Late quaternary history of Red Sea outflow

Abstract

A palaeoceanographic study is carried out on cores from the central Red Sea and western Gulf of Aden. Time stratigraphic frameworks are determined using oxygen isotope ratios, from the test of planktonic foraminifera Globigerinoides ruber, and combined with AMS ¹⁴C dates. Down-core variations in planktonic and benthic foraminifera, organic carbon content, and δ¹⁸O are used to deliver a comprehensive history of changes in the Red Sea basin during the late Quaternary. The short time span of the Gulf of Aden core 1006 and the presence of a redepositional event in core 1005 rendered both cores unsuitable for the study of glacial Red Sea outflow. Instead the study is focussed on changes in Red Sea circulation and deep water formation relying on evidence provided in the Red Sea cores.

The Red Sea is a marginal basin of the NW Indian Ocean. Today, water exchange with the open ocean only takes place across the shallow Hanish Sill at the Strait of Bab el Mandab. River inflow along with precipitation into the basin is negligible with respect to the high evaporation rate of 200 cm yr^-1. Thus, the basin is extremely sensitive to global climate change and sea level variation. Circulation in the basin is anti-estuarine, with a surface water inflow compensated by a subsurface outflow. Surface flow alters seasonally according to the monsoon. During the summer SW monsoon, northwesterly winds over the entire basin drive a south flowing surface current. At this time inflow into the basin continues as a shallow, subsurface current. During the winter NE monsoon, winds are northwesterly, north of 20^oN, driving a weak southward surface water flow, and southeasterly to the south, driving a strong northward surface water flow. The result is a zone of surface water convergence at around 25^oN which migrates south as intensity of the SW monsoon increases. Deep water is a 1:1 mixture of Red Sea surface water with Gulf of Suez outflow and is formed mainly in winter in the north of the basin, as well as the Gulf of Suez.

The waters of the present day Red Sea are oligotrophic, supporting a population of tropical-subtropical, spinose foraminifera dominated by Globigerinoides ruber (plate 1) in the south and Globigerinoides sacculifer (plate 2) in the north. The species distribution is controlled by their individual dietary requirements and the seasonal availability of food (mainly zooplankton), in turn controlled by the position of the current convergence zone and monsoonal intensity.

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