Late Quaternary paleo-oceanographic changes recorded by deep-sea benthos in the Western Mediterranean Ridge

Abstract

Paleoceanographic changes occurred in the Eastern Mediterranean in the last 400 000 y are reconstructed on the basis of a quantitative study of benthic foraminifers from the upper 9 m of a sediment core (KS 09) from the crestal area of the Mediterranean Ridge whose climatic record was already investigated through changes recorded in planktonic foraminiferal faunas, composition in stable isotopes, and clay mineralogy. Benthic foraminifers represent a minor constituent of the faunal assemblages, which are essentially planktonic. Species recorded are 92, with a maximum of 27 in a single sample. Twelve samples out of 69 do not contain any benthic form. Number of specimens recorded in a single sample ranges from 0 to 679. Peaks of abundance are always related to catastrophic paleoceanographic events. Peaks of abundance related to re-colonization of the bottom after a faunal annihilation were already known, but for the first time we discovered peaks of abundance in layers immediately predating the onset of stagnant conditions. They are limited to the lower part of the stratigraphic column older than approximately 200 000 y BP. A strong decrease in faunal density results from the frequently repeated (approximately 20 000 y cycles) stagnation of the later part of climatic zone V (here defined as «subzone V"») which correlates with isotopic stage 7. These stagnations were originated by density stratification resulting from excess meltwater from the North European and Siberian grounded ice-sheets. The composition of the fossil assemblages also record drastic changes. The best and most evenly represented species is the abyssal Articulina tubulosa. Species occasionally represented by 20 or more specimens are 15: they include Bulimina aculeata, B. exilis, Fursenkoina complanata Ellipsopolymorphina sp. and Cassidulinoides tenuis, which are present only in a limited number of samples characterized by quite unusual ecologic conditions. We recorded in abundance beneath the older sapropels (of zone V) Bulimina aculeata which is known to tolerate and prefer low oxygen levels, associated with large-sized Pyrgo like P. murrhyna, P. serrata, P. lucernula and P. depressa, whose preference for such habitats was previously unknown. The drastic downcore fluctuations in faunal density and the unexpected changes in the composition of the fossil assemblages display striking similarities with those recorded at exactly the same stratigraphic levels in Core Alb 189 from the Levantine Basin, some 1000 km to the East of Core KS 09. Spatial homogeneity versus temporal instability suggests large oceanographic changes, of more than local significance.