Evidence for Red Sea surface circulation from oxygen isotopes of modern surface waters and planktonic foraminiferal tests

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Hydrographic data and oxygen isotopic analyses performed on surface waters and planktonic foraminiferal tests, collected during early summer from two succeeding years (1984, 1985) throughout the Red Sea, reveal two different hydrographic regimes. In 1984 the summer “normal” situation prevailed where surface waters from the Red Sea flowed out into the Gulf of Aden, while in 1985 a reversed inflow current occurred. The higher temperatures and salinities observed in 1985 indicate high evaporation rates and increased aridity in the northern Red Sea and caused this inflow of Indian Ocean Surface Water which origins from the active upwelling region in the Arabian Sea. Lower salinities and lower oxygen isotopes were observed up to 18°N. The occurrence of Globorotalia menardii during 1985 with its surprisingly constant isotope values up to the Gulf of Suez indicates northward flowing surface currents for the entire Red Sea. Isotope values from Neogloboquadrina dutertrei (1985) indicate subsurface shell formation to about 50 m water depth. Oxygen isotope analysis on Globigerinoides ruber and Globigerinoides trilobus from the 1984 and 1985 tracks suggests that both species calcify in isotopic equilibrium with the surrounding water in the Gulf of Aden and in the northern Red Sea, while an offset from equilibrium values of up to −0.4‰ is found in the Red Sea. Occurrences of G. menardii in Red Sea sediments may be useful as a tool for detecting unusual hydrographic situations preserved in the sediment record, when subsurface water was brought to the surface by upwelling in the Arabian Sea and flowed into the Red Sea. As this process is triggered by high evaporation rates in the northern Red Sea region, the appearance of this species in Red Sea sediments may also indicate periods with extreme arid conditions.