Response of benthic foraminifera to stagnant episodes — A quantitative study of Core BAN 81-23, Eastern Mediterranean

Abstract

The core investigated was raised from the floor of the Ares Crater in the Western Mediterranean Ridge “Cobblestone Area 3” at 35°52.03′N, 20°47.46′E, at a water depth of 3026 m. Forty one samples were taken from beneath, within and above seven sapropel layers identified as S-1, S-3, S-4, S-5, S-6, S-7 and S-8 and two tephra layers, ranging in age from approximately 5000 to approximately 250,000 years B.P. Our study shows that the cyclically repeated stagnant episodes resulted in a strong reduction of the faunal density. Due to the basinal setting, some displaced elements were recorded in resedimented beds. Anomalinoides minimus displays peak abundances in post-sapropel layers thus supporting the interpretation that it is an opportunistic species, suitable for a rapid repopulation of the bottom after a stagnant episode. This taxon, along with Eponides tumidulus, Articulina tubulosa and to a lesser extent Miliolinella subrotunda dominates the autochthonous deep-water fauna. Other taxa are recorded in high percentages only at definite stratigraphic levels: they are Epistominella vitrea (beneath S-6) and Cassidulina laevigata (beneath S-1). We consistently recorded a sparse faunule within the sapropels, consisting of shallow-water epiphytic rotaliforms (interpreted as allochthonous, transported by floating algae), associated with Bolivina spp. and Anomalinoides minimus. The occurrence of Bolivina spp. and of buliminids in general within the sapropels is accounted for by their capability of tolerating very low oxygen levels. The topmost part of the multilayered sapropel S-6 in particular is rich in Bulimina exilis, Cassidulinoides tenuis and Ellipsopolymorphina sp. This sapropel, which unlike the other ones, originated during glacial conditions, is preceded by peak abundances of faunal density: a strong increase in productivity of the bottom-living fauna is inferred, and supported by similar findings in a nearby plateau core, where drastic changes in density and diversity are recorded.