Late glacial to recent deep-sea benthic foraminifera from the northeastern Atlantic (Cadiz Gulf) and western Mediterranean (Alboran Sea): Paleooceanographic results

Abstract

Deep-sea benthic foraminifera of the region of water exchange between the Mediterranean and the Atlantic Ocean, at the Gibraltar Strait, show significant water-mass relationships for the past 18,000 years. Cores used in this benthic foraminifera analysis were selected, according to the context of the distribution of various deep bottom water masses, on each side of the Gibraltar Strait: 1. (1) on the Atlantic side of the Strait, two cores (KS 8228, KS 8229) are located in the present day NADW, and one (KC 8221) in the upper part of the present Mediterranean outflow water, 2. (2) on the Mediterranean side of Gibraltar, two cores (KC 8241, SU 8107) are located in the deep water mass, and one (KS 8230) in the intermediate water mass. In the two deep basins (1500–2800 m depth in the Gulf of Cadiz, 1200–1300 m depth in the Alboran Sea), the paleooceanographic changes appear to be in an opposite way for the past 18,000 years. The Alboran basin shows a paleooceanographic evolution from a well-oxygenated, nutrient-rich environment at about 18,000 yr B.P. to a nutrient-poor, oxygen-depleted environment from 13,000 to the present time; moreover, for the time-span synchronous with the well-known development of sapropels in the eastern Mediterranean basins between 10,000 and 7000 yr B.P., the faunal assemblage shows most unusual characteristics implying drastic environmental conditions. Conversely, in the Gulf of Cadiz, the environment pass as from a biotope occupied by an oxygen-depleted, nutrient-poor water mass at about 18,000 years B.P. to a biotope occupied by NADW since the Younger Dryas; this agrees with previous data obtained in the northeastern Atlantic Ocean (Caralp, 1987). In the epibathyal zones (550–800 m depth), on both sides of the Gibraltar Strait, paleohydrographic changes do not seem so important. According to the present and very late Holocene assemblages, which are similar on both sides of the Strait with a strong east-west flow, two other stratigraphic episodes have shown the same conditions of water exchanges: the end of the isotopic stage 2 and the Younger Dryas. Conversely, during the last glacial maximum, the Bølling-Allerød and the lower Holocene, westward water fluxes were probably lower. At no time, the hypothesis of a reversal or a stop in the east-west exchanges between Mediterranean and Atlantic Ocean may be justified.