Aplanktonic zones in the Red Sea

Abstract

We use micropalaeontological and stable isotope results for a series of cores from north to south through the Red Sea, to assess temporal and spatial patterns of change in planktonic foraminiferal faunas leading up to the remarkable full-glacial Red Sea aplanktonic zones. Aplanktonic zones reflect salinities in the Red Sea in excess of the lethal 49 p.s.u. limit, caused by reduced exchange transport through the Strait of Bab el Mandab due to glacial sea-level lowering. Concerning the last glacial cycle, aplanktonic conditions began at 39 ka BP in the north, where salinities eventually reached 55–57 p.s.u. Paradoxically, planktonic faunas are reported to have survived the last glacial maximum in the northern Gulf of Aqaba, suggesting a poorly understood freshwater dilution at that time. Aplanktonic conditions (S≥49 p.s.u.) reached the south-central Red Sea by 22 ka BP, while planktonic foraminiferal faunas continued, albeit in very low abundances and limited diversity (S≤45 p.s.u.), in the southernmost Red Sea. During marine isotope stage (MIS) 6, the 49 p.s.u. isohaline appears to have resided in the central Red Sea, between cores KL11 and MD921017. We observe a systematic sequence of species disappearances before all glacial maxima of the last 500 kyr. Absence of a logical relationship with sea levels suggests that the disappearance sequences are not related to a general salinity increase. Instead, we argue that the sequences were driven by complex reorganisations in hydrography (stratification), productivity (food availability) and subsurface oxygenation (reproduction-habitats). The onset is marked by dramatic basin-wide expansion of conditions that today are restricted to only the southern Red Sea, suggesting an expansion of the dominance of NE monsoonal circulation over the entire Red Sea. This expansion occurred 15 or more kyr before the aplanktonic zones of MIS-12, 6 and 2, and also before MIS-10 and 8, which never reach the aplanktonic stage. Regarding the last glacial cycle, this event occurred as early as 75 ka BP (MIS-4/5 boundary). After this major climatic reorganisation, we reconstruct progressive intensification of the new conditions, especially marked by northward expansion and intensification of the subsurface oxygen minimum zone (OMZ). During the last glacial cycle, a shallow and very distinct OMZ affected the central Red Sea as early as 68 ka BP, and the north as late as 55 ka BP. The OMZ expansion/intensification appears to have been interrupted by episodes of increased ventilation.