Abstract

The Gulf of Aqaba, a slightly hypersaline body of partially enclosed marine water, forms the northern segment of the Red Sea. The Gulf occupies a narrow tectonic valley, about 10 to 26 km wide, bounded by fault planes which form submarine slopes that are virtual precipices and by shores which are equally steep. Near the reef communities, on the shelf of the northern gulf, salinities, which vary seasonally, range from 41.5 to 43.0 per thousand . The climate is hot and dry. Evaporation exceeds rainfall by 157 mm per year; the deficit is made up by inflow of waters from the Red Sea (normal salinity 38 to 40 per thousand ). North and northeast winds blow steadily along the Gulf ("chimney effect"), and waves and currents on the northwestern coast studied are predominantly from these directions. Around the margin of the Gulf of Aqaba is a narrow shelf(1/4 to 1 or 2 km wide) on which the water is <100 fathoms deep. Numerous reefs line the shelf. A fringing reef and the narrow lagoon, 10 to 50 m wide, landward of the reef were included in this study. Spurs and grooves dominate the seaward and lagoonal sides of the fringing reef. Live coral groups grow on the seaward side of the reef and along edges of the spurs. The spurs and grooves make an angle of about 25 degrees with the reef trend enabling the coral colonies to grow northeastward into the predominant direction from which the waves and currents flow. Coralline algae dominate the reef, particularly on the reef flat and backreef. Most of the reef is now dead, except on the seaward side or near spurs. The sedimentary deposits and organisms form a series of zones which vary as depth changes. Beachrock and fossil reef rock line the shore; terrigenous rubble from wadis paves the shallow-water lagoon on landward side and the brown alga Padina pavonica (L.) Thivy is attached to pebbles and boulders at depths of < nearly equal 40 cm below low tide. With increasing water depth terrigenous debris decreases and gives way to boulder-sized coral rubble, probably produced by storms. Near the reef, carbonate sand carpets the floor of the lagoon. Like reefs in other parts of the world the reef community is zoned ecologically. Carbonate sands dominate in the grooves between the spurs and on the seaward side of the reef. Similarly, sand is dominant near patch reefs. The ubiquitous carbonate sands near reef communities are less well sorted than nearby carbonate beach sands. Reef-derived sands contain material in the <62mu and <125mu fractions, sizes that are not present in nearby carbonate beach sands. Fine-grained particles taken into suspension along the beach are removed seaward into deeper water. The mineral composition of the reef-builders and reef-derived carbonate sediments is identical with that of other areas, such as Bermuda, the Bahamas, and Florida. The major-element composition of the insoluble fraction of the carbonate sands reflects the amount of terrigenous debris derived from the Precambrian Arabo-Nubian massif. Molluscan shells are composed of low-strontium aragonite and corals of high-strontium aragonite. All aragonite with <0.1 per thousand Mg is considered a low-magnesium aragonite, whereas that with >1 per thousand is termed high-magnesium aragonite. Molluscan shells in the area studied are composed of low-magnesium aragonite and skeletons of corals of high-magnesium aragonite. The magnesium content of the carbonate sands reflects the relative abundance of corals, coralline algae, and molluscs. Approximately linear relationships can be shown between barium and magnesium, barium and manganese, magnesium and manganese, iron and manganese, and iron and barium for coral, coralline algal, and molluscan skeletons, and carbonate sands derived from them. By comparison with molluscs, the skeletons of corals tend to be slightly enriched in barium, manganese, and iron, and those of coralline algae are enriched even more. The carbon and oxygen isotopic composition of calcareous skeletons and carbonate sands overlaps with that of comparable Quaternary material from other areas; slight differences from those described from other areas are, however, noted in 16 O enrichment of corals and 18 O enrichment of a barnacle, reflecting the effect of taxonomic position on isotopic fractionation.

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