Sedimentation, organic geochemistry and diagenesis of cores from the axial zone of the southern Red Sea: relationships to rift dynamics and climate

Abstract

Systematic study of sedimentary structures, textures, clay mineralogy, sedimentary dynamics, organic matter and lithified carbonate crusts within a 16 m core from the Suakin Deep (central Red Sea, depth 1969 m), is interpreted in terms of rift dynamics and global climatic change. The presence of 12 turbiditic sand bodies composed of planktonic skeletons mixed with material eroded from deep submarine scarps bordering the axial zone may relate to instability associated with the opening of the rift. More subtle expressions of rift dynamics include Fe and Mn oxides and certain smectitic clays precipitated from hydrothermal fluids arising within the axial zone.

Clay minerals and other silicates (quartz, feldspars, amphiboles and mica) are related to terrigenous sources during periods of delta activity on the Sudanese and adjacent shorelines. The geochemical properties of four organic-rich ‘sapropels’ also indicate terrigenous sources, the autochthonous marine contribution being minor. These geochemical studies also indicate that organic components, although supplied via oxic waters, have been deposited under highly saline, reducing conditions, suggesting a stratified water column. Associated arago- nite crusts have positive δ¹³C ( + 3‰) and δ¹⁸O ( + 6‰) isotopic properties confirming high salinities which have also favoured the precipitation of aragonitic spherulites and other diagenetic fabrics.

Comparison of this core from the Suakin Deep with published data from the axial zone confirms the widespread distribution of sapropels and lithified crusts whose age is in the order of 15 000 to 20 000 y BP. Furthermore, the presence of similar crusts overlain by sediment enriched in organic matter, also dated as 15 000 y BP, from the Sudanese Shelf (600 m) indicates that these phenomena are not confined to the axial zone. Cores from the Gulf of Aden, on the contrary, do not contain sapropels or carbonate crusts.

Conditions favouring the development of sapropels and carbonate crusts are related to elevated bottom- water salinities. Because the sapropels and crusts are dated as 15 000–20 000 y BP, they correspond to the end of the last glacial period. In agreement with published data, the authors conclude that the lowering of global sea level has resulted in the partial separation of the Red Sea and Indian Ocean by the Bab-el-Mandab sill, thus stimulating overall restriction in the Red Sea. Subsequent sea-level rise, possibly aided by continental run-off, has resulted in a stratified water column with reducing sea-floor conditions favouring preservation of sapropels. Thus, the sediments of the axial zone of the Red Sea record both rift dynamics and global climatic change.