Aplanktonic zones in the Red Sea

We use micropalaeontological and stable isotope results for a series of cor es from north to south through the Red Sea, to assess temporal and spatial patterns of change in planktonic foraminiferal faunas leading up to the rem arkable full-glacial Red Sea aplanktonic zones. Aplanktonic zones reflect s alinities 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 glac ial sea-level lowering. Concerning the last glacial cycle, aplanktonic cond itions 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 poorl y understood freshwater dilution at that time. Aplanktonic conditions (S gr eater than or equal to 49 p.s.u.) reached the south-central Red Sea by 22 k a BP, while planktonic foraminiferal faunas continued, albeit in very low a bundances and limited diversity (S less than or equal to 45 p.s.u.). in the southernmost Red Sea. During marine isotope stage (MIS) 6, the 49 p.s.u. i sohaline appears to have resided in the central Red Sea, between cores KL11 and MD921017. We observe a systematic sequence of species disappearances b efore all glacial maxima of the last 500 kyr. Absence of a logical relation ship with sea levels suggests that the disappearance sequences are not rela ted to a general salinity increase. Instead, we argue that the sequences we re driven by complex reorganisations in hydrography (stratification), produ ctivity (food availability) and subsurface oxygenation (reproduction-habita ts). The onset is marked by dramatic basin-wide expansion of conditions tha t today are restricted to only the southern Red Sea, suggesting an expansio n of the dominance of NE monsoonal circulation over the entire Red Sea. Thi s 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 s tage. Regarding the last glacial cycle, this event occurred as early as 75 ka BP (MIS-4/5 boundary). After this major climatic reorganisation, we reco nstruct progressive intensification of the new conditions, especially marke d by northward expansion and intensification of the subsurface oxygen minim um zone (OMZ). During the last glacial cycle, a shallow and very distinct O MZ 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 interr upted by episodes of increased ventilation. (C) 2000 Elsevier Science B.V. All rights reserved.