Preliminary paleoecological observations on benthic foraminiferal assemblages from an eastern equatorial Atlantic deep-sea core

Quantitative analyses of benthic foraminiferal assemblages were carried out on the late Pleistocene - Holocene succession of the core S13-33 (PRIMAR project, Leg 13, eastern equatorial Atlantic, Lat.: 0°51'S, Long.: 12°45'W, water depth: 3500 m). The 503 cm thick succession is constituted by a “Globigerina ooze”, which changes in colour at 356 cm from the top from dark grey in the lower part to light grey in the upper part. Planktonic foraminiferal assemblages occurring from the base to the top of the succession correlate with those of the five Ericson & Wollin (1968) climatic zones, going from V zone-upper part (late Pleisto¬cene) to Z zone (Holocene) included. Benthic foraminiferal assemblages are mainly represented from the base to the top by Epistominella exigua, Eponides pusillus and Globocassidulina subglobosa, and subordinately by Cibicidoides wuellerstorfi, Cibicidoides spp., Gyroidina spp., Melonis barleeanum, Oridorsalis umbonatus and Uvigerina auberiana. The predominance of E. exigua, E. pusillus and G. subglobosa, opportunistic species adapted to seasonal pulses of food supply (phytodetritus) in a food-limited environment, indicates that the bottom water masses were characterized by a relatively low organic matter content mainly constituted by phytodetritus, which originates in the euphotic zone and settles rapidly to the seafloor following the spring bloom. The abundance curve of C. wuellerstorfi shows a slightly increasing trend from the base to the top suggesting a correspondent slight intensification of the bottom water circulation. This hypothesis is also supported by the patterns of Cibicidoides spp., oxic species, and of Gyroidina spp. and M. barleeanum, suboxic species. U. auberiana trend suggests a clearly subordinate influence of upwelling on the composition of benthic foraminiferal assemblages with respect to that of spring bloom. The curve of the total number of benthic specimens per gram (BN) shows average values mainly ranging from 371 to 1088, and it is characterized by some significant positive peaks occurring in the lower part of the succession, at about 200 cm and at 59.5 cm. The benthic foraminiferal assemblages trends point out that during the late Pleistocene - Holocene in the investigated area the bottom paleoecological conditions were mostly influenced by the spring bloom and subordinately by upwelling phenomena, and were characterized by a generally low organic matter content mainly constituted by phytodetritus. Moreover the changes observed in the abundance curves of E. exigua, G. subglobosa and U. auberiana and in the BN pattern testify that such conditions slightly differentiate in the dark grey coloured lower part (core bottom to 356 cm), where a relative decrease of phytodetritus and a moderate increase of surface productivity occur, with respect to the light grey coloured middle and upper parts of the succession (356 cm to core top), where the surface productivity is even more subordinated. Nevertheless in this last interval two episodes of relative increase of surface productivity, due to discontinuous intensified upwelling phenomena, may be inferred by two concomitant peaks of U. auberiana and of BN.