs 1983). Organic carbonand sulphur contents of modem and ancient sediments have been used to assess theredox conditions of depositional environments (Berner 1981a, b, Maynard 1982).

Geologic Setting

Shanaab Bay lies about 210 km north from Port Sudan on the Sudanese Red Seacoast between lat. 21 °20' and 21°23'N and long. 37°00' and 37°05'p (Fig. 1). The bayis an elongated body of water extending in E- W direction with cross branches run-ning N-S. Such a morphology of the bay was developed from two systems of faults atright angles to one another (Crossland 1907). East -West extension of the bay is about6 km, while the width ranges from a minimum of 0.25 km to a maximumof2.80km.The bay covers an area of about 6.3 km2. The bay can be divided into two sectors: theeastern sector and the western sector. The western sector is further divided into threesegments. Modern reefs are actively building in Shanaab Bay almost paralleling the5m contour (Fig. 1). Raised modern coral limestone (up to 3m) surrounds the east-ern sector and the southern segment of the western sector. The western and thenorthwestern segments of the western sector of the bay are bordered by non-calcare-ous gravel deposits and quartz sandstones and lenses of conglomerate and rounded

igneous pebbles (Fig. 2).

Organic Maner and Carbonale Contents of Bolt om Sediments 43

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45Organic Matter and Carbonate Contents of Bottom Sediments.

Climate and Hydrographic Conditions

The climate is hot (45°C in summer) and arid. The average annual precipitation isabout 5 cm (Morcos 1.970). Water temperatures vary between 20 and 35°C. Salinityranges between 39% at the mouth of the bay and 41 % near the end of the westernsector. During winter, wind blows mainly from N-NNE attaining its maximumstrength (50-80 km/hr) in January and February. During summer, wind directionsvary but often from NW and SSE. The only freshwater source to the bay is the sea-sonal stream (K. Shanaab) that drains into the weste{n segments of the western sec-tor of the bay (Fig. 2) and contributes large amounts of terrigenous material during

rainy periods.

Material and Methods

Bottom samples from 38 stations in the bay (Fig. 1) were collected with a Petersongrab sampler, using a small outboard-engine boat. Sampling stations were located byboat-to-shore triangulation with a sextant. For the determination of organic matter,soluble salts were removed from samples by washing with distilled water. About 100gm of each sample were transferred to a previously weighed beaker and dried to aconstant weight in an oven at 70°C. The exact weight was recorded. The organic con-tent was determined by loss on ignition method (oven temperature = 550°C) (Galleand Runnels 1960, Dean 1974) with an accuracy of 0.01 mg and precision of +0.03mg. The percent weight of organic matter was then calculated. The samples fromwhich organic matter was removed were used for the determination of acid-solublecarbonates (on organic matter-free basis) by weight loss after leaching with 1NHCL(Jackson 1973) with an accuracy of 0.01 mg and precision of ::to.03 mg. The weightpercent of acid-soluble carbonates was then calculated. The mean diameter wascomputed using the formulae proposed by Friedman (1962). The correlation of thecarbonate and organic matter contents of the 38 samples with the corresponding val-ues of depth and mean diameter was computed using Pearson correlations.

Results

Carbonate content o( the bay sediments varies from 2% to 98% with an average of62%. The areal distribution of carbonate contents is shown in Fig. 3. In the westernsector, there is a gradual increase of carbonate content from the coast towards thecentre. On the other hand, in the eastern sector low carbonate contents are found inthe central basin and gradually increase in values towards the store. However, theentire extreme eastern end, where the bay is joined to the open sea, is characterizedby highest carbonate concentration. In the western sector, very low values of carbo-nate contents (2-50%) are recorded in the southern segment. The carbonate con-tents do not show any significant correlation, either with the depth (r = 0.15) or themean size of the sediment (r = 0.44).

Organic matter ranges between 1.18% and 5.88% with an average of2. 76%.. Theareal distribution of organic matter content in the bay is shown in Fig. 4. Almostthroughout the bay there is a gradual increase of organic matter from coast towards

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the centre where concentrations more than 4 % are encountered. A great part of thewestern sector is characterized by low « 2%) organic matter contents. In the east-ern end of the bay, organic matter increases from 2%, in the shallow regions, toabout 4%, in the deeper areas. Organic matter contents show moderate correlationwith depth (r = 0.61) and mean size of the sediment (r = 0.68).

Discussion

The carbonate content of Shanaab Bay sediments is similar to that of DongonabBay (Farah 1982). The low average carbonate content in Shanaab Bay is due todilu-tion by non-carbonate terrigenous material. NW wind and seasonal freshwater dis-charge carry considerable amounts of sand, silt and clay particles from the desert toShan~ab Bay. This is well documented in the very low carbonate contents in thenearshore areas of the western sector. The terrigenous input dilutes the carbonateresulting from the disintegration of skeletal fragments. However, such a dilution bythe terrigenous material gradually decreases towards the deeper regions as less andless non-calcareous sediments are carried offshore. Erosion of carbonate-richcoastline and transport of carbonate-rich sand (rom adjacent shores and shelf contri-bute to the high carbonate content of the shallow parts of the eastern sector as well asthe deeper parts of the eastern end of the bay. However, the fines transported fromthe shallow western sector and by the wind dilute the carbonate content of the deeper

areas.In addition to skeletal fragments, physico-chemical precipitation of calcium carbo-

nate also contributes to the carbonate content in Shanaab Bay. It has been shownthat high salinity, high temperature and shallow depth are the optimum conditionsfor physico-chemical precipitation of calcium carbonate (Illing 1954, Cloud 1962).The high salinity (= 40%), high water temperature (20-35°C) and shallow depth (av-erage 10.8m) of Shanaab Bay match with the optimum conditions for carbonate pre-

cipitation.Sediments deposited in Shanaab Bay are a mixture of land derived detritus, either

eroded from shore line or carried in by wind and freshwater discharge, and fragmentsof corals, mollusks, echinoderms, coralline algae, foraminifera as well as pellets andooids. Terrigenous mate-rial transported by wind and freshwater is poor in organiccarbon because the sediments are brought from a desert where very little or no veg-etationis found. So, the organic matter in Shanaab Bay is largely derived from thesources within the bay. Organic matter content in shallow areas « 10m) is generally.low « 2.5%) and dependent on depth. Further increase in depth causes gradual in-crease in organic matter content (except at the eastern end of the bay, stations 2, 6,8). The increase in organic matter with increase in depth is due to the accumulationof fine particles in deep areas. 1he association of organic carbon with fine particles(silt and clay) has been reported by several workers (Trask 1939, Kuenen 1950, Mar-shall 1972, Hobbs 1983, Lamere Hennessee eta/. 1986). But in the eastern end of thebay as the decrease in the mean size offshore is not pronounced, the increase in or-ganic matter with depth is not as significant as that in the western sector. Not only

49Organic Matter and Carbonate Contents of Bottom Sediments.

that, but the western sector is restricted with less water exchange and perhaps thedeeper areas are anaerobic leading to high organic matter, while the eastern sectordue to its proximity to the open sea is a relatively high energy environment with morewater exchange and more oxidation of organic matter.

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51Organic Mattert/nd Carbonate Contents of Bottom Sediments.

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