Geology and Geochemistry of Reefs, Carbonate Sediments, and Waters, Gulf of Aqaba (Elat), Red Sea

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 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.